A novel cryptic exon in intron 2 of the human dystrophin gene evolved froman intron by acquiring consensus sequences for splicing at different stages of anthropoid evolution

The dystrophin gene, which is mutated in Duchenne muscular dystrophy, is th us the largest human gene. A full spectrum of splicing of the dystrophin tr anscript has not been elucidated yet, though more than 10 alternative splic ings have been identified in the 5' region of the dystrophin gene. In this study, two novel dystrophin transcripts containing a 140-nucleotide inserti on precisely between exons 2 and 8 or between exons 2 and 18 were identifie d in skeletal muscle. The genomic region corresponding to and surrounding t his 140-nucleotide sequence was sequenced to reveal that the insertion poss essed a branch point and both acceptor and donor splice site consensus sequ ences perfectly. Therefore, the 140-bp insertion sequence was considered to be a novel exon. The novel exon was mapped to intron 2 and was designated exon 2a. Reverse-transcription PCR screening for transcripts containing exo n 2a in 12 human tissues revealed its presence in 3 of them, including skel etal muscle. Phylogenetic studies disclosed that exon 2a evolved from intro n DNA by the progressive acquisition of nucleotide substitutions in ancestr al hominoids. (C) 2000 Academic Press.