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

Genotypic and Phenotypic Spectrum and Pathogenesis of WNT1 Variants in a Large Cohort of Patients With OI/Osteoporosis

CONTEXT

Mutations in WNT1 can cause rare inherited disorders such as osteogenesis imperfecta (OI) and early-onset osteoporosis (EOOP). Owing to its rarity, the clinical characteristics and pathogenic mechanism of WNT1 mutations remain unclear.

OBJECTIVE

We aimed to explore the phenotypic and genotypic spectrum and treatment responses of a large cohort of patients with WNT1-related OI/OP and the molecular mechanisms of WNT1 variants.

METHODS

The phenotypes and genotypes of patients and their responses to bisphosphonates or denosumab were evaluated. Western blot analysis, quantitative polymerase chain reaction, and immunofluorescence staining were used to evaluate the expression levels of WNT1, total β-catenin, and type I collagen in the tibial bone or skin from one patient.

RESULTS

We included 16 patients with 16 mutations identified in WNT1, including a novel mutation. The types of WNT1 mutations were related to skeletal phenotypes, and biallelic nonsense mutations or frameshift mutations could lead to an earlier occurrence of fragility fractures and more severe skeletal phenotypes. Some rare comorbidities were identified in this cohort, including cerebral abnormalities, hematologic diseases, and pituitary adenoma. Bisphosphonates and denosumab significantly increased the spine and proximal hip BMD of patients with WNT1 mutations and reshaped the compressed vertebrae. We report for the first time a decreased β-catenin level in the bone of patient 10 with c.677C > T and c.502G > A compared to the healthy control, which revealed the potential mechanisms of WNT1-induced skeletal phenotypes.

CONCLUSION

Biallelic nonsense mutations or frameshift mutations of WNT1 could lead to an earlier occurrence of fragility fractures and a more severe skeletal phenotype in OI and EOOP induced by WNT1 mutations. The reduced osteogenic activity caused by WNT pathway downregulation could be a potential pathogenic mechanism of WNT1-related OI and EOOP.

Type XV osteogenesis imperfecta: A novel mutation in the WNT1 gene, c.620G >A (p.R207H), is associated with an inner ear deformity

The Wnt signaling pathway is vital in encouraging bone growth. WNT1 gene mutations have been identified as the major cause of type XV osteogenesis imperfecta (OI). Described here is a case of complex heterozygous WNT1 c.620G>A (p.R207H) and c.677C >T (p.S226L) OI caused by a novel mutation at locus c.620G >A (p.R207H). The female patient had type XV OI, distinguished by poor bone density, frequent fractures, a small stature, skull softening, lack of dentine hypoplasia, a brain malformation, and obvious blue sclera. A CT scan of the temporal bone revealed abnormalities of the inner ear, necessitating a hearing aid 8 months after birth. There was no family history of such disorders in the proband's parents. The proband inherited complex heterozygous WNT1 gene variants c.677C>T (p.S226L) and c.620G>A (p.R207H) from her father and mother, respectively. Presented here is a case of OI with inner ear deformation caused by c.620G>A (p.R207H), which is a novel WNT1 site mutation. This case broadens the genetic spectrum of OI and it provides a rationale for genetic testing of mothers and a medical consultation to estimate the risk of fetal illness.

Got WNTS? Insight into bone health from a WNT perspective

WNT signaling, essential for many aspects of development, is among the most commonly altered pathways associated with human disease. While initially studied in cancer, dysregulation of WNT signaling has been determined to be essential for skeletal development and the maintenance of bone health throughout life. In this review, we discuss the role of Wnt signaling in bone development and disease with a particular focus on two areas. First, we discuss the roles of WNT signaling pathways in skeletal development, with an emphasis on congenital and idiopathic skeletal syndromes and diseases that are associated with genetic variations in WNT signaling components. Next, we cover a topic that has long been an interest of our laboratory, how high and low levels of WNT signaling affects the establishment and maintenance of healthy bone mass. We conclude with a discussion of the status of WNT-based therapeutics in the treatment of skeletal disease.

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.

Genetic Parameter Estimation and Whole Sequencing Analysis of the Genetic Architecture of Chicken Keel Bending

Bone health is particularly important for high-yielding commercial layer chickens. The keel of poultry is an extension of the abdomen side of the sternum along the sagittal plane and is one of the most important bones. In this study, the keel phenotype of White Leghorns laying hen flocks showed significant individual differences. To clarify its genetic mechanism, we first estimated the heritability of keel bend (KB) in White Leghorn, recorded the production performance of the chicken flock, examined the blood biochemical indexes and bone quality in KB and keel normal (KN) chickens, and performed whole-genome pooled sequencing in KB and KN chickens. We then performed selection elimination analysis to determine the genomic regions that may affect the keel phenotypes. The results show that KB is a medium heritability trait. We found that cage height had a significant effect on the KB (p < 0.01). At 48 weeks, there were significant differences in the number of eggs, the number of normal eggs, and eggshell strength (p < 0.05). The content of parathyroid hormone was lower (p < 0.01) and that of calcitonin was higher (p < 0.01) in KB chickens than in KN chickens. The differences in bone mineral density, bone strength, and bone cortical thickness of the humerus and femur were extremely significant (p < 0.01), with all being lower in KB chickens than in KN chickens. In addition, the bones of KB chickens contained more fat organization. A total of 128 genes were identified in selective sweep regions. We identified 10 important candidate genes: ACP5, WNT1, NFIX, CNN1, CALR, FKBP11, TRAPPC5, MAP2K7, RELA, and ENSGALG00000047166. Among the significantly enriched Kyoto Encyclopedia of Genes and Genomes pathways found, we identifed two bone-related pathways, one involving “osteoclast differentiation” and the other the “MAPK signaling pathway.” These results may help us better understand the molecular mechanism of bone traits in chickens and other birds and provide new insights for the genetic breeding of chickens.

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.

Competitive blocking of LRP4-sclerostin binding interface strongly promotes bone anabolic functions

Induction of bone formation by Wnt ligands is inhibited when sclerostin (Scl), an osteocyte-produced antagonist, binds to its receptors, the low-density lipoprotein receptor-related proteins 5 or 6 (LRP5/6). Recently, it was shown that enhanced inhibition is achieved by Scl binding to the co-receptor LRP4. However, it is not clear if the binding of Scl to LRP4 facilitates Scl binding to LRP5/6 or inhibits the Wnt pathway in an LRP5/6-independent manner. Here, using the yeast display system, we demonstrate that Scl exhibits a stronger binding affinity for LRP4 than for LRP6. Moreover, we found stronger Scl binding to LRP6 in the presence of LRP4. We further show that a Scl mutant (SclN93A), which tightly binds LRP4 but not LRP6, does not inhibit the Wnt pathway on its own. We demonstrate that SclN93A competes with Scl for a common binding site on LRP4 and antagonizes Scl inhibition of the Wnt signaling pathway in osteoblasts in vitro. Finally, we demonstrate that 2 weeks of bi-weekly subcutaneous injections of SclN93A fused to the fragment crystallizable (Fc) domain of immunoglobulin (SclN93AFc), which retains the antagonistic activity of the mutant, significantly increases bone formation rate and enhances trabecular volumetric bone fraction, trabecular number, and bone length in developing mice. Our data show that LRP4 serves as an anchor that facilitates Scl-LRP6 binding and that inhibition of the Wnt pathway by Scl depends on its prior binding to LRP4. We further provide evidence that compounds that inhibit Scl-LRP4 interactions offer a potential strategy to promote anabolic bone functions.

Osteogenesis Imperfecta: Mechanisms and Signaling Pathways Connecting Classical and Rare OI Types

Osteogenesis imperfecta (OI) is a phenotypically and genetically heterogeneous skeletal dysplasia characterized by bone fragility, growth deficiency, and skeletal deformity. Previously known to be caused by defects in type I collagen, the major protein of extracellular matrix, it is now also understood to be a collagen-related disorder caused by defects in collagen folding, posttranslational modification and processing, bone mineralization, and osteoblast differentiation, with inheritance of OI types spanning autosomal dominant and recessive as well as X-linked recessive. This review provides the latest updates on OI, encompassing both classical OI and rare forms, their mechanism, and the signaling pathways involved in their pathophysiology. There is a special emphasis on mutations in type I procollagen C-propeptide structure and processing, the later causing OI with strikingly high bone mass. Types V and VI OI, while notably different, are shown to be interrelated by the interferon-induced transmembrane protein 5 p.S40L mutation that reveals the connection between the bone-restricted interferon-induced transmembrane protein-like protein and pigment epithelium-derived factor pathways. The function of regulated intramembrane proteolysis has been extended beyond cholesterol metabolism to bone formation by defects in regulated membrane proteolysis components site-2 protease and old astrocyte specifically induced-substance. Several recently proposed candidate genes for new types of OI are also presented. Discoveries of new OI genes add complexity to already-challenging OI management; current and potential approaches are summarized.

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.

New Structural and Single Nucleotide Mutations in Type I and Type II Collagens in Taiwanese Children With Type I and Type II Collagenopathies

Collagenopathy is a rare genetic condition characterized by abnormality in either collagen structure or metabolism. Variations in its clinical presentations highlight diversity in the genetic causes and potential existence of concurrent mutations. Through whole exome sequencing (WES) complemented with multiplex ligation-dependent probe amplification, we identified the genetic etiologies for six cases with osteogenesis imperfecta (OI) in COL1A1 (p.T1298N, p.Q1280Pfs^∗51, and p.G557Vfs^∗23) and COL1A2 (c.1-1677_133-441del) as well as three cases with spondyloepiphyseal dysplasia congenita in COL2A1 (p.G1041S, p.G654S, and p.G441A). Co-occurrence of COL1A1 and WNT1 mutations was found in a patient with a mild OI phenotype but severe osteoporosis. These findings extended the pathogenic variant spectrum of COL1A1, COL1A2, and COL2A1 for type I and type II collagenopathies. Although WES provides a fast and accurate method to identify the genetic causes in most of the patients with type I and type II collagenopathies, its limitation of detecting CNVs because of variable capturing uniformity should be kept in mind when interpreting the results. Taken together, we demonstrate that multiple genetic characterizing technologies can provide an accurate and efficient molecular diagnostic of new genetic variants in disease-causing genes that are compatible with clinical phenotypes.

Whole Genome Level Analysis of the Wnt and DIX Gene Families in Mice and Their Coordination Relationship in Regulating Cardiac Hypertrophy

The Wnt signaling pathway is an evolutionarily conserved signaling pathway that plays essential roles in embryonic development, organogenesis, and many other biological activities. Both Wnt proteins and DIX proteins are important components of Wnt signaling. Systematic studies of Wnt and DIX families at the genome-wide level may provide a comprehensive landscape to elucidate their functions and demonstrate their relationships, but they are currently lacking. In this report, we describe the correlations between mouse Wnt and DIX genes in family expansion, molecular evolution, and expression levels in cardiac hypertrophy at the genome-wide scale. We observed that both the Wnt and DIX families underwent more expansion than the overall average in the evolutionarily early stage. In addition, mirrortree analyses suggested that Wnt and DIX were co-evolved protein families. Collectively, these results would help to elucidate the evolutionary characters of Wnt and DIX families and demonstrate their correlations in mediating cardiac hypertrophy.

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.

Experimental agents to improve fracture healing: utilizing the WNT signaling pathway

The process of bone healing largely recapitulates bone development in the embryo and ideally achieves complete restoration of bone shape and structure. However, because successful fracture healing requires tight interactions of numerous cell types and signaling molecules, any disruption of this highly coordinated processes can result in delayed healing or even non-union formation. The rate of fracture healing complications in orthopedic patients is reported to be 5-20%. Therefore, there is a need for new therapeutic strategies to improve fracture healing in patients with healing complications. One treatment strategy would include the easy and safe application of a pharmacological agent inducing osteoanabolic effects during fracture healing. One potential promising molecular target is the osteoanabolic WNT signaling pathway. This pathway plays an important role during embryonic bone development, homeostasis, mechanotransduction, development of osteoporosis and bone regeneration. This review focuses on preclinical studies targeting WNT signaling molecules to accelerate fracture healing. The three main investigated antagonists of the WNT signaling pathway, which can be blocked experimentally by antibodies, are Sclerostin, Dickkopf-1 and Midkine. Treating animals with antibodies against these proteins enhanced bone formation in the fracture callus. This indicates a therapeutic potential for these antibodies to accelerate fracture healing in patients with orthopedic complications.

Bone material properties and response to teriparatide in osteoporosis due to WNT1 and PLS3 mutations

CONTEXT

Patients with osteoporosis-associated WNT1 or PLS3 mutations have unique bone histomorphometric features and osteocyte-specific hormone expression patterns.

OBJECTIVE

To investigate the effects of WNT1 and PLS3 mutations on bone material properties.

DESIGN

Transiliac bone biopsies were evaluated by quantitative backscattered electron imaging, immunohistochemistry, and bone histomorphometry.

SETTING

Ambulatory patients.

PATIENTS

Three pediatric and eight adult patients with WNT1 or PLS3 mutations.

INTERVENTION

Bone mineralization density distribution and osteocyte protein expression was evaluated in 11 patients and repeated in six patients who underwent repeat biopsy after 24 months of teriparatide treatment.

MAIN OUTCOME MEASURE

Bone mineralization density distribution and protein expression.

RESULTS

Children with WNT1 or PLS3 mutations had heterogeneous bone matrix mineralization, consistent with bone modeling during growth. Bone matrix mineralization was homogenous in adults and increased throughout the age spectrum. Teriparatide had very little effect on matrix mineralization or bone formation in patients with WNT1 or PLS3 mutations. However, teriparatide decreased trabecular osteocyte lacunae size and increased trabecular bone FGF23 expression.

CONCLUSION

The contrast between preserved bone formation with heterogeneous mineralization in children and low bone turnover with homogenous bone mineral content in adults suggests that WNT1 and PLS3 have differential effects on bone modeling and remodeling. The lack of change in matrix mineralization in response to teriparatide, despite clear changes in osteocyte lacunae size and protein expression, suggests that altered WNT1 and PLS3 expression may interfere with coupling of osteocyte, osteoblast, and osteoclast function. Further studies are warranted to determine the mechanism of these changes.

Signaling Pathways in Bone Development and Their Related Skeletal Dysplasia

Bone development is a tightly regulated process. Several integrated signaling pathways including HH, PTHrP, WNT, NOTCH, TGF-β, BMP, FGF and the transcription factors SOX9, RUNX2 and OSX are essential for proper skeletal development. Misregulation of these signaling pathways can cause a large spectrum of congenital conditions categorized as skeletal dysplasia. Since the signaling pathways involved in skeletal dysplasia interact at multiple levels and have a different role depending on the time of action (early or late in chondrogenesis and osteoblastogenesis), it is still difficult to precisely explain the physiopathological mechanisms of skeletal disorders. However, in recent years, significant progress has been made in elucidating the mechanisms of these signaling pathways and genotype–phenotype correlations have helped to elucidate their role in skeletogenesis. Here, we review the principal signaling pathways involved in bone development and their associated skeletal dysplasia.

Signaling pathways affected by mutations causing osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous connective tissue disorder characterized by bone fragility and skeletal deformity. To maintain skeletal strength and integrity, bone undergoes constant remodeling of its extracellular matrix (ECM) tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. There are at least 20 recognized OI-forms caused by mutations in the two collagen type I-encoding genes or genes implicated in collagen folding, posttranslational modifications or secretion of collagen, osteoblast differentiation and function, or bone mineralization. The underlying disease mechanisms of non-classical forms of OI that are not caused by collagen type I mutations are not yet completely understood, but an altered ECM structure as well as disturbed intracellular homeostasis seem to be the main defects. The ECM orchestrates local cell behavior in part by regulating bioavailability of signaling molecules through sequestration, release and activation during the constant bone remodeling process. Here, we provide an overview of signaling pathways that are associated with known OI-causing genes and discuss the impact of these genes on signal transduction. These pathways include WNT-, RANK/RANKL-, TGFβ-, MAPK- and integrin-mediated signaling as well as the unfolded protein response.

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.

Consanguineous-derived homozygous WNT1 mutation results in osteogenesis imperfect with congenital ptosis and exotropia

Background

Wnt signaling pathway plays an important role in promoting ostergenesis. WNT1 mutations have been considered as a major cause of ostergenesis imperfect (OI). We identified an OI patient with pathogenic consanguineous‐derived homozygous WNT1 missense mutation.

Methods

We designed and applied a panel of known 261 genes associated with hereditary bone diseases for targeted next‐generation sequencing to examine clinically diagnosed OI patients. Detected mutations were confirmed by Sanger sequencing.

Results

The female proband presented with severe OI with low bone density, multiple long bone fractures, short stature, and absence of dentinogenesis imperfect and brain malformation. She had congenital ptosis and exotropia with her left eye, and absence of blue sclera. The proband came from a consanguineous family and had a homozygous WNT1 missense mutation (c.677C>T, (p.S226L)). In addition, three other compound heterozygous mutations (c.1729C>T in FKBP10, c.1958A>C in FGFR3, c.760G>C in TRPV4) were also detected in her family members.

Conclusion

We report the first identified case of consanguineous derived homozygous WNT1 mutation leading to severe osteogenesis imperfecta with congenital ptosis and exotropia.

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.

Mendelian bone fragility disorders

Mendelian bone fragility disorders are caused by genetic variants that can be inherited in an autosomal dominant, autosomal recessive or X-linked manner and have a large detrimental effect on bone strength. As a rule, the more damaging the genetic defect is, the earlier the first fracture will occur, typically during bone development. This review focusses on conditions where bone fragility is the most conspicuous characteristic, of which osteogenesis imperfecta (OI) is the best-known disorder. The large majority of individuals with an OI phenotype have disease-causing dominant variants in COL1A1 or COL1A2, the genes coding for collagen type I. Interestingly, large sequencing databases indicate that there are about 10 times more carriers of COL1A1/COL1A2 variants that should lead to OI than there are individuals with a diagnosis of OI. It is possible that at least some of these variants lead to incomplete OI phenotypes and are diagnosed as osteoporosis during adulthood. Apart from mutations affecting collagen type I production, biallelic mutations in LRP5 and WNT1 can cause very rare and severe bone fragility disorders. Heterozygous pathogenic variants in these genes are much more common and can cause the clinical picture of primary osteoporosis. As sequencing studies are more widely performed in adults with bone fragility disorders, evidence is emerging that what appears as primary osteoporosis in fact can be due to mutations in bona fide OI genes. The distinction between OI and primary osteoporosis is therefore likely to blur in future.

Molecular mechanisms and clinical manifestations of rare genetic disorders associated with type I collagen

Type I collagen is an important structural protein of bone, skin, tendon, ligament and other connective tissues. It is initially synthesized as a precursor form, procollagen, consisting of two identical pro-α1(I) and one proα2(I) chains, encoded by COL1A1 and COL1A2, respectively. The N- and C- terminal propeptides of procollagen are cleavage by N-proteinase and C-proteinase correspondingly, to form the central triple helix structure with Gly-X-Y repeat units. Mutations of COL1A1 and COL1A2 genes are associated with osteogenesis imperfecta, some types of Ehlers-Danlos syndrome, Caffey diseases, and osteogenesis imperfect/Ehlers- Danlos syndrome overlapping diseases. Clinical symptoms caused by different variations can be variable or similar, mild to lethal, and vice versa. We reviewed the relationship between clinical manifestations and type I collagen - related rare genetic disorders and their possible molecular mechanisms for different mutations and disorders.

Splice receptor-site mutation c.697-2A>G of the COL1A1 gene in a Chinese family with osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a genetic disorder characterized by bone fragility and blue sclerae, which are mainly caused by a mutation of the COL1A1 or COL1A2 genes that encode type I procollagen. Mutations in the splice site of type I collagen genes are one of the mutations that cause OI and usually lead to a mild or moderate OI phenotype. A heterozygous A to G point mutation in intron 9 at the -2 position of the splice receptor site of COL1A1 was identified in a family with type I or IV OI. Three affected individuals in four generations of one family all presented with several clinical symptoms. They all had pectus carinatum, flat feet, gray-blue sclerae, and normal stature, teeth, hearing, and vision. Forearm fractures, small joint dislocations, and muscle weakness were all present in the patient's father and grandmother, who presented with a moderate type IV phenotype. The 10-year-old proband with type I OI had suffered a fracture twice, but had no history of joint dislocation or skin hyperextensibility. Charting the family helped to identify clinical symptoms in patients with mutations at the N-terminal of type I collagen genes.

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.

Ptosis as a unique hallmark for autosomal recessive WNT1-associated osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder, mainly characterized by bone fragility and low bone mass. Defects in the type I procollagen-encoding genes account for the majority of OI, but increasingly more rare autosomal recessive (AR) forms are being identified, which are caused by defects in genes involved in collagen metabolism, bone mineralization, or osteoblast differentiation. Bi-allelic mutations in WNT1 have been associated with a rare form of AR OI, characterized by severe osteoporosis, vertebral compression, scoliosis, fractures, short stature, and variable neurological problems. Heterozygous WNT1 mutations have been linked to autosomal dominant early-onset osteoporosis. In this study, we describe the clinical and molecular findings in 10 new patients with AR WNT1-related OI. Thorough revision of the clinical symptoms of these 10 novel patients and previously published AR WNT1 OI cases highlight ptosis as a unique hallmark in the diagnosis of this OI subtype.