Clinical variability in calpainopathy: what makes the difference?

Limb girdle muscular dystrophies (LGMD) are a heterogeneous group of genetic disorders characterised by progressive weakness of the pelvic and shoulder girdle muscles and a great variability in clinical course. LGMD2A, the most prevalent form of LGMD, is caused by mutations in the calpain-3 gene (CAPN-3). More than 100 pathogenic mutations have been identified to date, however few genotype : phenotype correlation studies, including both DNA and protein analysis, have been reported. In this study we screened 26 unrelated LGMD2A Brazilian families (75 patients) through Single-Stranded Conformation Polymorphism (SSCP), Denaturing high-performance liquid chromatography (DHPLC) and sequencing of abnormal fragments which allowed the identification of 47 mutated alleles (approximately 90%). We identified two recurrent mutations (R110X and 2362-2363AG > TCATCT) and seven novel pathogenic mutations. Interestingly, 41 of the identified mutations (approximately 80%) were concentrated in only 6 exons (1, 2, 4, 5, 11 and 22), which has important implications for diagnostic purposes. Protein analysis, performed in 28 patients from 25 unrelated families showed that with exception of one patient (with normal/slight borderline reduction of calpain) all others had total or partial calpain deficiency. The effects of type of mutation, amount of calpain in the muscle, gender and ethnicity of affected patients on clinical course (age of onset and ascertainment) were analysed. Interestingly, it was observed that, on average, African-Brazilian calpainopathy patients are more severely affected than Caucasians.