Emerging Landscape of Osteogenesis Imperfecta Pathogenesis and Therapeutic Approaches

Abstract

Osteogenesis imperfecta (OI) is an uncommon genetic disorder characterized by shortness of stature, hearing loss, poor bone mass, recurrent fractures, and skeletal abnormalities. Pathogenic variations have been found in over 20 distinct genes that are involved in the pathophysiology of OI, contributing to the disorder’s clinical and genetic variability. Although medications, surgical procedures, and other interventions can partially alleviate certain symptoms, there is still no known cure for OI. In this Review, we provide a comprehensive overview of genetic pathogenesis, existing treatment modalities, and new developments in biotechnologies such as gene editing, stem cell reprogramming, functional differentiation, and transplantation for potential future OI therapy.

Osteogenesis imperfecta (OI) is a rare congenital connective tissue illness that mostly affects the skeletal system and has an incidence of 1/15 to 1/20,000 in infants.¹ It is characterized by features such as easy fractures, joint laxity, hypodontia, blue sclera, and hearing loss. Additionally, OI may impact various organs including the eyes, ears, heart, and skin.² Clinically, OI is characterized by generalized low bone mass, recurrent fractures following minor trauma, bone deformities, and various associated features.³

Collagen, a vital protein present in various tissues, plays a crucial role in imparting strength and structure to bones. OI can arise from genetic mutations in at least 20 distinct genes that are associated with the synthesis, processing, or regulation of collagen. The most prevalent genes linked to OI are COL1A1 and COL1A2, responsible for encoding the two chains of type I collagen.⁴ Mutations in COL1A1 and COL1A2 account for most OI cases. Nevertheless, with the advancement of our understanding of the genetic basis of OI, mutations in additional genes have also been identified (summarized in Figure 1).

Figure 1.

Genetic mutations and OI phenotypes of variable severity. (A) Wordart of gene mutations related to osteogenesis imperfecta. COL1A1 and COL1A2 mutations are the most common cause of OI. (B) The severity of OI varies mutations in the COL1A1 gene tend to be more severe than mutations in the COL1A2 gene. The mildest forms of OI generally result from heterozygous mutations that lead to the loss of mRNA from one COL1A1 allele. OI types I through IV are caused by mutations in the COL1A1 or COL1A2 genes. These genes carry instructions to produce type 1 collagen. Collagen is the major protein of bone and connective tissue. OI type VII is caused by recessive mutations in the CRTAP gene. Loss of function mutations in CRTAP, P3H1, and PPIB have been identified in patients with severe recessive OI.

The diagnosis of OI typically relies on clinical presentation and is confirmed through genetic testing, including prenatal genetic testing for at-risk pregnancies. While medications, surgical procedures, and other interventions can provide partial relief for specific symptoms, a cure for OI remains elusive. Addressing the underlying genetic defects and improving bone formation, emerging technologies such as gene editing and stem cell therapies show substantial promise for more targeted and potentially curative treatments. This Review offers an overview of the clinical and genetic aspects of OI, shedding light on current therapeutic approaches and potential future advancements.

OI Typology

The clinical manifestations of OI can vary in severity, ranging from mild to severe. Originally proposed by Sillence et al. in 1979, there are four classifications (I–IV) based on distinct characteristics.¹ However, the understanding of OI has evolved, and currently, there are at least 22 recognized types (type I to XXII), with new types being identified as more patients with OI are discovered.⁵⁻¹⁰ As outlined in Table 1, osteoporosis, blue sclera, frequent fractures, and a lack of structurally normal collagen characterize type I, which is the most common and moderate form of OI. Notably, it does not involve abnormalities of the bones. In contrast, types II–IV OI are characterized by structural anomalies in type I collagen.^11,12

Table 1. Classification of Osteogenesis Imperfecta.

Mutated gene	OI type	Severity	Localization	Clinical characteristics
Impairment of collagen synthesis and structure
COL1A1 or COL1A2	I	Mild	Matrix	Normal stature, blue sclera, hearing loss, late onset
	II	Lethal		Perinatal lethality
	III	Severe		Severe progressive malformations
	IV	Mild to moderate		Moderately severe, white sclera, short stature, bone deformity
Compromise bone mineralization
IFITM5	V	Normal to severe	Plasma	Normal-to-severe skeletal deformities, endosteal ossification, radial dense bands and radial head dislocation, normal-to-blue sclera, sometimes hearing loss
SERPINF1	VI	Moderate to severe	Matrix	Moderate-to-severe skeletal deformity, the presence of osteoid, fish-scale appearance of lamellar bone pattern, childhood onset
Abnormal collagen post-translation modification
CRTAP	VII	Lethal	ER	Severe rhizomania with white sclera
P3H1	VIII	Severe to lethal	ER	Severe rhizomania with white sclera
PPIB	IX	Moderate to lethal	ER	Severe bone deformity with gray sclera
Compromised collagen processing and cross-linking
SERPINH1	X	Severe	ER-Golgi	Severe skeletal deformity, blue sclera, dentinogenesis imperfecta, skin abnormalities, inguinal hernia
FKBP10	XI	Moderate to severe	ER	Mild-to-severe skeletal deformity, normal-to-gray sclera, congenital contractures
BMP1	XII	Moderate to severe	Matrix	Mild-to-severe skeletal deformity, umbilical hernia
PLOD2	No type	Moderate to severe	ER	Moderate-to-severe skeletal deformities, progressive joint contractures
Altered osteoblast differentiation and function
SP7	XIII	Mild to moderate	Nucleus	Severe skeletal deformity, delayed tooth eruption, facial hypoplasia
TMEM38B	XIV	Moderate to severe	ER-Golgi	Severe bone deformity, normal-to-blue sclera
WNT1	XV	Moderate to severe	Matrix	Severe skeletal abnormalities, white sclera, possible neurological defects
MBTPS2	XVI	Moderate to severe	ER-Golgi	Moderate-to-severe skeletal deformity, light blue sclera, scoliosis, pectoral deformities
CREB3L1	XVII	Severe	ER-Golgi	Severe bone deformities
SPARC	XVIII	Moderate to severe	Matrix	Progressive severe bone fragility
Other OI types
TENT5A	XIX	Moderate to severe	Unknown	Congenital limb arch, wormy bone, blue sclera, severe osteoporosis, mineralization disorder
MESD	XX	Mild to severe	ER	Progressive deformity, early onset of osteoporosis
CCDC134	XXI	Moderate to lethal	ER-Golgi	Severe bone fragility, impaired mineralization
KDELR2	XXII	Severe	ER-Golgi	Progressive deformity, low bone density, bone fragility, short stature

Type II OI, characterized by extreme severity, often leads to perinatal death. Major symptoms include frequent fractures, severe osteoporosis, bead-like alterations in the ribs, and poor bone mineralization.¹³ In type III OI, individuals experience severe progressive skeletal abnormalities and extremely low height, representing the most severe form of the disorder seen in those who survive the newborn period. With appropriate medical care, support, and advancements in treatment, many individuals with type III OI can now survive into adulthood. However, it is important to note that the likelihood of fractures increases as patients age, those who do face similar symptoms to type I OI.¹⁴ In type IV OI, patients also exhibit symptoms akin to type I, including normal sclera, but they experience more pronounced adult bone shortening, resulting in mild-to-moderate bone abnormalities and short stature.¹⁵

Type V OI is relatively uncommon and shares many clinical characteristics with type IV.^15,16 Type V exhibits distinctive characteristics, including the formation of hyperplastic callus. Following fractures or surgical interventions, individuals with type V experience an abnormal proliferation of callus tissue around the affected bones. This callus can be notably larger and bulkier compared to what is typically observed in other types of OI. The X-ray appearance of type V should be distinguished from chondrosarcoma. Individuals with type V OI are also more prone to developing calcification of the interosseous membrane in the forearm, leading to clinical symptoms such as secondary radial head dislocation and restricted forearm rotation.¹⁷

Type VI OI is characterized by abnormal bone mineralization and the presence of ichthyosis-like bone buildup in the bone tissue.¹⁸⁻²⁰ Patients with type VII OI are more likely to exhibit short humerus and femurs, brittle bones, and skeletal abnormalities, while those with type VIII OI face significant challenges in bone formation and mineralization.^21,22 Type IX may manifest bowed limbs, spinal curvature, and other progressive bone abnormalities. A distinctive characteristic of this subtype is the presence of white sclera, often observed in individuals with type IX.²³⁻²⁵ The exceptionally rare type X presents clinical symptoms such as hydrocephalus, widespread bone loss leading to diminished muscular strength, and thoracic scoliosis.²⁶

Type XI of OI is characterized by worm-like bones in the skull, brittle fractures, joint contractures, and symptoms of scoliosis.^27,28 Moderately severe type XII presents symptoms such as hearing loss, facial hypoplasia, a collapsed nasal bridge, and a large forehead.^29,30 In type XIII, in addition to repeated fractures and skeletal abnormalities, there is an increase in bone mineral density (BMD), elevated bone fragility, radial head dislocation, and radial and ulnar curvature.^31,32 Type XIV individuals are more prone to repeated fractures, poor bone density, minor bone deformities, growth retardation, and may experience hearing loss or dentin hypoplasia.^33,34 Types XV and XVI represent severe and syndromic mutations, with type XV potentially manifesting drooping eyelids, high palatal arches, craniosynostosis, or mental impairment. Type XVI is rare and often results in infancy mortality, akin to type II OI.³⁵⁻³⁷

Both type XVII and type XVIII OI patients share a history of repeated fractures and exhibit a clinical severity ranging from moderate to severe.^38,39 Type XVII is characterized by impaired lower-extremity muscular strength and delayed speech or motor development.^40,41 On the other hand, type XVIII OI presents congenital lower extremity curvature, vertebral collapse, and numerous fractures. Patients with type XX OI have mutations associated with the WNT1 signaling pathway, similar to those seen in type XV patients.⁴²

Due to their altered genes and the distinct presentations of patients, they are categorized differently. Type XX individuals commonly exhibit progressive abnormalities and early-onset osteoporosis.⁴³ Type XXI and type XXII OI are recently discovered recessive forms, sharing many characteristics, especially severe bone fragility. Type XXI is characterized by low bone mineral density, while type XXII has a slow rate of bone mineralization and increasing abnormalities. The categorization of different types of OI remains a topic of debate, even as the number of recognized OI typologies continues to grow. One challenge lies in developing a classification system that accommodates newly identified types without compromising existing standards.⁴⁴

OI Pathogenesis

Autosomal dominant and autosomal recessive inheritance are now recognized as the two primary types of OI inheritance. Mutations in type I collagen are linked to the dominant inheritance process. The genetic traits display variability, contributing to the complexity of this autosomal recessive genetic disorder (Table 2).

Table 2. Summary of Current Clinical Therapeutic Approaches for OI.

Treatment	Vendor	Administration	Outcome	Notes
Physical rehabilitation and therapy
Surgical treatment	NA	Stabilization and orthopedic treatment of fractures and deformities	Improve patient’s spinal curvature stability	Correction of deformity and prevention of fracture recurrence
Rehabilitation treatment	NA	Increased strength and movement	Improved muscle strength	Necessary therapy in OI
Anti-resorptive treatments
Bisphosphonates	Pamidronate (Procter & Gamble), Alendronate (Merck), Ristorante (Celgene), Zoledronic acid (Novartis)	Infusion or oral	Anti-resorptive, inhibition of osteoclast activity	Mainstay of therapy in OI and may cause pain, allergic reactions, gastrointestinal symptoms, esophagitis, mandibular necrosis, fractures, hypocalcemia, etc.
Denosumab	Prolia (Amgen)	Subcutaneous injection	Anti-resorptive, anti-RANKL antibody, inhibition of osteoclast activity	Jaw necrosis and hypocalcemia may occur
Anabolic treatments
Teriparatide	Forteo (Eli Lilly & Co.)	Subcutaneous injection	Anabolic, recombinant human parathyroid hormone	Therapy limited to 24 months, not approved in children
Sclerostin antibody	Romosozumab (Amgen Inc.), Blosozumab (Eli Lilly & Co.), BSP804 (Novartis)	Subcutaneous injection or infusion	Anabolic, anti-sclerostin (an inhibitor of bone formation)	Twelve months of continuous use, not approved in stroke patient

Impaired Synthesis and Structure of Collagen

Currently, 80–85% of OI cases (type I–IV) are attributed to autosomal dominant inheritance. This is typically a result of a missense mutation in the COL1A1 or COL1A2 gene, leading to a glycine (Gly) substitution. Gly is essential for the formation of the triple helix in type I collagen.⁴⁵ Most of this triple helix structure consists of two 1(I) chains and one 2(I) chains, surrounded by the N- and C-terminal globular pre-peptides, acting as precursors in collagen production. The central Gly-Xaa-Yaa triplet generates an uninterrupted sequence of small Gly side chains (Figure 2).

Figure 2.

Schematic diagram of high-risk gene mutations encoding proteins involved in collagen biosynthesis. Classical OI is primarily caused by mutations in the genes COL1A1 and COL1A2, which encode the two chains of type I collagen, a crucial protein in bone formation. The molecular mechanisms involved in OI related to type I procollagen include PTM in ER and intracellular trafficking from ER to Golgi.

The triple-helix collagen structure is conceptually straightforward, despite the intricate synthesis of type I procollagen, involving crucial steps such as folding, transport, secretion, and integrated post-translational modifications (PTMs). During this process, the procollagen chain converges at its C-pre-peptide and folds toward the N-terminus. Proline (Pro) and lysine residues within the helical sections of both chains undergo hydroxylation by prolyl 4-hydroxylase 1 (LH1). These hydroxylysine residues can subsequently undergo glycosylation.⁴⁴

In the endoplasmic reticulum (ER), post-translationally modified prolyl 3-hydroxylase 1 (P3H1), cartilage-associated protein (CRTAP), and cyclophilin B (CyPB) form heterotrimeric complexes. Mutations in these proteins can lead to various forms of OI (refer to Table 1). Mutations in the N-terminal residues of the triple helix are often associated with a non-fatal phenotype, while mutations in the C-terminal helix region can result in lethal or intermediate outcomes. A common misfolded structure leading to OI in type I collagen involves the substitution of glycine in the helical structural domain.⁴⁶

Point mutations in Gly disrupt the correct folding of the type I collagen peptide chain into a triple helix, consequently changing the structure of the collagen. This disruption leads to a skeletal phenotype that causes moderate-to-severe OI. The severity of OI symptoms is influenced by the specific amino acid changes and the location of the Gly mutation. Notably, mutations in COL1A1 are often more deleterious than those in COL1A2. Given that alpha1 collagen chains constitute two-thirds of each collagen fiber complex, there exists a distinct genotype-phenotype relationship for each alpha chain. This underscores the intricate interplay between the genetic mutations and the resulting clinical manifestations in OI.⁴⁷⁻⁴⁹

Substituting charged amino acids or branching side chains at the two crucial ligand-binding sites near the carboxyl terminus of the 1(I) chains destabilizes the helix and disrupts the architecture of the extracellular matrix. This substitution has significant consequences for the interplay between collagen monomers and non-collagen matrix proteins. While proteoglycan-binding sites are sensitive to replacement, it is often not as detrimental to the 2(I) chain.⁵⁰ Unfortunately, predicting the phenotypic repercussions of a specific allele mutation is currently challenging and lacks a high degree of accuracy. The intricate nature of these interactions and the variety of potential mutations contribute to the difficulty in precisely anticipating the outcomes of specific genetic alterations.

Defects in Ossification and Mineralization

The advancements in human genome sequencing and high-throughput DNA sequencing technologies have led to the identification of additional pathogenic genes associated with OI. In OI types V and VI, a significant deficiency occurs in endochondral bone ossification or mineralization. Type V is specifically linked to mutations in the interferon-induced transmembrane protein 5 (IFITM5) gene, resulting in abnormal matrix mineralization. The precise mechanism by which IFITM5 regulates collagen mineralization remains unknown.⁵¹ Type V OI often arises from an autosomal dominant mutation in the 5′-UTR of IFITM5, leading to the addition of 5 amino acids to the N-terminal end and providing functional gain. The elucidation of these genetic factors enhances our understanding of the diverse molecular mechanisms underlying OI pathogenesis.

Interactions between IFITM5 and FKBP prolyl isomerase 11 (FKBP11) are known to take place on the extracellular surface of the plasma membrane or intracellularly.⁵² Palmitoylation sites enable IFITM5 to bind to the cell membrane in a normal osteoblast. However, the expression of interferon-induced genes is controlled by CD9’s interaction with the CD81-FKBP11 complex,⁵³ and this interaction is hindered by the presence of the IFITM5 mutations associated with type V OI, particularly those at cysteines S50 and S51, allowing IFITM5 to bind to FKBP11. While it has not been established that the skeletal development of IFITM5 knockout mice is abnormal, it is noteworthy that mice with the same IFITM5 mutation are embryonic lethal. This suggests that, during the bone remodeling process, IFITM5 mutations may impede the formation of new bone, emphasizing the intricate role of these molecular interactions in skeletal development.⁵⁴

Type VI OI is consistently linked to recessive mutations in the Serpin family F member 1 (SERPINF1) gene. The SERPINF1 gene plays a crucial role in the development and remodeling of bones, encoding proteins essential for the folding, secretion, and PTMs of collagen. The impact of SERPINF1 is particularly notable in its influence on the expression of pigment epithelium-derived factor (PEDF). PEDF, a fatty acid transport protein, binds to collagen in the matrix, exerting an anti-angiogenic effect. Studies indicate that PEDF binds to type I collagen, and modifications to the amino acid residues on the attached collagen can inhibit PEDF’s anti-angiogenic activities.^55,56 One of the mutations associated with type VI OI involves the IFITM5 gene. When this mutation occurs at the palmitoylation sites, specifically at S40, S50, and S51, it leads to poor palmitoylation of IFITM5 at S50 and S51 and its sequestration in the Golgi apparatus. In vitro culture systems have shown that osteoblasts carrying the S40L mutation experience impaired mineralization, reduced levels of PEDF secretion, and decreased SERPINF1 expression. Interestingly, this mutation also diminishes collagen expression during osteoblast formation.

The patient exhibits a genetic mutation that is atypical for type V but leads to a type VI phenotype. Despite the complementary effects of IFITM5 and PEDF on osteoblast mineralization and development, these proteins do not directly interact and are not involved in the synthesis, folding, cross-linking, processing, or synthesis of collagen. Consequently, the precise mechanism by which IFITM5 and PEDF induce these modifications remains unclear. The unique genetic mutation in this case highlights the complexity of the molecular interactions contributing to the observed phenotype and underscores the need for further research to unravel the intricate pathways involved in OI.⁴⁴

Abnormal PTMs of Collagen

PTMs play a vital role in the formation and function of collagen, including type I collagen, which is frequently implicated in OI. An instance of an aberrant PTM linked to OI is the hydroxylation of Pro and Lys residues. The hydroxylation of these amino acids is crucial for the correct formation of collagen’s triple-helix structure. In the case of type VII, type VIII, and type IX OI, recessive mutations are found in three distinct components encoding the collagen prolyl 3-hydroxylation complex—P3H1, CRTAP, and CyPB (encoded by PPIB).^22,57−60 These three proteins form a heterotrimeric complex in the ER, facilitating the folding of the triple helix and the PTM of the pre-collagen chain. The primary target of this complex is the modification of the Pro986 residue in collagen. Deletion of any one of these three genes results in the absence of complex activity. Double allelic mutations in CRTAP lead to type VII OI, which behaves differently from types II/III. The loss of CRTAP leads to excessive modification of the type I collagen helical region by LH1 and P4H1, delaying triple helix folding and preventing hydroxylation of proline 986 in the 1(I) chains. This mutation typically results in catastrophic bone dysplasia, and in surviving patients, severe growth abnormalities and popcorn-like calcified epiphyses are observed—the same signs of bone dysplasia seen in mice.^61,62 It is noteworthy that while CRTAP and P3H1 share the same N-terminal structural domain, the catalytic hydroxylation structural domain is absent from CRTAP. This highlights the role of these proteins in the collagen synthesis process and underscores the severe consequences of mutations in these components.

Carriers of P3H1-NULL mutations display symptoms similar to those resulting from CRTAP deletion. Individuals with P3H1 mutations have been shown to exhibit significantly higher collagen expression in the 1(I) chains. This increased collagen expression might be a potential cause of type VIII OI, particularly with dental abnormalities, as P3H1 is the only protein splice variant that contains the “KDEL” ER retention signal.⁶³

CyPB, a 21 kDa protein, was initially identified as a cell-binding protein for the immunosuppressive drug cyclosporin A (CsA). The diverse roles of these proteins in the intricate process of collagen synthesis highlight the complexity of OI pathogenesis and the importance of understanding the specific functions of each component within the collagen prolyl 3-hydroxylation complex. The conserved PPIB gene encodes a family of intracellular and/or secreted proteins and belongs to the procyclophilin gene family. CyPB operates in conjunction with P3H1 and CRTAP within the collagen prolyl 3-hydroxylation complex. Deficits in CyPB can result in reduced 3-prolyl hydroxylation and post-translational over-modification. Recent studies have shown that mutations in PPIB lead to type IX OI, this outcome is attributed to the disruption of the function of the P3H1/CRTAP/CyPB complex.^25,64 Understanding the intricate interactions within this complex is crucial for unraveling the molecular mechanisms underlying type IX OI and advancing potential therapeutic interventions.

Disruption of Collagen Processing and Cross-Linking

The protein products of SERPINH1 and FKBP10, known as HSP47 and FKBP65, respectively, function as molecular chaperones residing in the ER. These chaperones play a crucial role in preventing the premature development of ER protofibrils, occurring prior to collagen triple helix interactions.⁶⁵ Additionally, they assist in guiding the appropriately folded collagen into the Golgi apparatus, contributing to the stabilization of collagen folding. This process facilitates the correct assembly of the collagen triple helix.^27,66,67 In SERPINH1, only four distinct missense variations have been identified thus far. These mutations are in the structural serine-type endopeptidase inhibitor domain of filamentous protein H1, which is essential for the chaperone function ensuring proper transport of type I procollagen. Mutations such as L50R and R405H impair the physiological activity and function of the serine structural domain, leading to severe impairment of HSP47 function. Consequently, these mutations result in misfolded pre-collagen molecules, contributing to the development of OI.⁶⁶ Furthermore, HSP47, FKBP65, and immunoglobulin heavy chain binding protein (BiP) can form a complex in the ER. BiP plays a role in controlling the formation of lysine hydroxylase 2 (LH2), and mutations in SERPINH1 result in abnormal type I collagen.⁶⁸ In addition, FKBP65 in mutant cells is also affected by abnormal activity and low HSP47 protein levels, further highlighting the interplay among these molecular components in collagen synthesis and proper folding.⁶⁵

Mutations in FKBP65 can lead to a spectrum of overlapping symptoms, including Bruck syndrome, Kuskokwim syndrome, and varying degrees of severity in OI. Cells lacking FKBP65 exhibit significant lysine residue hydroxylation below the peptide’s C-terminal hydroxylation, a process typically catalyzed by PLOD2.⁶⁵ However, it remains unknown how FKBP65 reduces PLOD2 activity. This process may be associated with the loss of function of the chaperone protein FKBP65 due to mutation, hindering the correct folding of PLOD2 and thereby reducing PLOD2 activity.⁶⁹

Furthermore, mutations in BMP1, a BMP1-like protease discovered in bone extracts, can impact collagen synthesis and processing, dental matrix protein processing, and bone reconstruction. Mutations affecting the C peptide cleavage sites of BMP1 influence collagen fiber intra- and intermolecular cross-linking and catalytic lyase activity, resulting in a unique phenotype in patients with BMP1 mutant OI. BMP1’s processing of type I and type III collagen c-pre-peptides significantly affect extracellular matrix production and structure.⁷⁰

Interestingly, BMP1-associated OI presents a distinctive scenario with normal or significantly elevated bone mineral density (BMD), complicating the relationship between BMD and fracture risk, which contrasts with most cases of OI. The presence of pC collagen, a mature collagen with linked c-pre-peptide, affects the protofibrillar structure and leads to aberrant bone hypermineralization and paradoxical hyperosteogenic OI.⁷¹ Understanding these diverse genetic and molecular mechanisms contributing to OI emphasizes the complexity of the disorder and the need for detailed investigations into each component’s function.

Altered Osteoblast Differentiation

Recent studies have established a connection between OI and genes involved in osteoblast formation.^44,72 Loss-of-function mutations in the SP7 gene cause OI type XII. The SP7 gene encodes the zinc finger transcription factor osterix protein, primarily generated by osteoblasts.⁷³ Osterix fosters the maturation of pre-osteoblasts into osteoblasts with functional properties. Mice deficient in SP7 exhibit insufficient osteoblast growth and proliferation, along with reduced expression of osteoblast markers, highlighting the essential role of SP7 in bone formation.⁷⁴

Defects in the TMEM38B-encoded TRICB-type trimeric intracellular cation channel B (TRICB, also known as TM38B) have been identified in patients with moderate OI.^67,75 Recent studies indicate that TRIC-B cation-specific channels in the ER membrane, encoded by TMEM38B, are implicated in the OI phenotype.⁷⁶ Knockout mice lacking TMEM38B showed decreased mortality and bone volume during the embryonic stage.⁷⁷ TRIC-B channels are proposed to play a crucial role in regulating Ca²⁺ homeostasis and intracellular storage in the ER, contributing to the understanding of the molecular mechanisms underlying OI.

The TMEM38B mutation not only affects interactions with calcium reticulum proteins but also influences the interactions of Procyclin B and protein disulfide bond isomerase with calcium reticulum proteins. This mutation also alters the PTMs of collagen. The reduced calcium release resulting from the mutation inhibits mesenchymal stem cell (MSC) differentiation into osteoblasts and deactivates Ca²⁺-dependent kinases. Atypical calcium levels can prompt rapid rearrangement of the ER membranes, activating folding proteins and leading to ER stress and cytotoxicity, ultimately affecting bone matrix secretion. The pathophysiology of TMEM38B mutant OI may be linked to modifications in ER metabolic pathways, either intracellularly or extracellularly.

The reduced calcium flow kinetics across the ER in OI fibroblasts and osteoblasts lacking TRICB emphasize the crucial role of TRICB in maintaining proper calcium dynamics within the ER. The absence of TRICB contributes to the altered cellular processes observed in TMEM38B mutant OI. Understanding these molecular interactions not only sheds light on the intricate mechanisms underlying OI but also provides potential avenues for therapeutic intervention. Identifying and targeting key components involved in calcium regulation within the ER could hold promise for developing strategies to address the pathophysiology of OI associated with TMEM38B mutations.^78,79

WNT1 plays a crucial role in regulating the WNT1-induced catenin signaling pathway, which is essential for osteoblast development and maintaining intraosseous homeostasis (Table 1). In the conventional WNT/catenin pathway, WNT ligands activate Frizzled (Fzd)/LRP5 or low-density lipoprotein receptor-related protein 6 (LRP6), leading to reduced degradation of catenin and downstream cascades.⁸⁰ Mutations in the WNT1 gene result in reduced bone mass, increased susceptibility to fractures, and potential neurological issues in OI patients.⁸¹⁻⁸³ Early-onset osteoporosis is associated with heterozygous mutations, while severe OI is linked to severe homozygous mutations in WNT1.⁸⁴

Mutations in MESD, a gene encoding the ER chaperone of WNT receptors LRP5 and LRP6,^85,86 have been connected to recessive OI (type XX). MESD knockout mice exhibit inhibition of LRP5 and LRP6 trafficking and embryonic death.⁸⁷MESD mutations in OI fibroblasts impact ER retention, suggesting a critical role in the initial phases of bone development.⁸⁵

The regulated intramembrane proteolysis (RIP) signaling pathway is integral to the stress response, growth, and differentiation processes in the ER One of the transcription factors activated by RIP is OASIS (Old Astrocyte Specifically Induced Substance), a member of the cyclic adenylate response element binding protein/activating transcription factor (CREB/ATF) gene family, encoded by cAMP response element binding protein 3-like 1 (CREB3L1).⁸⁸ OASIS possesses a putative hydrophobic transmembrane structural region at its C-terminus, a characteristic feature shared by transcription factors triggered by regulated intramembrane protein hydrolysis.^89,90 After translocation to the Golgi membrane via the RIP pathway, the N-terminal cytoplasmic structural domain of OASIS is released and transferred to the nucleus to induce specific gene expressions. Mutations in CREB3L1 have been associated with severe osteomalacia, spontaneous fractures, and impaired type I collagen production in bone,⁹¹ indicating its importance in bone health. Aside from OASIS/CREB3L1, other substrates of RIP signaling linked to OI include cholesterol regulatory element-binding proteins (SREBP).⁹² SREBPs, which regulate cholesterol metabolism, undergo processing by class 2 RIPs in a two-stage cleavage process mediated by site-1 and site-2 proteases (S1P and S2P).

Cysteine-rich acidic secretory protein (SPARC) is involved in the folding and synthesis of type XVII collagen. Mutations affecting SPARC residues, Arg166 and Glu263, disrupt the affinity of SPARC for type I collagen.⁹³ Despite its role as a secreted protein that binds to collagen in the extracellular space, SPARC may also function as an intracellular chaperone for type I collagen. Mutations in the SPARC protein led to excessive alterations in the collagen alpha chain, like types I–IV OI, causing a delay in the formation of the collagen type I triple helix. Mice lacking SPARC exhibit progressive osteoporosis and intervertebral disc illness.⁹⁴

MBTPS2, also known as S2P, is a multi-domain, highly hydrophobic metalloprotease that plays a crucial role in OI type X (OI X). It possesses a zinc-binding motif called HEXXH,⁹⁵ which is essential for its function. In vivo, S2P controls protein translocation from the ER to the Golgi membrane, especially when unfolded proteins are retained in the ER.⁹⁶ This process involves cleavage by S1P and S2P.⁹⁷

Mutations in MBTPS2 result in various skeletal dysplasia traits and are associated with OI X.⁴⁴ Patients with S2P mutations often present with symptoms such as low stature, long bone and rib fractures, and short height.⁹⁵ Interestingly, these symptoms differ from the typical indications of OI, which include skeletal fragility. Skin disorders such as pruritus, photophobia, and follicular ichthyosis are commonly associated with OI X. Individuals with S2P mutations may exhibit aberrant collagen cross-linking, weaker bones, and reduced hydroxylation of Lys87 in the collagen chain,³⁵ contributing to the unique clinical features observed in OI X patients.

Other Mechanisms

Recent studies suggest that TENT5A, a new type of active non-canonical polymerase, may play a role in OI. TENT5A is known to regulate the maturation of myofibers, maintain myoblast stability, and influence the proliferation and migration of myogenic cells. Its impact on OI remains uncertain, but there are indications that TENT5A might have adverse effects on OI by inhibiting the maturation of type I collagen fibers in the body.⁹⁸

Mutations in the double allele KDELR2 have been identified in six individuals with numerous fractures, short stature, long bone curvatures, chest deformities, and a history of fractures starting in childhood.⁹⁹KDELR2 encodes a member of the KDEL receptor family, localized to the ER, Golgi complex, and intermediate ER-Golgi compartment.¹⁰⁰ The KDEL motif of the KDEL receptor facilitates the recovery of ER-resident proteins from the acidic Golgi environment to the neutral pH in the ER.¹⁰¹ In KDELR2 mutant patient fibroblasts, HSP47 was mislocalized, leading to decreased HSP47 and FKBP65 levels,⁹⁹ reduced collagen assembly, and mass. The decline in FKBP65 in KDELR2-deficient cells might be related to a decrease in HSP47. Although HSP47 is a key component in the pathogenic pathway of OI, KDELR2 mutations do not seem to prevent reverse Golgi to ER transport from occurring.^65,66

Recent discoveries have identified three cases of OI type XXII in Morocco with a homozygous mutation in CCDC134.¹⁰²CCDC134, also known as MAPK/ERK skeletal dysplasia, plays a role in the phosphorylation of mitogen-activated protein kinase (MAPK) by extracellular signal-regulated kinase (ERK) or c-Jun N-terminal kinase (JNK). Despite lacking dentinogenesis imperfecta, patients with CCDC134 mutations exhibit short stature and multiple fractures.¹⁰³CCDC134 is implicated in reducing osteoblast differentiation during bone formation and promoting ERK1/2 phosphorylation through the MAPK/ERK signaling pathway. Patient fibroblasts and osteoblasts with CCDC134 mutations show reduced type I collagen, decreased COL1A1 and OPN expression, and increased Erk1/2 phosphorylation. Mouse studies further support the critical role of CCDC134 in embryonic development.¹⁰⁴

OI Diagnosis

Key indicators for diagnosing OI include recurrent fractures, fractures at unusual sites, and low-stress fractures, sometimes even occurring spontaneously or in utero. Beyond skeletal manifestations, OI is a systemic disorder with a range of extraosseous features. These include short stature, blue-gray sclera (the white part of the eye), relative macrocephaly (larger head size in proportion to the body), dental anomalies, distinctive facial or chest structures, hearing loss, muscle weakness, and occasional cardiovascular or respiratory complications. Chronic bone pain is also common, reflecting the skeletal structural and functional challenges associated with OI. The severity and spectrum of manifestations can vary, necessitating accurate diagnosis for tailored medical management and genetic counseling.

Short Stature

Severe short stature is a characteristic feature of different forms of OI, particularly types III and IV, signifying moderate-to-severe OI. Earlier research has highlighted that individuals with OI often exhibit lower birthweights compared to the general population.^105,106 The influence of diminished stature on daily life is substantial, and the prevalence of fractures tends to rise with age in individuals with common type I OI, while decreasing in the pediatric population.¹⁰⁷

Short stature is a prominent characteristic, especially in severe cases of OI. Some milder forms, like type I OI, may be identified prenatally, while others manifest only after birth or in early childhood. Genetic testing is utilized to confirm clinical screening for OI, which is sometimes part of the investigation for unexplained fractures in infants and children. In a mouse model of OI, Scheiber et al. discovered that dysfunctional hypertrophic chondrocytes led to dwarfism.¹⁰⁸ The mechanism behind this involves excessively large ERs in these cells, including osteoblasts, which could stress the ER and hinder chondrocyte development.

Craniofacial and Dental Issues

All individuals with OI studied so far have exhibited abnormal craniofacial structural development. Recent cross-sectional research delved into the natural history of craniofacial abnormalities in OI. The findings suggest challenges in determining which types of OI lead to airway obstruction, with individuals with moderate-to-severe OI frequently presenting craniofacial, airway, and face malformations.¹⁰⁹ Recognizing this risk factor is crucial for effective OI treatment.¹¹⁰ Conversely, dental abnormalities are more common in severe OI patients, often involving dentinogenesis imperfecta (brittle or discolored teeth), missing teeth, ectopic teeth, and dental misbites.¹¹¹ These anomalies impact quality of life and are associated with functional restrictions. Animal research, utilizing microcomputed tomography (CT) and morphometry, compared the craniofacial phenotypes of Col1a1(Jrt)/+ mice and wild-type mice.¹¹² The results revealed decreased dentin matrix and mineral density, along with larger periodontal ligament gaps in Col1a1(Jrt)/+ mice.^110,112 These craniofacial and dental phenotypes underscore the importance of early oral health screening and dental treatment to enhance the quality of life for OI patients, though further research in additional OI mouse models is necessary.

Hearing Loss

Hearing loss is a recognized extra-skeletal symptom of OI, most commonly observed in individuals with OI types I through IV. The assumed mechanism involves altered genes related to COL1A1/COL1A2, resulting in abnormal inner ear mineralization and otosclerosis.⁸⁸ Hearing loss in people with OI is categorized into three types: conductive, sensorineural, and mixed. Conductive hearing loss is more prevalent in children and adolescents, while adults are more likely to experience mixed or sensorineural hearing loss.^113−115 Research suggests that hearing loss symptoms are less common in young OI patients, with the likelihood of hearing loss increasing steadily with age.¹¹⁵ The degree of hearing loss is not correlated with the type of OI or mutation genes. OI individuals are more prone to hearing loss in conjunction with skull fractures, often associated with conductive hearing loss due to OI footplate fixation or stirrup foot fractures.¹¹⁶ The exact cause of sensorineural hearing loss in OI is unknown but may be linked to cochlear hair cell atrophy, vascular striae, and cochlear capsule microfractures.¹¹⁷

The involvement of the round window in both conductive hearing loss and sensorineural hearing loss with age is noted.¹¹⁵ Alkaline phosphatase, recently linked to hearing loss in the general population, is elevated in the OI population with hearing loss, making it important for diagnosis and progression evaluation of hearing impairment.¹¹⁸ Regardless of the type of hearing loss, it often occurs bilaterally and symmetrically. Early-onset hearing loss with moderate-to-severe OI has been reported in some cases, but the majority of patients experience milder hearing impairment with adult onset, showing no significant association with OI type or severity.¹¹³

Muscle Weakness

Patients with various types of OI typically manifest muscular weakness, reduced muscle mass, and muscle hypofunction. This is attributed to the presence of type I collagen in tendons, ligaments, and the connective tissue surrounding muscle fibers. Studies indicate that muscular strength deficiencies are more pronounced in individuals with moderate and severe OI, and there is a correlation between muscular function and OI severity.^119,120 However, it is noteworthy that restricted activity and persistent under-exercise may also contribute to muscle abnormalities through relative muscle atrophy.

Interestingly, a study by Pouliot-Laforte et al. revealed that children with type I OI did not meet daily physical activity recommendations similar to their healthy peers, yet differences in muscle function persisted between the two groups.¹¹⁹ This suggests that decreased physical activity may not be the primary cause of muscle weakness in children with OI. Additionally, alterations in signal transduction and mechanical transduction between muscle and bone may potentially be linked to muscle disorders. However, much remains to be understood about these systems, and the exact cause of OI-related muscle weakness is still unclear.

In a mouse model of OI (syngeneic G610C OI mice model + /G610C), Jeong et al. evaluated activity levels and hindlimb muscle function.¹²¹ Compared to wild-type (WT) mice, the model mice did not show muscle lesions or altered activity levels. They were also able to withstand and complete an 8-week treadmill training program without fracturing their long hindlimb bones. Recent research has shown that oim/oim mice, another OI model, exhibit altered energy metabolism, supported by increased energy consumption and mitochondrial dysfunction in skeletal muscle.¹²² Dysfunction in the gastrocnemius muscle mitochondria of oim/oim mice may lead to muscle dysfunction and reduced physical activity.¹²³ The hypothesis that energy metabolism is disrupted in OI aligns with the prevalence of aberrant muscle function and skeletal muscle weakening in OI patients, considering skeletal muscle accounts for around 20% of the basal metabolic rate.^123−125 Further research is needed to confirm if mitochondrial dysfunction is a common occurrence in OI models.

Pulmonary Function

Another extra skeletal symptom of OI that significantly impacts morbidity and mortality is pulmonary problems. Despite limited comprehensive studies on pulmonary function in OI, respiratory illness mortality is reported to be three times higher than average in individuals with OI.^126,127

Pulmonary dysfunction in OI can manifest as early as infancy, as evidenced by Yimgang et al.’s study on 77 OI neonates, which noted 17 occurrences of respiratory problems during the neonatal period.¹²⁸ The traditional understanding is that skeletal anomalies such as rib and spinal fractures, kyphosis, and short stature lead to diaphragmatic constriction, impacting breathing and altering lung function.⁷ These physical anomalies result in pulmonary compression, inefficient coughing, poor airway clearance (increasing infection risk), and reduced blood oxygen levels. Ultimately, these skeletal abnormalities limit alveolar ventilation, and the deficiency of type I collagen, which provides elastic strength crucial for respiratory mechanics, further damages lung structure and function.¹²⁹

The progressive reduction in lung function in OI is associated with a higher risk of lung infection, potentially leading to respiratory failure and mortality.^7,49 Collagen and elastin, essential for supporting respiratory mechanics, play a crucial role in lung function, and their abnormalities in OI impact lung structure and function. It has been suggested that physicians assess lung function in OI patients during initial examinations to determine the extent of lung involvement and provide innovative treatments for lung care.¹³⁰

Some mouse models of OI have shown initial pathological changes in the lungs.¹³¹ For instance, the Aga2 mouse model exhibited altered gene expression in both cardiac and pulmonary primary fibroblasts, potentially leading to severe cardiopulmonary issues and neonatal mortality. In another study, Col1a1 Jrt/+ mice displayed emphysema and changes to the diaphragm.¹³² A recent study using the Crtap KO mouse model of recessive OI revealed abnormalities in type I collagen in the lung, suggesting primary lung abnormalities that impair the respiratory system in addition to osteoporosis.¹³³

Cardiovascular Abnormalities

OI not only increases bone brittleness but is also associated with cardiovascular issues, although these heart-related problems may not be as well-known or common as bone-related symptoms.^134,135

Comparatively, OI patients may exhibit certain cardiovascular abnormalities.^136,137 Enlarged left atrium and ventricle, dilation of the major pulmonary artery, and lower left ventricular ejection percentage have been observed in OI patients. These findings suggest a potential association between OI and anatomical and functional alterations in the cardiovascular system. Rare instances of abrupt mortality from left ventricular rupture in asymptomatic OI patients highlight the possibility that reduced collagen type 1 in OI may lead to decreased tensile strength in the heart muscle and potential cardiac problems.¹³⁸ Alternatively, decreased levels of collagen type 1 in OI may be involved in the expansion of atria and ventricles, potentially leading to atrial arrhythmias and heart failure.¹³⁹ The likelihood of experiencing these arrhythmias has not been fully assessed. A study by Bonilla Jiménez et al. found no statistically significant difference in the incidence of cardiac anomalies between healthy individuals and those with OI, although OI patients had larger aortic roots.¹³⁵ According to a systematic literature review by Ashournia et al., individuals with OI seem to be more likely than healthy individuals to develop heart disease.¹³⁹ Their review, spanning over 45 years of medical literature, included case reports, case series, and cross-sectional studies involving 499 people. Valvular disease and enlarged aortic diameter were the most diagnosed cardiovascular issues in patients with OI. Emerging evidence, especially in children with OI,^138,140 suggests a possible link between OI and cardiovascular problems.

Research has indicated that children with OI have a greater frequency of cardiovascular problems than children in the general population.^141,142 These anomalies may consist of functional deficits, structural flaws, and aberrant valve behavior.¹³⁸ Studies have also explored the relationship between OI genotypes and cardiovascular problems in children, considering that different gene mutations can cause different types of OI. Understanding how genotype and cardiovascular phenotype are related may aid in managing and advancing the understanding of the illness.¹³⁸ However, the observation that valve anomalies are less frequently seen in adults with OI than in children raises the possibility that patients with OI may develop cardiovascular illness as they age, necessitating evaluation of cardiovascular structure and function during long-term follow-up. Further evidence is required to establish the connection between the severity of OI and cardiac anomalies.

Ocular Anomalies

The eye and visual system are frequently affected in OI patients, with skeletal, auditory, dental, and other malformations extensively described in published papers.^143−145 Retinal detachment, optic neuropathy, glaucoma, corneal abnormalities, and other issues with the eyes and visual pathways have been observed in people with OI. Ocular tissues such as the cornea and sclera also contain type I collagen fibers, impacting almost every part of the eye due to OI.

Specifically, decreased corneal and scleral thickness may result in ocular complications, including blue sclera, which is correlated with the degree of altered type I collagen production in OI.^146,147 Patients with OI may experience serious ocular complications due to the weakening of collagen fibers in the cornea and sclera, leading to an elevated risk of eye injuries. A thorough literature analysis by Treurniet et al. revealed a higher likelihood of corneal and scleral rupture following mild trauma, with these areas being the most seriously impacted in individuals with OI.¹⁴⁸

A nationwide register-based cohort study conducted in Denmark examined the risk of ocular disorders in people with OI. The study found that individuals with OI are more likely to develop cataracts, glaucoma, vitreous hemorrhages, retinal detachments/ruptures, retinal diseases, and optic nerve abnormalities.¹⁴⁹ Reduced central corneal thickness (CCT) is another ocular trait observed in OI patients. Research has investigated the relationship between blue sclera and CCT, suggesting that type I collagen deficiencies, responsible for maintaining the structural integrity of the cornea, contribute to the lower CCT in OI patients.¹⁵⁰

In a cross-sectional case-control study, the clinical significance of blue sclera and the impact of OI on CCT were evaluated. OI patients showed significantly lower CCT values than the control group, and eyes with blue sclera in the OI group had even lower CCT values than eyes without blue sclera.¹⁵¹ Ocular symptoms in individuals with OI can significantly affect visual function and overall quality of life,^148,152 emphasizing the importance of protective eyewear and routine ophthalmologist monitoring to prevent unintentional eye injuries and address ocular issues. In summary, ocular features and problems in individuals with OI can impact many parts of the eye, underscoring the need for comprehensive eye care in OI management.

Genetic Diagnosis

In clinical practice, the severity of OI is often classified into four categories: mild, moderate deformation, severe deformation, and lethal. This categorization is depicted in Figure 1.^48,153,154 The use of high-risk genetic categorization is crucial, especially since most OI cases result from pathogenic mutations in the COL1A1 or COL1A2 genes. Advanced genetic diagnostic techniques, such as second-generation sequencing, are now commonly employed for assessing large samples from clinical patients. This is particularly important in cases where there is a family history or prenatal ultrasound abnormalities, as shown in the diagnostic process outlined in Figure 3. High-throughput gene sequencing can be utilized to identify pathogenic genes and enhance the accuracy of a positive diagnosis in high-risk individuals.^155,156 Current approach to OI gene diagnosis involves simultaneous targeted next-generation sequencing (NGS) of all 23 OI-associated genes. This comprehensive sequencing approach aims to identify pathogenic variants associated with OI, providing a more thorough understanding of the genetic basis of the condition.^157,158

Figure 3.

Flowchart of OI diagnosis and potential treatment strategy. Clinical evaluation and specialized testing are used to diagnose OI. A review of the patient’s family history, a physical exam to look for clinical indicators (such as blue sclera), and laboratory tests, including genetic testing to confirm the presence of OI-associated gene mutations should all be carried out. Bone density, fractures, and deformities are some frequent conditions that radiological examinations such as X-rays are used to diagnose. The OI treatment plan is customized for each patient based on the kind and severity of their disease. A multidisciplinary approach is frequently used, including advice from experts such as orthopedic surgeons, genetic counselors, physical therapists, and others.

Current and Future Therapy in OI

The primary goal of OI treatment is to improve the patient’s quality of life, and the current approach is centered around symptomatic and maintenance therapy. Treatment for OI is typically personalized, considering factors such as the patient’s age, the severity of the condition, and individual needs. Unfortunately, there is currently no cure for OI.

Therapeutic approaches used in OI treatment often borrow from strategies employed in treating osteoporosis or osteolytic bone metastases. These methods primarily involve bone anti-resorption or anabolic actions to increase bone mass and enhance bone structure. It is crucial to recognize that these techniques do not directly address the underlying genetic defect in OI patients. However, increasing bone mass significantly reduces the likelihood of fractures, diminishes pain intensity, and improves overall bone quality. Recent reports suggest that gene therapy holds substantial promise as a potential future treatment for OI. Gene therapy aims to target and correct the genetic defects in OI patients, providing a more targeted and potentially curative approach.

The objectives of OI therapies encompass various aspects, including improved mobility, self-care, functional independence, and overall quality of life. These therapeutic strategies involve a multidisciplinary approach, encompassing bone-modifying agents, muscle strengthening, exercise rehabilitation, as well as orthopedic stabilization and the treatment of fractures and abnormalities. Each patient’s treatment plan is customized to address their specific needs and challenges.⁷²

Stabilization and Orthopedic Treatment of Fractures and Deformities

In individuals with unstable fractures or delayed healing post-fractures, osteotomy of the long bone and intramedullary rod implantation become essential procedures to correct deformities and prevent recurrent fractures. Various methods are currently employed for fixing long bones after osteotomies, including Fassier–Duval, Baily–Dubow/Sheffield, and non-retractable Rush nails.^159,160 Notably, Fassier–Duval rods offer the advantage of being inserted through the skin, minimizing damage during the procedure.¹⁵⁹ In cases where basal bone impingement leads to compression due to hindbrain protrusion and cerebrospinal fluid blockage, shunts or occipito cervical fusion decompression may be utilized to alleviate these issues (refer to Table 3 and Figure 4 for details).

Table 3. Summary of Current Clinical Trials for Osteogenesis Imperfecta.

Treatment	Identifier	Administration	Study start and completion	No. of patients	Status
Device: Invisalign	NCT04815564	Surgery	Aug 2022 to Jul 2026	57	Recruiting
Device: telescoping intramedullary nail	NCT04694144	Surgery	Dec 2020 to Mar 2024	22	Recruiting
Whole body vibration	NCT06010134	Physical therapy	May 2023 to Sep 2023	60	Recruiting
Bisphosphonates	NCT04115774	Administered	Jun 2013 to Feb 2032	5000	Recruiting
Setrusumab vs bisphosphonates	NCT05768854	Intravenous infusion	Jun 2023 to Jun 2026	66	Recruiting
Setrusumab	NCT05125809	Intravenous infusion	Feb 2022 to Mar 2026	219	Recruiting
Setrusumab	NCT03118570	Intravenous Infusion	Sep 2017 to Nov 2020	112	Complete
Zoledronic Acid + Teriparatide	NCT03735537	Infusion	Nov 2016 to Apr 2023	380	Recruiting
Zoledronic acid	NCT00982124	Infusion	Oct 2007 to Apr 2016	14	Completed
Zoledronic acid	NCT00131118	Intravenous infusion	Jul 2004 to May 2007	127	Complete
Zoledronic acid	NCT00063479	Infusion	Jun 2003 to May 2007	158	Complete
Risedronate oral tablet	NCT04152551	Oral	Nov 2019 to Nov 2024	100	Recruiting
Risedronate sodium (Actonel)	NCT00106028	Oral	Nov 2004 to Mar 2010	143	Complete
SAR439459 (TGF-β)	NCT05231668	Intravenous infusion	Aug 2022 to Dec 2024	24	Recruiting
Romosozumab	NCT04545554	Subcutaneous injection	Jan 2021 to Mar 2023	25	Completed
Fresolimumab	NCT03064074	Infusion	Nov 2017 to Jul 2022	11	Complete
Denosumab	NCT01799798	Subcutaneous injection	Feb 2013 to Jan 2015	10	Complete
High-dose vitamin D	NCT01713231	Oral	Sep 2012 to Jul 2014	60	Completed
Alendronate or Pamidronate	NCT00159419	Oral or intravenous administration	Aug 1999 to Aug 2008	18	Complete
Pamidronate	NCT00005901	Infusion	Jun 2000 to Mar 2015	34	Complete
Alendronate	NCT02303873	Oral	Mar 2007 to Aug 2014	99	Complete
Teriparatide	NCT00131469	Oral	Jun 2005 to Jan 2011	79	Complete
Humatrope	NCT00001305	Subcutaneous injection	Nov 1991 to May 2017	79	Complete
BPS804	NCT01417091	Infusion	Jun 2011 to Dec 2012	2	Complete
Mesenchymal stem cells (MSCs)	NCT02172885	Infusion	Apr 2014 to Dec 2018	10	Complete
Bone marrow cell transplantation	NCT00705120	Transplantation	Nov 1995 to Oct 2007	9	Complete
Mesenchymal stromal cells	NCT01061099	Infusion	Feb 2010 to Feb 2014	5	Complete
Bone marrow transplant	NCT00187018	Infusion	Mar 2004 to Aug 2007.	9	Complete
Bone marrow-derived mesenchymal stromal cells	NCT05559801	Infusion	Aug 2023 to Oct 2026	12	Not yet recruiting
BOOST cells	NCT03706482	Intravenous administration	Aug 2019 to Dec 2031	18	Active, not recruiting

Figure 4.

Chemical structure of bisphosphonates. The chemical structures of the different generations of bisphosphonates used in human bone disease clinical applications are depicted.

Muscle Strengthening and Exercise Rehabilitation

Physical therapy for OI is specifically designed to maximize improvements in the patient’s motor skills and daily life activities. These rehabilitation programs are personalized for children with OI, with a primary focus on enhancing strength and mobility to achieve optimal outcomes. Research has demonstrated positive effects of physical training programs for OI children, leading to improvements in peak oxygen consumption, maximum work capacity, and muscular strength after three months of low-resistance training.¹⁶¹ It is crucial to emphasize the importance of consistent and appropriately intense exercise, as the benefits may diminish after six months of no training, highlighting the need for ongoing physical therapy to maintain and further enhance outcomes in individuals with OI.¹⁶¹

Bone Anti-resorptive Therapy

In addition to directly addressing fractures, appropriate pharmaceutical treatments play a crucial role in improving bone mass and reducing fragility in patients with OI. The primary pharmacological therapy for pediatric OI patients involves bisphosphonates, which function by inhibiting osteoclast activity and bone resorption.^107,162 Examples of bisphosphonates include pamidronate, alendronate, ibandronate, risedronate, and zoledronic acid. These drugs can effectively reduce the risk of fractures, delay bone discomfort, and enhance bone density by interfering with the mevalonate pathway, decreasing osteoclast activity, and inducing their death.^44,107

While bisphosphonates generally have few adverse effects and are effective in treating osteoporosis to increase bone density,^107,162 their efficacy in treating severe OI is still debated.¹⁶³ Some studies have shown a decrease in OI fractures following bisphosphonate treatment,^164−166 while others found no significant difference compared to a placebo group. Adverse effects of bisphosphonates may include temporary hypocalcemia and acute phase infusion responses.^166,167

In addition to bisphosphonates, two other strategies are employed in clinical OI treatment. Denosumab, an anti-RANKL antibody, blocks the interaction of RANK ligand to its receptor RANK. This action leads to a decrease in the differentiation of pre-osteoclasts and the survival of osteoclasts, ultimately reducing bone resorption.¹⁶⁸ Despite the occurrence of adverse events such as serious infection, cellulitis, eczema, and malignancy in postmenopausal women,¹⁶⁹ denosumab shows efficacy in mitigating bone resorption. Notably, initial trials involving children with OI treated with denosumab demonstrated an increase in areal bone mineral density and improved mobility. Furthermore, there was a significant and reversible suppression of bone resorption, and no severe side effects were observed by parents during this 2-year treatment period.¹⁷⁰ Another alternative strategy is to inhibit cathepsin K, an enzyme released by osteoclasts with the primary function of breaking down demineralized collagen fibers.¹⁷¹ Mature osteoclasts adhere to the bone surface and disintegrate hydroxyapatite by creating an acidic environment. Current treatment for postmenopausal women with poor bone mineral density involves the use of the cathepsin K inhibitor Odanacatib.¹⁷² This approach maintains the connection between bone resorption and creation since Odanacatib, unlike Denosumab, does not impact osteoclast development. However, it is regrettable that Merck announced in 2016 the shelving of future development for Odanacatib due to an elevated risk of cardiovascular illness in patients.¹⁷³

While bisphosphonates remain a common treatment for OI, ongoing research is exploring additional therapeutic options for moderate-to-severe cases, including Denosumab and other potential interventions. The choice of treatment depends on factors such as the patient’s response to therapy and the severity of their condition.

Bone Anabolic Therapy

Growth hormone supplementation was initially considered for children with severe OI due to its beneficial effects on bone strength in growth hormone-deficient kids. However, its use in OI is limited because, compared to bisphosphonates, growth hormone only modestly increases bone mineral density and may have negative effects on a child’s growth and development if used excessively.¹⁷⁴

Teriparatide, a human parathyroid hormone (PTH), has shown dramatic improvements in bone remodeling, formation, and density, reducing fracture risk in people with type I OI.¹⁷⁵ However, its clinical use is currently restricted to adults with OI, and treatment duration should be less than two years due to potential risks,¹⁷⁶ including the induction of osteosarcoma in youngsters.

Sclerostin inhibitors,^177,178 such as Romosozumab,^179,180 Blosozumab, and BSP804,¹⁸¹ target sclerostin to boost bone density in OI patients.^61,182,183 Studies have shown increased bone formation and bone mass in OI patients receiving these inhibitors, but there is a rapid loss of bone mass when the medication is stopped, which can be mitigated with bisphosphonates. Romosozumab, in particular, has been associated with an increased risk of cardiovascular events,¹⁸⁴ requiring further research to understand its long-term effects.

The TGF–β signaling pathway has been linked to bone mass^3,108,185 and fragility^18,185 in OI. Inhibiting this pathway using TGF–β inhibitory antibodies has shown promise in enhancing bone mass in OI mice,¹⁸⁶ but more research is needed to assess the safety and effectiveness of these medications.

The BMP signaling system, crucial for bone formation, has also been explored in OI treatment.^187−190 Recombinant BMP-2-loaded silk fibroin microspheres have shown promise in enhancing bone growth in OI-modeled animals.¹⁹¹ Chemical modalities that act as mimetics or activators of BMP signaling are being explored, with some promising examples showing potential for bone regeneration, such as ventromorphin,¹⁹² PD-407824,¹⁹³ and the newly discovered FK506 derivatives.¹⁹⁰

Traditional screening methods that use immortalized cell lines may overlook promising chemical hits and targets, as the non-developmental microenvironment lacks genuine BMP pathway regulation.^189,194 To overcome this limitation, researchers such as Wesseler et al. have employed innovative approaches. For instance, mesoderm patterning of embryonic stem cells (ESCs) allowed the identification of 4H-chromen-4-ones as a novel osteogenic BMP activator chemotype. These compounds potentiated BMP signaling outputs through negative TGFβ feedback loops, offering a unique mechanism of action.

Triazolo[1,5-c]quinazolines, discovered by the same team, serve as novel in vitro and in vivo active BMP amplifiers. These compounds stimulate BMP signaling via increased and sustained availability of BMP-Smad proteins, relying on a minimal amount of BMP input.¹⁹⁵

Carbazolomaleimides 2, another discovery by Wesseler et al., act as non-canonical BMP synergy. They precisely control Id gene expression through GSK3 inhibition and elevated β-catenin levels, operating in a submicromolar, SMAD-independent, but highly BMP-dependent manner.¹⁸⁸

While these developments offer exciting possibilities for modulating the BMP pathway and potentially serving as treatment alternatives for conditions like OI,¹⁸⁷ it is important to note that these compounds are still in the preclinical stage. Clinical trials and FDA approval would be necessary to establish their safety, efficacy, and suitability for human use. Further research is needed to explore their potential applications and address regulatory considerations before they can be considered as therapeutic options for OI or other related conditions.

ER Stress-Targeted Therapy

In osteoblasts and fibroblasts, the intracellular retention of mutant collagen leads to ER stress, presenting a potential therapeutic strategy for OI.^196−198 An emerging approach involves the use of the FDA-approved chemical chaperone, 4-phenylbutyric acid (4-PBA), which acts as a molecular chaperone to facilitate protein folding and prevent accumulation in the ER.^108,199 In several studies, 4-PBA, known for its histone deacetylase inhibitor properties and its impact on cellular mitochondrial oxidative stress, has shown promise as a therapeutic agent. When applied to dominant and recessive OI zebrafish models, 4-PBA demonstrated an ability to improve cellular homeostasis, resulting in increased collagen secretion, reduced ER size, improved skeletal mineralization in juvenile fish, and mitigation of skeletal deformities in adult fish within the OI group.^200−202 Treatment with 4-PBA in fibroblasts from individuals with dominant OI carrying α1(I) mutations led to enhanced collagen folding and secretion, coupled with a decrease in apoptosis.²⁰¹ Notably, 4-PBA promoted Atg5 gene expression and cellular autophagy. Unexpectedly, 4-PBA was found to enhance osteogenic gene expression and mineralization while suppressing apoptotic cell and unfolded protein response (UPR) gene expression.¹⁹⁶ This novel treatment approach aims to address the underlying cellular homeostasis, potentially reducing the severity of the OI phenotype. Given its direct impact on lowering intracellular collagen buildup, 4-PBA holds promise as a common component in the treatment of both dominant and recessive forms of OI.

Stem Cell Therapy

Adult Stem Cell-Based Therapy

Mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs) are crucial types of adult stem cells residing in bone marrow. HSCs are responsible for generating all blood cells, while MSCs have the capability to differentiate into either fat or bone tissues. These stem cell types collaborate in the repair and regeneration of damaged tissues. MSCs, through differentiation, can give rise to osteoblasts, chondrocytes, and osteocytes, contributing to the regeneration of bone tissue. Previous research, as summarized in Table 4, highlights the success of bone marrow cell transplantation as an effective technique for treating OI and its associated congenital bone fragility.^203−207

Table 4. Summary of OI Patient Treatments Based on MSC Transplantation.

Mutation	OI type	HLA	Receipt	Origins	Transplantation	Outcome	Ref
COL1A2	I	HLA-mismatched	Embryos	Male hfMSCs derived from 10 weeks gestation fetal liver	Prenatal transplantation 6.5 × 106 MSC	Psychomotor development was normal	(207)
COL1A2	III	HLA-mismatched	Embryos, 8 years of age	Male hfMSCs derived from 10 weeks gestation fetal liver	Prenatal transplantation 6.5 × 106 MSC and postnatal transplantation 2.8 × 106 at 8 years	Patient’s ability to walk was improved	(205)
COL1A1 and COL1A2	IV	HLA-mismatched	Embryos, 1 year and 6 months of age	Male hfMSCs derived from 7 weeks and 3 days gestation fetal liver	Prenatal transplantation 30 × 106 MSC and postnatal transplantation 10 × 106 at 1 year and 6 months	Patient started to walk shortly after the transplantation	(205)
COL1A2	Unknown	HLA-matched	13-month-old girl	Allogeneic bone marrow transplantation	Postnatal transplantation	Accelerated growth after treatment	(206)
COL1A2	Unknown	HLA-matched	13-month-old boy	Allogeneic bone marrow transplantation	Postnatal transplantation	Accelerated growth after treatment	(206)
COL1A2	Unknown	HLA-matched	17-month-old boy	Allogeneic bone marrow transplantation	Postnatal transplantation	Accelerated growth after treatment	(206)

Reports indicate that patients with OI who underwent human leukocyte antigen (HLA)-matched sibling bone marrow transplants experienced increased growth rates, reduced fracture rates, and improved systemic mineral content. While these outcomes are not always enduring, they suggest the potential efficacy of transplantation therapy in managing OI.^203,208,209 Götherström et al. described two cases, including a female newborn with typical OI characteristics who received MSCs at 31 weeks of gestation. After bisphosphonate therapy, symptoms improved, but growth slowed, and fractures became more frequent around the age of six. A second MSC injection at 8 years and 2 months led to gradual height increase. Another patient, at 26 weeks gestation, had several repaired fractures and shortening of long bones. An MSC injection at 31 weeks prevented fractures for 7 weeks. A second MSC injection at 19 months allowed the patient to resume growth and walking.^203,204

Similar positive outcomes were observed by Ramesh et al. in an HLA-matched bone marrow transplantation study involving five severe OI patients.²⁰⁹ Investigations into the effects of human or murine MSCs, particularly in utero transplantation, have shown promising findings.^210,211 For example, Li et al. injected mouse MSCs into the femur of OI murine mice.²¹², demonstrating that MSCs developed into osteoblasts and supported new bone production. Transplanting MSCs into the uterine horns of another OI model mouse prevented perinatal mortality and enhanced bone matrix production.²¹³

HSCs have the capacity for multidirectional differentiation and self-renewal.^214−216 Transplanting HSCs, taken from bone marrow or peripheral blood, can restore functional osteoblasts.²¹⁷ Hematopoietic niches, divided into osteoblastic and vascular niches, play a role in HSCs formation and expansion.²¹⁸ Studies such as Kang et al.’s work with HSCs transplantation into oim mice indicate that HSCs can differentiate into osteoblasts and osteoclasts, highlighting their potential therapeutic role in OI.²¹⁴

Although MSCs and HSCs are a promising research target for tissue repair due to their ability to differentiate and low immune rejection rate. However, long-term safety concerns and potential side effects need thorough evaluation, necessitating close monitoring of patients receiving adult stem cell therapy.

Pluripotent Stem Cell-Based Therapy

ESCs initially characterized in the early 1980s, are pluripotent stem cells derived from the inner cell mass of early-stage embryos.^219,220 Their unique property of pluripotency allows them to differentiate into various cell types, including osteoblasts responsible for bone formation. In a study by Buttery et al., the in vitro differentiation of murine ESCs (mESCs) into osteoblasts was successfully induced using dexamethasone, β-phosphoglycerol, and vitamin C.²²¹ Co-culturing fetal rat osteoblasts with mESCs resulted in a 5-fold increase in the number of bone nodules compared to controls, as demonstrated by Phillips et al.²²² This suggests that substances released by osteoblasts may play a role in encouraging ESCs to differentiate into osteoblasts.

Karp et al. demonstrated that cultivating ESCs in monolayers, induced with various doses of dexamethasone (Dex), β-phosphoglycerol, and vitamin C, for 4–5 days resulted in more bone nodules than the formation of embryoid bodies (EBs). This approach was deemed easier and more effective without the need for EB development.^223,224In vivo osteogenesis is triggered by cytokines, and to replicate these conditions in vitro, several substances, including 1,25-dihydroxyvitamin D3, Dex, and retinoic acid (RA), must be added.²²⁵ Phillips et al.’s research on EBs with RA over 2–5 days showed a low level of alkaline phosphatase (ALP) expression on day 5, reaching a maximum on day 15.²²² Osteocalcin progressively increased, peaking 5–15 days after induction. Kawaguchi et al. demonstrated that BMP-4 plays a role in promoting ESC differentiation into osteoblasts.²²⁶

Induced pluripotent stem cells (iPSCs), another major type of pluripotent stem cell, are obtained by reprogramming somatic cells through the transfer of four transcription factors (Oct4, Sox2, Klf4, and c-Myc).^227,228 iPSCs, like ESCs, can differentiate into various cell types, including osteoblasts (summarized in Table 5). Traditional methods involve developing iPSCs in embryoid bodies and inducing differentiation with retinoic acid, ascorbic acid, β-glycerophosphate, dexamethasone, bone morphogenetic proteins, and vitamin D3.^229−232 Kao et al. showed that the addition of resveratrol to the inducers could efficiently induce osteogenic differentiation of iPSCs, reducing the toxicity of dexamethasone and lowering the tumorigenicity of the cells.²³³ Tashiro et al. transduced the Runx2 or PPAR gamma genes into mouse iPSCs using adenoviral vectors, enhancing the cells’ ability to differentiate.²³⁴ Extremely low-frequency electromagnetic fields and an optimal osteoconductive environment further aided the osteogenic potential of iPSCs differentiation.²³⁵ Kim et al. successfully generated iPSC lines from the peripheral blood mononuclear cells of a patient with OI type I, providing a valuable cellular modeling platform for OI and a resource for drug screening.²³⁶ There is no doubt that corrected patient-derived iPSCs show great potential for OI in vivo therapies by using patient-specific cells.

Table 5. Summary of Osteoblasts Differentiated from ESC and iPSC Cells.

Cell source	Differentiation medium	Differentiation methods	Characterizations	Ref
mESCs	l-Ascorbate phosphate (50 g/mL) and α-glycerophosphate (10 mM) (osteogenic medium)	Forming EBs differentiation	Osteocalcin, Cbfa1, and osterix; formation of bone nodules	(259)
Fetal mouse osteoblasts and mESCs	Ascorbic acid phosphate (50 mg/mL) and β-glycerophosphate (10 mM) (mineralization media) cultured with 1027 M all-trans retinoic acid	Forming EBs differentiation	BMP-2 expression increased; bone mineral deposition and osteoblast marker genes including osteocalcin and alkaline phosphatase were up-regulated	(222)
hESC line H9	α-MEM containing and FBS supplemented with 10–8 M dexamethasone (DEX), 50 μg/mL AA, and 5 mM β-gP	Mesoderm induction: 10% serum in α-MEM or direct plating of the hESC	Increased number of osteoblasts; formation of bone nodules and matrix	(223)
mESCs line D3 and miPSC line	β-Glycerophosphate (10 mM), ascorbic acid (50 μg/mL), and either 1,25-dihydroxy vitamin D3 or dexamethasone (5 × 10–8 M	Forming EBs differentiation	Increased expression of early osteogenic marker RUNX2 and late marker SPARC and osteocalcin	(260)
mESCs line	50 mM BGP, 50 μg/mL ascorbate 2-phosphate, and 10 μM Dex	Forming EBs differentiation	Formation of mineralized bone nodules; production of osteoblast extracellular matrix	(261)
WT iPSC lines 201B7 and skin fibroblasts	1% Non-essential amino acids, 0.1 mM 2-mercaptoethanol, 2 mM Gluta-MAX, 10 mM glycerol-2-phosphate, 1 nM dexamethasone, and 50 μg/mL l-ascorbic acid 2-phosphate sesquimagnesium salt hydrate	Patient’s skin fibroblasts were reprogrammed into iPSCs with Yamanaka factors, and iPSCs were induced to differentiate into osteoblasts	Osteoblasts and osteocytes observed in both WT iPSCs and iPSCs reprogrammed by skin fibroblasts; greater expression of osteoblasts in WT	(262)
hiPSCs 414C2 and 409B2	2 mM Gluta-MAX, 10 mM glycerol-2-phosphate, 1 nM Dex, 0.1 mM 2-mercaptoethanol, 50 μg/mL l-ascorbic acid 2-phosphate sesquimagnesium salt hydrate, and 1% non-essential amino acids	hiPSCs were directly induced to differentiate into osteoblasts on type I collagen gel with osteogenic differentiation medium	Differentiation of hiPSCs into osteoblastics and osteocytes through nodule formation	(263)
Mouse gingival fibroblast-derived iPSCs	0.01 μM Dex, 10 mM β-glycerophosphate, 50 μg/mL ascorbate-2-phosphate, and 1% antibiotic–antimycotic solution	iPSCs were directly induced to differentiate into osteoblasts and shear stress was applied to the cells during the differentiation	Shear stress (0.5 Pa) enhancement of the osteogenic differentiation of iPSCs, partly mediated by Cx43 and Erk1/2 signals	(264)
Mouse femur bone marrow and mouse gingival fibroblast	2 mM l-Glutamine, 1 × 10–4 M non-essential amino acids, 1 × 10–4 M 2-mercaptoethanol, 50 U of penicillin, and 50 μg/mL streptomycin	Forming EBs differentiation	Production of robustly mineralized bone nodules that contained abundant calcium phosphate with hydroxyapatite crystal formation; increased expression of osteogenic marker genes	(265)
hiPSC line ATCCACS-1011	1 mM l-Glutamine, 1% non-essential amino acids, 4 ng/mL basic fibroblast growth factor, 0.1 mM β-mercaptoethanol, and low-intensity pulsed ultrasound (LIPUS)	Forming EBs differentiation	LIPUS accelerated the mesenchymal differentiation of EB cells and produced the mineralized nodule and osteoblasts	(266)
WT iPSC lines 201B7	2 mM Gluta-MAX, 10 mM glycerol-2-phosphate, 1 nM Dex, 0.1 mM 2-mercaptoethanol, 50 μg/mL l-ascorbic acid 2-phosphate sesquimagnesium salt hydrate, and 1% non-essential amino acids and RA	iPSCs were directly induced to differentiate into osteoblast	Osteogenesis differentiates into osteoblast-like and osteocyte-like cells, which produce human bone tissue when implanted into the skull defects of mice and form osteoid nodules within 10 days	(267)
miPSCs	50 mg/L Ascorbic acid, 10–2 M β-glycerophosphate, 10–4 M dexamethasone, and biomimetic nanofibers of hydroxyapatite/collagen/chitosan	Forming EBs, and then iPSCs were induced to iPSC-MSCs, and iPSC-MSCs directly differentiated into osteoblasts	Expression of osteogenic genes Runx2, Ocn, Alp and Col significantly up-regulated and effectively promoted bone regeneration in mice with skull defects	(268)
hiPSCs BC line and hUCMSCs	100 nM Dexamethasone, 10 mM β-glycerophosphate, 0.05 mM ascorbic acid, and 10 nM 1α,25-dihydroxyvitamin D	Forming EBs differentiation into hiPSC-derived MSCs	Osteogenic genes upregulated; mineral synthesis by cells increased with time in vitro and promoted bone regeneration in mice	(269)

Gene Therapy

Gene therapy is a ground-breaking approach aimed at treating or preventing diseases by manipulating the genetic material within an individual’s cells. The fundamental objective of gene therapy is to introduce, modify, or repair genes, with the goal of correcting genetic disorders or providing therapeutic benefits. This innovative field holds immense promise and has the potential to revolutionize the treatment landscape for various conditions.

Gene Editing

Zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR-Cas9 represent three generations of gene-editing technologies,²³⁷ each of these technologies has played a crucial role in advancing the field of gene editing, including their potential applications in treating genetic disorders like OI. Currently, CRISPR-Cas9 is the most popular and widely adopted gene-editing technology in the field of genetic disorders, despite its clinical applications are still limited.²³⁸

The development of COL1A1 mutant iPSCs, a crucial cell line facilitating research into the causes and treatments of OI disorders, was achieved through the application of CRISPR-Cas9 tools by Hosseini Far et al.²³⁹ In a parallel study, McGowan et al. harnessed the power of CRISPR-Cas9 genome editing to enhance osteoblast development and promote bone healing in a zebrafish model. Their approach involved the modification of the Wnt16 gene, leading to increased efficacy in stimulating osteoblast development and improving bone healing when the edited gene was introduced at the zebrafish egg stage.²⁴⁰ Furthermore, Jung et al. pioneered the development of an iPSC cell line derived from an OI patient, employing CRISPR-Cas9 to rectify the defective COL1A1 gene.²⁴¹

Freitas et al. have succeeded in differentiation of MSCs into osteoblasts by employing gene-editing techniques to induce the overexpression of the BMP9 protein, a crucial regulator in bone development.²⁴² Leveraging the precision of the CRISPR-Cas9 system, the researchers achieved a targeted enhancement of BMP9 expression, highlighting the potential for advanced genome editing technologies in steering cellular differentiation toward therapeutic outcomes. The advent of CRISPR-based genome editing has revolutionized our ability to manipulate the human genome precisely and effectively. This breakthrough technology opens avenues for accurately correcting gene mutations that underlie various diseases, as illustrated in Figure 5. The implications are far-reaching, offering the prospect of personalized and precise gene therapies when integrated with iPSCs. This powerful combination holds promise for the future of tailored and effective gene therapy interventions.

Figure 5.

Overview of manufacturing processes for stem cell-based allogeneic and autologous cell therapy. Different methods are used in the manufacturing procedures for stem cell-based allogeneic and autologous cell treatments to create cellular products for therapeutic use that are adapted to the demands of the patient. Both autologous and allogeneic cell treatments offer a lot of potential for treating OI. The source of the cells makes a significant difference; while autologous employs the patient’s own cells, allogeneic uses donor cells. A useful technique for producing patient-specific stem cells for disease modeling as well as genetically altered patient-specific cells for individualized cell treatment is provided by iPSC reprogramming, osteoblast differentiation, and direct transdifferentiation. Common and well-known allogeneic cell sources that can serve as donors for several patients are depicted.

Gene Silencing

In gene therapy for OI, a strategy involves the inactivation or silencing of defective genes. This approach aims to mitigate the severity of severe type OI by employing anti-sense nucleic acids, such as siRNA, shRNA, anti-sense RNA, anti-sense DNA, and nucleases. Lindahl et al. demonstrated allele-dependent silencing of collagen type I using 3′UTR indels, resulting in a significant reduction in the production of COL1A1 and COL1A2 proteins. This mutation-independent therapeutic approach holds promise for OI treatment.²⁴³

Rousseau et al. utilized siRNA to effectively silence COL1A1 in Brtl mice, showcasing its success in reducing COL1A1 expression and lowering HSP47 expression, a protein known to be elevated in OI patients.²⁴⁴ In another study, Chamberlain et al. employed adeno-associated virus (AAV) vectors to inactivate mutant COL1A2 genes in OI mesenchymal stem cells, demonstrating the successful targeting of both type I collagen genes responsible for OI.²⁴⁵ Maruelli et al. explored gene silencing therapy using three distinct COL1A2 silencing siRNAs, showing that Col1a2-siRNA-3554 effectively reduced COL1A2 expression while maintaining mineralization in mouse embryonic fibroblasts.²⁴⁶

While gene silencing therapy presents a novel approach for OI treatment, challenges remain, including the need for improved efficiency, specificity, and the selection of appropriate delivery vectors.²²⁰ Additionally, the potential necessity for repeated injections in the clinic may impact patient compliance, like stem cell treatments. The in vivo data on OI gene silencing is limited, primarily stemming from in vitro studies, highlighting the need for further research and development in this promising therapeutic avenue.^247−249

Gene Therapy

Gene addition therapy stands out as one of the main strategies for treating OI through gene therapy. This approach aims to enhance or correct gene mutations leading to protein dysfunction. Liu et al. utilized retroviral transfection to introduce the human COL1A1 gene into adipocyte-derived MSCs from mice lacking this gene, resulting in significantly increased bone formation in OI animals.²⁵⁰ Oyama et al. retrovirally transfected bone marrow stromal cells into nude mice, demonstrating in vivo expression and bone development after 6 weeks. This technique also enhanced the osteogenic ability when applied to oim mice, suggesting the potential of introducing normal genes into the OI model using these cells as vectors to improve osteogenic potential.²⁵¹ Additionally, Liu et al. used lentiviral vectors encoding the NELL1 gene, markedly boosting the expression of genes associated with osteogenesis and enhancing osteoblast differentiation.²⁵²

While viral vectors for gene therapy present challenges such as immunological reactions and safety concerns, ongoing research is actively addressing these issues. Gene therapy holds promise in treating various genetic disorders, including OI, by modifying patients’ genes. The use of non-viral or nanoparticle-based vectors in gene therapy is gaining prominence as an alternative to viral vectors.^253−258 Moreover, MSCs, often used for in vivo injection due to their osteoblast differentiation capacity, offer a method for systemic distribution. Genetically modified MSCs can home to bone following systemic administration, presenting a potential approach for disseminating transgenes to treat diseases affecting the entire skeleton, such as OI.

Perspective and Conclusion

OI has undergone significant advancements in recent decades, transforming from a condition with an unidentified etiology to one with a well-documented genetic foundation. Current therapeutic approaches for OI aim to mitigate the risk of fractures, manage symptoms, and enhance bone mass. Commonly prescribed medications include denosumab, synthetic parathyroid hormone, bisphosphonates, and pediatric growth hormone. However, these treatments often face challenges such as low efficacy, treatment resistance, or adverse effects in some individuals.^3,107,185

The understanding of OI’s molecular genetics has progressed considerably, leading to the identification of new treatment targets and the initiation of clinical trials. While there is currently no cure for OI, patient care has improved. Existing pharmaceutical therapies primarily focus on increasing bone density and shaping, addressing matrix abnormalities that contribute significantly to brittle bones in patients. The future of OI treatment may involve molecular chaperones, bone formation stimulators, gene editing, and stem cell-based cell therapy. Stem cell technology holds promise for OI treatment; however, most protocols for PSC differentiation currently lack chemically defined and xeno-free media. Challenges persist in developing robust, reproducible, scalable, and Good Manufacturing Practice (GMP)-compliant manufacturing processes. Before translating these advancements to OI therapy in clinical settings, further fundamental and applied research is necessary.

Author Contributions

^# Y.S., L.L. and H.T.L. contributed equally.

The authors declare no competing financial interest.