Vacuolar myopathy caused by CASQ1 p.Asp244His: pathogenic evidence from two unrelated Chinese families.

Calsequestrin-1 (CASQ1)-related myopathy is a rare skeletal muscle disorder caused by mutations in CASQ1 gene, which encodes a major calcium-buffering protein of the sarcoplasmic reticulum (SR). It is characterized histopathologically by tubular aggregates or optically empty vacuoles, predominantly affecting type II muscle fibers. In this study, we report two unrelated Chinese patients presenting with late-onset, slowly progressive muscle weakness, fatigue, and myalgia. Both had mildly to moderately elevated serum creatine kinase levels. Muscle biopsies revealed typical optically empty vacuoles primarily in type II fibers. Whole-exome sequencing identified an identical heterozygous CASQ1 variant, c.730G > C (p.Asp244His), located at a highly conserved residue. In vitro expression of the mutant CASQ1 in HeLa cells confirmed its aggregation tendency, suggesting impaired protein folding or calcium handling. Immunofluorescence revealed abnormal aggregation of CASQ1 protein around the edge of vacuoles, co-localized with SQSTM1/p62, and the endoplasmic reticulum (ER) stress marker PERK. Our findings support the pathogenic role of the p.Asp244His variant and provide further insights into CASQ1-related myopathy in Asian populations.