Transcriptional activation of the non-muscle, full-length dystrophin isoforms in Duchenne muscular dystrophy skeletal muscle.

Despite promoter tissue specificity, up-regulation of the brain and Purkinje cell type dystrophin isoforms was described in skeletal muscle of X-linked dilated cardiomyopathy (XLDCM) and BMD affected individuals. An extended population of 11 Duchenne muscular dystrophy (DMD) and 11 Becker muscular dystrophy (BMD) patients was investigated to determine whether ectopic muscle expression of the two full-length non-muscular isoforms is a common event in dystrophinopathies and if it has functional significance. Up-regulation of the two non-muscle-specific isoforms was detected in four DMD patients but in none of the BMD affected individuals or non-dystrophic controls. This is the first report of an expression of these two isoforms in DMD skeletal muscle. Ectopic expression is not confined to regenerating or revertant fibers and does not correlate with age at biopsy, clinical phenotype, cardiac involvement, deletion size or location. We consider that muscle ectopic expression of the brain and Purkinje cell-type isoforms has no favorable prognostic significance in DMD and BMD patients.