Syntaxin 18 Defects in Human and Zebrafish Unravel Key Roles in Early Cartilage and Bone Development.

SNARE proteins comprise a conserved protein family responsible for catalyzing membrane fusion during vesicle traffic. Syntaxin18 (STX18) is a poorly characterized endoplasmic reticulum (ER)-resident t-SNARE. Recently, together with TANGO1 and SLY1, its involvement was shown in ER to Golgi transport of collagen II during chondrogenesis. We report a fetus with a severe osteochondrodysplasia in whom we identified a homozygous substitution of the highly conserved p.Arg10 to Pro of STX18. CRISPR/Cas9-mediated Stx18 deficiency in zebrafish reveals a crucial role for Stx18 in cartilage and bone development. Furthermore, increased expression of multiple components of the Stx18 SNARE complex and of COPI and COPII proteins suggests that Stx18 deficiency impairs antero- and retrograde vesicular transport in the crispant stx18 zebrafish. Taken together, our studies highlight a new candidate gene for a recessive form of osteochondrodysplasia, thereby possibly broadening the SNAREopathy phenotypic spectrum and opening new doors toward future research avenues. © 2023 American Society for Bone and Mineral Research (ASBMR).

Bi-allelic premature truncating variants in LTBP1 cause cutis laxa syndrome.

Latent transforming growth factor ß (TGFß)-binding proteins (LTBPs) are microfibril-associated proteins essential for anchoring TGFß in the extracellular matrix (ECM) as well as for correct assembly of ECM components. Variants in LTBP2, LTBP3, and LTBP4 have been identified in several autosomal recessive Mendelian disorders with skeletal abnormalities with or without impaired development of elastin-rich tissues. Thus far, the human phenotype associated with LTBP1 deficiency has remained enigmatic. In this study, we report homozygous premature truncating LTBP1 variants in eight affected individuals from four unrelated consanguineous families. Affected individuals present with connective tissue features (cutis laxa and inguinal hernia), craniofacial dysmorphology, variable heart defects, and prominent skeletal features (craniosynostosis, short stature, brachydactyly, and syndactyly). In vitro studies on proband-derived dermal fibroblasts indicate distinct molecular mechanisms depending on the position of the variant in LTBP1. C-terminal variants lead to an altered LTBP1 loosely anchored in the microfibrillar network and cause increased ECM deposition in cultured fibroblasts associated with excessive TGFß growth factor activation and signaling. In contrast, N-terminal truncation results in a loss of LTBP1 that does not alter TGFß levels or ECM assembly. In vivo validation with two independent zebrafish lines carrying mutations in ltbp1 induce abnormal collagen fibrillogenesis in skin and intervertebral ligaments and ectopic bone formation on the vertebrae. In addition, one of the mutant zebrafish lines shows voluminous and hypo-mineralized vertebrae. Overall, our findings in humans and zebrafish show that LTBP1 function is crucial for skin and bone ECM assembly and homeostasis.

Loss-of-Function Variants in EFEMP1 Cause a Recognizable Connective Tissue Disorder Characterized by Cutis Laxa and Multiple Herniations.

Hereditary disorders of connective tissue (HDCT) compromise a heterogeneous group of diseases caused by pathogenic variants in genes encoding different components of the extracellular matrix and characterized by pleiotropic manifestations, mainly affecting the cutaneous, cardiovascular, and musculoskeletal systems. We report the case of a 9-year-old boy with a discernible connective tissue disorder characterized by cutis laxa (CL) and multiple herniations and caused by biallelic loss-of-function variants in EFEMP1. Hence, we identified EFEMP1 as a novel disease-causing gene in the CL spectrum, differentiating it from other HDCT.

New insights on the clinical variability of FKBP10 mutations.

To date 45 autosomal recessive disease-causing variants are reported in the FKBP10 gene. Those variant were found to be associated with Osteogenesis Imperfecta (OI) for which the hallmark phenotype is bone fractuers or Bruck Syndrome (BS) where bone fractures are accompanied with contractures. In addition, a specific homozygous FKBP10 mutation (p.Tyr293del) has been described in Yup'ik Inuit population to cause Kuskokwim syndrome (KS) in which contractures without fractures are observed. Here we present an extended Palestinian family with 10 affected individuals harboring a novel homozygous splice site mutation, c.391+4A > T in intron 2 of the FKBP10 gene, in which the three above mentioned syndromes segregate as a result of skipping of exon 2 and absence of the FKBP65 protein. At the biochemical level, Hydroxylysyl pyridinoline (HP)/lysyl pyridinoline (LP) values were inversely correlated with OI phenotypes, a trend we could confirm in our patients. Our findings illustrate that single familial FKBP10 mutations can result in a phenotypic spectrum, ranging from fractures without contractures, to fractures and contractures and even to only contractures. This broad intra-familial clinical variability within one single family is a new finding in the field of bone fragility.