Homozygosity by descent for a COL1A2 mutation in two sibs with severe osteogenesis imperfecta and mild clinical expression in the heterozygotes

The missing link between genetic association and regulatory function

The genetic basis of most traits is highly polygenic and dominated by non-coding alleles. It is widely assumed that such alleles exert small regulatory effects on the expression of cis-linked genes. However, despite the availability of gene expression and epigenomic datasets, few variant-to-gene links have emerged. It is unclear whether these sparse results are due to limitations in available data and methods, or to deficiencies in the underlying assumed model. To better distinguish between these possibilities, we identified 220 gene-trait pairs in which protein-coding variants influence a complex trait or its Mendelian cognate. Despite the presence of expression quantitative trait loci near most GWAS associations, by applying a gene-based approach we found limited evidence that the baseline expression of trait-related genes explains GWAS associations, whether using colocalization methods (8% of genes implicated), transcription-wide association (2% of genes implicated), or a combination of regulatory annotations and distance (4% of genes implicated). These results contradict the hypothesis that most complex trait-associated variants coincide with homeostatic expression QTLs, suggesting that better models are needed. The field must confront this deficit and pursue this 'missing regulation.'

Homozygous HESX1 and COL1A1 Gene Variants in a Boy with Growth Hormone Deficiency and Early Onset Osteoporosis

We report on a patient born to consanguineous parents, presenting with Growth Hormone Deficiency (GHD) and osteoporosis. SNP-array analysis and exome sequencing disclosed long contiguous stretches of homozygosity and two distinct homozygous variants in HESX1 (Q6H) and COL1A1 (E1361K) genes. The HESX1 variant was described as causative in a few subjects with an incompletely penetrant dominant form of combined pituitary hormone deficiency (CPHD). The COL1A1 variant is rare, and so far it has never been found in a homozygous form. Segregation analysis showed that both variants were inherited from heterozygous unaffected parents. Present results further elucidate the inheritance pattern of HESX1 variants and recommend assessing the clinical impact of variants located in C-terminal propeptide of COL1A1 gene for their potential association with rare recessive and early onset forms of osteoporosis.

Reproductive options for families at risk of Osteogenesis Imperfecta: a review

Background

Osteogenesis Imperfecta (OI) is a rare genetic disorder involving bone fragility. OI patients typically suffer from numerous fractures, skeletal deformities, shortness of stature and hearing loss. The disorder is characterised by genetic and clinical heterogeneity. Pathogenic variants in more than 20 different genes can lead to OI, and phenotypes can range from mild to lethal forms. As a genetic disorder which undoubtedly affects quality of life, OI significantly alters the reproductive confidence of families at risk. The current review describes a selection of the latest reproductive approaches which may be suitable for prospective parents faced with a risk of OI. The aim of the review is to alleviate suffering in relation to family planning around OI, by enabling prospective parents to make informed and independent decisions.

Main body

The current review provides a comprehensive overview of possible reproductive options for people with OI and for unaffected carriers of OI pathogenic genetic variants. The review considers reproductive options across all phases of family planning, including pre-pregnancy, fertilisation, pregnancy, and post-pregnancy. Special attention is given to the more modern techniques of assisted reproduction, such as preconception carrier screening, preimplantation genetic testing for monogenic diseases and non-invasive prenatal testing. The review outlines the methodologies of the different reproductive approaches available to OI families and highlights their advantages and disadvantages. These are presented as a decision tree, which takes into account the autosomal dominant and autosomal recessive nature of the OI variants, and the OI-related risks of people without OI.

The complex process of decision-making around OI reproductive options is also discussed from an ethical perspective.

Conclusion

The rapid development of molecular techniques has led to the availability of a wide variety of reproductive options for prospective parents faced with a risk of OI. However, such options may raise ethical concerns in terms of methodologies, choice management and good clinical practice in reproductive care, which are yet to be fully addressed.

A family with homozygous and heterozygous p.Gly337Ser mutations in COL1A2

Osteogenesis imperfecta (OI) is commonly caused by monoallelic mutations in COL1A1 or COL1A2. Biallelic mutations are extremely rare. Only five previous reports have identified seven OI patients with homozygous mutations in COL1A2. OI is a genetically and phenotypically heterogeneous disorder which challenges an establishment of genotype-phenotype correlation. Notably, more than thirty patients with OI possess the heterozygous mutation, p.Gly337Ser, in COL1A2. Their clinical severity ranges from mild OI type I to severe types III and IV. Here, we report a 17-year-old Thai female with recurrent bone fractures, short stature, blue sclerae, triangular face, missing teeth, dentinogenesis imperfecta (DI), skeletal deformities, and scoliosis. She was diagnosed with OI type III. Her parents were second-cousin-once-removed. The father was a professional Thai boxer. Both had normal bone mineral density, no history of bone fractures, and only teeth problems. They were diagnosed with DI without OI. Whole exome sequencing identified that the proband harbored the homozygous mutation, c.1009G > A (p.Gly337Ser), in exon 19 of COL1A2 while her parents were heterozygous for this mutation. This study reports the eighth child with OI and the homozygous mutation in COL1A2; and the first two individuals with the heterozygous p.Gly337Ser mutation in COL1A2 causing an isolated DI without OI.

New Insights Into Monogenic Causes of Osteoporosis

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

Autosomal Recessive Osteogenesis Imperfecta Caused by a Novel Homozygous COL1A2 Mutation

Osteogenesis imperfecta (OI) is a skeletal dysplasia characterized by brittle bones and extraskeletal manifestations. The disease phenotype varies greatly. Most commonly, OI arises from monoallelic mutations in one of the two genes encoding type I collagen, COL1A1 and COL1A2 and is inherited as an autosomal dominant trait. Here, we describe a consanguineous family with autosomal recessive OI caused by a novel homozygous glycine substitution in COL1A2, NM_000089.3: c.604G>A, p.(Gly202Ser), detected by whole-genome sequencing. The index patient is a 31-year-old Greek woman with severe skeletal fragility. She had mild short stature, low bone mineral density of the lumbar spine and blue sclerae. She had sustained multiple long bone and vertebral fractures since childhood and had been treated with bisphosphonates for several years. She also had an affected sister with similar clinical manifestations. Interestingly, the parents and one sister, all carriers of the COL1A2 glycine mutation, did not have manifestations of OI. In summary, we report on autosomal recessive OI caused by a homozygous glycine-to-serine substitution in COL1A2, leading to severe skeletal fragility. The mutation carriers lacked OI manifestations. This family further expands the complex genetic spectrum of OI and underscores the importance of genetic evaluation for correct genetic counselling.

Fracture of mandible during yawning in a patient with osteogenesis imperfecta

Osteogenesis imperfecta is a genetic disorder characterised by fragility and multiple fractures of bones. Clinical signs and symptoms vary depending on the type of disease. Fractures of facial bones are rare compared with load-bearing long bones. We report a case of fracture of the mandible during yawning which was managed by open reduction and internal fixation.

A new osteogenesis imperfecta with improvement over time maps to 11q

Osteogenesis imperfecta (OI) is basically divided into four clinical types, I-IV. Type IV clearly represents a heterogeneous group of disorders. Here we describe two OI patients in the same family. They would typically be classified as having type IV, but are distinguishable from other OI type IV patients by the improving and resolving course of their disease. Mutation screening did not identify mutations affecting glycine codons or splice sites in the coding regions of the two collagen I genes. Genome-wide screening of DNA samples from the two homozygous patients identified one region of high concordance of homozygosity on chromosome 11 on the long arm (11q23.3-11q24.1).

Epidural hematoma formation following trivial head trauma in a child with osteogenesis imperfecta

Osteogenesis imperfecta (OI) represents a rare heterogeneous group of inherited disorders characterized by low bone mass, increased bone fragility, and other connective tissue manifestations. This condition can contribute to dramatic complications after a seemingly insignificant injury. A large epidural hematoma that developed in a child with OI after a trivial fall highlights the importance of close monitoring in these patients. After an injury that occurred several months prior to the head trauma the authors describe, this child had been placed in foster care because it was believed that his skeletal injuries were caused by nonaccidental injury. Subsequent genetic analysis confirmed that the child was heterozygous for the missense mutation c767G>T,pG256V at exon 16 of COLIA2, consistent with OI, and the foster care order was overturned. The authors review the literature concerning OI, its relationship to head injury, and the importance of genetic analysis in its diagnosis.

COL7A1 mutation G2037E causes epidermal retention of type VII collagen

COL7A1 glycine substitution (GS) mutations result in dominant and recessive dystrophic epidermolysis bullosa (DDEB and RDEB). Here, we report a DDEB family in which retention of type VII collagen by epidermal keratinocytes was observed for a female proband. Mutational analysis detected a GS mutation, G2037E, in the proband and her affected father. To demonstrate direct association of G2037E and type VII collagen retention we introduced this mutated COL7A1 gene into cultured keratinocytes using retroviral methods. This mutation was dominant, so we transferred a 1:1 mixture of wild-type (unaffected) and G2037E-mutated COL7A1, together, in addition to the unaffected gene or the mutated gene alone. The increase in type VII collagen cytoplasmic staining in the G2037E/wild transfectant cell samples was compared with that for control/wild-type cells. Intracellular collagen VII staining in the G2037E (alone)-transfected cells was even stronger than for the G2037E/wild transfection sample. These results indicate that the G2037E COL7A1 mutation leads to increased epidermal retention of type VII collagen in vivo, and also suggests that homozygotes carrying this dominant GS mutation may have more severe phenotypes than heterozygotes. This study furthers our understanding of GS COL7A1 mutations in DEB.

Osteogenesis imperfecta type VII maps to the short arm of chromosome 3

We have identified a novel form of autosomal recessive osteogenesis imperfecta (OI) in a small First Nations community from northern Quebec. Mutation screening of the COL1A1/COL1A2 genes revealed no detectable mutations, and type I collagen protein analyses were also normal. By linkage analysis, we mapped this unique autosomal recessive variant of osteogenesis imperfecta to chromosome 3p22-24.1. Based on the assumption of a founder effect, genome-wide screening was performed on a DNA sample pooled from seven affected individuals. Familial as well as historical recombinations identified within an extended haplotype of 19 markers localized the disease between markers D3S2324 and D3S1561, separated by <5 cM. Based on chromosomal localization to 3p22-24.1, the transforming growth factor-beta receptor 2 gene and the parathyroid hormone/parathyroid hormone-related peptide receptor were tested, but were excluded as being associated with the phenotype. This study excludes type I collagen mutations in the pathogenesis of the disease and assigns this form of OI to a locus other than the ones containing the type I collagen genes.

Osteogenesis imperfecta type VII: an autosomal recessive form of brittle bone disease

Osteogenesis imperfecta (OI) is a heritable disease of bone with low bone mass and bone fragility. The disease is generally classified into four types based on clinical features and disease severity, although recently fifth and sixth forms have also been reported. Most forms of OI are autosomal dominant. Rarely, autosomal recessive disease has been described. We report the clinical, radiological, and histological features of four children (age 3.9-8.6 years at last follow-up; all girls) and four adults (age 28-33 years; two women) with a novel form of autosomal recessive OI living in an isolated First Nations community in northern Quebec. In keeping with the established numeric classification for OI forms, we have called this form of the disease OI type VII. The phenotype is moderate to severe, characterized by fractures at birth, bluish sclerae, early deformity of the lower extremities, coxa vara, and osteopenia. Rhizomelia is a prominent clinical feature. Histomorphometric analyses of iliac crest bone samples revealed findings similar to OI type I, with decreased cortical width and trabecular number, increased bone turnover, and preservation of the birefringent pattern of lamellar bone. The disease has subsequently been localized to chromosome 3p22-24.1, which is outside the loci for type I collagen genes. The underlying genetic basis for the disease remains to be determined.

Vascular thrombohemorrhagic disorders: hereditary and acquired

This review has summarized the more important diseases that may be accompanied by or lead to a disorder of hemostasis or thrombosis via alterations of the vasculature. It is to be stressed that the vascular component of hemostasis is often overlooked by clinicians caring for patients with disorders of hemostasis and thrombosis. It should be appreciated that the vasculature is intricately related to the coagulation protein system and to platelets when involved in thrombohemorrhagic diatheses. Although many vascular disorders may lead to hemorrhage or thrombosis, it must be appreciated that often it is impossible to discern between a primary vascular defect/damage and a defect that has been induced by platelet activation/dysfunction or procoagulant abnormalities.

Novel COL1A1 mutation (G559C) [correction of G599C] associated with mild osteogenesis imperfecta and dentinogenesis imperfecta

A genotype-phenotype analysis of a three-generation family segregating for an autosomal-dominant osteogenesis imperfecta (OI) variant is reported here. The family was ascertained through the presentation of a proband concerned about discoloration of her teeth, found to be dentinogenesis imperfecta (DGI). Examination of 36 family members identified 15 individuals with DGI. Linkage studies were performed for genetic markers from candidate intervals known to contain genes responsible for DGI on chromosomes 4q, 7q, and 17q. Conclusive evidence for linkage of DGI was obtained to genetic markers on chromosome 17q21-q22 (DLX-3, Z(max) = 5.34, theta = 0.00). All DGI-affected family members shared a common haplotype, which was not present in individuals without DGI. Haplotype analysis sublocalized the gene to a 5-cM genetic interval that contained the collagen 1 alpha 1 (COL1A1) gene. More than 150 different COL1A1 gene mutations have been associated with various forms of OI, and five of these have been associated with DGI and type IV OI. After excluding these five mutations, mutational analysis was performed on the remaining exons including intron--exon boundaries, which resulted in identification of a Gly559Cys mutation in exon 32, present in all DGI-affected family members. Clinical features segregating with this G559C mutation included hyperextensible joints, joint pain and an increased propensity for bone fractures with moderate trauma. This is the first report of joint pain associated with a COL1A1 mutation and DGI. The mild skeletal features and reduced penetrance of the non-dental findings illustrate the importance of genetic evaluations for families with a history of DGI.

Osteogenesis imperfecta: perspectives and opportunities

The last 2 years have seen additions proposed to the very limited armamentarium of treatments for osteogenesis imperfecta. These include the use of bisphosphonates to decrease bone resorption, growth hormone to augment growth and collagen production, and bone marrow transplantation to create chimeras at the level of the collagen production unit in bone. Although there are optimistic proponents for each strategy, the lack of well-controlled studies and the absence of clearly defined objectives for therapy hinder clear assessment.

The Human Collagen Mutation Database 1998

The collagens are a large and diverse family of proteins which are found in the extracellular matrix. In common with one another, the 19 known collagen types have triple-helical domains of variable length but they differ with respect to their overall size and the nature and location of their globular domains. Collagen mutations lead to heritable defects of connective tissues and mutation data for collagen types I and III are presented here. The mutation data are accessible on the world wide web at http://www.le.ac.uk/genetics/collagen/

Autosomal dominant postaxial polydactyly, nail dystrophy, and dental abnormalities map to chromosome 4p16, in the region containing the Ellis-van Creveld syndrome locus

We have studied a four-generation family with features of Weyers acrofacial dysostosis, in which the proband has a more severe phenotype, resembling Ellis-van Creveld syndrome. Weyers acrofacial dysostosis is an autosomal dominant condition with dental anomalies, nail dystrophy, postaxial polydactyly, and mild short stature. Ellis-van Creveld syndrome is a similar condition, with autosomal recessive inheritance and the additional features of disproportionate dwarfism, thoracic dysplasia, and congenital heart disease. Linkage and haplotype analysis determined that the disease locus in this pedigree resides on chromosome 4p16, distal to the genetic marker D4S3007 and within a 17-cM region flanking the genetic locus D4S2366. This region includes the Ellis-van Creveld syndrome locus, which previously was reported to map within a 3-cM region between genetic markers D4S2957 and D4S827. Either the genes for the condition in our family and for Ellis-van Creveld syndrome are near one another or these two conditions are allelic with mutations in the same gene. These data also raise the possibility that Weyers acrofacial dysostosis is the heterozygous expression of a mutation that, in homozygous form, causes the autosomal recessive disorder Ellis-van Creveld syndrome.