Rolfsmeier M L, Dixon M J, Pessoa-Brandão L, Pelletier R, Miret J J, Lahue R S
Eppley Institute for Research in Cancer and Allied Diseases, University of Nebraska Medical Center, Omaha, Nebraska 68198-6805, USA.
Genetics. 2001 Apr;157(4):1569-79. doi: 10.1093/genetics/157.4.1569.
Trinucleotide repeat (TNR) instability in humans is governed by unique cis-elements. One element is a threshold, or minimal repeat length, conferring frequent mutations. Since thresholds have not been directly demonstrated in model systems, their molecular nature remains uncertain. Another element is sequence specificity. Unstable TNR sequences are almost always CNG, whose hairpin-forming ability is thought to promote instability by inhibiting DNA repair. To understand these cis-elements further, TNR expansions and contractions were monitored by yeast genetic assays. A threshold of approximately 15--17 repeats was observed for CTG expansions and contractions, indicating that thresholds function in organisms besides humans. Mutants lacking the flap endonuclease Rad27p showed little change in the expansion threshold, suggesting that this element is not altered by the presence or absence of flap processing. CNG or GNC sequences yielded frequent mutations, whereas A-T rich sequences were substantially more stable. This sequence analysis further supports a hairpin-mediated mechanism of TNR instability. Expansions and contractions occurred at comparable rates for CTG tract lengths between 15 and 25 repeats, indicating that expansions can comprise a significant fraction of mutations in yeast. These results indicate that several unique cis-elements of human TNR instability are functional in yeast.
人类中的三核苷酸重复序列（TNR）不稳定性受独特的顺式元件调控。其中一个元件是阈值，即最小重复长度，它会导致频繁突变。由于阈值尚未在模型系统中得到直接验证，其分子本质仍不确定。另一个元件是序列特异性。不稳定的TNR序列几乎总是CNG，其形成发夹的能力被认为通过抑制DNA修复来促进不稳定性。为了进一步了解这些顺式元件，通过酵母遗传分析监测了TNR的扩增和收缩。观察到CTG扩增和收缩的阈值约为15 - 17个重复序列，这表明阈值在除人类之外的生物体中也起作用。缺乏瓣状核酸内切酶Rad27p的突变体在扩增阈值上变化不大，这表明该元件不会因瓣状加工的存在与否而改变。CNG或GNC序列会频繁发生突变，而富含A - T的序列则稳定得多。这种序列分析进一步支持了TNR不稳定性的发夹介导机制。对于15至25个重复序列的CTG片段长度，扩增和收缩以相当的速率发生，这表明扩增在酵母突变中可能占很大比例。这些结果表明，人类TNR不稳定性的几个独特顺式元件在酵母中具有功能。
