Fluorescence in situ hybridization analysis of the replication properties of the myotonic dystrophy protein kinase (DMPK) gene region

Myotonic dystrophy (DM) is caused by an expansion of a CTG repeat sequence in the 3' noncoding region of a protein kinase gene (DMPK) at 19q13.3. We u sed in situ hybridization to analyse the replication timing of the genomic region containing DMPK in fibroblasts and myoblasts from controls and myoto nic dystrophy patients. In this method the relative proportion of singlet t o doublet hybridization signals is used to infer the relative time of repli cation of specific loci or regions. Our results show that in cells from nor mal individuals approximately 65% of signals appear as doublers, indicating early replication. In DM patients with a number of CTG repeats ranging fro m about 600-1800 we observed a significant increase of singlet-doublets com pared to the background level. These results suggest the existence of repli cation alternations and/or structural differences between the normal and mu tant alleles induced by the presence of the DM mutation.