Gellibolian R, Bacolla A, Wells R D
Center for Genome Research, Institute of Biosciences and Technology, Texas A & M University, Texas Medical Center, 2121 Holcombe Blvd., Houston, Texas 77030-3303, USA.
J Biol Chem. 1997 Jul 4;272(27):16793-7. doi: 10.1074/jbc.272.27.16793.
The variance of writhe, the contribution of writhe to supercoiling, and the free energies of supercoiling were calculated for (CTG.CAG)n and (CGG.CCG)n triplet repeat sequences (TRS) by statistical mechanics from the bending and torsional moduli previously determined. Expansions of these sequences are inherited by non-mendelian transmission and are linked with several hereditary neuromuscular diseases. The variance 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 supercoiling 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, respectively. 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 local minimum at approximately 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 repeats, full mutations), which lead to disease, are observed in their offspring. Therefore, the length corresponding to the local minimum in free energy of supercoiling correlates with the genetic breakpoint between premutation and full mutation. We propose that (a) TRS instability is mediated by DNA mispairing caused by the accumulation of supercoiling within the repeats, and (b) the expansions that take place at the premutation to full mutation threshold are associated with increased mispairing caused by the optimal partitioning of writhe within the TRS at this length.
通过统计力学，利用先前确定的弯曲和扭转模量，计算了（CTG.CAG）n和（CGG.CCG）n三联体重复序列（TRS）的扭曲方差、扭曲对超螺旋的贡献以及超螺旋的自由能。这些序列的扩增通过非孟德尔遗传传递，并与几种遗传性神经肌肉疾病相关。TRS的扭曲方差比随机B-DNA的大。对于随机B-DNA、（CGG）n和（CTG）n，扭曲对超螺旋的贡献分别为70%、78%和79%，而在10千碱基对长度时超螺旋的自由能分别为1040.RT、760.RT和685.RT。这些数据表明，TRS是超螺旋分配的优先位点。TRS与随机B-DNA之间超螺旋自由能差异的计算显示，在大约520个碱基对处有一个局部最小值。人类医学遗传学研究表明，在脆性X或强直性肌营养不良基因位点携带多达180 - 200个TRS拷贝（540 - 600个碱基对，前突变）的个体通常无症状，而在其后代中观察到导致疾病的大扩增（>200个重复，完全突变）。因此，超螺旋自由能局部最小值对应的长度与前突变和完全突变之间的遗传断点相关。我们提出：（a）TRS的不稳定性是由重复序列内超螺旋积累导致的DNA错配介导的；（b）在前突变到完全突变阈值处发生的扩增与在该长度下TRS内扭曲的最佳分配导致的错配增加有关。
