Simultaneous analysis of expression of the three myotonic dystrophy locus genes in adult skeletal muscle samples: the CTG expansion correlates inversely with DMPK and 59 expression levels, but not DMAHP levels

The causative mutation in the majority of cases of myotonic dystrophy has b een shown to be the expansion of a CTG trinucleotide repeat, but the mechan ism(s) by which this repeat leads to the very complex symptomatology in thi s disorder remains controversial. We have developed a highly sensitive and quantifiable assay, based on competitive RT-PCR, to test the hypothesis tha t the expansion disrupts the expression of the genes in its immediate vicin ity, DMPK, 59 and DMAHP, In order to avoid cell culture-induced artifacts w e performed these experiments using adult skeletal muscle biopsy samples an d analysed total cytoplasmic poly(A)(+) mRNA levels for each gene simultane ously, as this is more physiologically relevant than allele-specific levels , There was considerable overlap between the expression levels of the three genes in myotonic dystrophy patient samples and samples from control indiv iduals, However, in the myotonic dystrophy samples we detected a strong inv erse correlation between the repeat size and the levels of expression of DM PK and 59, This is the first report of a possible effect of the CTG expansi on on gene 59, Our results indicate that whilst a simple dosage model of ge ne expression in the presence of the mutation is unlikely to be sufficient in itself to explain the complex molecular pathology in this disease, the r epeat expansion may be a significant modifier of the expression of these tw o genes.