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.

Mice with a mutation (D244G, DG) in calsequestrin 1 (CASQ1), analogous to a human mutation in CASQ1 associated with a delayed onset human myopathy (vacuolar aggregate myopathy), display a progressive myopathy characterized by decreased activity, decreased ability of fast twitch muscles to generate force and low body weight after one year of age. The DG mutation causes CASQ1 to partially dissociate from the junctional sarcoplasmic reticulum (SR) and accumulate in the endoplasmic reticulum (ER). Decreased junctional CASQ1 reduces SR Ca2+ release. Muscles from older DG mice display ER stress, ER expansion, increased mTOR signaling, inadequate clearance of aggregated proteins by the proteasomes, and elevation of protein aggregates and lysosomes. This study suggests that the myopathy associated with the D244G mutation in CASQ1 is driven by CASQ1 mislocalization, reduced SR Ca2+ release, CASQ1 misfolding/aggregation and ER stress. The subsequent maladaptive increase in protein synthesis and decreased protein aggregate clearance are likely to contribute to disease progression.

To identify and characterize patients with calsequestrin 1 (CASQ1)-related myopathy. Patients selected according to histopathologic features underwent CASQ1 genetic screening. CASQ1-mutated patients were clinically evaluated and underwent muscle MRI. Vacuole morphology and vacuolated fiber type were characterized. Twenty-two CASQ1-mutated patients (12 families) were identified, 21 sharing the previously described founder mutation (p.Asp244Gly) and 1 with the p.Gly103Asp mutation. Patients usually presented in the sixth decade with exercise intolerance and myalgias and later developed mild to moderate, slowly progressive proximal weakness with quadriceps atrophy and scapular winging. Muscle MRI (n = 11) showed a recurrent fibrofatty substitution pattern. Three patients presented subclinical cardiac abnormalities. Muscle histopathology in patients with p.Asp244Gly showed vacuoles in type II fibers appearing empty in hematoxylin-eosin, Gomori, and nicotinamide adenine dinucleotide (NADH) tetrazolium reductase stains but strongly positive for sarcoplasmic reticulum proteins. The muscle histopathology of p.Gly103Asp mutation was different, showing also NADH-positive accumulation consistent with tubular aggregates. We report the clinical and molecular details of the largest cohort of CASQ1-mutated patients. A possible heart involvement is presented, further expanding the phenotype of the disease. One mutation is common due to a founder effect, but other mutations are possible. Because of a paucity of symptoms, it is likely that CASQ1 mutations may remain undiagnosed if a muscle biopsy is not performed.