The GAA triplet-repeat sequence in Friedreich ataxia shows a high level of somatic instability in vivo, with a significant predilection for large contractions
The GAA triplet-repeat sequence in Friedreich ataxia shows a high level of somatic instability in vivo, with a significant predilection for large contractions
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DOI：
10.1093/hmg/11.18.2175
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发表时间：
2002-09-01
影响因子：
3.5
通讯作者：
Bidichandani, SI
中科院分区：
文献类型：
--
作者：
Sharma, R;Bhatti, S;Bidichandani, SI
Friedreich共济失调通常是由FRDA基因第一内含子中的GAA三联体重复序列(GAA-TR)的大量扩张引起的。我们使用小池聚合酶链式反应分析了携带12个不同扩展等位基因的受试者外周血DNA中7190个FRDA分子的体细胞变异性，这些等位基因的大小从241到1105个三胞胎不等。扩展的等位基因显示出体细胞变异性随长度的增加，突变载量从47%到78%不等。我们注意到在长等位基因(>；500三联体)中存在强烈的收缩偏向，这表明大收缩的频率是扩张的4倍。一些收缩是非常大的；在所有评分的体细胞突变中，类似于5%涉及原始等位基因长度的50%的收缩，0.29%涉及完全回复到正常/前突变长度(小于或等于60个三胞体)。这些观察结果与强直性肌营养不良的CTG三联体扩张型重复序列的强烈扩张偏向形成鲜明对比。从15个正常等位基因(8-25个三联体)分析的>；6000个个体FRDA分子中没有检测到体细胞变异。发现一个有44个不间断GAA重复的前突变等位基因是不稳定的，大小从6到113个三联体，从而建立了26到44个GAA三联体之间的体细胞不稳定阈值。对携带9个扩展等位基因(132-933个三联体)的连续传代的淋巴母细胞系的另外7850个FRDA分子的分析显示，突变载量非常低，从0%到6.2%不等。我们的数据表明，Friedreich共济失调患者的GAA-TR等位基因高度可变，在体内具有自然收缩的趋势，这些特性取决于多种因素，包括DNA序列、三重重复长度和未知的细胞类型特定因素。
Friedreich ataxia is commonly caused by large expansions of a GAA triplet-repeat (GAA-TR) sequence in the first intron of the FRDA gene. We used small-pool PCR to analyze somatic variability among 7190 individual FRDA molecules from peripheral blood DNA of subjects carrying 12 different expanded alleles, ranging in size from 241 to 1105 triplets. Expanded alleles showed a length-dependent increase in somatic variability, with mutation loads ranging from 47% to 78%. We noted a strong contraction bias among long alleles (>500 triplets), which showed a 4-fold higher frequency of large contractions versus expansions. Some contractions were very large; of all somatic mutations scored, similar to5% involved contractions of >50% of the original allele length, and 0.29% involved complete reversion to the normal/premutation length (less than or equal to60 triplets). These observations contrast sharply with the strong expansion bias seen in expanded CTG triplet repeats in myotonic dystrophy. No somatic variability was detected in >6000 individual FRDA molecules analyzed from 15 normal alleles (8-25 triplets). A premutation allele with 44 uninterrupted GAA repeats was found to be unstable, ranging in size from 6 to 113 triplets, thus establishing the threshold for somatic instability between 26 and 44 GAA triplets. Analysis of an additional 7850 FRDA molecules from serially passaged lymphoblastoid cell lines carrying nine expanded alleles (132-933 triplets) showed very low mutation loads, ranging from 0% to 6.2%. Our data indicate that expanded GAA-TR alleles in Friedreich ataxia are highly mutable and have a natural tendency to contract in vivo, and that these properties depend on multiple factors, including DNA sequence, triplet-repeat length and unknown cell-type-specific factors.