Intrafamilial variability of myoclonic dystonia in a large French family carrying a novel SGCE variant

Myoclonus–dystonia syndrome (MDS) is an autosomal dominant movement disorder most caused by pathogenic variants in SGCE, an imprinted gene subject to maternal silencing. While numerous pathogenic variants have been reported, the extent and determinants of intrafamilial variability remain incompletely understood. We investigated a large French family in which ten individuals across three generations presented with myoclonic jerks, dystonia, or combined phenotypes. Trio whole-exome sequencing performed in the proband, her affected brother, and her affected father identified a heterozygous SGCE variant, NM₀03919. 3: c. 406T>G, predicting a p. Cys136Gly substitution in a highly conserved cysteine-rich extracellular domain of ε-sarcoglycan. The variant segregated with disease in all clinically affected individuals for whom DNA was available and was absent from population databases. The phenotypic spectrum ranged from mild, stress-induced myoclonus in females to early-onset myoclonus with cervical dystonia and alcohol responsiveness in males, consistent with imprinting effects and sex-dependent modifiers. Variant interpretation using ACMG/AMP criteria (PM1, PM2, PM5, PP1-strong, PP2, PP3) supports a pathogenic classification. This previously unreported SGCE missense variant expands the mutational spectrum of MDS and further illustrates the substantial intrafamilial variability of SGCE-related disease.