Loss of TANGO1 leads to absence of bone mineralization

ABSTRACT TANGO1 (transport and Golgi organization‐1 homolog) encodes a transmembrane protein, which is located at endoplasmic reticulum (ER) exit sites where it binds bulky cargo, such as collagens, in the lumen and recruits membranes from the ER‐Golgi intermediate compartment (ERGIC) to create an export route for cargo secretion. Mice lacking Mia3 (murine TANGO1 orthologue) show defective secretion of numerous procollagens and lead to neonatal lethality due to insufficient bone mineralization. Recently, aberrant expression of truncated TANGO1 in humans has been shown to cause a mild‐to‐moderate severe collagenopathy associated with dentinogenesis imperfecta, short stature, skeletal abnormalities, diabetes mellitus, and mild intellectual disability. We now show for the first time that complete loss of TANGO1 results in human embryonic lethality with near‐total bone loss and phenocopies the situation of Mia3−/− mice. Whole‐exome sequencing on genomic DNA (gDNA) of an aborted fetus of Indian descent revealed a homozygous 4‐base pair (4‐bp) deletion in TANGO1 that is heterozygously present in both healthy parents. Parental fibroblast studies showed decreased TANGO1 mRNA expression and protein levels. Type I collagen secretion and extracellular matrix organization were normal, supporting a threshold model for clinical phenotype development. As such, our report broadens the phenotypic and mutational spectrum of TANGO1‐related collagenopathies, and underscores the crucial role of TANGO1 for normal bone development, of which deficiency results in a severe‐to‐lethal form of osteochondrodysplasia. © 2021 American Society for Bone and Mineral Research © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research

Ehlers-Danlos syndrome, hypermobility type, is linked to chromosome 8p22-8p21.1 in an extended Belgian family

Joint hypermobility is a common, mostly benign, finding in the general population. In a subset of individuals, however, it causes a range of clinical problems, mainly affecting the musculoskeletal system. Joint hypermobility often appears as a familial trait and is shared by several heritable connective tissue disorders, including the hypermobility subtype of the Ehlers-Danlos syndrome (EDSHT) or benign joint hypermobility syndrome (BJHS). These hereditary conditions provide unique models for the study of the genetic basis of joint hypermobility. Nevertheless, these studies are largely hampered by the great variability in clinical presentation and the often vague mode of inheritance in many families. Here, we performed a genome-wide linkage scan in a unique three-generation family with an autosomal dominant EDS-HT phenotype and identified a linkage interval on chromosome 8p22-8p21.1, with a maximum two-point LOD score of 4.73. Subsequent whole exome sequencing revealed the presence of a unique missense variant in the LZTS1 gene, located within the candidate region. Subsequent analysis of 230 EDS-HT/BJHS patients resulted in the identification of three additional rare variants. This is the first reported genome-wide linkage analysis in an EDS-HT family, thereby providing an opportunity to identify a new disease gene for this condition

A Novel Splice Variant in the N-propeptide of COL5A1 Causes an EDS Phenotype with Severe Kyphoscoliosis and Eye Involvement

Background: The Ehlers-Danlos Syndrome (EDS) is a heritable connective tissue disorder characterized by hyperextensible skin, joint hypermobility and soft tissue fragility. The classic subtype of EDS is caused by mutations in one of the type V collagen genes (COL5A1 and COL5A2). Most mutations affect the type V collagen helical domain and lead to a diminished or structurally abnormal type V collagen protein. Remarkably, only two mutations were reported to affect the extended, highly conserved N-propeptide domain, which plays an important role in the regulation of the heterotypic collagen fibril diameter. We identified a novel COL5A1 N-propeptide mutation, resulting in an unusual but severe classic EDS phenotype and a remarkable splicing outcome. Methodology/Principal Findings: We identified a novel COL5A1 N-propeptide acceptor-splice site mutation (IVS6-2A>G, NM_000093.3_c.925-2A>G) in a patient with cutaneous features of EDS, severe progressive scoliosis and eye involvement. Two mutant transcripts were identified, one with an exon 7 skip and one in which exon 7 and the upstream exon 6 are deleted. Both transcripts are expressed and secreted into the extracellular matrix, where they can participate in and perturb collagen fibrillogenesis, as illustrated by the presence of dermal collagen cauliflowers. Determination of the order of intron removal and computational analysis showed that simultaneous skipping of exons 6 and 7 is due to the combined effect of delayed splicing of intron 7, altered pre-mRNA secondary structure, low splice site strength and possibly disturbed binding of splicing factors. Conclusions/Significance: We report a novel COL5A1 N-propeptide acceptor-splice site mutation in intron 6, which not only affects splicing of the adjacent exon 7, but also causes a splicing error of the upstream exon 6. Our findings add further insights into the COL5A1 splicing order and show for the first time that a single COL5A1 acceptor-splice site mutation can perturb splicing of the upstream exon

A homozygous CREB3L1 missense mutation expands the mutational spectrum of CREB3L1-related osteogenesis imperfecta

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Expanding the genotypic and phenotypic spectrum of the B3GAT3 linkeropathy

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A homozygous pathogenic missense variant broadens the phenotypic and mutational spectrum of CREB3L1-related osteogenesis imperfecta

The cyclic adenosine monophosphate (AMP) responsive element binding protein 3-like 1 (CREB3L1) gene codes for the endoplasmic reticulum stress transducer old astrocyte specifically induced substance (OASIS), which has an important role in osteoblast differentiation during bone development. Deficiency of OASIS is linked to a severe form of autosomal recessive osteogenesis imperfecta (OI), but only few patients have been reported. We identified the first homozygous pathogenic missense variant (p.(Ala304Val)) in a patient with lethal OI, which is located within the highly conserved basic leucine zipper domain, four amino acids upstream of the DNA binding domain. In vitro structural modeling and luciferase assays demonstrate that this missense variant affects a critical residue in this functional domain, thereby decreasing the type I collagen transcriptional binding ability. In addition, overexpression of the mutant OASIS protein leads to decreased transcription of the SEC23A and SEC24D genes, which code for components of the coat protein complex type II (COPII), and aberrant OASIS signaling also results in decreased protein levels of SEC24D. Our findings therefore provide additional proof of the potential involvement of the COPII secretory complex in the context of bone-associated disease

The clinical and mutational spectrum of B3GAT3 linkeropathy : two case reports and literature review

Background: Proteoglycans are large and structurally complex macromolecules which can be found in abundancy in the extracellular matrix and on the surface of all animal cells. Mutations in the genes encoding the enzymes responsible for the formation of the tetrasaccharide linker region between the proteoglycan core protein and the glycosaminoglycan side chains lead to a spectrum of severe and overlapping autosomal recessive connective tissue disorders, collectively coined the glycosaminoglycan linkeropathies'. Results: We report the clinical findings of two novel patients with a complex linkeropathy due to biallelic mutations in B3GAT3, the gene that encodes glucuronosyltransferase I, which catalyzes the addition of the ultimate saccharide to the linker region. We identified a previously reported c.667G>A missense mutation and an unreported homozygous c.416C>T missense mutation. We also performed a genotype and phenotype-oriented literature overview of all hitherto reported patients harbouring B3GAT3 mutations. A total of 23 patients from 10 families harbouring bi-allelic mutations and one patient with a heterozygeous splice-site mutation in B3GAT3 have been reported. They all display a complex phenotype characterized by consistent presence of skeletal dysplasia (including short stature, kyphosis, scoliosis and deformity of the long bones), facial dysmorphology, and spatulate distal phalanges. More variably present are cardiac defects, joint hypermobility, joint dislocations/contractures and fractures. Seven different B3GAT3 mutations have been reported, and although the number of patients is still limited, some phenotype-genotype correlations start to emerge. The more severe phenotypes seem to have mutations located in the substrate acceptor subdomain of the catalytic domain of the glucuronosyltransferase I protein while more mildly affected phenotypes seem to have mutations in the NTP-sugar donor substrate binding subdomain. Conclusions: Loss-of-function mutations in B3GAT3 are associated with a complex connective tissue phenotype characterized by disproportionate short stature, skeletal dysplasia, facial dysmorphism, spatulate distal phalanges and -to a lesser extent- joint contractures, joint hypermobility with dislocations, cardiac defects and bone fragility. Based on the limited number of reported patients, some genotype-phenotype correlations start to emerge

The N‐terminal p.(Ser38Cys) TIMP3 mutation underlying Sorsby fundus dystrophy is a founder mutation disrupting an intramolecular disulfide bond

Sorsby fundus dystrophy (SFD) is a macular degeneration caused by mutations in TIMP3, the majority of which introduce a novel cysteine. However, the exact molecular mechanisms underlying SFD remain unknown. We aimed to provide novel insights into the functional consequences of a distinct N-terminal mutation. Haplotype reconstruction in three SFD families revealed that the identified c.113C>G, p.(Ser38Cys) mutation is a founder in Belgian and northern French families with a late-onset SFD phenotype. Functional consequences of the p.(Ser38Cys) mutation were investigated by high-resolution Western blot analysis of wild type and mutant TIMP3 using patient fibroblasts and in vitro generated proteins, and by molecular modeling of TIMP3 and its interaction partners. We could not confirm a previous hypothesis on dimerization of mutant TIMP3 proteins. However, we identified aberrant intramolecular disulfide bonding. Our data provide evidence for disruption of the established Cys36-Cys143 disulfide bond and formation of a novel Cys36-Cys38 bond, possibly associated with increased glycosylation of the protein. In conclusion, we propose a novel pathogenetic mechanism underlying the p.(Ser38Cys) TIMP3 founder mutation involving intramolecular disulfide bonding. These results provide new insights into the pathogenesis of SFD and other retinopathies linked to mutations in TIMP3, such as age-related macular degeneration