Compound heterozygous variants in GOSR2 associated with congenital muscular dystrophy: A case report

Compound Heterozygous Variants of GOSR2 Associated With Congenital Muscular Dystrophy and Progressive Myoclonus Epilepsy: A Case Report

Objectives

The GOSR2 gene is a Golgi vesicle transport gene that encodes for the Golgi SNAP receptor complex member 2 protein. This protein mediates transport between the medial and trans-Golgi compartments. The homozygous missense variant in the GOSR2 gene, c.430G>T, has been associated with progressive myoclonus epilepsy (PME). There have been reports suggesting that compound heterozygous GOSR2 variants are associated with the congenital muscular dystrophy (CMD) phenotype.

Methods

In this article, we report a pediatric case with congenital hypotonia, motor delay, elevated creatine kinase, and abnormal muscle biopsy consistent with CMD who subsequently developed PME. Whole-exome sequencing identified pathogenic compound heterozygous variants in the GOSR2 gene, one of which was the previously described PME-related c.430G>T(p.Gly144Trp), and a novel variant, c.22dup(p.Thr8fs).

Result

To our knowledge, this is a novel case of compound heterozygous variants in GOSR2 associated with both CMD and PME phenotypes.

Discussion

This case adds to the expanding clinical phenotype of GOSR2-related neurologic diseases.

Progressive Myoclonus Epilepsy: A Scoping Review of Diagnostic, Phenotypic and Therapeutic Advances

The progressive myoclonus epilepsies (PME) are a diverse group of disorders that feature both myoclonus and seizures that worsen gradually over a variable timeframe. While each of the disorders is individually rare, they collectively make up a non-trivial portion of the complex epilepsy and myoclonus cases that are seen in tertiary care centers. The last decade has seen substantial progress in our understanding of the pathophysiology, diagnosis, prognosis, and, in select disorders, therapies of these diseases. In this scoping review, we examine English language publications from the past decade that address diagnostic, phenotypic, and therapeutic advances in all PMEs. We then highlight the major lessons that have been learned and point out avenues for future investigation that seem promising.

Novel Genetic and Phenotypic Expansion in GOSR2-Related Progressive Myoclonus Epilepsy

Biallelic variants in the Golgi SNAP receptor complex member 2 gene (GOSR2) have been reported in progressive myoclonus epilepsy with neurodegeneration. Typical clinical features include ataxia and areflexia during early childhood, followed by seizures, scoliosis, dysarthria, and myoclonus. Here, we report two novel patients from unrelated families with a GOSR2-related disorder and novel genetic and clinical findings. The first patient, a male compound heterozygous for the GOSR2 splice site variant c.336+1G>A and the novel c.364G>A,p.Glu122Lys missense variant showed global developmental delay and seizures at the age of 2 years, followed by myoclonus at the age of 8 years with partial response to clonazepam. The second patient, a female homozygous for the GOSR2 founder variant p.Gly144Trp, showed only mild fine motor developmental delay and generalized tonic-clonic seizures triggered by infections during adolescence, with seizure remission on levetiracetam. The associated movement disorder progressed atypically slowly during adolescence compared to its usual speed, from initial intention tremor and myoclonus to ataxia, hyporeflexia, dysmetria, and dystonia. These findings expand the genotype-phenotype spectrum of GOSR2-related disorders and suggest that GOSR2 should be included in the consideration of monogenetic causes of dystonia, global developmental delay, and seizures.

BET1 variants establish impaired vesicular transport as a cause for muscular dystrophy with epilepsy

BET1 is required, together with its SNARE complex partners GOSR2, SEC22b, and Syntaxin-5 for fusion of endoplasmic reticulum-derived vesicles with the ER-Golgi intermediate compartment (ERGIC) and the cis-Golgi. Here, we report three individuals, from two families, with severe congenital muscular dystrophy (CMD) and biallelic variants in BET1 (P1 p.(Asp68His)/p.(Ala45Valfs*2); P2 and P3 homozygous p.(Ile51Ser)). Due to aberrant splicing and frameshifting, the variants in P1 result in low BET1 protein levels and impaired ER-to-Golgi transport. Since in silico modeling suggested that p.(Ile51Ser) interferes with binding to interaction partners other than SNARE complex subunits, we set off and identified novel BET1 interaction partners with low affinity for p.(Ile51Ser) BET1 protein compared to wild-type, among them ERGIC-53. The BET1/ERGIC-53 interaction was validated by endogenous co-immunoprecipitation with both proteins colocalizing to the ERGIC compartment. Mislocalization of ERGIC-53 was observed in P1 and P2's derived fibroblasts; while in the p.(Ile51Ser) P2 fibroblasts specifically, mutant BET1 was also mislocalized along with ERGIC-53. Thus, we establish BET1 as a novel CMD/epilepsy gene and confirm the emerging role of ER/Golgi SNAREs in CMD.

Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation

The Golgi is the central organelle of the secretory pathway and it houses the majority of the glycosylation machinery, which includes glycosylation enzymes and sugar transporters. Correct compartmentalization of the glycosylation machinery is achieved by retrograde vesicular trafficking as the secretory cargo moves forward by cisternal maturation. The vesicular trafficking machinery which includes vesicular coats, small GTPases, tethers and SNAREs, play a major role in coordinating the Golgi trafficking thereby achieving Golgi homeostasis. Glycosylation is a template-independent process, so its fidelity heavily relies on appropriate localization of the glycosylation machinery and Golgi homeostasis. Mutations in the glycosylation enzymes, sugar transporters, Golgi ion channels and several vesicle tethering factors cause congenital disorders of glycosylation (CDG) which encompass a group of multisystem disorders with varying severities. Here, we focus on the Golgi vesicle tethering and fusion machinery, namely, multisubunit tethering complexes and SNAREs and their role in Golgi trafficking and glycosylation. This review is a comprehensive summary of all the identified CDG causing mutations of the Golgi trafficking machinery in humans.

Congenital muscular dystrophies: What is new?

Congenital muscular dystrophies (CMDs) are a group of inherited conditions defined by muscle weakness occurring before the acquisition of ambulation, delayed motor milestones, and characterised by muscle dystrophic pathology. A large number of genes - at least 35- are responsible for CMD phenotypes, and it is therefore not surprising that CMDs comprise a wide spectrum of phenotypes, with variable involvement of cardiac/respiratory muscles, central nervous system, and ocular structures. The identification of several new genes over the past few years has further expanded both the clinical and the molecular spectrum underlying CMDs. Comprehensive gene panels allow to arrive at a final diagnosis in around 60% of cases, suggesting that both new genes, and unusual mutations of the currently known genes are likely to account for the remaining cases. The aim of this review is to present the most recent advances in this field. We will outline recent natural history studies that provide additional information on disease progression, discuss recently discovered genes and the current status of the most promising therapeutic options.

Myopathy can be a key phenotype of membrin (GOSR2) deficiency

T1-weighted, cross-sectional MR images showing shoulder girdle, abdominal, paraspinal, gluteal and thigh muscles almost completely replaced by fat, whereas lower leg muscles are almost unaffected i a patient who is compound heterozygous for pathogenic variants in GOSR2.