Expression of four alternative dystrophin transcripts in brain regions regulated by different promoters

Cognitive impairment occurs in one-third of patients with Duchenne muscular dystrophy, a lethal X-linked, recessive disease caused by mutations in the dystrophin gene which is expressed in both brain and muscle, the two transcripts having alternative first exons. Previous reports have indicated that the 'brain-type' dystrophin transcript predominates in brain. Using in situ hybridisation with antisense oligonucleotides, expression of four distinct mRNAs in specific brain areas is demonstrated here; the 14 kb muscle-type and brain-type transcripts were found to coexist in cortical and hippocampal neurons and two new transcripts have been identified in dentate gyrus and cerebellar Purkinje neurons, respectively. The latter has a novel first exon which was isolated and sequenced from mouse and human, and which would encode a protein with a different amino-terminus from the known muscle- and brain-type isoforms. Mapping in human located this exon in a large intron between the muscle-type promoter and second exon of the dystrophin gene. This finding of four alternative transcripts regulated by different promoters in brain reveals a new complexity to dystrophin expression that may have important insights for mental retardation mechanisms.