Genotype-phenotype analysis of a rare type of osteogenesis imperfecta in four Chinese families with WNT1 mutations

Osteoporosis Caused by Monoallelic Variant of WNT1 Gene in Four Pediatric Patients

Pediatric patients of autosomal dominant early onset osteoporosis conferred by heterozygous mutation in the WNT1 (OMIM: 615221) were rarely reported, and therapy in pediatrics is relatively inexperienced. The clinical and genotypic characteristics and treatment process of four children with osteoporosis caused by WNT1 monoallelic variation were analyzed. The patients admitted from June 2023 to January 2024. All patients presented multiple vertebral compression fracture, two of them experienced recurrent peripheral fragility fractures. The age of the first fractures occur between 2 years and 12 years. Lumber BMD by dual-energy X-ray absorptiometry were decreased (height adjusted z score of -8.06 to -3.50). Four monoallelic variants in WNT1 (c.505G>T, c.616G>A, c.677C>T and c. 506G>A with transcript ID. NM_005430.4) were identified in the probands, and relatives carrying mutations presented with a bone phenotype, consistent with autosomal dominant inheritance. Novel variant c.616G>A was analyzed by 3D protein structural modeling. Subsequent to the treatment of zelodronic acid on all four patients, lumbar BMD improvement by 0.061-0.251 g/cm2. Our data showed that the age of onset of osteoporosis by monoallelic variants in WNT1 is significantly earlier than the age of onset in the general population. Severe osteoporosis is also exhibited in pediatric patients, not just in aging patients with WNT1 variant. Zoledronic acid treatment is effective in short-term observation for pediatric patients with improvement of bone pain and BMD, and no more facture during treatment.

Emerging Landscape of Osteogenesis Imperfecta Pathogenesis and Therapeutic Approaches

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

In silico analyses of Wnt1 nsSNPs reveal structurally destabilizing variants, altered interactions with Frizzled receptors and its deregulation in tumorigenesis

Wnt1 is the first mammalian Wnt gene, which is discovered as proto-oncogene and in human the gene is located on the chromosome 12q13. Mutations in Wnt1 are reported to be associated with various cancers and other human diseases. The structural and functional consequences of most of the non-synonymous SNPs (nsSNPs), present in the human Wnt1 gene, are not known. In the present work, extensive bioinformatics analyses are used to screen 292 nsSNPs of Wnt1 for predicting pathogenic and harmless polymorphisms. We have identified 10 highly deleterious nsSNPs among which 7 are located within the highly conserved areas. These 10 nsSNPs are also predicted to affect the post-translational modifications of Wnt1. Further, structure based stability analyses of these 10 highly deleterious nsSNPs revealed 8 variants as highly destabilizing. These 8 highly destabilizing variants were shown to have high BC score and high RMSIP score from normal mode analyses. Based on the deformation energies, obtained from the normal mode analyses, variants like G169A, G169S, G331R and G331S were found to be unstable. Molecular Dynamics (MD) simulations revealed structural stability and fluctuation of WT Wnt1 and its prioritized variants. RMSD remained fluctuating mostly between 4 and 5 Å and occasionally between 3.5 and 5.5 Å ranges. RMSF in the CTD region (residues 330-360) of the binding pocket were lower compared to that of WT. Studying the impacts of nsSNPs on the binding interface of Wnt1 and seven Frizzled receptors have predicted substitutions which can stabilize or destabilize the binding interface. We have found that Wnt1 and FZD8-CRD is the best docked complex in our study. MD simulation based analyses of wild type Wnt1-FZD8-CRD complex and the 8 prioritized variants revealed that RMSF was higher in the unstructured regions and RMSD remained fluctuating in the region of 5 Å ± 1 Å. We have also observed differential Wnt1 gene expression pattern in normal, tumor and metastatic conditions across different tissues. Wnt1 gene expression was significantly higher in metastatic tissues of lungs, colon and skin; and was significantly lower in metastatic tissues of breast, esophagus and kidney. We have also found that Wnt1 deregulation is associated with survival outcome in patients with gastric and breast cancer. Furthermore, these computationally screened highly deleterious nsSNPs of Wnt1 can be analyzed in population based genetic studies and may help understand the Wnt1 associated diseases.

Understanding Musculoskeletal Disorders Through Next-Generation Sequencing

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An insight into musculoskeletal disorders through advancements in next-generation sequencing (NGS) promises to maximize benefits and improve outcomes through improved genetic diagnosis.

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The primary use of whole exome sequencing (WES) for musculoskeletal disorders is to identify functionally relevant variants.

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The current evidence has shown the superiority of NGS over conventional genotyping for identifying novel and rare genetic variants in patients with musculoskeletal disorders, due to its high throughput and low cost.

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Genes identified in patients with scoliosis, osteoporosis, osteoarthritis, and osteogenesis imperfecta using NGS technologies are listed for further reference.

Osteogenesis imperfecta in 140 Turkish families: Molecular spectrum and, comparison of long-term clinical outcome of those with COL1A1/A2 and biallelic variants

BACKGROUND

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous group of diseases characterized by increased bone fragility and deformities. Although most patients with OI have heterozygous mutations in COL1A1 or COL1A2, 17 genes have been reported to cause OI, most of which are autosomal recessive (AR) inherited, during the last years. The aim of this study is to determine the mutation spectrum in Turkish OI cohort and to investigate the genotype-phenotype correlation.

METHODS

150 patients from 140 Turkish families with OI phenotype were included in this study. Mutations in OI-related genes were identified using targeted gene panel, MLPA analysis for COL1A1 and whole exome sequencing. 113 patients who had OI disease-causing variants were followed for 1-20 years.

RESULTS

OI disease-causing variants were detected in 117 families, of which 62.4% in COL1A1/A2, 35.9% in AR-related genes. A heterozygous variant in IFITM5 and a hemizygous in MBTPS2 were also described, one in each patient. Eighteen biallelic variants (13 novel) were identified in nine genes (FKBP10, P3H1, SERPINF1, TMEM38B, WNT1, BMP1, CRTAP, FAM46A, MESD) among which FKBP10, P3H1 and SERPINF1 were most common. The most severe phenotypes were in patients with FKBP10, SERPINF1, CRTAP, FAM46A and MESD variants. P3H1 patients had moderate, while BMP1 had the mild phenotype. Clinical phenotypes were variable in patients with WNT1 and TMEM38B mutations. We also found mutations in ten genes (PLS3, LRP5, ANO5, SLC34A1, EFEMP2, PRDM5, GORAB, OCRL1, TNFRSF11B, DPH1) associated with diseases presenting clinical features which overlap OI, in eleven families.

CONCLUSION

We identified disease-causing mutations in 83.6% in a large Turkish pediatric OI cohort. 40 novel variants were described. Clinical features and long-term follow-up findings of AR inherited OI types and especially very rare biallelic variants were presented for the first time. Unlike previously reported studies, the mutations that we found in P3H1 were all missense, causing a moderate phenotype.

Case report: Early-onset osteoporosis in a patient carrying a novel heterozygous variant of the WNT1 gene

The WNT1 gene is crucial for bone development and homeostasis. Homozygous mutations in WNT1 cause severe bone fragility known as osteogenesis imperfecta type XV. Moreover, heterozygous WNT1 mutations have been found in adults with early-onset osteoporosis. We identified a 35 year-old Caucasian woman who experienced multiple vertebral fractures two months after her second pregnancy. There was no history of risk factors for secondary osteoporosis or family history of osteoporosis. Dual-energy X-ray absorptiometry confirmed a marked reduction of bone mineral density (BMD) at the lumbar spine (0.734 g/cm², Z-score -2.8), femoral neck (0.48 g/cm², Z-score -3.5), and total hip (0.589 g/cm², Z-score -3.0). Blood tests excluded secondary causes of bone fragility. Genetic analysis revealed a heterozygous missense mutation (p.Leu370Val) in the WNT1 gene. Varsome classified it as a variant of uncertain significance. However, the fact that the Leucine residue at position 370 is highly conserved among vertebrate species and the variant has a very low allelic frequency in the general population would exclude the possibility of a polymorphism. The patient was treated for two years with teriparatide therapy associated with calcium and vitamin D supplements. During the follow-up period she did not report further clinical fractures. After 24 months of teriparatide, BMD increased at lumbar spine (+14.6%), femoral neck (+8.3%) and total hip (+4.9%) compared to baseline. We confirm that the heterozygous WNT1 mutation could cause a variable bone fragility and low turnover osteoporosis. We suggest that teriparatide is one of the most appropriate available therapies for this case.

Effects of WNT1 c.110 T>C and c.505G>T mutations on osteoblast differentiation via the WNT1/β-catenin signaling pathway

Background

WNT1 c.110 T>C and c.505G>T missense mutations have been identified in patients with osteogenesis imperfecta (OI). Whether these mutations affect osteoblast differentiation remains to be determined. This study aimed to investigate the effects of WNT1 c.110 T>C and c.505G>T mutations on osteoblast function, gene expression, and pathways involved in OI.

Methods

Empty vector (negative control), wild-type WNT1, WNT1 c.110 T>C, WNT1 c.505G>T, and WNT1 c.884C>A (positive control) mutant plasmids were constructed and transfected into preosteoblast (MC3T3-E1) cells to investigate their effect on osteoblast differentiation. The expressions of osteoblast markers, including BMP2, RANKL, osteocalcin, and alkaline phosphatase (ALP), were determined using quantitative real-time polymerase chain reaction (RT-qPCR), western blotting (WB), enzyme-linked immunosorbent assay, and ALP staining assay, respectively. The mRNA and protein expression levels of WNT1 or the expression levels of the relevant proteins involved in the WNT1/β-catenin signaling pathway were also determined using RT-qPCR, WB, and immunofluorescence (IF) assays after the different plasmids were transfected into MC3T3-E1 cells.

Results

Compared with those in the wild-type group, in the mutation groups, the mRNA and protein expression levels of BMP2 were suppressed, the expressions of osteocalcin and ALP were inhibited, and the mRNA and protein expression levels of RANKL were enhanced in MC3T3-E1 cells. WB and IF assays revealed that the protein expression levels of WNT1 in MC3T3-E1 cells were downregulated in the mutation groups compared with those in the wild-type WNT1 group. Furthermore, the expression levels of nonphosphorylated β-catenin (non-p-β-catenin) and phosphorylated GSK-3β (p-GSK-3β) were downregulated in the mutation groups compared with those in the wild-type group. However, no significant changes in the expression level of non-p-β-catenin or p-GSK-3β were observed in the mutation groups.

Conclusions

WNT1 c.110 T>C and c.505G>T mutations may alter the proliferation and osteogenic phenotype of MC3T3-E1 linked to the progression of OI via the inhibition of the WNT1/β-catenin signaling pathway. This is the first study to confirm the effect of WNT1 c.110 T>C and c.505G>T missense mutations on osteoblast differentiation and propose a new molecular mechanism for OI development.

Comprehensive Genetic Analysis of 128 Candidate Genes in a Cohort With Idiopathic, Severe, or Familial Osteoporosis

Context

The genetic bases of osteoporosis (OP), a disorder with high heritability, are poorly understood at an individual level. Cases of idiopathic or familial OP have long puzzled clinicians as to whether an actionable genetic cause could be identified.

Objective

We performed a genetic analysis of 28 cases of idiopathic, severe, or familial osteoporosis using targeted massively parallel sequencing.

Design

Targeted sequencing of 128 candidate genes was performed using Illumina NextSeq. Variants of interest were confirmed by Sanger sequencing or SNP array.

Patients and Setting

Thirty-seven patients in an academic tertiary hospital participated (54% male; median age, 44 years; 86% with fractures), corresponding to 28 sporadic or familial cases.

Main Outcome Measure

The identification of rare stop-gain, indel, splice site, copy-number, or nonsynonymous variants altering protein function.

Results

Altogether, we identified 28 variants of interest, but only 3 were classified as pathogenic or likely pathogenic variants: COL1A2 p.(Arg708Gln), WNT1 p.(Gly169Asp), and IDUA p.(His82Gln). An association of variants in different genes was found in 21% of cases, including a young woman with severe OP bearing WNT1, PLS3, and NOTCH2 variants. Among genes of uncertain significance analyzed, a potential additional line of evidence has arisen for GWAS candidates GPR68 and NBR1, warranting further studies.

Conclusions

While we hope that continuing efforts to identify genetic predisposition to OP will lead to improved and personalized care in the future, the likelihood of identifying actionable pathogenic variants in intriguing cases of idiopathic or familial osteoporosis is seemingly low.

Genotypic and Phenotypic Analysis in Chinese Cohort With Autosomal Recessive Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a rare heritable skeletal disorder which is mainly caused by defected type I collagen. Autosomal recessive OI (AR-OI) is caused by mutations of genes that are responsible for type I collagen modification and folding, and is often associated with more severe phenotypes. Due to the limited number of recessive OI patients, it has been difficult to study the mutation spectrum as well as the correlation of genotype and phenotype. This study recruited a Chinese cohort of 74 AR-OI families, aiming to establish the mutation spectrum and to examine the genotypic and phenotypic correlation. We identified 82 variants including 25 novel variants and 57 HGMD reported variants in these AR-OI patients, using whole exome sequencing/panel sequencing combined with Sanger sequencing. Pathogenic mutations were found at WNT1 (n = 30, 40.54%), SERPINF1 (n = 22, 29.73%), FKBP10 (n = 10, 13.51%), CRTAP (n = 3, 4.05%), P3H1 (n = 3, 4.05%), SERPINH1 (n = 2, 2.70%), SEC24D (n = 3, 4.05%), and PLOD2 (n = 1, 1.35%) respectively. Thus, WNT1 represents the most frequent pathogenic gene of AR-OI in Chinese population. The most common clinical manifestations of AR-OI patients include walking problem (72.86%), scoliosis (65.28%) and frequent fractures (fractures ≥2/year) (54.05%). Interestingly, ptosis represents a unique phenotype of patients carrying WNT1 variants, and it was rare in patients harboring other pathogenic genes. Our study expanded the mutation spectrum of AR-OI and enriched the knowledge of genotypic and phenotypic correlation in Chinese cohort with AR-OI.

Comprehensive bioinformatic analysis of Wnt1 and Wnt1-associated diseases

Wnt1 is the first member of the Wnt family that was identified. It is phylogenetically conserved and essential for oncogenesis and multiple developmental processes. This study has summarized diseases and mutations related to Wnt1. Wnt1 is involved in various cancers, genetic type XV osteogenesis imperfecta, osteoporosis, and neurological diseases. The expression of Wnt1 in normal tissues and different types of cancers and the potential survival of cancer were analyzed using experiment-based bioinformatic analysis. Systematic analysis indicated that abnormal expression of Wnt1 is significantly associated with cancers, such as kidney renal carcinoma, hepatocellular carcinoma, thyroid carcinoma, head and neck squamous cell carcinoma, and uterine corpus endometrial carcinoma. GeneMANIA and STRING predicted that 32 proteins were involved with Wnt1 in Wnt signaling pathways and sorting and secretion of Wnts. These interacting molecules significantly co-occurred according to cBioPortal analysis. Thirty-three genes with an alteration frequency of more than 50% were observed in several cancers like esophageal squamous cell carcinoma, melanoma, and non-small cell lung cancer. Functional and experiment-based bioinformatics indicated that Wnt1 may act as a target of a potential biomarker for various types of human cancers. Wnt1 and other Wnt1-related proteins and signaling pathways may be ways to treat osteoporosis.

NOVEL MUTATIONS IN THE WNT1, TMEM38B, P4HB, AND PLS3 GENES IN FOUR UNRELATED CHINESE FAMILIES WITH OSTEOGENESIS IMPERFECTA

OBJECTIVE

Osteogenesis imperfecta (OI) is a group of heritable fragile bone diseases, and the majority are caused by pathogenic variants in the COL1A1 and COL1A2 genes. We sought to identify the genetic causes and phenotypes of OI in Chinese patients without COL1A1 or COL1A2 mutations.

METHODS

Twenty-three patients who were diagnosed with sporadic OI but did not carry COL1A1/2 mutations were recruited, and their genomic DNA was analyzed using targeted next-generation sequencing of rare OI-related genes. The resulting damaging mutations in the probands and their parents were verified using Sanger sequencing. Moreover, the efficacy of long-term bisphosphonate treatment was evaluated in proband 1.

RESULTS

Compound heterozygous variants in the WNT1 and TMEM38B genes were identified in proband 1 and proband 2, respectively. A heterozygous mutation in the P4HB gene was identified in proband 3, and a hemizygous mutation in PLS3 was identified in proband 4. The unaffected parents of the probands (except the father of proband 4) with mutations in the WNT1, TMEM38B, and PLS3 genes were heterozygous carriers of each of the variants, respectively. Notably, proband 3 had the characteristic exophthalmos, flat nasal bridge and flat, wide forehead. None of the patients presented with dentinogenesis imperfecta or hearing loss. Furthermore, bisphosphonates exerted beneficial effects on proband 1, who carried the WNT1 mutations, by increasing bone mineral density Z-score, reshaping the compressed vertebrae and decreasing the fracture risk.

CONCLUSION

We identified novel mutations and expanded the spectrum of phenotypes and genotypes of the extremely rare disorder OI.

ABBREVIATIONS

BMD = bone mineral density; MIM = Mendelian Inheritance in Man; OI = osteogenesis imperfecta; PDI = protein disulfide isomerase.

New Insights Into Monogenic Causes of Osteoporosis

Osteoporosis, characterized by deteriorated bone microarchitecture and low bone mineral density, is a chronic skeletal disease with high worldwide prevalence. Osteoporosis related to aging is the most common form and causes significant morbidity and mortality. Rare, monogenic forms of osteoporosis have their onset usually in childhood or young adulthood and have specific phenotypic features and clinical course depending on the underlying cause. The most common form is osteogenesis imperfecta linked to mutations in COL1A1 and COL1A2, the two genes encoding type I collagen. However, in the past years, remarkable advancements in bone research have expanded our understanding of the intricacies behind bone metabolism and identified novel molecular mechanisms contributing to skeletal health and disease. Especially high-throughput sequencing techniques have made family-based studies an efficient way to identify single genes causative of rare monogenic forms of osteoporosis and these have yielded several novel genes that encode proteins partaking in type I collagen modification or regulating bone cell function directly. New forms of monogenic osteoporosis, such as autosomal dominant osteoporosis caused by WNT1 mutations or X-linked osteoporosis due to PLS3 mutations, have revealed previously unidentified bone-regulating proteins and clarified specific roles of bone cells, expanded our understanding of possible inheritance mechanisms and paces of disease progression, and highlighted the potential of monogenic bone diseases to extend beyond the skeletal tissue. The novel gene discoveries have introduced new challenges to the classification and diagnosis of monogenic osteoporosis, but also provided promising new molecular targets for development of pharmacotherapies. In this article we give an overview of the recent discoveries in the area of monogenic forms of osteoporosis, describing the key cellular mechanisms leading to skeletal fragility, the major recent research findings and the essential challenges and avenues in future diagnostics and treatments.

Novel WNT1 mutations in children with osteogenesis imperfecta: Clinical and functional characterization

INTRODUCTION

Biallelic mutations in WNT1 can give rise to a rare form of moderate to severe OI. Here we report on 12 children (age 2 to 16 years; 5 girls) with biallelic WNT1 mutations.

METHODS

Genomic DNA was analyzed either by targeted next-generation sequencing or Sanger sequencing. Mutations were modeled on the WNT1 protein structure. The in vitro functional effect of WNT1 mutations on WNT signaling was assessed in HEK293 cells using the topflash reporter assay system.

RESULTS

All patients had lower extremity deformities and vertebral compression fractures. Seven individuals had upper extremity deformities. Intellectual development appeared normal in 11 children, but was clearly impaired in a 3-year old boy. Ptosis was noted in 7 patients. Height z-scores varied widely, from -7.2 to +1.5. A total of 11 disease-causing WNT1 variants (7 missense mutations, 4 mutations leading to premature termination codons) were identified, of which 9 were novel. Three-dimensional protein modeling suggested that each of the missense mutations led to structural modifications. Functional in vitro studies revealed that all observed missense mutations led to decreased ability of WNT1 to induce WNT signaling via the canonical WNT pathway.

CONCLUSIONS

The reported biallelic WNT1 variants cause loss of WNT1 function and lead to a severe bone fragility phenotype with conspicuous involvement of the spine.

Novel Mutations in PLOD2 Cause Rare Bruck Syndrome

Bruck syndrome is a rare autosomal recessive form of osteogenesis imperfecta (OI), which is mainly characterized by joint contractures and recurrent fragility fractures. Mutations in FKBP10 and PLOD2 were identified as the underlying genetic defects of Bruck syndrome. Here we investigated the phenotypes and the pathogenic mutations of three unrelated Chinese patients with Bruck syndrome. Clinical fractures, bone mineral density (BMD), bone turnover biomarkers, and skeletal images were evaluated in detail. The pathogenic mutations were identified by targeted next-generation sequencing and subsequently confirmed by Sanger sequencing and cosegregation analysis. We also evaluated the effects of zoledronic acid on bone fracture incidence and BMD of the patients. Three patients had congenital joint contractures, recurrent fragility fractures, camptodactyly, clubfoot, scoliosis, but without dentinogenesis imperfecta and hearing loss. Five novel heterozygous mutations were detected in PLOD2, including three heterozygous missense mutations (c.1138C>T, p.Arg380Cys; c.1153T>C, p.Cys385Arg; and c.1982G>A, p.Gly661Asp), one heterozygous nonsense mutation (c.2038C>T, p.Arg680X), and one heterozygous splice-site mutation (c.503-2A>G). Their parents were all heterozygous carriers of these mutations in PLOD2. No clear genotype-phenotype correlations were found in these patients with PLOD2 mutations. Z-score of BMD was significantly increased, but scoliosis progressed and new bone fractures occurred during the treatment of zoledronic acid. Our findings expanded the spectrum of gene mutations of Bruck syndrome.

Complex heterozygous WNT1 mutation in severe recessive osteogenesis imperfecta of a Chinese patient

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder with a predominately autosomal-dominant inheritance pattern. Recessive forms of OI are rare and involve many different causative genes. WNT1 mutations were found to cause either autosomal-recessive OI or dominantly inherited early-onset osteoporosis. Here we describe a 32-year-old boy with severe osteopenia and deformity of the extremities. The relative long thumb and ring finger are obvious. We identified a novel combination of complex heterozygous WNT1 mutation of c.397 A>T (p.Ala133Thr) and c.506dupG (p.Cys170Leufs*) in the proband, both parents and young brother were shown to be heterozygous asymptomatic carriers of the mutation. This is the eleventh family and the thirteenth patient we have ever found in China. Mutation of c.397 A>T (p.Ala133Thr) was found for the third time following our previous findings in two individual families with four patients in total, and may be a hotspot mutation in Chinese WNT1-related OI patients. In silico programs supported the damaging effects for both mutations. The three-D structure demonstrated the severely destroyed stability of WNT1. Serum levels of WNT1, LRP5, and β-catenin were decreased, while higher levels of GSK-3β were detected. The molecular mechanisms of the complex heterozygous mutations need further study.

Recent Discoveries in Monogenic Disorders of Childhood Bone Fragility

PURPOSE OF REVIEW

This review summarizes our current knowledge on primary osteoporosis in children with focus on recent genetic findings.

RECENT FINDINGS

Advances in genetic research, particularly next-generation sequencing, have found several genetic loci that associate with monogenic forms of inherited osteoporosis, widening the scope of primary osteoporosis beyond classical osteogenesis imperfecta. New forms of primary osteoporosis, such as those related to WNT1, PLS3, and XYLT2, have identified defects outside the extracellular matrix components and collagen-related pathways, in intracellular cascades directly affecting bone cell function. Primary osteoporosis can lead to severe skeletal morbidity, including abnormal longitudinal growth, compromised bone mass gain, and noticeable fracture tendency beginning at childhood. Early diagnosis and timely care are warranted to ensure the best achievable bone health. Future research will most likely broaden the spectrum of primary osteoporosis, hopefully provide more insight into the genetics governing bone health, and offer new targets for treatment.

Osteocyte-specific WNT1 regulates osteoblast function during bone homeostasis

Mutations in WNT1 cause osteogenesis imperfecta (OI) and early-onset osteoporosis, identifying it as a key Wnt ligand in human bone homeostasis. However, how and where WNT1 acts in bone are unclear. To address this mechanism, we generated late-osteoblast-specific and osteocyte-specific WNT1 loss- and gain-of-function mouse models. Deletion of Wnt1 in osteocytes resulted in low bone mass with spontaneous fractures similar to that observed in OI patients. Conversely, Wnt1 overexpression from osteocytes stimulated bone formation by increasing osteoblast number and activity, which was due in part to activation of mTORC1 signaling. While antiresorptive therapy is the mainstay of OI treatment, it has limited efficacy in WNT1-related OI. In this study, anti-sclerostin antibody (Scl-Ab) treatment effectively improved bone mass and dramatically decreased fracture rate in swaying mice, a model of global Wnt1 loss. Collectively, our data suggest that WNT1-related OI and osteoporosis are caused in part by decreased mTORC1-dependent osteoblast function resulting from loss of WNT1 signaling in osteocytes. As such, this work identifies an anabolic function of osteocytes as a source of Wnt in bone development and homoeostasis, complementing their known function as targets of Wnt signaling in regulating osteoclastogenesis. Finally, this study suggests that Scl-Ab is an effective genotype-specific treatment option for WNT1-related OI and osteoporosis.