HAIRPINS ARE FORMED BY THE SINGLE DNA STRANDS OF THE FRAGILE-X TRIPLET REPEATS - STRUCTURE AND BIOLOGICAL IMPLICATIONS

Inordinate expansion and hypermethylation of the fragile X DNA triplet repeat, (GGC)(n) .(GCC)(n), are correlated with the ability of the in dividual G- and C-rich single strands to form hairpin structures. Two- dimensional NMR and gel electrophoresis studies show that both the G- and C-rich single strands form hairpins under physiological conditions . This propensity of hairpin formation is more pronounced for the C-ri ch strand than for the G-rich strand. This observation suggests that t he C-rich strand is more likely to form hairpin or ''slippage'' struct ure and show asymmetric strand expansion during replication. NMR data also show that the hairpins formed by the C-rich strands fold in such a way that the cytosine at the CpG step of the stem is C . C paired. T he presence of a C . C mismatch at the CPG site generates local flexib ility, thereby providing analogs of the transition to the methyltransf erase. In other words, the hairpins of the C-rich strand act as better substrates for the human methyltransferase than the Watson-Crick dupl ex or the G-rich strand. Therefore, hairpin formation could account fo r the specific methylation of the CpG island in the fragile X repeat t hat occurs during inactivation of the FMR1 gene during the onset of th e disease.