Myotonic dystrophy or DM1 is the most common form of inherited neuromuscular disorder in adults with a global incidence of about 1 in 8500 individuals (1). The genetic basis for this autosomal dominant disease is an expanded trinucleotide repeat (CTG)n situated in the 3'-untranslated region (3'-UTR) of the dystrophy myotonic protein kinase (DMPK) gene (2-3). DM1 patients have from 50 to 1000 CTG repeats in the mild and classic forms of the disease and up to several thousand repeats in the severe congenital form. There is usually a good correlation between the size of the expansion, the age of onset and the severity of the disease (4,5). The (CTG)n > 50 expansion is unstable and expands progressively in successive generations of affected families, providing a molecular support for the phenomenon of anticipation which is observed in DM1 (6). Moreover, CTG repeats have been shown to have different sizes in different tissues of the same patient, with one of the largest repeat lengths being present in skeletal muscle (6,7).
Specific Aspect Of Cell Pathology
The exact physiopathological mechanism leading to DM1 is still not thoroughly understood. The nuclear retention of the mutant DMPK RNA containing large CUG repeats as discrete foci seems to play a key role in the development of the pathology (8,9). This dominant RNA mutation could cause a gain of function at the RNA level possibly by impairing the normal functions of proteins involved in RNA processing (10,11). However, the decreased level of the DMPK protein in DM1 skeletal muscle and myoblasts (12,13), and/or alterations in the expression of the neighbouring genes (DMWD and DMAHP) by the CTG expansions (14,15) could also be involved in the molecular mechanism of DM1.
The proliferating CDM cells are typically spindle shaped and have a very similar morphology to the proliferating cells isolated from age-matched control biopsies. The myogenicity of the muscle cell cultures was calculated from the number of cells expressing desmin, which is only expressed in myogenic cells (16,17). The myogenic purity between CDM and control cultures is slightly different (P < 0.05, t-test) and ranged from 77 to 86% for control cultures and 58 to 69% for the CDM cultures. The mean doubling time for each cell strain was calculated at the early stage of the culture and ranged from 37 to 47 h in most of the cultures except for patient 3, in which the doubling time was much longer (84 h). This mean doubling time progressively increased during the time the cells were maintained in culture and finally stopped when the cells reached proliferative senescence.
Distribution of DMPK mutant RNA
Intracellular localization of DMPK mutant RNA was analysed in differentiated CDM cultures. Using in situ hybridization, we showed that mutant transcripts were detected as discrete foci in the CDM myotube nucle. No cytoplasmic transcripts containing an expanded CUG repeat were detected in the cytoplasm of the CDM myotubes. Moreover, no foci were detected either in the nuclei or in the cytoplasm of control ...