To explore the clinical phenotype and genetic etiology of a child with Ehlers-Danlos syndrome, spondylodysplastic type 2 (EDSSPD2). A child who was admitted to the Children's Medical Center of the Affiliated Hospital of Guangdong Medical University in July 2024 for "delayed motor development for 1 and a half year" was selected as the study subject. Clinical data of the child was collected, including medical history, family history, and results of auxiliary examinations. Peripheral venous blood samples were collected from the child and his two brothers and both parents. Genomic DNA was extracted from the child and his family members and subjected to whole-exome sequencing (WES) and copy number variation (CNV) analysis. Sanger sequencing was used to verify the parental origin of the candidate variants. Multiple protein function prediction software tools, including SIFT, PolyPhen-2, and REVEL, were used to assess the impact of candidate variants on the protein function. Based on protein database information from UniProt, a two dimensional structural schematic of the target protein was generated. The pathogenicity of the variants was classified based on the guidelines from the American College of Medical Genetics and Genomics (ACMG). Relevant literature on the B3GALT6 gene variants leading to EDSSPD2 was retrieved from CNKI, Wanfang Data Knowledge Service Platform, and PubMed databases. The procedures followed in this study were reviewed and approved by the Medical Ethics Committee of Affiliated Hospital of Guangdong Medical University (Ethics No.：PJ2021-097). The proband was a 2-year-old male with an onset in infancy. The main clinical manifestations included loose skin, scoliosis and kyphosis, generalized hypermobility of joints, and motor developmental delay. WES has revealed two compound heterozygous variants of the B3GALT6 gene (NM_080605.4): c.766C>T (p.Arg256Trp) and c.962G>A (p.Cys321Tyr). Sanger sequencing verification showed that the c.766C>T and c.962G>A variants were respectively derived from his phenotypically normal father and mother. Bioinformatics analysis showed that for the c.766C>T (p.Arg256Trp) variant, the Arg256 site is located within the galactosyltransferase catalytic domain (GalT domain) of the β3GalT6 protein. According to the ACMG guidelines, the c.766C>T variant was classified as a likely pathogenic (PS3+PM2_supporting+PM3+PP3), and the c.962G>A was classified as a variant of unknown significance (PM2_Supporting+PM3+PP3). By following the pre-set literature retrieval strategy, a total of 12 articles related to B3GALT6 gene variants were identified (11 English and 1 Chinese), which involved a total of 71 patients. Among these, 4 reports (involving 20 patients) involved B3GALT6 gene variants leading to EDSSPD2. Among the 18 live-born EDSSPD2 patients (including the proband in this study), common clinical manifestations have included scoliosis (88.9%, 16/18), generalized hypotonia (83.3%, 15/18), and soft and lax skin (66.7%, 12/18). Some patients already showed skeletal abnormalities on prenatal ultrasound scan (22.2%, 4/18), while a few presented with cervical instability (16.7%, 3/18). One child had deceased at 18 months of age due to hypoxia caused by tracheomalacia and tracheal compression due to scoliosis. Among the 23 reported EDSSPD2 related B3GALT6 variant sites, missense variants were the most common (78.3%, 18/23), followed by nonsense variants (21.7%, 5/23). Above finding has enriched the clinical and mutational spectra of EDSSPD2. Early genetic testing has important clinical value for the diagnosis, differential diagnosis, and genetic counseling of this disease.

Loss of ZIP13 causes Ehlers-Danlos syndrome spondylodysplastic type 3 involving connective tissue dysplasias associated with a reduction in muscular strength. However, ZIP13 role in skeletal muscle homeostasis, particularly for the regulation of muscle satellite cells (MuSCs), remains poorly understood. In this study, we investigated Zip13-knockout (KO) mice and found a reduction in MuSCs of Zip13-KO mice, in which the quiescent and activated phase balances were disrupted. To clarify the physiological role and dynamics of ZIP13 expression in MuSCs, we generated Zip13-GFP knock-in (KI) mice encoding GFP at the Zip13 locus, which showed that ZIP13 contributes to the phase balance regulation of quiescent and activated MuSCs and their functions. Indeed, Zip13-KO mice exhibited delayed recovery from skeletal muscle injury, indicating ZIP13 requirement for proper skeletal muscle regeneration. Moreover, GFP expression was reduced in the MuSCs of homozygous Zip13-GFP KI mice whose intact ZIP13 expression was perturbed, suggesting that positive feedback mechanisms exist to maintain ZIP13 expression. Altogether, our results illustrate that ZIP13 might be positively involved in skeletal muscle regeneration by controlling the quiescent/activated phase balance of MuSCs through autoregulatory ZIP13 expression, and that newly generated Zip13-GFP KI mice would be useful for investigating the roles and dynamics of ZIP13-expressing cells.

Biallelic variants in the SCL35D1 gene have been originally associated with a severe skeletal dysplasia called "Schneckenbecken dysplasia" because of the resemblance of the pelvic shape to a snail. More recently, SLC35D1 variants have been associated with much milder phenotypes of skeletal dysplasia. Our report describes one such individual with a novel SLC35D1 variant. A 17-year-old male with a coarse face and short stature was referred to our clinic. On his radiographic imaging, shortness of the long bones and metaphyseal flaring were detected. Using a clinical exome panel, we discovered a novel homozygous missense variant in the SLC35D1 gene, c.899G>T (p.Gly300Val). We identified a biallelic variant that was causative for a mild skeletal dysplasia and showed its phenotypic effects. Our observation confirms the existence of nonlethal skeletal dysplasias associated with biallelic SLC35D1 variants and suggests the existence of a phenotypic spectrum.

The linkeropathies are a group of rare disorders, characterized by overlapping clinical features involving the skeletal and connective tissues. Each "linker" gene encodes an enzyme responsible for the addition of glycosaminoglycan chains to proteoglycans via a common tertrasaccharine linker region. The original descriptions of the autosomal recessive B3GALT6-related disorder showed that the associated clinical features are pleiotropic, spanning the skeletal dysplasia (Spondyloepimetaphyseal dysplasia with joint laxity) (SEMD-JL1) and connective tissue disorder (Ehlers-Danlos syndrome) (EDS spondylodysplastic Type 2) spectrum. Here, we describe three patients with biallelic B3GALT6 variants: each had different clinical presentations, and the two older patients initially received alternative clinical diagnoses (Larsen syndrome and Osteogenesis imperfecta, respectively). We describe the clinico-radiological features of these patients to highlight the spectrum of disease associated with the B3GALT6-linkeropathy.

Biallelic mutations in B3GALT6, coding for a galactosyltransferase involved in the synthesis of glycosaminoglycans (GAGs), have been associated with various clinical conditions, causing spondyloepimetaphyseal dysplasia with joint laxity type 1 (SEMDJL1 or SEMDJL Beighton type), Al-Gazali syndrome (ALGAZ), and a severe progeroid form of Ehlers-Danlos syndrome (EDSSPD2). In the 2017 Ehlers-Danlos syndrome (EDS) classification, Beta3GalT6-related disorders were grouped in the spondylodysplastic EDSs together with spondylodysplastic EDSs due to B4GALT7 and SLC39A13 mutations. Herein, we describe a patient with a previously unreported homozygous pathogenic B3GALT6 variant resulting in a complex phenotype more severe than spondyloepimetaphyseal dysplasia with joint laxity type 1, and having dural ectasia and aortic dilation as additionally associated features, further broadening the phenotypic spectrum of the Beta3GalT6-related syndromes. We also document the utility of repeating sequencing in patients with uninformative exomes, particularly when performed by using "first generations" enrichment capture methods.