Stiffness, Facial Dysmorphism, and Skeletal Abnormalities: Schwartz-Jampel Syndrome 1A

Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients

Dyssegmental dysplasia (DD) is a severe skeletal dysplasia comprised of two subtypes: lethal Silverman-Handmaker type (DDSH) and nonlethal Rolland-Desbuquois type (DDRD). DDSH is caused by biallelic pathogenic variants in HSPG2 encoding perlecan, whereas the genetic cause of DDRD remains undetermined. Schwartz-Jampel syndrome (SJS) is also caused by biallelic pathogenic variants in HSPG2 and is an allelic disorder of DDSH. In SJS and DDSH, 44 and 8 pathogenic variants have been reported in HSPG2, respectively. Here, we report that five patients with DDRD carried four pathogenic variants in HSPG2: c.9970 G > A (p.G3324R), c.559 C > T (p.R187X), c7006 + 1 G > A, and c.11562 + 2 T > G. Two patients were homozygous for p.G3324R, and three patients were heterozygous for p.G3324R. Haplotype analysis revealed a founder haplotype spanning 85,973 bp shared in the five patients. SJS, DDRD, and DDSH are allelic disorders with pathogenic variants in HSPG2.

[The Schwartz-Jampel syndrome]

The Schwartz-Jampel syndrome (SJS, OMIM #255800) is an ultra-rare genetic disease characterized by myotonic manifestations combined with bone and cartilage abnormalities. Following an autosomal recessive mode of inheritance, its prevalence is more significant in highly-inbred areas. The unraveling of the HSPG2 gene encoding a protein of the basal lamina enabled a better nosological delineation of the syndrome. The diagnosis is usually strongly suspected at the clinical level and then confirmed by molecular biology. To date, the treatment remains essentially symptomatic.

Novel HSPG2 Gene Mutation Causing Schwartz-Jampel Syndrome in a Moroccan Family: A Literature Review

Schwartz-Jampel syndrome type 1 (SJS1) is a rare autosomal recessive musculoskeletal disorder caused by various mutations in the HSPG2 gene encoding the protein perlecan, a major component of basement membranes. We report a novel splice mutation HSPG2(NM_005529.7):c.3888 + 1G > A and a known point mutation HSPG2(NM_005529.7):c.8464G > A, leading to the skipping of exon 31 and 64 in mRNA, respectively, in a Moroccan child with clinical features suggestive of SJS1 and carrying two compound heterozygous mutations in the HSPG2 gene detected by next-generation sequencing. Both parents harboured one mutation. Real-time and immunostaining analysis revealed down-regulation of the HSPG2 gene and a mild reduction in the protein in the muscle, respectively. We reviewed all genetically characterized SJS1 cases reported in literature, confirming the clinical hallmarks and unspecific instrumental data in our case. The genotype-phenotype correlation is very challenging in SJS1. Therapy is mainly focused on symptom management and several drugs have been administered with different efficacy.Here, we report the second case with spontaneous improvement.

Evaluation of Human-Induced Pluripotent Stem Cells Derived from a Patient with Schwartz–Jampel Syndrome Revealed Distinct Hyperexcitability in the Skeletal Muscles

Schwartz–Jampel syndrome (SJS) is an autosomal recessive disorder caused by loss-of-function mutations in heparan sulfate proteoglycan 2 (HSPG2), which encodes the core basement membrane protein perlecan. Myotonia is a major criterion for the diagnosis of SJS; however, its evaluation is based solely on physical examination and can be challenging in neonates and young children. Furthermore, the pathomechanism underlying SJS-related myotonia is not fully understood, and effective treatments for SJS are limited. Here, we established a cellular model of SJS using patient-derived human-induced pluripotent stem cells. This model exhibited hyper-responsiveness to acetylcholine as a result of abnormalities in the perlecan molecule, which were confirmed via comparison of their calcium imaging with calcium imaging of satellite cells derived from Hspg2−/−-Tg mice, which exhibit myotonic symptoms similar to SJS symptoms. Therefore, our results confirm the utility of creating cellular models for investigating SJS and their application in evaluating myotonia in clinical cases, while also providing a useful tool for the future screening of SJS therapies.

A Novel Pathogenic HSPG2 Mutation in Schwartz-Jampel Syndrome

Schwartz–Jampel syndrome is a rare autosomal recessive disease caused by mutation in the heparan sulfate proteoglycan 2 (HSPG2) gene. Its cardinal symptoms are skeletal dysplasia and neuromuscular hyperactivity. Herein, we identified a new pathogenic mutation site (NM_005529.6:c.1125C>G; p.Cys375Trp) of HSPG2 leading to Schwartz–Jampel syndrome by whole-exome sequencing. This mutation carried by the asymptomatic parents was previously registered in a single-nucleotide polymorphism database of the National Institutes of Health as a coding sequence variant rs543805444. The pathogenic nature of this missense mutation was demonstrated by in silico pathogenicity assessment, clinical presentations, and cellular function of primary fibroblast derived from patients. Various in silico software applications predicted the mutation to be pathogenic [Sorting Intolerant From Tolerant (SIFT), 0; Polyphen-2, 1; CADD (Combined Annotation Dependent Depletion), 23.7; MutationTaster, 1; DANN (deleterious annotation of genetic variants using neural networks); 0.9]. Needle electromyography revealed extensive complex repetitive discharges and multiple polyphasic motor unit action potentials in axial and limb muscles at rest. Short exercise test for myotonia showed Fournier pattern I. At cellular levels, mutant primary fibroblasts had reduced levels of secreted perlecan and impaired migration ability but normal capability of proliferation. Patients with this mutation showed more neuromuscular instability and relatively mild skeletal abnormality comparing with previously reported cases.