Genetic Skeletal Disorders with Defects in Glycosaminoglycan Biosynthesis

Abstract

Background

Proteoglycans are a major component of the connective tissue matrix, which consists of a core protein and covalently attached glycosaminoglycan (GAG) chains, which are highly sulfated polysaccharides with a tetrasaccharide linker for the core protein attachment. Impaired synthesis or degradation of GAG causes genetic disorders. In the 1950s, deficient lysosomal GAG degradation was discovered in mucopolysaccharidoses. In the 1990s, a defective enzyme for GAG synthesis was implicated in a variant of Ehlers-Danlos syndrome and an impaired GAG sulfation in diastrophic dysplasia. Newer studies have uncovered that abnormal GAG synthesis causes a large group of genetic skeletal disorders with joint and skin abnormalities.

Summary

The prototype of this group includes diastrophic dysplasia and Desbuquois dysplasia. The former is attributed to abnormal GAG sulfation, while the latter to impaired GAG chain elongation. Defective linker formation causes distinctive phenotypes termed linkeropathy. Moreover, there remain many disorders with defective GAG synthesis, in which the phenotypes are poorly documented and thus the clinical suspicion and even interpretation of molecular findings are challenging. Here, we attempt to review the skeletal manifestations of abnormal GAG synthesis disorders, based on our own experiences and previous reports. Each disorder has distinct clinical and radiological features, but they share some common skeletal manifestations, such as distal humeral hypoplasia, misshapen proximal femora, accelerated carpal ossification, and malsegmentation of the short tubular bones.

Key Message

Awareness of the phenotypic similarities and differences among this group of disorders facilitates our clinical and genetic practices for affected individuals.

Plain Language Summary

Proteoglycans are scaffold proteins in the extracellular matrix that carry long, highly sulfated sugar chains called glycosaminoglycans (GAGs). When the making or sulfating of these GAG chains goes wrong, cartilage and other connective tissues lose strength, causing genetic skeletal dysplasias with bone, joint, and sometimes skin/visceral complications. This review outlines disorders caused by (1) defective GAG sulfation, (2) defective GAG chain elongation, and (3) defects in the four-sugar “linker” that tethers GAGs to core proteins and briefly comments on diseases from abnormal core proteins. Among these conditions, common findings include epiphyseal dysplasia, characteristic hip changes (e.g., “monkey-wrench” proximal femur), accelerated carpal ossification, and joint laxity or, less often, restriction (radioulnar involvement is common). Clubfeet, spinal malalignment, and EDS-like skin features may occur; the severity varies widely, and all disorders are inherited in an autosomal recessive manner. Because many entities are rare and phenotypes overlap, genotype-phenotype correlations remain incomplete, and diagnosis can be challenging. Recognizing the common imaging and clinical patterns helps guide targeted genetic testing and management. Key examples include SLC26A2-related diastrophic dysplasia, a classic sulfate-transport defect characterized by hitchhiker thumb, clubfeet, and kyphoscoliosis; animal studies suggest N-acetylcysteine may ameliorate undersulfation, and the allelic spectrum spans from mild autosomal-recessive multiple epiphyseal dysplasia (AR-MED) to lethal achondrogenesis type 1B (ACG1B). PAPSS2 deficiency produces prominent vertebral changes reminiscent of brachyolmia. Defects in CHST3, CHST11, GPAPP, or CHST14 disrupt the sulfotransferase pathway and yield phenotypes that include recessive Larsen-like chondrodysplasia and musculocontractural Ehlers-Danlos syndrome. CANT1-related Desbuquois dysplasia frequently shows strikingly advanced carpal bone age with the characteristic “monkey-wrench” configuration of the proximal femora. Linkeropathies due to XYLT1, B4GALT7, B3GALT6, or B3GAT3 feature joint laxity with skin and craniofacial findings, alongside diverse spinal and hip abnormalities. Finally, EXT1/EXT2 haploinsufficiency underlies hereditary multiple exostoses through impaired heparan-sulfate polymerization.

Introduction

The extracellular matrix contains specific glycoproteins termed proteoglycans (PGs) that are composed of a core protein and covalently attached side chains of highly sulfated polysaccharides termed glycosaminoglycans (GAGs). Impairment in either their synthesis or degradation leads to genetic disorders with skeletal and connective tissue phenotypes. The relation between mucopolysaccharidoses and impaired GAG degradation was elucidated in the 1950s. In 1990, a deficient enzyme of GAG synthesis was discovered in a progeroid syndrome now known as Ehlers-Danlos syndrome spondylodysplasia type 1 [1]. In 1994, impairment of a transporter needed in GAG sulfation was found to cause a well-known skeletal dysplasia, diastrophic dysplasia (DTD) [2].

Since then, advances in molecular technology have revealed that a large group of skeletal dysplasias are related to defective synthesis of GAGs and core proteins [3, 4]. In the latest 2023 nosology of genetic skeletal disorders, the disorders of defective GAG synthesis are classified into the groups 4 and 5, while those of abnormal core protein assembly into group 7 [5]. Here we review the clinical and radiological findings of skeletal disorders due to abnormal biosynthesis of GAGs, along with the genetic, molecular, and pathogenic information (Table 1). We also briefly comment on disorders with abnormal core proteins at the end of this review. All disorders that we address here are inherited in an autosomal recessive manner. Since most disorders are rare, the genotype-phenotype correlation remains incomplete, and there remain many disorders whose phenotypes are poorly documented. Thus, the clinical diagnosis and even interpretation of molecular genetic findings are currently challenging.

Table 1.

Molecular, clinical, and radiological summary of genetic skeletal disorders with abnormal GAG synthesis

​	Gene name	Protein name	Alternative protein name	Protein function	Joint abnormalities	Other clinical findings	Skeletal radiological findings
Disorders of impaired GAG sulfation (OMIM)
DTD (222600)	SLC26A2 or DTDST	Solute carrier family 26 member 2	DTD sulfate transporter	Sulfate transporter	Mostly restricted but joint laxity may be seen (e.g., hitchhiker’s thumb)	Short stature; progressive kyphoscoliosis; cystic hematoma of the auricle	Metadiaphyseal broadening with flat epiphyses; “mushroom-shaped” proximal femora; “snow melting on a roof appearance” of the metacarpals; distal humeral hypoplasia (Y shaped on the frontal view and tapered on the lateral view); phalangeal segmental anomalies; mildly advanced carpal ossification; cervical kyphosis
AR-MED (226900)	SLC26A2 or DTDST	Solute carrier family 26 member 2	DTD sulfate transporter	Sulfate transporter	Not overt but clubfeet are seen	Normal height	Similar to, but milder than, those of DTD; double-layered patella
AO2 (256050)	SLC26A2 or DTDST	Solute carrier family 26 member 2	DTD sulfate transporter	Sulfate transporter	Restriction of large joints; hitchhiker thumb	Lethal	Similar to, but more severe than, those of DTD; platyspondyly with coronal clefts that is distinctive from DTD; distal femoral hypoplasia is seen
ACG1B (600972)	SLC26A2 or DTDST	Solute carrier family 26 member 2	DTD sulfate transporter	Sulfate transporter	Restriction of large joints	Lethal and commonly stillborn; hydrops fetalis with extremely short limbs noted in the 2nd trimester	Severe ossification defect in the axial and appendicular bones; rudimentary vertebral ossification; triangular-shaped or stellate long bones; a paraglider-like appearance of the ilia; “cobra sign” of neural arches
PAPSS2-related SEMD (AR-BO) (612847)	PAPSS2	3′-Phosphoadenosine 5′-phosphosulfate synthase 2	Phosphoadenosine phosphosulfate synthetase 2	Synthesis of PAPS (a universal sulfate donor)	Not overt	Postnatal short-trunked short stature; precocious puberty	Severe platyspondyly; dumbbell deformity of long bones (at younger ages); striations of the proximal femora
Chondrodysplasia with congenital joint dislocation, CHST3 related (143095)	CHST3	Carbohydrate sulfotransferase 3	Chondroitin 6-O-sulfotransferase 1	Chondroitin 6-O-sulfation	Congenital joint dislocations and clubfeet	Short stature; cardiac valvular diseases	Bifid distal humerus; multiple coronal clefts; cobra sign of neural arches
OCBMD, CHST11-related (618167)	CHST11	Carbohydrate sulfotransferase 11	Chondroitin 4-O-sulfotransferase 1	Chondroitin 4-O-sulfation	Not overt	Mild short stature	Similar to type C brachydactyly, associated with hyperphalangy of the index and middle finger
Chondrodysplasia with congenital joint dislocation, IMPAD 1-related (614078)	BPNT2 or IMPAD1	3′(2′),5′-bisphosphate nucleotidase 2 or inositol monophosphatase domain-containing protein 1	GPAPP	Hydrolysis of PAP to AMP	Congenital dislocations; radial deviation of the thumb with ulnar deviation of the index finger and lateral deviation of the 5th toe	Facial dysmorphism (mid-face hypoplasia and micrognathia with cleft palate); short-limbed short stature	Malformation of the thumb, index finger, and 5th toe (delta-shaped proximal phalanx, shortening of the middle phalanx and anomalous epiphysis of the metacarpal, and longitudinally splitting or delta-shaped metatarsal, respectively)
EDSMC1, CHST14-related (601776)	CHST14	Carbohydrate sulfotransferase 14	Dermatan 4-sulfotransferase 1	Conversion from CS to DS	Distal arthrogryposis and hypermobility of large joints	Facial dysmorphism (open fontanel, blue sclera, blepharophimosis, downward slanting palpebral fissures, micrognathia); Ehlers-Danlos syndrome-like skin; marfanoid habitus; ocular abnormalities	Severe kyphoscoliosis; dolichospondyly; arachnodactyly; slender short tubular bones
Spondylo-epi-metaphyseal dysplasia with hyposulfatemia, SLC13A1-related (not cited in OMIM)	SLC13A1	Solute carrier family 13, member 1	​	Renal and intestinal sulfate transporter	Mild restriction of large joints	Normal height to mild short stature; mild restriction of large joints	Platyspondyly; ovoid vertebral bodies with anterior tongue; striking metaphyseal flaring; delayed epiphyseal ossification
Chondrodysplasia with hypomyelinating leukodystrophy, SLC35B2-related (620269)	SLC35B2	Solute carrier family 35, member B2	​	Golgi membrane PAPS transporter	Dislocation and scoliosis	Psychomotor delay; callosal hypoplasia; Pierre-Robin sequence; severe short stature	Cervical hypoplasia; mild epiphyseal dysplasia; metaphyseal broadening; flat acetabula; advanced carpal ossification
Neurofascioskeletal syndrome with or without renal agenesis, HS2ST1-related (619194)	HS2ST1	HS 2-O-sulfotransferase 1	​	Sulfation of HS	Flexion contractures	Facial abnormalities (hypertelorism, upslanted palpebral fissures, broad nasal tip, wide mouth, etc.); developmental delay; callosal hypoplasia; renal agenesis; brachydactyly with broad finger tips and toes	Broadening of long bones; short middle phalanges; broad distal phalanges
Short stature, amelogenesis imperfecta, and skeletal dysplasia with scoliosis, SLC10A7-related (618363)	SLC10A7	Solute carrier family 10, member 7	​	Transporter with a yet unidentified substrate	Joint laxity and scoliosis	Short stature; facial abnormalities (flat face, micrognathia, and cleft palate); amelogenesis imperfecta	Mild spondylar dysplasia; broad femora with prominent lesser trochanters; flat epiphyses; advanced carpal ossification
Disorders of impaired GAG chain polymerization
Temtamy preaxial brachydactyly syndrome, CHSY1-related (605282)	CHSY1	CS synthase 1	Chondroitin synthase 1	Catalysis for elongation of the disaccharide unit of CS	Joint laxity	Delayed motor and mental development; short stature; micrognathia; dental anomalies; sensorineural hearing loss	Hitchhiker-like thumbs and great toes and hyperphalangy and delta-shaped proximal phalanges of the index and middle fingers
Schneckenbecken dysplasia (269250)	SLC35D1	Solute carrier family 35, member D1	UPD-GluUA/UDP-GalNAc dual transporter	Golgi membrane transporter for UDP sugars (UDP-GluUA and UDP-GalNAc)	Not overt	Lethal; resembling ACG1B	Snail-shaped ilia; platyspondyly with dorsal underossification; interpedicular narrowing of the lumbar spine
Arthrogryposis, impaired intellectual development, and seizures, SLC35A3-related (615553)	SLC35A3	Solute carrier family 35, member 3	UDP-GlcNAc transporter	Golgi transporter of UDP-GlcNAc	Distal arthrogryposis	Microcephaly; intellectual disability; seizure	Vertebral malsegmentation with scoliosis; malalignment of distal limbs; camptodactyly
DBQD (251450)	CANT1	Calcium activated nucleotidase 1	Soluble calcium-activated nucleotidase 1	Hydrolysis of UDP to UMP	Joint laxity	Mid-face hypoplasia; short stature with short limbs varying in severities	Advanced carpal ossification; monkey wrench femora; flat epiphyses especially in the proximal femora; hyperphalangy of the index finger and bifid thumbs (type1)
SDJLABA, CSGALNACT1-related (618870)	CSGALNACT1	CS N-acetylgalactosaminyltransferase 1	β1,4-N-acetylgalactosaminyltransferase 1	Catalysis for the 1st step of elongation of CS and DS	Joint laxity	Similar to but milder than DBQD type 2	Monkey wrench femora; coronal cleft; advanced bone age
SEMD with immune deficiency and intellectual disability, EXTL3-related (617425)	EXTL3	Exostosin-like glycosyltransferase 3	α1,4-N-acetylglucosaminyltransferase-I	Catalysis for the 1st step of elongation of HS	Not overt	Developmental delay; hepatorenal cysts; combined immunodeficiency; short stature	Severe spondylo-epi-metaphyseal dysplasia (platyspondyly with kyphoscoliosis, broad metaphyses, and brachydactyly)
HME types 1 and 2 (133700 and 133701)	EXT1 EXT2	Exostosin glycosyltransferase 1	Exostosin 1	HS elongation	Not overt	​	Multiple exostoses
		Exostosin glycosyltransferase 2	Exostosin 2
Disorders of linker region formation (linkeropathy)
Baratela-Scott syndrome, XYLT1 related (615777)	XYLT1	Xylotransferase 1	UDP-D-xylose: PG core protein β-D-xylosyltransferase 1	Linker region formation (1st step)	Joint laxity	Similar to DBQD type 2; flat face with synophrys; short stature	Monkey wrench femora; advanced carpal ossification; brachymetacarpia
Ehlers-Danlos syndrome, spondylodysplastic type 1, B4GALT7-related (130070)	B4GALT7	β1,4-galactosyltransferase 7	β1,4-galactosyltransferase-I	Linker region formation (2nd step)	Severe joint laxity	Flat face with protruding eyes, deep nasolabial folds, and microstomia; short stature; thin and overextended skin	Advanced carpal ossification; broad metaphyses; radioulnar synostosis; arachnodactyly
SEMDJL Beighton type, B3GALT6-related (271640)	B3GALT6	β1,3-galactosyltransferase 6	β1,3-galactosyltransferase-II	Linker region formation (3rd step)	Severe joint laxity	Facial dysmorphism with blue sclera; elastic skin; progressive kyphoscoliosis	Platyspondyly; iliac hypoplasia; monkey wrench femora; metaphyseal flaring; advanced carpal ossification
Ehlers-Danlos syndrome, spondylodysplastic type 2 (615349)	B3GALT6	β1,3-galactosyltransferase 6	β1,3-galactosyltransferase-II	Linker region formation (3rd step)	Severe joint laxity	Thin hair; wrinkled skin: prolonged wound healing; joint laxity is more severe than SEMDJL Beighton	Similar to SEMDJL Beighton type; osteoporosis may be seen
Al-Gazali syndrome (609465)	B3GALT6	β1,3-galactosyltransferase 6	β1,3-galactosyltransferase-II	Linker region formation (3rd step)	Congenital contracture (arthrogryposis)	Semilethal disorder; corneal opacity	Overtubulation of long bones; radioulnar synostosis; arachnodactyly
Lethal Desbuquois-like dysplasia (not available)	FAM20B	Family with sequence similarity 20, member B	GAG xylosylkinase	Increased activity of β1,3-galactosyltransferase 6 via phosphorylation of the xylose residue of the linker region	Multiple joint dislocations	Lethal disorder; reported in only one family	Desbuquois-like
Multiple joint dislocations, short stature, craniofacial dysmorphisms, and skeletal dysplasia, with or without heart defects, B3GAT3-related (245600)	B3GAT3	β1,3-glucuronyltransferase 3	β1,3-glucuronosyltransferase-I	Linker region formation (the last step)	Dislocation and kyphoscoliosis	Remarkable phenotypic diversity; flat face; cardiovascular abnormalities	Bowed long bones, metaphyseal widening; arachnodactyly; radioulnar synostosis; bone fragility; craniosynostosis

OCBMD, osteochondrodysplasia, brachydactyly, and overlapping malformed digits.

Structure and Synthetic Pathway of PGs

PGs are crucial for water retention in the extracellular matrix, which secures compressive strength. They also play an important role in embryonic morphogenesis and pre- and postnatal growth and maintenance of many tissues and organs. PGs encompass a diverse group of glycoproteins that are composed of a core protein and GAG side chains consisting of a linker region of four sugars (tetrasaccharide linker) and a long repeat of a pair of sugars (disaccharide unit) (Fig. 1). The polysaccharide chain is highly sulfated.

Fig. 1.

Composition of proteoglycan (PG). PGs are composed of a core protein and covalently attached side chains of highly sulfated polysaccharides termed glycosaminoglycans (GAGs). GAGs consist of a linker region of four sugars (tetrasaccharide linker) and a long repeat of a pair of sugars (disaccharide unit). Main GAGs include chondroitin sulfate (CS), dermatan sulfate (DS), and heparan sulfate (HS). The tetrasaccharide linker is attached to a serine residue of a core protein, and its formation is catalyzed by four glycosyltransferases (XYLT, B4GALT7, B3GALT6, and B3GAT3). The linker is universal to CS, DS, and HS, while each GAG has a specific disaccharide unit. GluUA and IdoUA are interchangeable via epimerization and reverse epimerization. Interestingly, binding of the first sugar of a disaccharide unit to the linker is catalyzed by a specific enzyme for CS and DS (CSGALNACT1) and the other for HS (EXTL3). Further polymerization of disaccharide units and sugar sulfation is catalyzed by many enzymes (CHSY1 and EXTL1/2, etc.).

PGs and their core proteins are given the same taxonomical terms, such as aggrecan, biglycan, and perlecan. Each type of PG is found in a specific tissue. For example, aggrecan is exclusively found in the cartilage. Main GAGs include chondroitin sulfate (CS), dermatan sulfate (DS), and heparan sulfate (HS). CS is mainly found in the cartilage, DS in the skin, tendons, blood vessels, and heart, and HS in the cell surfaces, basement membranes, and brain. However, both DS and CS have a pivotal role in cartilage homeostasis. The tetrasaccharide linker is universal to CS, DS, and HS, while each of CS, DS, and HS has a specific disaccharide unit.

GAG synthesis occurs in the Golgi apparatus (Fig. 2). Linker formation and disaccharide chain elongation are catalyzed by glycosyltransferases using a type of sugar donor (UDP sugar) synthesized in the cytoplasm, while GAG sulfation is conducted by sulfotransferases using a universal sulfate donor (PAPS). Core proteins are synthesized in the rough endoplasmic reticulum, transported into the Golgi apparatus, and bound to the linker. It is notable that DS can be derived from CS by epimerization and can be reconverted by reverse epimerization.

Fig. 2.

Scheme for synthesis of proteoglycan (PG). Assembly of PG begins with the synthesis of a core protein in the rough endoplasmic reticulum, which is transferred to the Golgi apparatus and bound to GAG chains composed of highly sulfated sugars. GAG chain formation is accomplished by consecutive glycosylation that requires a sugar donor (UDP sugar). (1) UDP sugar is synthesized in the cytoplasm and translocated to the Golgi apparatus by a UDP sugar/UMP exchanger (an antiporter with UMP). (2) Golgi-glycosyltransferases disassemble UDP sugar and transfer the liberated sugar to a GAG chain. (3) UDP is hydrolyzed to UMP and phosphate by CANT1, and UDP may inhibit glycosyltransferases. (4) GAG sulfation is conducted by transport of extracellular sulfate into cells, intracytoplasmic formation of a universal sulfate donor (PAPS), and PAPS translocation into the Golgi apparatus; Golgi sulfotransferases (CHST3, CHST11, CHST14, etc.) disassemble PAPS and transfer the liberated sulfate to a GAG chain. (5) PAP is hydrolyzed to AMP and phosphate by GPAPP, and PAP inhibits sulfotransferases.

Phenotypic Consequences of Abnormal GAG Synthesis

Abnormal GAGs impair the integrity of the connective tissue matrix and are responsible for the multiplicity of clinical symptoms in GAG synthesis disorders. Affected individuals present with bone, joint, and skin manifestations and occasionally visceral abnormalities.

Abnormal cartilage matrices interfere with endochondral skeletal development and early skeletal morphogenesis. Affected individuals present with short stature and occasionally have distinctive digital anomalies. The other diagnostic skeletal features are variable but include epiphyseal dysplasia, distal humeral hypoplasia, and a monkey wrench appearance of the proximal femora. Interestingly, accelerated carpal ossification is frequently seen.

Defective soft tissue matrices disturb joint and skin homeostasis. The joint abnormality usually manifests as joint laxity but occasionally as a joint restriction or even joint synostosis. The radioulnar joint is almost always affected. Clubfeet and spinal malalignment are frequent. The skin abnormality is similar to that of Ehlers-Danlos syndrome but variable in severity. In addition, facial dysmorphism, intellectual disability, and other morbidities, such as cardiovascular and ocular complications, may be present.

We discuss the skeletal phenotypes of disorders resulting from impairment of GAG sulfation, GAG chain elongation, and formation of the tetrasaccharide linker. It is assumed that the steps of the GAG synthetic pathway are recapitulated by the gradient of disease phenotypes. However, the presumption is not necessarily true, and the phenotypic diversity is more significant than expected. Nonetheless, we believe that most disorders share some common features, implying a common pathogenesis.

Group 1: Disorders of Impaired GAG Sulfation

Sulfation is essential for many biological processes, including hormone regulation via cytosolic sulfotransferases and extracellular matrix synthesis via Golgi sulfotransferases. Impaired sulfation of GAGs in the Golgi apparatus leads to abnormal matrix properties, which ultimately cause bone and joint manifestations and occasionally skin abnormalities. The GAG sulfation pathway comprises (1) transport of extracellular sulfate into the cell via a membrane transporter (SLC26A2 formerly termed DTDST) that acts as a sulfate-chloride antiporter or exchanger, (2) cytosolic activation of sulfate to 3′-phosphoadenosine 5′-phosphosulfate (PAPS) via an activating enzyme PAPS synthetase 2 (PAPSS2), (3) transport of PAPS into the Golgi apparatus via Golgi membrane transporters (SLC35B2 and SLC35B3), (4) sulfation of GAGs with conversion of PAPS to 3′-phosphoadenosine 5′-phosphate (PAP) via Golgi sulfotransferases (CHST3, CHST11, CHST14, etc.), and (5) hydrolysis of PAP to AMP and phosphate via a Golgi resident phosphoadenosine phosphate phosphatase (GPAPP, also known as IMPAD1 or BPNT2). Sulfation disorders are discussed in order of the synthetic pathway (Fig. 2).

DTD (SLC26A2 Related)

DTD is a well-known skeletal dysplasia recognizable at birth and most common among GAG sulfation disorders. The clinical characteristics of DTD were described in 1960, but defective sulfate transport due to mutations in the SLC26A2 gene was discovered about 30 years later. DTD is prevalent in Finland due to a founder mutation (c.−26+2T>C) [2]. The genetic spectrum of DTD has been well elucidated. Four common mutations (p.Arg279Trp, c.−26+2T>C, p.Arg178Ter, and p.Cys653Ser) account for approximately 65% of disease alleles in DTD [6].

Affected individuals show severe short stature (the average adult height: 136 cm for males and 129 cm for females) [7]. Joint abnormalities are primarily restrictive. However, some joints are lax. Hypermobile, proximally set, abducted thumbs manifest with a distinctive physical finding referred to as “hitchhiker’s thumb.” Clubfeet and progressive kyphoscoliosis are prevalent. Cystic swelling of the auricle and cleft palate are common syndromic components [8]. The histological hallmark of DTD is premature degeneration of resting chondrocytes with fibrotic foci in the cartilage matrix, which accounts for abnormal endochondral bone growth, premature closure of growth plates, epiphyseal dysplasia, and joint deformity seen in DTD.

The radiological characteristics include stunted long bones with metadiaphyseal broadening and generalized epiphyseal dysplasia (Fig. 3). The proximal femora are particularly broad, the lesser trochanters are protuberant, and the capital femoral epiphyses are especially flat and delayed in ossification. The appearance of the proximal femora is coined “mushroom deformity.” Premature closure of the growth plate is sometimes seen and causes metaphyseal chevron deformity of long bones. The distal humeri appear tapered on the lateral view and Y shaped on the frontal view. This distinctive elbow deformity is reminiscent of the chevron deformity of other long bones. Elbow dislocation is always seen. Carpal ossification is mildly advanced.

Fig. 3.

DTD (a severe phenotype). a–f Neonate. The thorax is mildly narrow. The vertebral bodies are slightly flat. The distal femora are mildly constricted. The distal humerus is tapered on the lateral view and Y shaped on the frontal view. The elbow joint is dislocated. The short tubular bones are short and broad. Hitchhiker’s thumb is seen along with an ovoid-shaped 1st metacarpal. Severe cervical kyphosis is evident along with hypoplasia of the mid-cervical vertebral bodies.

The first metacarpals are very short and ovoid shaped. Other short tubular bones are broad, along with flat epiphyses. Flat epiphyses lying on the sloping end of the distal metacarpal are compared to “snow melting on a roof.” The proximal and middle phalanges, particularly of the index and middle fingers, occasionally show malsegmentation, i.e., proximal symphalangism associated with delta-shaped, angel-shaped, and/or supernumerary phalanges (Fig. 4). The supernumerary phalanges are defined as “hyperphalangy” similar to that of brachydactyly type C. The vertebral bodies are almost normal in shape but may be mildly flat. Severe cervical kyphosis is a risk for myelopathy.

Fig. 4.

DTD (2 cases). a Two years. All short tubular bones are severely short and broad. Hitchhiker’s thumbs are seen. The 1st metacarpals are delta shaped. The middle fingers show osseous proximal symphalangism with an accessory ossicle at the proximal phalangeal base or the proximal segment of the hypersegmented proximal phalanx. The index and middle fingers are laterally deviated, while the ring and little fingers are medially deviated. b Early childhood. The pattern of the short tubular bones is similar to, but milder than, that of case a. The middle and ring fingers show hyperphalangy. A snow melting on a roof appearance of the metacarpals is seen.

Therapeutic Implications for DTD

Intracellular sulfate is mainly supplied by the transport of extracellular sulfate into the cell via SLC26A2. However, sulfate is also derived from the intracellular disassembly of sulfur-containing amino acids (cysteine and methionine) and other thiols. Therefore, N-acetylcysteine (NAC) attracted attention as a candidate for an alternative intracellular sulfate donor. NAC is a well-known drug with antioxidant properties commonly used as the antidote for acetaminophen intoxication and as a mucolytic expectorant. Prenatal and postnatal administration of NAC for an animal model of DTD (a Slc26a2 knock-in mouse) alleviated undersulfation of PGs in the cartilage matrix and improved the skeletal and cartilage alterations of the knock-in mice [9, 10]. Thus, NAC is a promising medicine that may be effective for bone and joint abnormalities in DTD.

Other SLC26A2-Related Skeletal Dysplasias

SLC26A2 mutations lead to a continuous spectrum of disorders, ranging from autosomal recessive multiple epiphyseal dysplasia (AR-MED), DTD, atelosteogenesis type 2 (AO2) to achondrogenesis type 1B (ACG1B) in increasing order of severity. The mildest phenotype of DTD merges with AR-MED, while the severest with AO2.

AR-MED was proposed as “multiple epiphyseal dysplasia without hitchhiker’s thumb, or overt short stature” and was also known as multiple epiphyseal dysplasia type 4 [11, 12]. AR-MED is the most prevalent phenotype in SLC26A2-related disorders, usually associated with p.Arg279Trp in homozygosity [13]. Short stature is not a feature (an average height of −1.23 SD), nor are severe joint manifestations. The skeletal changes are similar to those of mild DTD (Fig. 5). Mushroom-shaped proximal femora and snow melting on a roof appearance of the metacarpals are seen. However, epiphyseal flattening is not as marked as in DTD, and there is no accelerated carpal ossification [14]. A coronal cleft of the patella, termed double-layered patella, is a distinctive finding, which already attracted attention in a subset of MED a considerable time ago [15].

Fig. 5.

AR-MED. a–c Three years, 4 years, and 7 months. The long bones show metadiaphyseal broadening and epiphyseal flattening, particularly of the capital femoral epiphyses. The short tubular bones are broad, with a snow melting on a roof appearance of the metacarpals. The patella shows double ossification.

AO2 is a lethal disorder that initially attracted attention in terms of phenotypic similarities to AO1 and AO3 (FLNB-related lethal skeletal dysplasias). However, AO2 was proven to be allelic to DTD and indeed shows radiological similarities [16]. As with DTD, AO2 shows broad tubular bones, distal humeral hypoplasia, hitchhiker’s thumb, malsegmentation of short tubular bones, and cervical kyphosis. These findings appear much more severe than those of DTD (Fig. 6). In addition, the distal femora are hypoplastic, as are the distal humeri. Platyspondyly is severe and may be associated with multiple coronal clefts of the vertebral bodies.

Fig. 6.

AO2 (a mild phenotype) and ACG1B. a–c Neonate with AO. The thorax is narrow. The right distal humerus is tapered, while the left is Y shaped. The distal femora are constricted. Platyspondyly is severe. The elbow joint is dislocated. The short tubular bones are irregularly ossified. d Stillbirth with ACG1B at the second trimester. Ossification of the axial and appendicular skeleton is severely defective. The posterior neural arches show a cobra appearance.

ACG1B is one of the most severe lethal skeletal dysplasias, together with ACG1A (TRIP11 related) and ACG2 (COL2A1 related). ACG1B is caused by a complete loss of the SLC26A2 transporter function, and thus the manifestation is homogeneous [17, 18]. Extremely short limbs and fetal hydrops attract attention in the early second trimester. All bones show severe ossification defects (Fig. 6). Vertebral ossification is missing or rudimentary. Caudal ossification defects of the ilia manifest as a paraglider-like appearance. The long bones appear triangular or stellate in shape, but distal humeral tapering may be discernible. The short tubular bones are poorly ossified. These findings of ACG1B are similar to those of ACG1A. However, the appearance of the neural arches is different between the disorders. The interpedicular distances are wider in the cervical and lumbar spine than in the thoracic spine in ACG1B. This finding is termed “a cobra sign,” contrasting with the parallel orientation of the interpedicular distances of all spines in ACG1A.

Spondylo-Epi-Metaphyseal Dysplasia (PAPSS2 Related)

PAPSS2 deficiency impairs PAPS synthesis and creates a type of spondylo-epi-metaphyseal dysplasia (SEMD) better known as autosomal recessive brachyolmia (AR-BO). PAPS acts as a universal sulfate donor for sulfotransferase reactions, and thus, the phenotype of AR-BO is supposed to be reminiscent of that of DTD, but the presumption is not true. AR-BO is a late-onset skeletal dysplasia, and joint problems are not a major clinical issue. It is reported that p.Gly270Asp is a common pathogenic variant in the Northern European ethnicity [19].

Affected individuals present with postnatal short-trunked short stature, occasionally accompanied by precocious puberty due to deficiency of cytosolic sulfation of dehydroepiandrosterone [20]. The radiological hallmarks include severe platyspondyly with elongated vertebral bodies, irregular endplates, and a narrow lumbar spinal canal, as well as broad metaphyses of tubular bones (Fig. 7). The proximal femora show longitudinal striations. Epiphyseal dysplasia is not a cardinal feature [21]. At younger ages, the long bones manifest with dumbbell deformity resembling that of type 2 collagenopathy and type 11 collagenopathy [22, 23]. Dumbbell deformity becomes less striking with age.

Fig. 7.

AR-brachyolmia. a, b Adolescent boy. The vertebral bodies are flat and elongated. The spinal canal is narrow. The ilia are broad and short. The capital femoral epiphyses are mildly flat. The femoral necks are broad and show longitudinal striations.

Chondrodysplasia with Congenital Joint Dislocation (CHST3 Related)

CHST3-related chondrodysplasia is caused by a deficiency of chondroitin 6-O-sulfotransferase 1 encoded by the CHST3 gene [24]. This rare disorder is alternatively termed autosomal recessive Larsen syndrome, whereas autosomal dominant Larsen syndrome is a common skeletal dysplasia due to FLNB mutations.

As with dominant Larsen syndrome, recessive Larsen syndrome manifests with mild short stature with multiple congenital joint dislocations, and clubfeet; however, affected individuals do not show mid-face flattening characteristics of dominant Larsen syndrome. Cardiac valvular disease may be an essential syndromic component of recessive Larsen syndrome [25].

The radiological hallmarks are multiple coronal clefts of the vertebral bodies, increased intervertebral distance at Th12-L2 (another cobra sign), and bifid distal humerus resembling Y-shaped distal humeri resembling those of DTD (Fig. 8). The unique manifestations of the spine and distal humerus led to a previous disease name, humerospinal dysostosis [26]. The other features include cervical kyphosis, undertubulation of long bones, rounded epiphyses, and mildly accelerated carpal ossification. Kyphoscoliosis and disc degeneration are progressive with age, resulting in a short trunk. The late manifestation was previously termed spondyloepiphyseal dysplasia Omani type [27]. The final adult height was 110–139 cm in a large inbred affected family in Oman.

Fig. 8.

AR-Larsen dysplasia (2 cases). a, b At birth and 1 year. The knee joints are dislocated along with genu recurvatum. The hip joints are also dislocated. The long bones of the legs are undertubulated. c–e Three months and 10 months. The neural arches show a cobra sign. The lumbar vertebral bodies show multiple coronal clefts. The distal humerus is Y shaped. The humerus is undertubulated.

Osteochondrodysplasia, Brachydactyly, and Overlapping Malformed Digits (CHST11 Related)

Osteochondrodysplasia, brachydactyly, and overlapping malformed digits is caused by a deficiency of chondroitin 4-O-sulfotransferase 1 encoded by the CHST11 gene [28]. Very few affected individuals have been reported. They show mild short stature and distinctive digital malformations. The skeletal hallmarks are type C brachydactyly-like digital anomalies, including short, deviated index and middle fingers with hyperphalangy. The ring finger may have hyperphalangy but is less severely affected, and thus it should be the longest finger.

Chondrodysplasia with Congenital Joint Dislocation (GPAPP Type)

Chondrodysplasia with congenital joint dislocation GPAPP type is alternatively named IMPAD1-related chondrodysplasia. The name of this disorder is confusing because of renaming the causing gene. The phenotype is caused by impairment of GPAPP (Golgi 3′-phosphoadenosine 5′-phosphate 3′-phosphatase) encoded by the BPNT2 gene (formerly IMPAD1) that catalyzes the hydrolysis of PAP to AMP and phosphate. GPAPP deficiency causes an abnormal accumulation of PAP and leads to sulfotransferase inhibition [29]. Multiple involvement of sulfotransferases may account for the phenotypic multiplicity of this disorder.

Affected children show short-limbed short stature, facial dysmorphism (mid-face hypoplasia and micrognathia with cleft palate), and congenital dislocations. They also present with distinctive malalignment of the fingers and toes (i.e., radial deviation of the thumb resembling the hitchhiker’s thumb, ulnar deviation of the index finger, and lateral deviation of the 5th toe). The digital malformation is radiologically depicted as a delta-shaped proximal phalanx of the thumb, hypoplastic middle phalanx and anomalous metacarpal epiphysis of the index finger, a delta-shaped proximal phalanx of the great toe, and longitudinally splitting or delta-shaped metatarsal of the 5th toe (Fig. 9) [29]. Supernumerary carpal bones and carpal fusion may be seen. Metadiaphyseal undertubulation of long bones and capital femoral epiphyseal dysplasia can be seen.

Fig. 9.

GPAPP-related chondrodysplasia (2 cases) and CHSY1-related chondrodysplasia. a, b Different infants with GPAPP-related chondrodysplasia. The proximal phalanx of the thumb is delta shaped, and the middle phalanx of the index finger is hypoplastic. The proximal phalanx of the great toe is delta shaped. The metacarpal of the 5th great toe is rudimentary. c, d 28-Month-old child with CHSY1-related chondrodysplasia. The thumb is very short along with dysplastic metacarpal and proximal phalanx. The index and middle fingers show hyperphalangy. The great toe is extremely short along with double ossifications of the proximal phalanx and triple ossifications of the metatarsal. A supernumerary bone is seen between the 2nd and 3rd toes.

Ehlers-Danlos Syndrome: Musculocontractural Type 1 (CHST14 Related)

EDSMC1 is caused by a deficiency of dermatan 4-O-sulfotransferase 1 encoded by the CHST14 gene. This disorder was previously termed “adducted thumb, clubfoot, and progressive joint and skin laxity syndrome.” The pathogenesis is unique. The sulfation defect of DS accelerates conversion from DS to CS (reverse epimerization). The lack of DS is responsible for the clinical phenotype. The cardinal features are EDS-like skin and soft tissue manifestations, including hyperextensible skin with easy bruising, recurrent subcutaneous hematomas, and visceral fragility (cardiac valvular incompetence, gastrointestinal perforation, etc.), as well as joint manifestations (distal arthrogryposis, particularly adducted thumbs, clubfeet, and hypermobility of large joints) [30, 31]. Affected individuals also manifest with an abnormal face (open fontanel, blue sclera, blepharophimosis, downward slanting palpebral fissures, micrognathia, etc.), marfanoid habitus, and ocular abnormalities (microcornea, anterior chamber defect, and glaucoma). The skeletal phenotype comprises severe kyphoscoliosis with dolichospondyly and arachnodactyly with slender short tubular bones, which may be largely secondary to severe joint abnormalities. EDSMC2 is caused by defective conversion from CS to DS (epimerization), and the phenotype is much milder than that of EDSMC1 [30].

Newly Identified Disorders of Abnormal Sulfation

SLC13A1-related spondylo-epi-metaphyseal dysplasia is a bone dysplasia associated with hyposulfatemia and hypersulfatauria, which is caused by a deficiency of a renal and intestinal sulfate transporter. SLC13A1-mediated sulfate reabsorption in the renal proximal tubule is particularly important to maintain serum sulfate levels. SLC13A1 is a natrium-sulfate cotransporter in the brush-border membrane, which is coupled with SLC26A1, a sulfate-anion exchanger or antiporter in the basolateral membrane. It is tempting to assume that the low plasma sulfate levels lead to a defect of SLC26A2-mediated sulfate uptake into connective tissue cells, putative lack of cytoplasmic sulfate, and cartilage GAG undersulfation. Affected individuals present with normal height or mild short stature with a short trunk and mild restriction of large joints, probably due to large epiphyses, but not extraskeletal features. The skeletal alterations resemble those of AR-BO, including moderate platyspondyly with biconvex vertebral bodies and anterior tongue, striking metaphyseal flaring, and retarded epiphyseal ossification. The vertebral and metaphyseal changes become less striking with age. Intravenous NAC administration can normalize plasma sulfate levels.

SLC35B2-related chondrodysplasia with hypomyelinating leukodystrophy is the first recognized disorder of an abnormal Golgi membrane PAPS transporter [32]. The clinical hallmarks include severe psychomotor delay with hypomyelinating leukodystrophy and corpus callosum hypoplasia, Pierre-Robin sequence, severe short stature, and distal arthrogryposis and dislocations of large joints. The skeletal manifestations are reported to be hypoplastic cervical vertebrae (C3–C4), early onset of scoliosis, subluxation and dislocation of large joints, flat acetabula, mild epiphyseal dysplasia and metaphyseal broadening of long bones, and advanced carpal ossification. Given the severe neurological manifestation of this disorder, SLC35B2 is likely to be important in the central nervous system rather than the bone and joint.

HS2ST1-related neurofacioskeletal syndrome (with or without renal agenesis) is caused by a deficiency of HS 2-O-sulfotransferase 1 [33]. The clinical hallmarks include severe developmental delay with corpus callosal hypoplasia and facial dysmorphism (hypertelorism, upslanted palpebral fissures, broad nasal tip, wide mouth, etc.), commonly associated with uni- or bilateral renal agenesis. Affected individuals also present with micromelic short stature, kyphoscoliosis, flexion contractures of large joints, and brachydactyly with broad fingertips and toes. The skeletal changes comprise broadening of long bones, shortening of the middle phalanges, and broadening of the distal phalanges.

SLC10A7-related skeletal dysplasia with amelogenesis imperfecta is not a sulfation disorder, but is commented here, because the disorder manifests with skeletal alterations resembling those of DTD. The studies for Slc10a7 knock-out mice revealed reduced HS in the cartilage but not in the muscle. Neither structural alteration nor sulfation defect of HS was ascertained. HS was also decreased in patient fibroblasts. The pathogenic mechanism for the defective GAG synthesis remains undetermined. Affected individuals present with pre- and postnatal short stature, facial abnormalities (a flat, round face, micrognathia, and cleft palate), dislocation of large joints, and kyphoscoliosis. Hypomineralized amelogenesis imperfecta is a distinctive finding. The skeletal changes include mild spondylar dysplasia, broad femoral with prominent lesser trochanters, flat epiphyses, and advanced carpal ossification.

Group 2: Disorders of GAG Chain Elongation

Elongation of GAG polysaccharides (both tetrasaccharide linker and disaccharide units) is accomplished by consecutive glycosylation using a sugar donor (UDP sugar) (Fig. 1). UDP sugar is synthesized in the cytoplasm and transported into the Golgi apparatus, where its sugar is transferred to a GAG chain via Golgi glycosyltransferases and UDP (a by-product of Golgi glycosyltransferase reaction) is catabolized to UMP and phosphate.

Attachment of the first sugar of a disaccharide unit to the tetrasaccharide linker is catalyzed by GAG-specific glycosyltransferases. The glycosyltransferases catalyzing the initial step are different between CS/DS and HS, and the first sugars are GalNAc for CS/DS and GlcNAc for HS. Further elongation of disaccharide units is catalyzed by a number of glycosyltransferases.

We discuss the disorders with impairment of disaccharide unit elongation, sugar donor assembly, and binding of the first sugar to the linker. Other than Desbuquois dysplasia (DBQD) and hereditary multiple exostoses (HMEs), very few cases have been reported in this group of disorders.

Temtamy Preaxial Brachydactyly Syndrome (CHSY1 Related)

Temtamy preaxial brachydactyly syndrome is caused by defective CS synthase 1 encoded by the CHSY1 gene, which interferes with the elongation of the CS disaccharide unit (Fig. 1). The resultant phenotype resembles those of sulfation disorders of CS (CHST11- and GPAPP-related disorders).

Affected individuals manifest with intellectual disability, micrognathia, dental anomalies (talon cusps), sensorineural hearing loss, and short stature. The preaxial digital malformation is characteristic [34, 35]. The digital anomalies comprise hitchhiker-like thumbs and great toes (radial or medial deviation with very short, misshapen proximal phalanges and metacarpals/metatarsals) and type C brachydactyly-like phalangeal malsegmentation mainly affecting the 2nd and 3rd digits and less severely the 4th digit, i.e., hyperphalangy, delta-shaped phalanges, and proximal symphalangism (Fig. 9). Carpal and tarsal fusion and radioulnar synostosis may be seen.

Schneckenbecken Dysplasia (SLC35D1 Related)

Schneckenbecken dysplasia is a lethal skeletal dysplasia caused by mutations in the SLC35D1 gene encoding a Golgi membrane transporter for UDP sugars that are the components of the CS disaccharide unit (UDP-GluUA and UDP-GalNAc). Defects of the transporter function reduce the availability of sugar donors for CS elongation. Only a few SLC35D1 mutations are currently known in the lethal disorder [36, 37]. Recently, one family with a non-lethal SLC35D1-related skeletal dysplasia has been reported [38].

Schneckenbecken means “snail pelvis” in German, referring to a distinctive shape of the ilia resembling a snail (round ilia with medial bone projection) [36, 39]. The other hallmarks include platyspondyly with dorsal underossification of vertebral bodies, interpedicular narrowing of the lower lumbar spine, and mild dumbbell deformity of long bones. The overall pattern is similar to but milder than that of ACG.

Arthrogryposis, Impaired Intellectual Development, and Seizures (SLC35A3 Related)

SLC35A3-related disorder is caused by a defective Golgi transporter of UDP-GlcNAc for the disaccharide unit of HS. The clinical manifestation is distal arthrogryposis that can be associated with microcephaly, intellectual disability, and intractable seizures. The skeletal changes are composed of camptodactyly, malalignment of the distal limbs, and scoliosis due to multiple vertebral malsegmentation [40, 41]. Given the combination of severe neurologic symptoms and vertebral malformation, SLC35A3 is presumed to be important in brain development and spinal segmentation at the embryonic stage.

DBQD (CANT1 Related)

DBQD is the second most common disorder among GAG synthesis disorders, which is caused by mutations in the CANT1 gene. CANT1 disassembles UDP (a by-product of Golgi glycosyltransferase reaction) to UMP and phosphate. UMP is transported from the Golgi apparatus to the cytoplasm and recycled for intracytoplasmic UDP-sugar assembly [42]. The pathogenesis of DBQD remains elusive. Decreased UMP may hamper the recycling process for UDP-sugar synthesis. On the other hand, an abnormal accumulation of UDP may inhibit glycosyltransferase activities, as with sulfotransferase inhibition in GPAPP-related chondrodysplasia (Fig. 1).

The clinical hallmarks of DBQD include mid-face flattening, micromelic short stature (−4SD to −10SD), and severe joint laxity commonly associated with congenital dislocations, the constellation of which was often misdiagnosed with autosomal dominant Larsen syndrome. Affected individuals with DBQD show a wide range of clinical severity. Severely affected children may succumb to lung hypoplasia in infancy. The radiological findings are also diverse. However, there are two skeletal changes that aid in the diagnosis, i.e., markedly advanced carpal ossification and a monkey wrench appearance of the proximal femora (Fig. 10, 11). Accelerated carpal ossification is often very striking, and several carpal bones are ossified even in early infancy. Monkey wrench femora results from short, broad proximal femora with prominent lesser trochanters, which gives a diagnostic clue at an early age, but becomes less pronounced with age [43]. Other skeletal changes include metaphyseal broadening of long bones and epiphyseal dysplasia, particularly prominent in the proximal femoral epiphyses. Coronal clefts of vertebral bodies may be seen in severely affected individuals.

Fig. 10.

DBQD type 1 (2 cases). a–c At birth. The knee joints are dislocated with genu recurvartum. The elbow joints are dislocated as well. The lesser trochanters are protuberant. Multiple coronal clefts of the lumbar spine are seen. There are hyperphalangy of the index finger and duplication of the distal and proximal phalanges of the thumb. The short tubular bones are overtubulated. The triquetrum is prematurely ossified. d At birth. A monkey wrench appearance of the proximal femoral is apparent. Dislocation of the hip and knee joints is seen.

Fig. 11.

DBQD type 2 (2 cases). a, b Six months and 10 months. The lesser trochanters are protuberant. Dislocation of the hip joints is seen. Carpal ossification is strikingly advanced. c, d 2 years and 7 months. A monkey wrench appearance of the proximal femora is evident. The capital femoral epiphyses are flat. Carpal ossification is advanced. The metacarpals are short, while the phalanges are relatively elongated.

On radiological grounds, DBQD is subdivided into DBQD type 1 with hyperphalangy and DBQD type 2 without hyperphalangy. In general, DBQD1 tends to be clinically more severe than DBQD2 [44]. The hyperphalangy of DBQD1 usually occurs in the index finger but sometimes in the middle finger and is commonly accompanied by other digital anomalies, e.g., bifid distal phalanges and misshapen proximal phalanges of the thumb and interphalangeal subluxation. It is noteworthy that hyperphalangy of the index and middle fingers is a diagnostic clue to other malformation syndromes, e.g., Catel-Manzke syndrome, Chitayat syndrome, Kynureninase deficiency, and type C brachydactyly, as well as DTD, CHST11 deficiency, and CHSY1 deficiency [45–47]. DBQD2 is prevalent in the Korean and Japanese populations due to a founder effect (p. Val226Met) and is alternatively termed Kim variant [48, 49]. In Kim variant, the metacarpals are short, while the phalanges tend to be relatively elongated. The overall radiological pattern of DBQD2 is intimated by a few other GAG synthesis disorders (described later).

Skeletal Dysplasia with Joint Laxity and Advanced Bone Age (CSGALNACT1 Related)

SDJLABA is caused by impaired binding of the first sugar (GalNAc) of the disaccharide unit of CS/DS to the linker, which is catalyzed by CS N-acetylgalactosaminyltransferase 1. Its phenotype was reported as a bone dysplasia similar to, but milder than, that of DBQD2 [50]. The clinical features include mild facial dysmorphism, language delay, joint laxity, and micromelia. The skeletal changes comprise monkey wrench femora, advanced carpal bone age, coronal clefts of vertebral bodies, flat acetabula, and metadiaphyseal broadening of long bones; however, those changes are milder [51, 52].

SEMD with Immune Deficiency and Intellectual Disability (EXTL3 Related)

EXTL3-related SEMD is caused by impaired binding of the first sugar (GlcNAc) of the disaccharide unit of HS to the linker, catalyzed by exostosin-like glycosyltransferase 3. This disorder is a multisystem disorder involving the brain, immune system, liver, as well as bone [53, 54]. The clinical manifestations include severe short stature and developmental delay that may be complicated by craniosynostosis, hepatorenal cysts, and combined immunodeficiency varying in combination. The immunodeficiency can be accompanied by Omenn syndrome with skin changes. The skeletal phenotype is severe spondylo-epi-metaphyseal dysplasia. Joint symptoms are not prominent.

Hereditary Multiple Exostoses

It is surprising to know that HMEs are related to abnormal HS synthesis. HMEs are divided into types 1, 2, and 3. It is currently known that HME types 1 and 2 are related to heterozygous loss-of-function mutations in the EXT1 gene encoding exostosin glycosyltransferase 1 (EXT1) and the EXT2 gene encoding exostosin glycosyltransferase 2 (EXT2), respectively [55]. These enzymes are pivotal for polymerization of HS. However, it is thought that a deficiency of EXT1 and EXT2 by itself does not trigger the development of exostoses, unless a hitherto undefined “second hit” does occur.

Group 3: Disorders of Linker Formation

The assembly of the tetrasaccharide linker is catalyzed by four consecutive glycosyltransferases (Fig. 1). Deficiency of these enzymes causes a disorder termed “linkeropathy.” Since the linker is universal to CS, DS, and HS, linkeropathies might be expected to share common clinical and radiological findings. However, the phenotypic variability is more important than expected. The variations can be significant even in the same gene mutations [56].

All linkeropathies show severe joint laxity and skin elasticity and share common facial abnormalities (frontal bossing, mid-face flattening, micrognathia, etc.) in varying severity. Short stature tends to be severe and intellectual disability is not uncommon. On the other hand, the skeletal phenotypes are diverse. One skeletal phenotype is reminiscent of DBQD1 and DBQD2, another is a type of spondylo-epi-metaphyseal dysplasia, and the other is akin to Marfanoid syndrome or severe arthrogryposis syndrome. Additional clinical reports are necessary to define the phenotypic spectrum of linkeropathies. We discuss these disorders in order of the synthetic pathway.

Baratela-Scott Syndrome (XYLT1 Related)

Xylosyltransferases 1 and 2 are encoded by the XYLT1 and XYLT2 genes, respectively. These enzymes catalyze the first step of linker formation. XYLT2 deficiency causes a brittle bone syndrome termed spondylo-ocular syndrome with severe osteoporosis, cataracts, and cardiovascular abnormalities. On the other hand, XYLT1 deficiency causes a skeletal dysplasia resembling DBQD2. Thus, the phenotype was initially named “DBQD type 2”; however, the disorder is currently renamed Baratela-Scott syndrome [5, 57–60].

Affected individuals may show synophrys. As with DBQD2, Baratela-Scott syndrome radiologically shows monkey wrench femora and markedly advanced carpal ossification (Fig. 12). The long bones appear mildly broad. Metacarpal shortening is pronounced, while the phalanges are not as short as the metacarpals.

Fig. 12.

Baratela-Scott syndrome. a, b Two years and 3 months. A monkey wrench appearance of the proximal femora is evident. The capital femoral epiphyses are flat. The long bones show metadiaphyseal broadening. The elbow is dislocated. Carpal ossification is strikingly advanced.

Ehlers-Danlos Syndrome: Spondylodysplastic Type (B4GALT7 Related)

The B4GALT7 gene encodes for β1,4-galactosyltransferase 7. The phenotype of B4GALT7 deficiency was initially termed “progeroid Ehlers-Danlos syndrome,” which was renamed EDSSPD1 in the 2017 EDS nosology [61].

Affected individuals show a flat face with prominent eyes, deep nasolabial folds, and microstomia, giving an impression of a senile appearance. The skeletal manifestations vary in severity (Fig. 13). Mildly affected individuals show skeletal changes similar to those of DBQD2, including monkey wrench femora, advanced carpal ossification, and metaphyseal broadening of long bones. Radioulnar synostosis is often seen [62]. Severely affected individuals manifest with bowing and overtubulation of long bones and arachnodactyly, the constellation of which is superficially similar to Marfanoid syndromes.

Fig. 13.

B4GALT7-related chondrodysplasia (2 cases). a–c Mildly affected child at ages 1 month and 9 months. Mild arachnodactyly is seen, and carpal ossification is advanced. The proximal femora show a monkey wrench appearance. The long bones are broad. Radioulnar synostosis is seen. d Severely affected fetus (terminated in the second trimester). The long bones are overtubulated, and the femora are bowed. The elbow joints are malaligned. Arachnodactyly is seen.

A variant of EDSSPD1 is prevalent in Reunion Island in the Southern Indian Ocean due to a founder mutation (p.Arg270Cys). The phenotype was named “Larsen of Reunion Island syndrome” [63]. The disorder is associated with severe congenital dislocations, but its skeletal manifestation is identical to that of severe EDSSPD1. EDSSPD1-like phenotypes are reported in individuals with mutations in B3GALT6, encoding another glycosyltransferase for linker formation, and those with mutations in SLC39A13, encoding a zinc transporter termed ZIP13. The former is called EDSSPD2, while the latter is EDSSPD3.

Spondylo-Epi-Metaphyseal Dysplasia with Joint Laxity Beighton Type or Ehlers-Danlos Syndrome: Spondylodysplastic Type 2 (B3GALT6 Related)

The B3GALT6 gene encodes for β1,3-galactosyltransferase 6. The concept of linkeropathy started with the discovery of B3GALT6 mutations in SEMDJL Beighton type (SEMDJL-B) [64]. SEMDJL-B was first reported and is frequent in the Afrikaner population of South Africa due to a founder effect (Thr79Ala; Pro67Leu) [65].

Affected individuals may show blue sclera. Progressive kyphoscoliosis leads to severe physical morbidities. The skeletal changes are characterized by severe platyspondyly with an anterior projection of the vertebral bodies, broad ilia with short greater sciatic notches, prominent lesser trochanters, metaphyseal flaring of long bones, mildly advanced carpal ossification, and radioulnar dislocation (Fig. 14). The combination of severe platyspondyly and iliac hypoplasia is a unique finding that is not found in other linkeropathies.

Fig. 14.

SEMDJL Beighton type. a–c 10 months and 5 years and 10 months. The ilia are broad, and the greater sciatic notches are short. The lesser trochanters are protuberant. Platyspondyly with anterior tongue of the vertebral bodies is evident. Severe scoliosis becomes manifest with age.

Some individuals with B3GALT6 mutations show more severe skin manifestations (thin hair, wrinkled skin, prolonged wound healing, and scarring) and more marked joint laxity than those of SEMDJL-B. The phenotype was initially termed Ehlers-Danlos syndrome-like connective tissue disorder and is currently renamed EDSSPD2 [66]. However, the skeletal manifestation of EDSSPD2 is identical to that of SEMDJL-B.

Mutations of B3GALT6 were also reported in Al-Gazali syndrome, a semilethal arthrogryposis syndrome characterized by growth retardation, corneal opacity, and large joint contractures [67, 68]. Diaphyseal overtubulation, arachnodactyly, bone fragility, and radioulnar synostosis are documented in the disorder.

The activity of β1,3-galactosyltransferase 6 is enhanced by phosphorylation of the xylose residue of the linker region. The phosphorylation is catalyzed by GAG xylosylkinase encoded by the FAM20B gene. Deficiency of the xylosylkinase causes a severe skeletal dysplasia resembling severe cases of DBQD [69].

Multiple Joint Dislocations, Short Stature, Craniofacial Dysmorphisms, and Skeletal Dysplasia, with or without Heart Defects (B3GAT3 Related)

The B3GAT3 gene encodes for β1,3-glucuronosyltransferase 3. The phenotype of B3GAT3 mutations was initially reported as a Larsen dysplasia-like disorder associated with cardiac malformation [70]. Additional reports have revealed striking phenotypic variabilities in B3GAT3 mutations, ranging from lethal phenotypes to mild connective tissue diseases.

Affected individuals show severe mid-face flattening resembling that of autosomal dominant Larsen dysplasia. Notably, 60% of affected individuals have cardiovascular abnormalities (aortic root dilation, valvular disease, ASD, VSD, etc.). The skeletal hallmarks include modest platyspondyly, iliac flaring, metaphyseal broadening of long bones, and arachnodactyly (Fig. 15) [71, 72]. The skeletal phenotype of severely affected individuals was previously termed “pseudodiastrophic dysplasia” [73]. Other skeletal findings include craniosynostosis, radioulnar synostosis, and bone fragility. The constellation of craniosynostosis, arachnodactyly, and cardiac abnormalities sometimes seen in individuals with B3GAT3 mutations may be confused with Marfanoid syndromes, e.g., Antley-Bixler syndrome, neonatal Marfan syndrome, and Loeys-Dietz syndrome.

Fig. 15.

B3GAT3-related chondrodysplasia (2 cases). a–c Mildly affected child at age 32 months. Mild platyspondyly is seen. The ilia are flared. The proximal femora are broad. The elbow is dislocated. Arachnodactyly is seen. d–f Neonate with the phenotype of pseudodiastrophic dysplasia. There are mild platyspondyly, iliac flaring, metaphyseal flaring of long bones, femoral bowing, elbow dislocation, and malalignment of the short tubular bones.

Group 4: Disorders of Abnormal Core Protein

The discussion of disorders with abnormal core proteins is beyond this short review. In general, the major features of core protein abnormalities are generalized skeletal changes but not joint or skin abnormalities. Examples in this category include abnormalities of aggrecan, biglycan, and perlecan, encoded by the ACAN, BGN, and HSPG2 genes, respectively. Biallelic mutations of ACAN and hemizygous mutation of BGN lead to spondylo-epi-metaphyseal dysplasias, while biallelic HSPG2 mutations are responsible for Schwartz-Jampel syndrome and dyssegmental dysplasia [74–77].

Conclusion

We reviewed the manifestations of skeletal disorders with abnormal GAG biosynthesis. Awareness of the phenotypic uniqueness and overlapping features facilitates the diagnosis of this diverse group of disorders. Accurate diagnosis enhances our understanding of the large group of disorders.

Acknowledgments

We are grateful to Professors David Chitayat, Ok Hwa Kim, Nursel Elcioglu, and Stephen Robertson and Dr. Masanori Nishikawa, Naoya Morisada, and Ritsuko Pooh for providing us with the images.

Conflict of Interest Statement

Gen Nishimura was a member of the journal’s Editorial Board at the time of submission.

Funding Sources

This work was supported by European Commission-Next Generation EU, Mission 4/1 CUP F53D23005310006, and “Dipartimenti di Eccellenza 2023–2027” to A.R. for access to literature databases and full access to articles.

Author Contributions

Y.T., S.U., and P.O.S.K. raised the conceptual idea and collected the relevant images. A.H. and T.K. provided some images. Y.T. wrote the original manuscript with support from S.U. This work was supervised by G.N. (radiology), A.R. (biology), and A.S.F. and M.J. (perspective). All authors read and agreed to submit the manuscript. We contacted Dr. Maki Fukami for prior discussion regarding the scope of our work, and it was suggested that the content of our review article would be relevant and suitable for consideration in this journal.

Funding Statement

This work was supported by European Commission-Next Generation EU, Mission 4/1 CUP F53D23005310006, and “Dipartimenti di Eccellenza 2023–2027” to A.R. for access to literature databases and full access to articles.