HETEROGENEITY OF DM KINASE REPEAT EXPANSION IN DIFFERENT FETAL TISSUES AND FURTHER EXPANSION DURING CELL-PROLIFERATION IN-VITRO - EVIDENCE FOR A CAUSAL INVOLVEMENT OF METHYL-DIRECTED DNA MISMATCH REPAIR IN TRIPLET REPEAT STABILITY

We have analysed the mitotic behaviour of expanded CTG repeats in soma tic tissues and cultured somatic cells from myotonic dystrophy (DM) fe tuses using indirect and direct methods, Heterogeneity of expansions b etween fetal tissues was demonstrated in a 16 week old fetus whereas t here was no evidence for such a somatic heterogeneity in a 13 week old fetus, Dilution plating of cultured cells from an adult patient and a fetus resulted in isolation of clones showing single expanded restric tion fragments when the donor showed a heterogeneous smear of expansio ns or a single expanded fragment, During proliferation in vitro to 45 doublings, DM cells experienced highly synchronous further repeat expa nsion which first became evident at similar to 15 cell generations and reached a plateau of maximum expansion at similar to 200 days. When m athematically expressed as a function of culture time the dynamics of expansion of restriction fragments followed a sigmoid curve, This unst able behaviour of CTG repeat expansions in DM was compared to the mito tically stable patterns of full mutation in fragile X fetal tissues an d led to the hypothesis that methylation of CpGs within the repeat seq uence is a stabilizing factor of largely expanded CGG and GCC repeats allowing for efficient methyl-directed strand-specific DNA mismatch re pair.