Cloning and study on physiological role of new dystrophin isoform.

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At least four promoters L,C,M and P have been identified in 5'region of the dystrophin gene, and each promoter is expressed in a tissue-or development-specific manner, giving rise to multiple isoforms of dystrophin. In addition, alternative splicing functions to increase the number of dystrophin isoforms. Although cardiomyopathy is one of the most severe complications of Duchenne/Becker muscular dytrophies, no isoform specifically expressed in cardiac muscle has not been identified. We supposed that dystrophin isoform specific for cardiac muscle is produced by alternative splicing.In one case of Becker muscular dystrophy we identified a dystrophin transcript lacking exons 71 to 74 and having a sequence joining exon 70 to exon 75. From this case we could cloned the DNA fragment consisting of exons 70 and 75. By using this DNA fragment as a probe, we did a Northern blot analysis of mRNA obtained from various tissues. The result showed a very specific band is present in mRNA obtained from cardiac muscle. This suggested the presence of new dystrophin isoform specific for cardiac muscle. We are now on the way to clone the full length transcript of new dystrophin isoform. Elucidation of this transcript will facilitate understanding of pathophysiology of cardiomyopathy in DMD/BMD.