Splicing mutations in DMD/BMD detected by RT-PCR/PTT: detection of a 19AA insertion in the cysteine rich domain of dystrophin compatible with BMD.

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Abstract
We have used an RNA based mutation detection method to screen the total coding region of the dystrophin gene of a Duchenne and a Becker muscular dystrophy patient in whom DNA based mutation detection methods have so far failed to detect mutations. By RT-PCR and the protein truncation test (PTT) we could identify point mutations in both cases. DMD patient DL184.3 has a T-->A mutation in intron 59 at position -9, creating a novel splice acceptor site for exon 60. As a result seven intronic bases are spliced into the mRNA, causing a frameshift and premature translation termination 20 codons downstream. Since this patient had died and only fibroblasts were available, we applied MyoD induced myodifferentiation of stored fibroblasts to enhance muscle specific gene expression. With the results of this mutation analysis, prenatal diagnosis could subsequently be performed in this family. BMD patient BL207.1 carries a G-->C mutation at position +5 of intron 64, disrupting the splice donor consensus sequence and activating a cryptic splice donor site 57bp downstream. The inclusion of these 57 intronic bases in the mRNA leaves the reading frame open and results in the insertion of 19 amino acids into the cysteine rich domain of dystrophin. Interestingly, this insertion in a part of the dystrophin considered to interact with the dystrophin binding complex of the sarcolemma is apparently compatible with mild BMD-like clinical features. Both mutations reported are missed by analysis of multiplex PCR products designed for deletion screening of the coding region. Extrapolation from existing point mutation detection efficiencies by DNA and RNA based methods emphasises that RNA based methods are more sensitive and that most of the remaining undetected mutations may affect splice or branch sites or create cryptic splice sites.