Cannella Milena, Maglione Vittorio, Martino Tiziana, Ragona Giuseppe, Frati Luigi, Li Guo-Min, Squitieri Ferdinando
Neurogenetics Unit, IRCCS Neuromed, Pozzilli, IS, Italy.
BMC Med Genet. 2009 Feb 11;10:11. doi: 10.1186/1471-2350-10-11.
The expanded CAG repeat in the Huntington's disease (HD) gene may display tissue-specific variability (e.g. triplet mosaicism) in repeat length, the longest mutations involving mitotic (germ and glial cells) and postmitotic (neurons) cells. What contributes to the triplet mutability underlying the development of HD nevertheless remains unknown. We investigated whether, besides the increased DNA instability documented in postmitotic neurons, possible environmental and genetic mechanisms, related to cell replication, may concur to determine CAG repeat mutability. To test this hypothesis we used, as a model, cultured HD patients' lymphoblasts with various CAG repeat lengths.
Although most lymphoblastoid cell lines (88%) showed little or no repeat instability even after six or more months culture, in lymphoblasts with large expansion repeats beyond 60 CAG repeats the mutation size and triplet mosaicism always increased during replication, implying that the repeat mutability for highly expanded mutations may quantitatively depend on the triplet expansion size. None of the investigated genetic factors, potentially acting in cis to the mutation, significantly influence the repeat changes. Finally, in our experiments certain drugs controlled triplet expansion in two prone-to-expand HD cell lines carrying large CAG mutations.
Our data support quantitative evidence that the inherited CAG length of expanded alleles has a major influence on somatic repeat variation. The longest triplet expansions show wide somatic variations and may offer a mechanistic model to study triplet drug-controlled instability and genetic factors influencing it.
亨廷顿舞蹈症（HD）基因中扩展的CAG重复序列在重复长度上可能表现出组织特异性变异性（例如三联体镶嵌现象），其中最长的突变涉及有丝分裂细胞（生殖细胞和神经胶质细胞）和有丝分裂后细胞（神经元）。然而，导致HD发生的三联体变异性的原因仍然未知。我们研究了除了有丝分裂后神经元中记录到的DNA不稳定性增加之外，与细胞复制相关的可能的环境和遗传机制是否可能共同决定CAG重复序列的变异性。为了验证这一假设，我们使用具有不同CAG重复长度的HD患者培养淋巴细胞作为模型。
尽管大多数淋巴母细胞系（88%）即使在培养六个月或更长时间后也几乎没有或没有重复序列不稳定性，但在CAG重复序列超过60次的大扩展重复的淋巴细胞中，突变大小和三联体镶嵌现象在复制过程中总是增加，这意味着高度扩展突变的重复序列变异性可能在数量上取决于三联体扩展大小。所研究的任何可能顺式作用于突变的遗传因素都没有显著影响重复序列的变化。最后，在我们的实验中，某些药物控制了携带大CAG突变的两个易于扩展的HD细胞系中的三联体扩展。
我们的数据支持定量证据，即扩展等位基因的遗传CAG长度对体细胞重复序列变异有重大影响。最长的三联体扩展显示出广泛的体细胞变异，并可能为研究三联体药物控制的不稳定性及其影响因素提供一个机制模型。
