A novel cryptic exon in intron 3 of the dystrophin gene was incorporated into dystrophin mRNA with a single nucleotide deletion in exon 5.

The dystrophin gene, which is mutated in Duchenne muscular dystrophy, is the largest human gene. A full spectrum of the gene transcripts has not been fully elucidated yet, although two cryptic exons have so far been identified in the 5' region of the dystrophin gene. Here, a novel dystrophin mRNA containing a 62-nucleotide insertion between exons 3 and 4 was identified in lymphocytes from a Japanese Duchenne muscular dystrophy patient with a single nucleotide deletion in exon 5. The inserted 62-nucleotide sequence was found to be homologous to part of intron 3 and it was revealed that the insertion possessed branch point and both acceptor and donor splice site consensus sequences perfectly. Therefore, the 62-bp insertion sequence was considered to be a novel exon and was designated as exon 3a. However, this insertion was not present in the patient's muscle and 12 different normal tissues that were screened. The physiological role of the novel cryptic exon remains to be clarified.