Genetic Evaluation for Monogenic Disorders of Low Bone Mass and Increased Bone Fragility: What Clinicians Need to Know

Abstract

Purpose of Review

The purpose of this review is to outline the principles of clinical genetic testing and to provide practical guidance to clinicians in navigating genetic testing for patients with suspected monogenic forms of osteoporosis.

Recent Findings

Heritability assessments and genome-wide association studies have clearly shown the significant contributions of genetic variations to the pathogenesis of osteoporosis. Currently, over 50 monogenic disorders that present primarily with low bone mass and increased risk of fractures have been described. The widespread availability of clinical genetic testing offers a valuable opportunity to correctly diagnose individuals with monogenic forms of osteoporosis, thus instituting appropriate surveillance and treatment.

Summary

Clinical genetic testing may identify the appropriate diagnosis in a subset of patients with low bone mass, multiple or unusual fractures, and severe or early-onset osteoporosis and thus clinicians should be aware of how to incorporate such testing into their clinical practices.

Introduction

Disorders that manifest with decreased bone mass and increased risk of fractures are often encountered in the practice of both adult and pediatric medicine. Osteoporosis, the most common bone disease in the U.S., has been estimated to affect 1 in 8 individuals over the age of 50 years. An additional 43% of U.S. adults have low bone mass, a risk factor for fragility fractures [1]. The prevalence of osteoporosis and fragility fractures is low in healthy children but may be high in children with conditions that affect mobility [2, 3]. Fractures confer significant morbidity and the cost burden of treating osteoporosis-related fractures in the U.S. has been estimated to exceed $25 billion by 2025 [4]. Thus, appropriate screening, diagnostic evaluation, and treatment of individuals with osteoporosis are critical.

Osteoporosis is a multifactorial disorder that results from complex interactions between genetics, nutrition, hormones, physical activity, medication use, lifestyle habits, and comorbidities such as endocrine, gastrointestinal, and renal disorders [5]. Heritability assessments and genome-wide association studies have underscored the significant genetic contributions to the pathogenesis of osteoporosis [6–16]. Whereas all forms of osteoporosis have genetic contributions, monogenic forms of osteoporosis, i.e., osteoporosis caused by pathogenic variants with large effect sizes in single genes, are also not uncommonly encountered in clinical practice and such patients may present with early-onset disease, unusually low bone mass, and recurrent fractures. The rapid advances in and the widespread availability of molecular cytogenetic techniques and massively parallel sequencing (MPS) technologies over the past two decades have brought genetic testing into the mainstream practice of clinical medicine. However, healthcare providers are often faced with a bewildering array of clinical genetic testing options available for diagnostic evaluation of individuals with monogenic forms of osteoporosis including single gene and multiple gene panel-based sequencing assays, as well as genome-wide tests such as chromosome microarray analysis (CMA), exome sequencing (ES), and whole genome sequencing (WGS). Some pertinent questions that may arise in clinical practice include: “Which patients with low bone mass or increased bone fragility need clinical genetic testing?” “Which type of clinical genetic testing is appropriate and what type of genetic variants do these tests detect?” and “What counseling do patients need while undergoing clinical genetic evaluation?” In this review, we aim to provide a brief overview of the genetic contributions to the development of low bone mass and increased bone fragility, highlight the monogenic forms of low bone mass/early-onset osteoporosis, enumerate the sequencing technologies used for clinical genetic testing, and outline the basic principles of genetic evaluation and types of genetic variants that are reported on clinical genetic testing reports. This review is intended to be a practical resource for clinicians without specialized expertise in medical genetics who would want to perform genetic evaluation in individuals with suspected monogenic forms of osteoporosis.

Genetic architecture of bone mineral density and fracture risk

Previous studies have uncovered the significant contributions of genetic factors in determining bone mineral density, bone geometry, cortical and trabecular architecture, and fractures [6–20]. For example, in a twin study involving middle-aged women, 33% to 81% of variance in the cortical and medullary cross-sectional area, cortical porosity and trabecular volumetric bone mineral density at the distal tibia and radius were explicable by genetic factors [10]. However, heritability estimates have shown significant variation based on location and methodology of measurement of bone mineral density [10, 11, 21, 22]. With the increased availability of human genotyping data, genome-wide association studies (GWAS) have allowed us to understand the association between numerous genetic variants across the human genome and areal bone mineral density (aBMD), cortical and trabecular volumetric bone mineral density (vBMD), bone geometry, and risk for fractures [23–27]. Through large individual GWAS and meta-analyses of such GWAS, over 500 loci that confer risk for osteoporosis have been identified [14, 24, 28–32]. However, most of these commonly occurring variants or loci can cumulatively explain less than 20% of the variance in bone density [30–32]. Interestingly, rare genetic variants with large effect sizes including those in genes that are known to cause monogenic forms of osteoporosis have also been shown to be associated with low aBMD and fracture risk [33, 34]. The Osteoporotic Fractures in Men Study (MrOS) evaluated the burden of rare variants on osteoporotic fracture in men over 65 years of age and found that men with the lowest adjusted aBMD had three times as many rare variants in genes known to cause osteogenesis imperfecta (i.e., brittle bone disease) and two times as many rare variants in genes known to cause Ehlers Danlos syndrome compared to controls, suggesting that the rare pathogenic variants in genes known to cause monogenic disorders may be relevant in individuals with very low bone density [7, 9]. The genetic architecture of osteoporosis is highly complex and is likely determined by interaction of the cumulative effective size from numerous commonly occurring alleles with rare variants with larger effect sizes [7] [6, 35]. Furthermore, the interactions of environmental factors and other comorbidities have an important role in determining the bone mass and fracture risk. Providing a nuanced and comprehensive discussion of the genetic architecture of complex traits like osteoporosis is beyond the scope of this review and the readers are encouraged to refer to previously published high quality, comprehensive articles [6–8].

Whereas most diagnostic laboratories do not routinely offer clinical genotyping to assay for common or rare genetic variants that have been shown to associate with low BMD, the current wealth of information from GWAS has allowed for development and refinement of osteoporosis polygenic risk scores (PRS). These scores use an individual’s genetic information to estimate likelihood of fracture, secondary to the burden of known osteoporosis-associated variants [36–40]. In some studies, PRS have been used successfully to refine the risk of fracture thus allowing clinical decision making on screening [37, 39]. Interestingly, individuals with osteoporosis who have low PRS have been shown to be more likely to harbor rare pathogenic variants in osteoporosis risk genes [34]. However, the bias of genetic data from individuals with European ancestry in the existing GWAS can be a limitation while using PRS in a diverse population of individuals, particularly those with African ancestry [37, 38, 41, 42]. Whereas PRS is not typically used in patient care outside of specialized centers, the utility of calculating a PRS score may be twofold: quantifying burden of common risk variants with subsequent incorporation into existing clinical screening mechanisms and identifying patients with low bone density disease and low PRS scores who may then benefit from further clinical genetic testing for rare variants [34, 37]. The current utility of clinical genetic testing predominantly lies in identifying monogenic forms of low aBMD and fractures and occasionally identifying oligogenic contributors to early-onset or particularly severe disease.

Monogenic causes of low bone mass and increased bone fragility

Genetic skeletal disorders which are constitutional errors of bone development caused by pathogenic variants in single genes have an estimated prevalence of 1 in 5000 births [43]. The current nosology of genetic skeletal disorders categorizes 771 disorders caused by variants in 552 genes into 41 groups. The “Osteogenesis imperfecta and bone fragility group” comprising over 50 disorders represents the well-known monogenic forms of osteoporosis [44]. An abbreviated list of monogenic forms of osteoporosis has been outlined in Supplementary Table 1. Such forms of bone fragility should be suspected in patients who present with recurrent fragility fractures, unusually low bone mass, bone deformities, short stature, extraskeletal manifestations, and childhood-onset disease. However, milder forms of monogenic bone disorders may first manifest during adolescence or adulthood. There is significant phenotypic variability in monogenic forms of osteoporosis and the severity is not only influenced by the type of pathogenic variant but also by modifier genes, and genetic burden of many common variants; such complex interactions have been reviewed elsewhere [6]. In children and adults who had undergone genetic testing for evaluation of osteoporosis without a known cause, COL1A1 and COL1A2-related osteogenesis imperfecta was the most common diagnosis; hypophosphatasia and osteoporosis related to WNT1, LRP5, and PLS3 were other commonly uncovered diagnoses [45, 46].

The most common monogenic form of bone fragility is osteogenesis imperfecta (OI), a genetically and phenotypically heterogeneous group of disorders with an estimated prevalence of 1 in 10,000 to 20,000 live births [47]. Individuals with OI present with low aBMD and/or vBMD, recurrent fractures, bone deformities, scoliosis, dental abnormalities, hearing loss, blue-grey sclera, cardiopulmonary problems, joint laxity, chronic pain, and muscle weakness [23, 48–53]. The disease severity ranges from individuals with a few fractures to perinatal lethality [54]. Whereas the genetic classification of OI has resulted in multiple subtypes of OI, 90% of all OI is caused by pathogenic variants in COL1A1 and COL1A2 which encode α1 and α2 chains of type I collagen, the major matrix protein in bone [23, 44]. Clinically, Sillence classification is used to categorize OI based on phenotypic severity into mild, nondeforming (type I), severe perinatal (type II), progressively deforming (type III), and moderate form (type IV) [55]. When evaluating individuals with osteoporosis, OI should be suspected in individuals with blue sclera, hearing loss, dentinogenesis imperfecta, and a family history suggestive of autosomal dominant mode of transmission. Clinicians should note that variable expressivity and reduced penetrance may give the impression of the disorder “skipping generations” [56].

Hypophosphatasia (HPP) is caused by pathogenic variant(s) in ALPL which encodes a tissue nonspecific alkaline phosphatase [57]. HPP is characterized by defective bone and teeth mineralization and calcific arthritis. HPP severity ranges from perinatal lethality to children with fractures and long bone deformities to adults with only metatarsal fractures. Childhood-onset HPP, especially the more severe forms, are autosomal recessive. Adult forms of hypophosphatasia are typically transmitted in an autosomal dominant manner and should be suspected in individuals with fractures, osteomalacia, and reduced serum alkaline phosphatase activity [58, 59]. WNT1 (wingless-type mmtv integration site family, member 1) encodes a key ligand for the WNT pathway which plays a significant role in skeletogenesis and accrual of bone mass [60–64]. Biallelic pathogenic variants in WNT1 cause OI type XV, a disorder characterized by severe bone fragility and variable structural brain malformations including hypoplasia of pons, cerebellum, optic chiasm, and mesencephalic tectum [63, 65]. Monoallelic pathogenic variants in WNT1 can have variable presentation including individuals who present only with adult-onset fractures and low bone density [66–68]. LRP5 (Low density lipoprotein receptor-related protein 5) is an important regulator of bone mass [69]. Whereas biallelic pathogenic variants in LRP5 cause osteoporosis-pseudoglioma syndrome and exudative vireoretinopathy, monoallelic gain-of-function or loss-of-function variants can result in autosomal dominant forms of osteopetrosis and osteoporosis, respectively [70–73]. PLS3 (Plastin 3) localized to the X chromosome encodes for an actin-binding protein. Hemizygous males with PLS3 pathogenic variants may present with childhood-onset osteoporosis whereas heterozygous women can be affected variably ranging from having normal bone density and absence of fractures to early-onset osteoporosis and vertebral compression fractures [74].

In addition to these monogenetic forms of osteoporosis, low bone density and increased bone fragility are well-known manifestations of numerous genetic disorders that secondarily affect the skeleton. For example, individuals with neurofibromatosis type 1, heritable connective tissue disorders including Marfan syndrome, Loeys-Dietz syndrome, and Ehlers Danlos syndromes have low bone density and/or increased risk of fracture and may be at risk for more severe age-related osteoporosis [75–82]. However, bone fragility is typically not the primary manifestation in these conditions, thus allowing for recognition of their syndromic nature.

Genetic variants and genetic testing platforms

The genetic variations that cause monogenic disorders of low bone mass and increased bone fragility can be categorized into: single nucleotide variants (SNVs), insertions-deletions (indels), copy-number variants (CNVs), copy-neutral variants, and chromosomal abnormalities. SNVs result from a single base substitution at a genetic locus. SNVs can affect protein function by changing an encoded amino acid (missense variant), creating a premature stop codon (nonsense variant), or affecting the normal splicing of mRNA (splice site variant). Indels are insertions and/or deletions of nucleotides less than 100bp in length; within protein-coding exons of genes, indels in multiples of three can add or delete amino acids from a protein, while indels not in multiples of three can alter the reading frame (frameshift variant) resulting in downstream premature termination or change in amino acid sequence. CNVs, including deletions, duplications, and triplications, are structural variants that result in deviation from the normal diploid (N=2) state at a particular locus and can affect gene dosage and function. Copy-neutral variants such as inversions and balanced translocations are variants that affect the structure of specific genomic regions rather than copy-number. Chromosomal abnormalities, especially those involving sex chromosomes, including abnormal number (e.g., 45,X and 47,XXY) and chromosomal structure, are well-known causes of low bone mass.

Clinical genetic testing for monogenic forms of low bone mass and increased bone fragility is typically performed on DNA obtained from peripheral blood, saliva, or buccal swab. Frequently used sequencing and cytogenetic tests to detect causal variants are summarized in Table 1. For sequencing tests, most laboratories require 3–6 mL of blood collected in EDTA containing tubes. For karyotype analyses, 3–6 mL of blood in sodium heparin tubes is required. For buccal swabs and saliva collection, most laboratories provide kits with specific instructions for sample collection.

Table 1:

Genetic Testing platforms available for clinical testing

Genetic testing platform		Genomic regions interrogated	Type of genetic variations that can be detected	When to consider using for disorders of low bone mass and increased bone fragility
DNA Sequencing	Sanger sequencing	Exonic regions and exon-intron boundaries of single or few genes	SNV, indels	Suspicion of clinical diagnosis of a particular disorder is strong and only one to few genes need to be sequenced (e.g., ALPL sequencing in HPP)
	Gene panel testing using MPS	Exonic regions and exon-intron boundaries of few to hundreds of genes known to cause particular type of human diseases	SNV, indels, CNV of genes included in the panel	Clinical findings can narrow down the diagnostic category, but genetic heterogeneity necessitates interrogation of multiple genes (e.g., osteogenesis imperfecta gene panel in individuals with severe OI)
	Exome sequencing	Exonic regions and exon-intron boundaries of thousands of genes known to cause human diseases	SNV, indels, CNV of exonic regions of genome	Multisystem disorder; phenotype is not distinct to make a diagnosis or narrow down to a diagnostic category; undiagnosed diseases
	Whole genome sequencing	Whole genome	SNV, indels, CNV and other genomic rearrangements in coding and non-coding regions of genome	Multisystem disorder; phenotype is not sufficient to make a diagnosis or narrow down to a diagnostic category; undiagnosed diseases
	DNA genotyping microarray	Specific variants in few to hundreds of genes	Targeted assessment of specific variations in the genome	Polygenic risk scoring
Cytogenetic testing	Karyotype	Chromosomes	Chromosomal aneuploidy, large chromosomal deletions, duplications, and rearrangements	Hypogonadism and features suggestive of sex chromosome disorders (e.g., Turner syndrome and Klinefelter syndrome)
	Chromosomal Microarray	Genomic regions that harbor genes known to be associated with human diseases	Chromosomal aneuploidy, and CNV including small chromosomal deletions, duplications, and complex genomic rearrangements	Multisystem disorder, undiagnosed diseases

CNV - copy-number variation; HPP – hypophosphatasia; MPS - massively parallel sequencing; OI osteogenesis imperfecta

Sequencing-based genetic testing includes Sanger sequencing, targeted gene-panels, and genome-wide tests such as ES and WGS. Sanger sequencing technology uses labelled dideoxynucleotides to cause termination of chain elongation and subsequent electrophoresis to separate each terminated DNA fragment which provides a sequence readout of the template DNA fragment of interest. Though Sanger sequencing is highly accurate and can detect SNVs and indels, it does not detect CNVs. Because Sanger sequencing is not a high-throughput assay, it is typically used to sequence single or limited number of genes. Single gene testing is utilized when the clinical suspicion of a particular disorder is strong and only one or few genes need to be sequenced. The advent of MPS technologies have transformed the use of genetic testing in clinical practice [83]. While it is beyond the scope of this manuscript to review the MPS platforms and analysis pipelines used for MPS, generally, MPS technologies involve separation of DNA into 200–300 bp fragments then simultaneous amplification and sequencing of multiple fragments. MPS is a high-throughput assay that can detect SNVs, indels, and CNVs in a particular set of genes, the entire coding region of the genome or exome, or the entire genome. Gene panel testing involves sequencing of multiple genes that are known to cause similar or overlapping phenotypes and panels may consist of a subset of genes known to cause specific phenotypes (e.g., “osteogenesis imperfecta panel” and “abnormal bone mineralization panel”) or comprise hundreds of genes known to cause many monogenic forms of bone disorders (e.g., “skeletal dysplasia panel”). Gene panels have a high depth of coverage with every nucleotide being sequenced multiple times which can detect SNVs with high sensitivity and accuracy. Algorithms may use the high read count to calculate CNVs in the interrogated regions, but larger deletions or duplications and nucleotide repeat expansions are not reliably detected by gene panels. [84] Gene panel testing is typically used when clinical findings narrow down the diagnostic category and targeted genetic testing is desired.

ES involves capture, amplification, and sequencing of most of the ~20,000 human protein-coding genes (exome) using MPS. While ES detect SNVs, indels, and CNVs from most of the exome, clinical diagnostic laboratories dictate what type of variants are reported and most focus their analysis to genes directly related to or contributing to the phenotype of interest. ES cannot detect copy-neutral variations or nucleotide repeat expansions. ES is primarily used in diagnosing genetic forms of skeletal disorders when phenotypes are not distinct enough make a diagnosis or narrow down to a diagnostic category, in diagnosis of multisystem disorders, and in cases that remain undiagnosed after standard diagnostic evaluations.

WGS is the most comprehensive genetic testing assay currently available and can detect variants in both coding and non-coding regions of the genome. Unlike gene panels and ES, WGS does not involve a “capture” step and genomic DNA isolated from a patient sample is directly sequenced. Thus, WGS is less prone to bias attributable to under-representation of genes or portions of genes that are poorly captured in ES and gene panels. SNVs and indels are readily detected by WGS; furthermore, using bioinformatics tools, CNVs, chromosomal rearrangements, and nucleotide repeat expansions can also be detected [84]. WGS can detect non-coding variants and have reduced depth of coverage compared to gene panels or ES which may present challenges with GS interpretation, though this is simply addressed with increasing read depth albeit with increased cost. Like ES, WGS is used in the diagnosis of genetic forms of skeletal disorders with phenotypes that are not distinct enough to make a diagnosis, multisystem disorders, and undiagnosed diseases. These genetic tests typically report rare causal variants known to cause monogenic forms of disease and do not report commonly occurring SNVs that are associated with low bone density and/or fracture (e.g., common SNVs discovered to be associated by GWAS). While some diagnostic platforms, including direct-to-consumer testing, employ DNA genotyping microarrays to report on common SNVs associated with low bone density, these are not frequently or widely used during standard clinical genetic testing. With the increased resolution of WGS, phenotypic expansion, i.e., the association of new or variant phenotypes with different alleles within a gene, is increasing in both diagnostic and research studies. Proving pathogenicity can be challenging without a cohort of similar, newly associated phenotypes, and may depend on model organism-based functional studies which are still primarily in the research domain.

The most used clinical cytogenetic testing includes karyotype analysis and CMA. Karyotype is an arrangement of the chromosome pairs assembled from microscopic images of the cell nucleus. Karyotype analyses is typically used to detect chromosomal abnormalities, including missing or extra chromosomes (e.g., 45,X and 47,XXY, respectively), deletions, duplications, and inversions of large segments, and chromosomal translocations. CMA uses thousands to millions of locus-specific probes in one assay to assess for CNVs across the genome and can detect abnormal number of chromosomes, chromosomal deletions, duplications, and complex genomic rearrangements. Large rearrangements detected by cytogenetic tools are usually associated with more complex multi-system disorders, although higher resolution arrays can detect small intra-genic deletions that may be associated with isolated phenotypes.

Genetic evaluation for monogenic disorders of low bone mass and increased bone fragility

A genetic evaluation starts with a detailed medical and family history including taking a multigenerational pedigree. Recommendations for pedigree standards and nomenclature have been previously published [85]. Family history should focus on fracture history and clinical features that may point to specific genetic skeletal disorders such as hearing loss and dentinogenesis imperfecta in OI, and early loss of primary teeth and low plasma alkaline phosphatase activity in hypophosphatasia. Though opinions vary on when to consider genetic testing, generally, testing should be considered when patients present with short stature, recurrent fractures, fractures at unusual sites, multiple vertebral fractures, bone deformities, early-onset osteoporosis, very low areal bone mineral density (e.g., T-score < −3.0), and family history suggestive of a monogenic form of osteoporosis.

Diagnostic yield and accuracy are important considerations when ordering genetic testing. In genetic skeletal disorders, diagnostic yield is dependent on which patients are tested and what tests are used. Generally, WGS and ES provide the highest diagnostic yield for rare, novel, and pediatric monogenic diseases. In pediatric skeletal disease, the diagnostic yield of ES has been reported between 39% and 60% [84, 86–89]. MPS-based gene panel testing interrogating between 113 and 374 genes returned a molecular diagnosis in 42% of all individuals where skeletal dysplasia was suspected, though only 29% of adults received a diagnostic result [90]. Similarly, in a multicenter study of 394 adults, a skeletal disease gene panel detected pathogenic variants in ~21% of patients who presented with low BMD or clinically suspected bone fragility without a known secondary cause of osteoporosis. Variants in COL1A1, COL1A2, ALPL, LRP5, PLS3, and WNT1 were the most prevalent and the likelihood of detecting a causal variant correlated with positive family history of osteoporosis, number of peripheral fractures (> 2), and high body mass index [45].

As of January 2024, the National Center for Biotechnology Information’s Genetic Test Registry lists 200 genetic tests for the indication of osteoporosis [91]. Prior to initiating genetic testing, patients should be counseled about inheritance patterns, recurrence risks, type of genetic testing used, potential results, and implications of testing on management. The financial burden of genetic testing on patients must also be considered. While many government-funded and private insurance plans cover genetic testing in the U.S., the policies for the indications and types of tests covered vary significantly. Patients should be counseled on the 2008 federal law, Genetic Information Nondiscrimination Act (GINA), which protects individuals against discrimination based on their genetic information in health coverage and employment; however, this protection does not extend to life insurance or disability insurance.

The American College of Medical Genetics and Genomics (ACMG) in conjunction with the Association for Molecular Pathology have published guidelines for the reporting of genetic testing results [92]. The ACMG recommends the use of five descriptive categories of genetic variants: benign, likely benign, uncertain significance, likely pathogenic, and pathogenic, with the delineation of variants into these categories using stratified criteria. The “likely” designation clarifies a variant has a 90% level of certainty to be either benign (likely benign) or pathogenic (likely pathogenic). Variants of uncertain significance (VUS) are labeled as such if the variant does not meet the threshold for classification as benign/likely benign or pathogenic/likely pathogenic. While benign/likely benign and pathogenic/likely pathogenic variants are more easily integrated into clinical decision-making, VUS present a diagnostic challenge. When the number of interrogated genes is high, as in large gene panels, ES, and WGS, the likelihood of uncovering VUS increases [93, 94]. Patients should be informed that variants may be reclassified as more information becomes available; indeed, incorporation of data from functional assays like RNA-sequencing has been shown to not only improve diagnostic rates in patients with negative ES/WGS but also provide sufficient evidence for VUS reclassification to pathogenic [95]. ACMG guidelines outline a clinician’s duty to alert their patients of relevant genetic reclassifications and recommends patients be counseled on appropriate genetics follow-up with major life events, including pregnancy [92, 96]. Patients undergoing ES and WGS should be counseled on the possibility of uncovering variants in genes unrelated to their indication for genetic testing [97]. The genes reported as secondary findings are primarily associated with cancer predisposition, cardiomyopathies, and aortopathies and have been selected based on the direct implications positive results have on screening or prevention of complications. Approximately 1% of individuals undergoing ES is estimated to have a medically actionable incidental finding [98, 99]. Most U.S. clinical diagnostic laboratories follow ACMG recommendations for reporting secondary findings and while patients can typically “opt out” of receiving secondary findings, some laboratories may ask patients to “opt in” [98, 100]. Thus, understanding the policy of the clinical laboratory conducting the genetic test is necessary to accurately inform patients of their options.

Genetic testing has the potential to make an accurate diagnosis in a subset of individuals with increased bone fragility and make a significant impact on patients and families who undergo protracted periods of diagnostic odyssey. Accurate diagnosis offers “closure” to affected individuals and their families, has direct impact on management via institution of specific therapies or appropriate surveillance for extraskeletal manifestations, and may be an avenue for patients to receive the ever-increasing number of investigational therapies for these disorders [101, 102].

Conclusion

Low bone mass and increased bone fragility have a strong genetic component. Increased availability of genetic testing presents a challenge to clinicians in determining which patients may benefit from clinical genetic testing and which testing modality is most appropriate. Here, we have outlined when to suspect an underlying monogenic genetic disorder in patients with bone fragility or low bone mass, the strengths and limitations of sequencing technologies in detecting causative genetic variants, and important considerations when referring patients for genetic testing. Clinical genetic testing is likely to be diagnostic in a subset of patients who present with low bone mass or increased bone fragility and ultimately, as more osteoporosis risk loci are identified, an increasing number of patients will benefit from routine genetic screening and identification of osteoporosis risk.