Libby Randell T, Hagerman Katharine A, Pineda Victor V, Lau Rachel, Cho Diane H, Baccam Sandy L, Axford Michelle M, Cleary John D, Moore James M, Sopher Bryce L, Tapscott Stephen J, Filippova Galina N, Pearson Christopher E, La Spada Albert R
Department of Laboratory Medicine, University of Washington Medical Center, Seattle, WA, USA.
PLoS Genet. 2008 Nov;4(11):e1000257. doi: 10.1371/journal.pgen.1000257. Epub 2008 Nov 14.
At least 25 inherited disorders in humans result from microsatellite repeat expansion. Dramatic variation in repeat instability occurs at different disease loci and between different tissues; however, cis-elements and trans-factors regulating the instability process remain undefined. Genomic fragments from the human spinocerebellar ataxia type 7 (SCA7) locus, containing a highly unstable CAG tract, were previously introduced into mice to localize cis-acting "instability elements," and revealed that genomic context is required for repeat instability. The critical instability-inducing region contained binding sites for CTCF -- a regulatory factor implicated in genomic imprinting, chromatin remodeling, and DNA conformation change. To evaluate the role of CTCF in repeat instability, we derived transgenic mice carrying SCA7 genomic fragments with CTCF binding-site mutations. We found that CTCF binding-site mutation promotes triplet repeat instability both in the germ line and in somatic tissues, and that CpG methylation of CTCF binding sites can further destabilize triplet repeat expansions. As CTCF binding sites are associated with a number of highly unstable repeat loci, our findings suggest a novel basis for demarcation and regulation of mutational hot spots and implicate CTCF in the modulation of genetic repeat instability.
人类至少有25种遗传性疾病是由微卫星重复序列扩增引起的。在不同的疾病位点以及不同组织之间，重复序列的不稳定性存在显著差异；然而，调节这种不稳定性过程的顺式元件和反式因子仍不明确。先前已将来自人类7型脊髓小脑共济失调（SCA7）位点的基因组片段（包含高度不稳定的CAG序列）导入小鼠体内，以定位顺式作用的“不稳定元件”，结果表明基因组背景对于重复序列的不稳定性是必需的。关键的不稳定诱导区域包含CTCF的结合位点，CTCF是一种与基因组印记、染色质重塑和DNA构象变化相关的调节因子。为了评估CTCF在重复序列不稳定性中的作用，我们培育了携带具有CTCF结合位点突变的SCA7基因组片段的转基因小鼠。我们发现，CTCF结合位点突变会促进生殖系和体细胞组织中的三联体重复序列不稳定性，并且CTCF结合位点的CpG甲基化会进一步破坏三联体重复序列的扩增稳定性。由于CTCF结合位点与许多高度不稳定的重复序列位点相关，我们的研究结果为突变热点的划分和调控提供了新的依据，并表明CTCF参与了对遗传重复序列不稳定性的调节。
