Identification of myogenesis‐dependent transcriptional enhancers in promoter region of mouse γ‐sarcoglycan gene

Four sarcoglycan subunit proteins, α‐, β‐, γ‐ and δ‐sarcoglycans, form a complex on the skeletal muscle cell surface membrane and a gene defect in any one of them causes the loss or marked decrease of whole sarcoglycan complex, resulting in an autosomal recessive muscular dystrophy, sarcoglycanopathy. To characterize the regulation of sarcoglycan transcription during myocyte differentiation, we isolated the promoter regions for all sarcoglycan transcripts and measured the level of transcriptional activity of these promoter regions in the C2C12 skeletal muscle cell line. The promoters of γ‐sarcoglycan and one of two promoters of α‐sarcoglycan exhibited marked transcriptional activation following differentiation to myotubes. Then, we characterized the 1.5‐kb region of the γ‐sarcoglycan promoter by generating reporter‐constructs having various deletions and measuring their transcriptional activities. In this promoter, we identified a basal promoter region and two enhancer regions dependent on differentiation. We also showed that A/T‐rich and E box elements in the upstream enhancer region are essential for the activation of γ‐sarcoglycan transcription following myotube formation. Furthermore, from the identification of binding proteins to these elements together with the cotransfection experiments with the γ‐sarcoglycan promoter reporter construct and cDNAs encoding these binding factors to 10T1/2 fibroblast cell line, it was suggested that MyoD directs the transcription of γ‐sarcoglycan gene as one of the trans activators.