TRIPLET REPEAT INSTABILITY AND DNA TOPOLOGY - AN EXPANSION MODEL-BASED ON STATISTICAL-MECHANICS

The variance of writhe, the contribution of writhe to supercoiling, an d the free energies of supercoiling were calculated for (CTG.CAG)(n) a nd (CGG.CCG)(n) triplet repeat sequences (TRS) by statistical mechanic s from the bending and torsional moduli previously determined, Expansi ons of these sequences are inherited by non-mendelian transmission and are linked with several hereditary neuromuscular diseases, The varian ce of writhe was greater for the TRS than for random B-DNA, For random B-DNA, (CGG)(n), and (CTG)(n), the contribution of writhe to supercoi ling was 70, 78, and 79%, whereas the free energy of supercoiling at a length of 10 kilobase pairs was 1040.RT, 760.RT, and 685.RT, respecti vely, These data indicate that the TRS are preferential sites for the partitioning of supercoiling, Calculations of the differences in free energy of supercoiling between the TRS and random B-DNA revealed a loc al minimum at similar to 520 base pairs, Human medical genetic studies have shown that individuals carrying up to 180-200 copies of TRS (540 -600 base pairs, premutations) in the fragile X or myotonic dystrophy gene loci are usually asymptomatic, whereas large expansions (>200 rep eats, full mutations), which lead to disease, are observed in their of fspring, Therefore, the length corresponding to the local minimum in f ree energy of supercoiling correlates with the genetic breakpoint betw een premutation and full mutation, We propose that (a) TRS instability is mediated by DNA mispairing caused by the accumulation of supercoil ing within the repeats, and (b) the expansions that take place at the premutation to full mutation threshold are associated with increased m ispairing caused by the optimal partitioning of writhe within the TRS at this length.