Hirst M C, White P J
Institute of Molecular Medicine, The John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DS, UK.
Nucleic Acids Res. 1998 May 15;26(10):2353-8. doi: 10.1093/nar/26.10.2353.
The normal human FMR1 gene contains a genetically stable (CGG) n trinucleotide repeat which usually carries interspersed AGG triplets. An increase in repeat number and the loss of interspersions results in array instability, predominantly expansion, leading to FMR1 gene silencing. Instability is directly related to the length of the uninterrupted (CGG) n repeat and is widely assumed to be related to an increased propensity to form G-rich secondary structures which lead to expansion through replication slippage. In order to investigate this we have cloned human FMR1 arrays with internal structures representing the normal, intermediate and unstable states. In one replicative orientation, arrays show a length-dependent instability, deletions occurring in a polar manner. With longer arrays these extend into the FMR1 5'-flanking DNA, terminating at either of two short CGG triplet arrays. The orientation-dependent instability suggests that secondary structure forms in the G-rich lagging strand template, resolution of which results in intra-array deletion. These data provide direct in vivo evidence for a G-rich lagging strand secondary structure which is believed to be involved in the process of triplet expansion in humans.
正常人类FMR1基因包含一个遗传稳定的（CGG）n三核苷酸重复序列，该序列通常散布着AGG三联体。重复序列数量的增加和散布序列的缺失会导致阵列不稳定，主要是扩增，从而导致FMR1基因沉默。不稳定性与不间断的（CGG）n重复序列的长度直接相关，并且普遍认为与形成富含G的二级结构的倾向增加有关，这种二级结构通过复制滑动导致扩增。为了对此进行研究，我们克隆了具有代表正常、中间和不稳定状态内部结构的人类FMR1阵列。在一种复制方向上，阵列显示出长度依赖性不稳定性，缺失以极性方式发生。对于较长的阵列，这些缺失延伸到FMR1 5'侧翼DNA中，在两个短的CGG三联体阵列中的任何一个处终止。方向依赖性不稳定性表明富含G的滞后链模板中形成了二级结构，其解析导致阵列内缺失。这些数据为富含G的滞后链二级结构提供了直接的体内证据，据信该结构参与了人类三联体扩增过程。
