Skeletal characteristics associated with homozygous and heterozygous WNT1 mutations

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

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. 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.

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.

Osteoporosis related to WNT1 variants: a not infrequent cause of osteoporosis

Nearly 10% of subjects with severe idiopathic osteoporosis present pathogenic WNT1 mutations. Clinical characteristics include a family history of osteoporosis, early adulthood onset, and fragility fractures which may evolve to pseudoarthrosis. WNT1 should be genetically screened in these patients as the phenotype is often variable and therapeutic approaches may differ.

INTRODUCTION

Recent studies have shown that homozygous WNT1 gene mutations may be related to severe osteoporosis resembling osteogenesis imperfecta (OI). Conversely, heterozygous WNT1 mutations are linked to a milder phenotype of early-onset osteoporosis. Treatment with bisphosphonates is reported to be unsatisfactory. Our aim was to analyze the presence and prevalence of WNT1 mutations and the main associated clinical characteristics in subjects with primary early-onset osteoporosis.

METHODS

A cohort comprising 56 subjects (aged 19-60 years) with severe, early-onset osteoporosis was screened by massive parallel sequencing with a 23-gene panel. The gene panel included 19 genes known to cause OI (including the WNT1 gene), three genes related to osteoporosis, and the gene related to hypophosphatasia (ALPL).

RESULTS

We identified five patients (3 men) with heterozygous WNT1 variants. All presented severe osteoporosis with early fracture onset and a family history of fragility fractures. None presented a characteristic phenotype of OI or skeletal deformities. One patient was previously treated with bisphosphonates, presenting inadequate response to treatment and two developed pseudoarthrosis after upper arm fractures. All subjects were diagnosed in adulthood.

CONCLUSIONS

Nearly 1/10 adult subjects with severe idiopathic osteoporosis may present pathogenic WNT1 mutations. Clinical characteristics commonly include a family history of osteoporosis, onset in early adulthood, marked decrease in bone mass, and prevalent fractures, particularly vertebral. WNT1 should be genetically screened in these subjects as the phenotype is often variable and the therapeutic approach may differ. The role of WNT1 mutations in the development of pseudoarthrosis should also be elucidated.

Early-Onset Osteoporosis: Rare Monogenic Forms Elucidate the Complexity of Disease Pathogenesis Beyond Type I Collagen

Early-onset osteoporosis (EOOP), characterized by low bone mineral density (BMD) and fractures, affects children, premenopausal women and men aged <50 years. EOOP may be secondary to a chronic illness, long-term medication, nutritional deficiencies, etc. If no such cause is identified, EOOP is regarded primary and may then be related to rare variants in genes playing a pivotal role in bone homeostasis. If the cause remains unknown, EOOP is considered idiopathic. The scope of this review is to guide through clinical and genetic diagnostics of EOOP, summarize the present knowledge on rare monogenic forms of EOOP, and describe how analysis of bone biopsy samples can lead to a better understanding of the disease pathogenesis. The diagnostic pathway of EOOP is often complicated and extensive assessments may be needed to reliably exclude secondary causes. Due to the genetic heterogeneity and overlapping features in the various genetic forms of EOOP and other bone fragility disorders, the genetic diagnosis usually requires the use of next-generation sequencing to investigate several genes simultaneously. Recent discoveries have elucidated the complexity of disease pathogenesis both regarding genetic architecture and bone tissue-level pathology. Two rare monogenic forms of EOOP are due to defects in genes partaking in the canonical WNT pathway: LRP5 and WNT1. Variants in the genes encoding plastin-3 (PLS3) and sphingomyelin synthase 2 (SGMS2) have also been found in children and young adults with skeletal fragility. The molecular mechanisms leading from gene defects to clinical manifestations are often not fully understood. Detailed analysis of patient-derived transiliac bone biopsies gives valuable information to understand disease pathogenesis, distinguishes EOOP from other bone fragility disorders, and guides in patient management, but is not widely available in clinical settings. Despite the great advances in this field, EOOP remains an insufficiently explored entity and further research is needed to optimize diagnostic and therapeutic approaches. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Non-collagen pathogenic variants resulting in the osteogenesis imperfecta phenotype in children: a single-country observational cohort study

BACKGROUND/OBJECTIVES

In England, children (0-18 years) with severe, complex and atypical osteogenesis imperfecta (OI) are managed by four centres (Birmingham, Bristol, London, Sheffield) in a 'Highly Specialised Service' (HSS OI); affected children with a genetic origin for their disease that is not in COL1A1 or COL1A2 form the majority of the 'atypical' group, which has set criteria for entry into the service. We have used the data from the service to assess the range and frequency of non-collagen pathogenic variants resulting in OI in a single country.

METHODS

Children with atypical OI were identified through the HSS OI service database. All genetic testing for children with OI in the service were undertaken at the Sheffield Diagnostic Genetics Service. Variant data were extracted and matched to individual patients. This study was done as part of a service evaluation project registered with the Sheffield Children's Hospital Clinical Governance Department.

RESULTS

One hundred of 337 children in the HSS met the 'atypical' criteria. Eighty have had genetic testing undertaken; 72 had genetic changes detected, 67 in 13 genes known to be causative for OI. The most frequently affected genes were IFITM5 (22), P3H1 (12), SERPINF1 (8) and BMP1 (6).

CONCLUSION

Among children with more severe forms of OI (approximately one-third of all children with OI), around 20% have pathogenic variants in non-collagen genes. IFITM5 was the most commonly affected gene, followed by genes within the P3H1 complex. These data provide additional information regarding the likelihood of different genetic origins of the disease in children with OI, which may influence clinical care.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

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.

Wnt16 signaling in bone homeostasis and osteoarthristis

Wnts are secreted cysteine-rich glycoproteins involved in joint development and skeletal homeostasis and have been implicated in the occurrence of osteoarthritis. Over the past decade, Wnt16, a member of the Wnt family, has received widespread attention for its strong association with bone mineral density, cortical bone thickness, bone strength, and osteoporotic fracture risk. In recent years, further studies have shed light on the role of Wnt16 a positive regulator of bone mass and protective regulator of osteoarthritis progression. Transduction mechanisms and crosstalk involving Wnt16 signaling have also been illustrated. More importantly, local Wnt16 treatment has been shown to ease osteoarthritis, inhibit bone resorption, and promote new bone formation in bone defect models. Thus, Wnt16 is now a potential therapeutic target for skeletal diseases and osteoarthritis. This paper reviews our current understanding of the mechanisms by which Wnt16 signaling regulates bone homeostasis and osteoarthritis.

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.

Assessment of Wnt pathway selected gene expression levels in peripheral blood mononuclear cells (PBMCs) of postmenopausal patients with low bone mass

The purpose of the study was to assess the expression of selected genes of the Wnt pathway: APC, AXIN1, CTNNB1, DKK1, GSK3B, KREMEN1, SFRP1, and WNT1 in peripheral blood mononuclear cells (PBMC) of patients, selected in consideration of their bone mineral density (BMD), and the occurrence of low-energy fractures. The study involved 45 postmenopausal women, divided into four groups, according to BMD and fracture history. Measurements of laboratory parameters and RNA expression in PBMC cells were carried out in material, collected once at the inclusion visit. The densitometric examination was performed on all participants. In the analysis of the relative expression levels (RELs) of the studied genes in the entire population, we observed an overexpression for SFRP1 in 100% of samples and WNT1. In addition, the REL of DKK1, APC, and GSK3B genes were slightly elevated versus the calibrator. In contrast, CTNNB1 and AXIN1 presented with a slightly decreased RELs. Analysis did not show any significant differences among the groups in the relative gene expression levels (p < 0.05) of particular genes. However, we have observed quite numerous interesting correlations between the expression of the studied genes and BMD, the presence of fractures, and laboratory parameters, both in the whole studied population as well as in selected groups. In conclusion, the high level of CTNNB1 expression maintains normal BMD and/or protects against fractures. It also appears that the changes in expression levels of the Wnt pathway genes in PBMCs reflect the expected changes in bone tissue.

Osteogenesis imperfecta in children

Osteogenesis imperfecta (OI) is a disease characterised by altered bone tissue material properties together with abnormal micro and macro-architecture and thus bone fragility, increased bone turnover and hyperosteocytosis. Increasingly appreciated are the soft tissue changes, sarcopenia in particular. Approaches to treatment are now multidisciplinary, with bisphosphonates having been the primary pharmacological intervention over the last 20 years. Whilst meta-analyses suggest that anti-fracture efficacy across the life course is equivocal, there is good evidence that for children bisphosphonates reduce fracture risk, increase vertebral size and improve vertebral shape, as well as improving motor function and mobility. The genetics of OI continues to provide insights into the molecular pathogenesis of the disease, although the pathophysiology is less clear. The complexity of the multi-scale interactions of bone tissue with cellular function are gradually being disentangled, but the fundamental question of why increased tissue brittleness should be associated with so many other changes is unclear; ER stress, pro-inflammatory cytokines, accelerated senesence and altered matrix component release might all contribute, but a unifying hypothesis remains elusive. New approaches to therapy are focussed on increasing bone mass, following the paradigm established by the treatment of postmenopausal osteoporosis. For adults, this brings the prospect of restoring previously lost bone - for children, particularly at the severe end of the spectrum, the possibility of further reducing fracture frequency and possibly altering growth and long term function are attractive. The alternatives that might affect tissue brittleness are autophagy enhancement (through the removal of abnormal type I collagen aggregates) and stem cell transplantation - both still at the preclinical stage of assessment. Preclinical assessment is not supportive of targeting inflammatory pathways, although understanding why TGFb signalling is increased, and whether that presents a treatment target in OI, remains to be established.

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.

Wnt7a promotes the osteogenic differentiation of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) have the ability of differentiating into osteoblasts. Elucidating the molecular mechanisms of MSC differentiation into osteoblasts may provide novel therapeutic strategies for bone‑related diseases. Increasing evidence has confirmed that Wnt signaling plays the key role in osteoblast differentiation; however, the role of individual Wnt proteins in osteogenesis needs to be investigated. The present study thus aimed to explore the role of Wnt7a in bone formation. For this purpose, human bone‑derived MSCs were identified by flow cytometry and the cell differentiation potential, including osteogenic and adipogenic differentiation was examined. In order to explore the role of Wnt7a in MSC osteogenic differentiation, Wnt7a expression was measured at the mRNA and protein level following treatment with the osteogenic inducer, bone morphogenetic protein (BMP)4/7, and following the induction of osteogenic or adipogenic differentiation. The ectopic expression of Wnt7a in MSCs was confirmed and its influence on MSC osteogenic differentiation was detected using osteocyte markers and by Alizarin Red S staining. Mechanistically, the influence of Wnt7a on Runt‑related transcription factor 2 (RUNX2) expression was examined at the mRNA and protein level. The regulatory effects of Wnt7a on RUNX2 promoter activities were examined by promoter reporter assay, and by examining the binding of TCF1, a downstream target of Wnt, to the RUNX2 promoter by ChIP assay. The results revealed that the knockdown of Wnt7a in MSCs decreased the expression of osteocyte markers and inhibited osteogenic differentiation. In accordance, the overexpression of Wnt7a in MSCs increased the expression of osteocyte markers and promoted osteogenic differentiation. Mechanistically, the knockdown of Wnt7a in MSCs reduced RUNX2 expression and the overexpression of Wnt7a in MSCs promoted RUNX2 expression. Furthermore, it was confirmed that Wnt7a regulated RUNX2 promoter activities by promoter report assay, and by examining the binding of TCF1 to the RUNX2 promoter by ChIP assay. On the whole, the present study demonstrates that Wnt7a plays a key role in MSC differentiation into osteoblasts and the findings presented herein may provide a promising therapy target for bone‑related diseases.

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.

Bone biology: insights from osteogenesis imperfecta and related rare fragility syndromes

The limited accessibility of bone and its mineralized nature have restricted deep investigation of its biology. Recent breakthroughs in identification of mutant proteins affecting bone tissue homeostasis in rare skeletal diseases have revealed novel pathways involved in skeletal development and maintenance. The characterization of new dominant, recessive and X-linked forms of the rare brittle bone disease osteogenesis imperfecta (OI) and other OI-related bone fragility disorders was a key player in this advance. The development of in vitro models for these diseases along with the generation and characterization of murine and zebrafish models contributed to dissecting previously unknown pathways. Here, we describe the most recent advances in the understanding of processes involved in abnormal bone mineralization, collagen processing and osteoblast function, as illustrated by the characterization of new causative genes for OI and OI-related fragility syndromes. The coordinated role of the integral membrane protein BRIL and of the secreted protein PEDF in modulating bone mineralization as well as the function and cross-talk of the collagen-specific chaperones HSP47 and FKBP65 in collagen processing and secretion are discussed. We address the significance of WNT ligand, the importance of maintaining endoplasmic reticulum membrane potential and of regulating intramembrane proteolysis in osteoblast homeostasis. Moreover, we also examine the relevance of the cytoskeletal protein plastin-3 and of the nucleotidyltransferase FAM46A. Thanks to these advances, new targets for the development of novel therapies for currently incurable rare bone diseases have been and, likely, will be identified, supporting the important role of basic science for translational approaches.

Heterozygous WNT1 variant causing a variable bone phenotype

Osteogenesis imperfecta (OI) is a family of heritable disorders of bone fragility. Most individuals with OI have mutations in the genes encoding type I collagen; at least 17 other genes have been associated with OI. Biallelic loss-of-function mutations in WNT1 cause severe OI. Heterozygous missense variants in WNT1 are responsible for early-onset osteoporosis with variable bone phenotypes. Herein, we report a third-generation family with four affected individuals, some presenting with multiple low-impact fractures in childhood and others presenting with early-onset osteoporosis without a striking fracture history. A WNT1 variant (c. 1051 > C; p.Trp351Arg) was identified in the proband and segregated with a bone phenotype in three additional family members, consistent with autosomal dominant inheritance. In the proband, whole genome sequencing also revealed a de novo duplication (434 kb) of 22q11.2 that involves 25 genes, 4 of which are associated with human disease when haploinsufficient. Though smaller than the typical (1.5 Mb) 22q11.2 duplication, the duplication in the proband may be responsible for additional nonosseous aspects of his phenotype (hypotonia, developmental delay, small genitalia, strabismus, and depression in preadolescence). This case demonstrates the variability of bone phenotype conferred by a WNT1 variant and extends the spectrum of bone phenotypes associated with heterozygous WNT1 mutations.

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.

Autism and heritable bone fragility: A true association?

Objectives

Osteogenesis Imperfecta (OI) is a heterogeneous condition mainly characterised by bone fragility; intelligence is reported to be normal. However, a minority of children seen also show symptomology consistent with an ‘Autism Spectrum Disorder’. A joint genetics and psychology research study was undertaken to identify these patients using ‘Gold Standard’ research tools: Autism Diagnostic Inventory Revised (ADI-R); Autism Diagnostic Observation Schedule (ADOS) and undertake genetic analyses in them.

Method

A cohort of n = 7 children with autistic traits and severe/complex OI were recruited to the study. The study was set-up to explore whether there was a genetic link between bone fragility and autism in a sub-set of patients with bone fragility identified with autism traits in our complex/severe OI clinic. This was not set-up as a prevalence study but rather an exploration of genetics in association with ADI/ADOS confirmed ASD and bone fragility.

ADI& ADOS

Standardised tools were used to confirm autism diagnosis. ADI and ADOS were completed by the Clinical Psychologist; ADI comprises a 93 item semi-structured clinical review with a diagnostic algorithm diagnosing Autism; ADOS is a semi-structured assessment of socialisation, communication and play/imagination which also provides a diagnostic algorithm.

Exome sequencing

In patients recruited, those that fulfilled research criteria for diagnosis of autism using above tools were recruited to trio whole exome sequencing (WES).

Results

one patient had compound heterozygous variants in NBAS; one patient had a variant in NRX1; one patient had a maternally inherited PLS3 variant; all the other patients in this cohort had pathogenic variants in COL1A1/COL1A2.

Conclusions

Although, not set out as an objective, we were able to establish that identifying autism had important clinical and social benefits for patients and their families in ensuring access to services, appropriate schooling, increased understanding of behaviour and support.

Lay summary

It is important for clinicians looking after children with brittle bone disease, also referred to as Osteogenesis Imperfecta (OI) to be aware of early features of developmental delay/autistic traits especially with severe forms of OI as the emphasis is on their mobility and bone health. Ensuring appropriate assessment and access to services early-on will enable these patients to achieve their potential. Further investigations of genomics in bone fragility in relation to autism are required and dual diagnosis is essential for high quality clinical and educational provision.

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Osteogenesis Imperfecta is the commonest form of heritable bone fragility disorder with an incidence of 1 in 15,000 live births

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Intelligence is usually reported to be normal; however, this study describes association of autistic traits with OI

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It is important to undertake autism assessments early in case of clinical suspicion of ASD as children with OI would benefit from early educational intervention

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Early identification and clarification of diagnosis of ASD in children with OI will ensure that children are able to achieve their full potential.

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.

THE CRUCIAL ROLE OF THE WNT SYSTEM IN BONE REMODELLING

The maintenance of bone mass is critically dependent on the balance between bone formation by osteoblasts and bone resorption by osteoclasts, processes in which osteocytes play also an important role. The activities of these bone cells are regulated by a variety of endocrine and paracrine factors of which sex steroids, parathyroid hormone, 1.25(OH)2-vitamin D3, glucocorticoids, retinoids and thyroid hormones are the most well known systemic factors. To the long list of locally acting factors belong cytokines and growth factors. This list was extended some 15 years ago by the discovery of the very important role of the WNT signalling system for the maintenance of bone mass. The first evidence of its role was the findings that mutations in the LRP5 gene, encoding a co-receptor in WNT-signaling, could result in either gain or loss of bone mass, i.e. either high bone mass or osteoporosis. This was a most unexpected observation since no indications existed prior to this discovery that the WNT signalling system had a role in bone remodeling. Since then, many observations have been made demonstrating the important role of different WNTs in regulating bone formation and resorption. Interestingly, some of these findings have demonstrated that trabecular and cortical bone are regulated by different mechanisms. It is the aim of the present overview to give the readers an insight into the WNT signalling system and its role in bone remodeling.

Osteogenesis imperfecta

Skeletal deformity and bone fragility are the hallmarks of the brittle bone dysplasia osteogenesis imperfecta. The diagnosis of osteogenesis imperfecta usually depends on family history and clinical presentation characterized by a fracture (or fractures) during the prenatal period, at birth or in early childhood; genetic tests can confirm diagnosis. Osteogenesis imperfecta is caused by dominant autosomal mutations in the type I collagen coding genes (COL1A1 and COL1A2) in about 85% of individuals, affecting collagen quantity or structure. In the past decade, (mostly) recessive, dominant and X-linked defects in a wide variety of genes encoding proteins involved in type I collagen synthesis, processing, secretion and post-translational modification, as well as in proteins that regulate the differentiation and activity of bone-forming cells have been shown to cause osteogenesis imperfecta. The large number of causative genes has complicated the classic classification of the disease, and although a new genetic classification system is widely used, it is still debated. Phenotypic manifestations in many organs, in addition to bone, are reported, such as abnormalities in the cardiovascular and pulmonary systems, skin fragility, muscle weakness, hearing loss and dentinogenesis imperfecta. Management involves surgical and medical treatment of skeletal abnormalities, and treatment of other complications. More innovative approaches based on gene and cell therapy, and signalling pathway alterations, are under investigation.

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.

The brains of the bones: how osteocytes use WNT1 to control bone formation

WNT proteins drive the development and maintenance of many tissues, including bone. It is less clear which of the many WNT proteins act on bone or where these WNTs act in the skeleton; however, loss-of-function mutations in WNT1 cause bone fragility in children and adults. In this issue of the JCI, Joeng and colleagues demonstrate that bone formation is under the control of WNT1 produced by osteocytes, the cells that reside deep in the bone matrix and form dendritic networks. The implication of WNT1 in the control of bone formation identifies a potential new target for the treatment of low bone mass disorders, such as osteoporosis.

Phenotypic Spectrum in Osteogenesis Imperfecta Due to Mutations in TMEM38B: Unraveling a Complex Cellular Defect

CONTEXT

Recessive mutations in TMEM38B cause type XIV osteogenesis imperfecta (OI) by dysregulating intracellular calcium flux.

OBJECTIVES

Clinical and bone material phenotype description and osteoblast differentiation studies.

DESIGN AND SETTING

Natural history study in pediatric research centers.

PATIENTS

Eight patients with type XIV OI.

MAIN OUTCOME MEASURES

Clinical examinations included bone mineral density, radiographs, echocardiography, and muscle biopsy. Bone biopsy samples (n = 3) were analyzed using histomorphometry, quantitative backscattered electron microscopy, and Raman microspectroscopy. Cellular differentiation studies were performed on proband and control osteoblasts and normal murine osteoclasts.

RESULTS

Type XIV OI clinical phenotype ranges from asymptomatic to severe. Previously unreported features include vertebral fractures, periosteal cloaking, coxa vara, and extraskeletal features (muscular hypotonia, cardiac abnormalities). Proband lumbar spine bone density z score was reduced [median -3.3 (range -4.77 to +0.1; n = 7)] and increased by +1.7 (1.17 to 3.0; n = 3) following bisphosphonate therapy. TMEM38B mutant bone has reduced trabecular bone volume, osteoblast, and particularly osteoclast numbers, with >80% reduction in bone resorption. Bone matrix mineralization is normal and nanoporosity low. We demonstrate a complex osteoblast differentiation defect with decreased expression of early markers and increased expression of late and mineralization-related markers. Predominance of trimeric intracellular cation channel type B over type A expression in murine osteoclasts supports an intrinsic osteoclast defect underlying low bone turnover.

CONCLUSIONS

OI type XIV has a bone histology, matrix mineralization, and osteoblast differentiation pattern that is distinct from OI with collagen defects. Probands are responsive to bisphosphonates and some show muscular and cardiovascular features possibly related to intracellular calcium flux abnormalities.

Novel missense loss-of-function mutations of WNT1 in an autosomal recessive Osteogenesis imperfecta patient

Osteogenesis imperfecta (OI) is a heritable skeletal disorder characterized by bone fragility and low bone mass. Recently, loss-of-function mutations of WNT1 have been reported to be causative in OI or osteoporosis. We report an OI patient with novel compound heterozygous WNT1 missense mutations, p.Glu123Asp and p.Cys153Gly. Both mutations are found in the exon 3, and the p.Glu123Asp is the most proximal N-terminus missense mutation among the reported WNT1 missense mutations in OI patients. In vitro functional analysis reveals that while expression of wildtype WNT1 stimulates canonical WNT1-mediated β-catenin signaling, that of individual WNT1 mutant fails to do so, indicative of the pathogenic nature of the WNT1 variants. Although the pathogenic mechanism of WNT1 defects in OI has yet to be uncovered, these findings further contribute to the implications and importance of functional relevance of WNT1 in skeletal disorders.

Featured Article: Transcriptional landscape analysis identifies differently expressed genes involved in follicle-stimulating hormone induced postmenopausal osteoporosis

Osteoporosis is a disorder associated with bone tissue reorganization, bone mass, and mineral density. Osteoporosis can severely affect postmenopausal women, causing bone fragility and osteoporotic fractures. The aim of the current study was to compare blood mRNA profiles of postmenopausal women with and without osteoporosis, with the aim of finding different gene expressions and thus targets for future osteoporosis biomarker studies. Our study consisted of transcriptome analysis of whole blood serum from 12 elderly female osteoporotic patients and 12 non-osteoporotic elderly female controls. The transcriptome analysis was performed with RNA sequencing technology. For data analysis, the edgeR package of R Bioconductor was used. Two hundred and fourteen genes were expressed differently in osteoporotic compared with non-osteoporotic patients. Statistical analysis revealed 20 differently expressed genes with a false discovery rate of less than 1.47 × 10-4 among osteoporotic patients. The expression of 10 genes were up-regulated and 10 down-regulated. Further statistical analysis identified a potential osteoporosis mRNA biomarker pattern consisting of six genes: CACNA1G, ALG13, SBK1, GGT7, MBNL3, and RIOK3. Functional ingenuity pathway analysis identified the strongest candidate genes with regard to potential involvement in a follicle-stimulating hormone activated network of increased osteoclast activity and hypogonadal bone loss. The differentially expressed genes identified in this study may contribute to future research of postmenopausal osteoporosis blood biomarkers.

New Genetic Forms of Childhood-Onset Primary Osteoporosis

Recent developments in genetic technology have given us the opportunity to look at diseases in a new and more detailed way. This Mini Review discusses monogenetic forms of childhood-onset primary osteoporosis, with the main focus on osteoporosis caused by mutations in WNT1 and PLS3, two of the most recently discovered genes underlying early-onset osteoporosis. The importance of WNT1 in the accrual and maintenance of bone mass through activation of canonical WNT signaling was recognized in 2013. WNT1 was shown to be a key ligand for the WNT-signaling pathway, which is of major importance in the regulation of bone formation. More recently, mutations in PLS3, located on the X chromosome, were shown to be the cause of X-linked childhood-onset primary osteoporosis affecting mainly males. The function of PLS3 in bone metabolism is still not completely understood, but it has been speculated to have an important role in mechanosensing by osteocytes and in matrix mineralization. In this new era of genetics, our knowledge on genetic causes of childhood-onset osteoporosis expands constantly. These discoveries bring new possibilities, but also new challenges. Guidelines are needed to implement this new genetic knowledge to clinical patient care and to guide genetic investigations in affected families.

Skeletal Characteristics of WNT1 Osteoporosis in Children and Young Adults

WNT proteins comprise a 19-member glycoprotein family that act in several developmental and regenerative processes. In bone, WNT proteins regulate osteoblast differentiation and maintain bone health by activating the canonical WNT/β-catenin pathway. We reported a heterozygous missense mutation c.652T>G (p.C218G) in WNT1 exon 4 as the cause for severe early-onset, autosomal dominant osteoporosis. The initial study concerned a large Finnish family with 10 affected adults. Here we report clinical findings of the WNT1 osteoporosis in 8 children and young adults (median age 14 years; range 10 to 30 years) in two families, all with the p.C218G mutation in WNT1. Clinical assessments showed no apparent dysmorphia or features similar to typical osteogenesis imperfecta (OI). Biochemistry revealed no changes in parameters of calcium metabolism and bone turnover markers. Fracture frequencies varied, but all subjects had sustained at least one fracture and 4 had a pathological fracture history. Plain radiographs showed osteopenic appearance, loss in vertebral height, and thin diaphyses of the long bones. Bone densitometry showed the BMD to be below normal median in all subjects and the bone mass deficit seemed to be more severe in older participants. Bone histomorphometry revealed a low turnover osteoporosis in 2 subjects at ages 14 and 16 years. These findings are congruent with earlier findings in adult patients and indicate that WNT1 osteoporosis causes significant skeletal changes already in early childhood and impairs bone mass gain during pubertal years. Genetic testing of children or close relatives of affected individuals is recommended for appropriate preventive measures. © 2016 American Society for Bone and Mineral Research.

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

BACKGROUNDS

Osteogenesis imperfecta (OI) is a rare inherited disease characterized by increased bone fragility and vulnerability to fractures. Recently, WNT1 is identified as a new candidate gene for OI, here we detect pathogenic mutations in WNT1 and analyze the genotype-phenotype association in four Chinese families with OI.

METHODS

We designed a targeted next generation sequencing panel with known fourteen OI-related genes. We applied the approach to detect pathogenic mutations in OI patients and confirmed the mutations with Sanger sequencing and cosegregation analysis. Clinical fractures, bone mineral density (BMD) and the other clinical manifestations were evaluated. We also observed the effects of bisphosphonates in OI patients with WNT1 mutations.

RESULTS

Four compound heterozygous mutations (c.110T>C; c.505 G>T; c. 385G>A; c.506 G>A) in WNT1 were detected in three unrelated families. These four mutations had not been reported yet. A recurrent homozygous mutation (c.506dupG) was identified in the other two families. These patients had moderate to severe OI, white to blue sclera, absence of dentinogenesis imperfecta and no brain malformation. We did not observe clear genotype-phenotype correlation in WNT1 mutated OI patients. Though bisphosphonates increased BMD in WNT1 related OI patients, height did not increase and fracture continued.

CONCLUSIONS

We reported four novel heterozygous variants and confirmed a previous reported WNT1 mutation in four Chinese families with a clinical diagnosis of OI. Our study expanded OI spectrum and confirmed moderate to severe bone fragility induced by WNT1 defects.

The management of osteoporosis in children

This article reviews the manifestations and risk factors associated with osteoporosis in childhood, the definition of osteoporosis and recommendations for monitoring and prevention. As well, this article discusses when a child should be considered a candidate for osteoporosis therapy, which agents should be prescribed, duration of therapy and side effects. There has been significant progress in our understanding of risk factors and the natural history of osteoporosis in children over the past number of years. This knowledge has fostered the development of logical approaches to the diagnosis, monitoring, and optimal timing of osteoporosis intervention in this setting. Current management strategies are predicated upon monitoring at-risk children to identify and then treat earlier rather than later signs of osteoporosis in those with limited potential for spontaneous recovery. On the other hand, trials addressing the prevention of the first-ever fracture are still needed for children who have both a high likelihood of developing fractures and less potential for recovery. This review focuses on the evidence that shapes the current approach to diagnosis, monitoring, and treatment of osteoporosis in childhood, with emphasis on the key pediatric-specific biological principles that are pivotal to the overall approach and on the main questions with which clinicians struggle on a daily basis. The scope of this article is to review the manifestations of and risk factors for primary and secondary osteoporosis in children, to discuss the definition of pediatric osteoporosis, and to summarize recommendations for monitoring and prevention of bone fragility. As well, this article reviews when a child is a candidate for osteoporosis therapy, which agents and doses should be prescribed, the duration of therapy, how the response to therapy is adjudicated, and the short- and long-term side effects. With this information, the bone health clinician will be poised to diagnose osteoporosis in children and to identify when children need osteoporosis therapy and the clinical outcomes that gauge efficacy and safety of treatment.

The genetics of bone mass and susceptibility to bone diseases

Osteoporosis is characterized by low bone mass and an increased risk of fracture. Genetic factors, environmental factors and gene-environment interactions all contribute to a person's lifetime risk of developing an osteoporotic fracture. This Review summarizes key advances in understanding of the genetics of bone traits and their role in osteoporosis. Candidate-gene approaches dominated this field 20 years ago, but clinical and preclinical genetic studies published in the past 5 years generally utilize more-sophisticated and better-powered genome-wide association studies (GWAS). High-throughput DNA sequencing, large genomic databases and improved methods of data analysis have greatly accelerated the gene-discovery process. Linkage analyses of single-gene traits that segregate in families with extreme phenotypes have led to the elucidation of critical pathways controlling bone mass. For example, components of the Wnt-β-catenin signalling pathway have been validated (in both GWAS and functional studies) as contributing to various bone phenotypes. These notable advances in gene discovery suggest that the next decade will witness cataloguing of the hundreds of genes that influence bone mass and osteoporosis, which in turn will provide a roadmap for the development of new drugs that target diseases of low bone mass, including osteoporosis.

Two Rare Mutations in the COL1A2 Gene Associate With Low Bone Mineral Density and Fractures in Iceland

We conducted a genome-wide association study of low bone mineral density (BMD) at the hip and spine utilizing sequence variants found through whole-genome sequencing of 2636 Icelanders. We found two rare missense mutations, p.Gly496Ala and p.Gly703Ser, in the COL1A2 gene that associate with measures of osteoporosis in Icelanders. Mutations in COL1A2 are known to cause the autosomal dominant disorder osteogenesis imperfecta. Both variants associate with low BMD and with osteoporotic fractures. p.Gly496Ala (frequency of 0.105%) shows the strongest association with low BMD at the spine (p = 1.8 × 10(-7) , odds ratio [OR] = 4.61 [95% confidence interval (CI) 2.59, 8.18]), whereas p.Gly703Ser (frequency of 0.050%) is most strongly associated with low BMD at the hip (p = 1.9 × 10(-8) , OR = 9.34 [95% CI 4.28, 20.3]). Association with fractures was p = 2.2 × 10(-5) , OR = 3.75 (95% CI 2.03, 6.93) and p = 0.0023, OR = 4.32 (95% CI 1.69, 11.1), respectively. The carriers of these variants do not have signs of osteogenesis imperfecta other than low BMD, demonstrating that similar mutations in COL1A2 can affect skeletal phenotypes in more than one way.

Bone mass and mineralization in osteogenesis imperfecta

The main clinical features of osteogenesis imperfecta (OI) are low bone mass and high bone fragility. While the decrease in bone mass is generally regarded as an indicator of disease severity, bone fragility appears as the hallmark of the disorder. Bone has a multiscale hierarchical structural organization and is optimized to resist to fractures. In OI, modifications at the molecular level affect the total mechanical integrity of the bone. A specific characteristic in OI is that the bone matrix is abnormally high mineralized independently of the underlying mutation or clinical severity. The increased matrix mineralization affects bone material quality, leading to increased stiffness and brittleness and making bone prone to fractures. The purpose of this review is to give further insights on bone matrix mineralization in OI and to discuss advantages and pitfalls of invasive and noninvasive imaging techniques.

Recessive osteogenesis imperfecta caused by missense mutations in SPARC

Secreted protein, acidic, cysteine-rich (SPARC) is a glycoprotein that binds to collagen type I and other proteins in the extracellular matrix. Using whole-exome sequencing to identify the molecular defect in two unrelated girls with severe bone fragility and a clinical diagnosis of osteogenesis imperfecta type IV, we identified two homozygous variants in SPARC (GenBank: NM_003118.3; c.497G>A [p.Arg166His] in individual 1; c.787G>A [p.Glu263Lys] in individual 2). Published modeling and site-directed mutagenesis studies had previously shown that the residues substituted by these mutations form an intramolecular salt bridge in SPARC and are essential for the binding of SPARC to collagen type I. The amount of SPARC secreted by skin fibroblasts was reduced in individual 1 but appeared normal in individual 2. The migration of collagen type I alpha chains produced by these fibroblasts was mildly delayed on SDS-PAGE gel, suggesting some overmodification of collagen during triple helical formation. Pulse-chase experiments showed that collagen type I secretion was mildly delayed in skin fibroblasts from both individuals. Analysis of an iliac bone sample from individual 2 showed that trabecular bone was hypermineralized on the material level. In conclusion, these observations show that homozygous mutations in SPARC can give rise to severe bone fragility in humans.

The WNT system: background and its role in bone

WNTs are extracellular proteins that activate different cell surface receptors linked to canonical and noncanonical WNT signalling pathways. The Wnt genes were originally discovered as important for embryonic development of fruit flies and malignant transformation of mouse mammary cancers. More recently, WNTs have been implicated in a wide spectrum of biological phenomena and diseases. During the last decade, several lines of clinical and preclinical evidence have indicated that WNT signalling is critical for trabecular and cortical bone mass, and this pathway is currently an attractive target for drug development. Based on detailed knowledge of the different WNT signalling pathways, it appears that it might be possible to develop drugs that specifically target cortical and trabecular bone. Neutralization of a bone-specific WNT inhibitor is now being evaluated as a promising anabolic treatment for patients with osteoporosis. Here, we provide the historical background to the discoveries of WNTs, describe the different WNT signalling pathways and summarize the current understanding of how these proteins regulate bone mass by affecting bone formation and resorption.

Unique micro- and nano-scale mineralization pattern of human osteogenesis imperfecta type VI bone

Osteogenesis imperfecta (OI) is a heterogeneous group of inheritable connective tissue disorders characterized by mutation in genes involved in collagen synthesis and leading to increased bone fragility, low bone mass, impaired bone material properties and abnormally high bone matrix mineralization. Recessive OI type VI is caused by mutation in SERPINF1 leading to a loss-of-function of pigment epithelium-derived factor (PEDF) a collagen-binding protein with potent antiangiogenic activity. Affected patients develop a severe OI phenotype with a striking histological characteristic, rare in other OI types, of an excess of osteoid tissue and prolonged mineralization lag time. To get insights into matrix mineralization, we evaluated biopsies from 9 affected children by quantitative and by high-resolution backscattered electron imaging and assessed bone mineralization density distribution. Thickness, shape and arrangement of mineral particles were measured in a subset of 4 patients by synchrotron small angle X-ray scattering. Typical calcium content in the bone matrix was found to be increased compared to controls, even exceeding values found previously in OI patients with collagen-gene mutations. A main characteristic however, is the coexistence of this highly mineralized bone matrix with seams showing abnormally low mineral content. Atypical collagen fibril organization was found in the perilacunar region of young osteocytes, suggesting a disturbance in the early steps of mineralization. These observations are consistent with the presence of a heterogeneous population of mineral particles with unusual size, shape and arrangement, especially in the region with lower mineral content. The majority of the particles in the highly mineralized bone areas were less disorganized, but smaller and more densely packed than in controls and in previously measured OI patients. These data suggest that the lack of PEDF impairs a proper osteoblast-osteocyte transition and consequently affects the early steps of mineralization, downstream collagen assembly making OI type VI different from "classical" OI with mutations in collagen-type I encoding genes, despite the typical hypermineralization of the bone matrix.