Revised spectrum of mutations in sarcoglycanopathies

To define the spectrum of mutations in α-, β-, γ-, and δ-sarcoglycan (SG) genes, we analyzed these genes in 69 probands with clinical and biological criteria compatible with the diagnosis of autosomal recessive limb-girdle muscular dystrophy. For 48 patients, muscle biopsies were available and multiplex western blot analysis of muscle proteins showed significant abnormalities of α- and γ-SG. Our diagnostic strategy includes multiplex western blot, sequencing of SG genes, multiplex quantitative-fluorescent PCR and RT-PCR analyses. Mutations were detected in 57 patients and homozygous or compound heterozygous mutations were identified in 75% (36/48) of the patients with abnormal western blot, and in 52% (11/21) of the patients without muscle biopsy. Involvement of α-SG was demonstrated in 55.3% of cases (26/47), whereas γ- and β-SG were implicated in 25.5% (12/47) and in 17% (8/47) of cases, respectively. Interestingly, we identified 25 novel mutations, and a significant proportion of these mutations correspond to deletions (identified in 14 patients) of complete exon(s) of α- or γ-SG genes, and partial duplications (identified in 5 patients) of exon 1 of β-SG gene. This study highlights the high frequency of exonic deletions of α- and γ-SG genes, as well as the presence of a hotspot of duplications affecting exon 1 of the β-SG gene. In addition, protein analysis by multiplex western blot in combination with mutation screening and genotyping results allowed to propose a comprehensive and efficient diagnostic strategy and strongly suggested the implication of additional genes, yet to be identified, in sarcoglycanopathy-like disorders.