Calpain III mutation analysis of a heterogeneous limb-girdle muscular dystrophy population

Objective: To determine the frequency of calpain III mutations in a heterog eneous limb-girdle muscular dystrophy (LGMD) population. Background: Mutati ons of the calpain III gene have been shown to cause a subset of autosomal recessive LGMDs. Patient populations studied to date have been primarily of French and Spanish origin, in which calpain III may cause 30% of autosomal recessive MDs. The incidence of calpain III mutations in non-French/Spanis h MD patients has not been studied thoroughly. No sensitive and specific bi opsy screening methods for detecting patients with abnormal calpain III pro tein are available. Thus, detection of patients relies on direct detection of gene mutations. Methods: The authors studied the calpain III gene in 107 MD patient muscle biopsies exhibiting normal dystrophin. Muscle biopsy RNA was produced f'or each patient, and the entire calpain III complementary D NA was screened for mutations by reverse-transcriptase PCR/single-strand co nformation polymorphism using three different conditions. Results: The auth ors identified nine patients (eight unrelated) with causative mutations. Si x of the seven distinct mutations identified are novel mutations and have n ot been described previously. Conclusion: The results suggest that approxim ately 9.2% of patients in the heterogeneous population with an LGMD diagnos is will show mutations of the calpain III gene. Interestingly, two patients were heterozygous for a single mutation at the DNA level, whereas only the mutant allele was observed at the RNA level. This suggests that there are undetectable, nondeletion mutations that ablate expression of the calpain I II gene.