Bi-allelic Variants in TONSL Cause SPONASTRIME Dysplasia and a Spectrum of Skeletal Dysplasia Phenotypes.

SPONASTRIME dysplasia is an autosomal-recessive spondyloepimetaphyseal dysplasia characterized by spine (spondylar) abnormalities, midface hypoplasia with a depressed nasal bridge, metaphyseal striations, and disproportionate short stature. Scoliosis, coxa vara, childhood cataracts, short dental roots, and hypogammaglobulinemia have also been reported in this disorder. Although an autosomal-recessive inheritance pattern has been hypothesized, pathogenic variants in a specific gene have not been discovered in individuals with SPONASTRIME dysplasia. Here, we identified bi-allelic variants in TONSL, which encodes the Tonsoku-like DNA repair protein, in nine subjects (from eight families) with SPONASTRIME dysplasia, and four subjects (from three families) with short stature of varied severity and spondylometaphyseal dysplasia with or without immunologic and hematologic abnormalities, but no definitive metaphyseal striations at diagnosis. The finding of early embryonic lethality in a Tonsl-/- murine model and the discovery of reduced length, spinal abnormalities, reduced numbers of neutrophils, and early lethality in a tonsl-/- zebrafish model both support the hypomorphic nature of the identified TONSL variants. Moreover, functional studies revealed increased amounts of spontaneous replication fork stalling and chromosomal aberrations, as well as fewer camptothecin (CPT)-induced RAD51 foci in subject-derived cell lines. Importantly, these cellular defects were rescued upon re-expression of wild-type (WT) TONSL; this rescue is consistent with the hypothesis that hypomorphic TONSL variants are pathogenic. Overall, our studies in humans, mice, zebrafish, and subject-derived cell lines confirm that pathogenic variants in TONSL impair DNA replication and homologous recombination-dependent repair processes, and they lead to a spectrum of skeletal dysplasia phenotypes with numerous extra-skeletal manifestations.

Mutations in SNRPB, encoding components of the core splicing machinery, cause cerebro-costo-mandibular syndrome.

Cerebro-costo-mandibular syndrome (CCMS) is a developmental disorder characterized by the association of Pierre Robin sequence and posterior rib defects. Exome sequencing and Sanger sequencing in five unrelated CCMS patients revealed five heterozygous variants in the small nuclear ribonucleoprotein polypeptides B and B1 (SNRPB) gene. This gene includes three transcripts, namely transcripts 1 and 2, encoding components of the core spliceosomal machinery (SmB' and SmB) and transcript 3 undergoing nonsense-mediated mRNA decay. All variants were located in the premature termination codon (PTC)-introducing alternative exon of transcript 3. Quantitative RT-PCR analysis revealed a significant increase in transcript 3 levels in leukocytes of CCMS individuals compared to controls. We conclude that CCMS is due to heterozygous mutations in SNRPB, enhancing inclusion of a SNRPB PTC-introducing alternative exon, and show that this developmental disease is caused by defects in the splicing machinery. Our finding confirms the report of SNRPB mutations in CCMS patients by Lynch et al. (2014) and further extends the clinical and molecular observations.

Heterozygous FGFR1 mutation may be responsible for an incomplete form of osteoglophonic dysplasia, characterized only by radiolucent bone lesions and teeth retentions.

Non-ossifying fibromas are seen in different disorders recognizable by specific features. Indeed, osteoglophonic dysplasia (OD) is characterized by radiolucent bone lesions associated with severe short stature, dysmorphism and failure of dental eruption. This syndrome is caused by heterozygous activating mutations in the immunoglobulin-like D3 domain of the FGFR1 gene, encoding a tyrosine kinase. Here, we report three patients from the same family presenting with radiolucent bone lesions and teeth retentions. Exome sequencing allowed identification of a novel mutation c.917C > T, p. Pro306Leu in exon 7 of the FGFR1 gene. Our patients present with normal stature and no severe dysmorphism. This report describes a mild form of OD and expands the phenotype related to FGFR1 mutations. These findings emphasize the need to consider FGFR1 variants in the case of multiple non-ossifying bone lesions associated with dental eruption anomalies.

Homozygous Loss-of-Function Mutations in CCDC134 Are Responsible for a Severe Form of Osteogenesis Imperfecta.

Osteogenesis imperfecta (OI) is a primary bone fragility disorder with an estimated prevalence of 1 in 15,000 births. The majority of OI cases are inherited in an autosomal-dominant manner, while 5% to 10% have recessive or X-linked inheritance. Up to now, approximately 5% of OI cases remain without mutation demonstrated, supporting the involvement of other genes in the disease spectrum. By whole-exome sequencing, we identified a homozygous variant (c.2T>C) in CCDC134 gene in three patients from two unrelated families with severe bone fragility that did not respond to bisphosphonate treatment, short stature, and gracile long bones with pseudarthroses but no dentinogenesis imperfecta. CCDC134 encodes a secreted protein widely expressed and implicated in the regulation of some mitogen-activated protein kinases (MAPK) signaling pathway. Western blot and immunofluorescence analyses confirmed the absence of CCDC134 protein in patient cells compared with controls. Furthermore, we demonstrated that CCDC134 mutations are associated with increased Erk1/2 phosphorylation, decreased OPN mRNA and COL1A1 expression and reduced mineralization in patient osteoblasts compared with controls. These data support that CCDC134 is a new gene involved in severe progressive deforming recessive osteogenesis imperfecta (type III). © 2020 American Society for Bone and Mineral Research.

SLC10A7 mutations cause a skeletal dysplasia with amelogenesis imperfecta mediated by GAG biosynthesis defects.

Skeletal dysplasia with multiple dislocations are severe disorders characterized by dislocations of large joints and short stature. The majority of them have been linked to pathogenic variants in genes encoding glycosyltransferases, sulfotransferases or epimerases required for glycosaminoglycan synthesis. Using exome sequencing, we identify homozygous mutations in SLC10A7 in six individuals with skeletal dysplasia with multiple dislocations and amelogenesis imperfecta. SLC10A7 encodes a 10-transmembrane-domain transporter located at the plasma membrane. Functional studies in vitro demonstrate that SLC10A7 mutations reduce SLC10A7 protein expression. We generate a Slc10a7-/- mouse model, which displays shortened long bones, growth plate disorganization and tooth enamel anomalies, recapitulating the human phenotype. Furthermore, we identify decreased heparan sulfate levels in Slc10a7-/- mouse cartilage and patient fibroblasts. Finally, we find an abnormal N-glycoprotein electrophoretic profile in patient blood samples. Together, our findings support the involvement of SLC10A7 in glycosaminoglycan synthesis and specifically in skeletal development.