Bidichandani S I, Purandare S M, Taylor E E, Gumin G, Machkhas H, Harati Y, Gibbs R A, Ashizawa T, Patel P I
Department of Neurology, Baylor College of Medicine, TX 77030, USA.
Hum Mol Genet. 1999 Dec;8(13):2425-36. doi: 10.1093/hmg/8.13.2425.
The vast majority of Friedreich ataxia patients are homozygous for large GAA triplet repeat expansions in intron 1 of the X25 gene. Instability of the expanded GAA repeat was examined in 23 chromosomes bearing 97-1250 triplets in lymphoblastoid cell lines passaged 20-39 times. Southern analyses revealed 18 events of significant changes in length ranging from 69 to 633 triplets, wherein the de novo allele gradually replaced the original over 1-6 passages. Contractions and expansions occurred with equal frequency and magnitude. This behavior is unique in comparison with other large, non-coding triplet repeat expansions [(CGG)(n)and (CTG)(n)] which remain relatively stable under similar conditions. We also report a rare patient who, having inherited two expanded alleles, showed evidence of contracted GAA repeats ranging from nine to 29 triplets in DNA from two independent peripheral blood samples. The GAA triplet repeat is known to adopt a triplex structure, and triplexes in transcribed templates cause enhanced mutagenesis. The poly(A) tract and a 135 bp sequence, both situated immediately upstream of the GAA triplet repeat, were therefore examined for somatic mutations. The poly(A) tract showed enhanced instability when in cis with the GAA expansion. The 135 bp upstream sequence was found to harbor a 3-fold excess of point mutations in DNA derived from individuals homozygous for the GAA triplet repeat expansion compared with normal controls. These data are likely to have important mechanistic and clinical implications.
绝大多数弗里德赖希共济失调患者在X25基因第1内含子中存在大的GAA三联体重复序列扩增，且为纯合子。在传代20 - 39次的淋巴母细胞系中，对23条含有97 - 1250个三联体的染色体上扩增的GAA重复序列的不稳定性进行了检测。Southern分析显示有18个长度发生显著变化的事件，变化范围为69至633个三联体，其中新生等位基因在1 - 6次传代过程中逐渐取代了原始等位基因。收缩和扩增出现的频率和幅度相同。与其他大型非编码三联体重复序列扩增（(CGG)(n)和(CTG)(n)）相比，这种行为是独特的，后者在类似条件下保持相对稳定。我们还报告了1例罕见患者，该患者继承了两个扩增的等位基因，在两份独立的外周血样本的DNA中显示出GAA重复序列收缩的证据，收缩范围为9至29个三联体。已知GAA三联体重复序列会形成三链结构，转录模板中的三链体会导致诱变增强。因此，对紧邻GAA三联体重复序列上游的聚腺苷酸序列和一个135 bp的序列进行了体细胞突变检测。当聚腺苷酸序列与GAA扩增处于顺式时，显示出增强的不稳定性。与正常对照相比，在GAA三联体重复序列扩增纯合子个体的DNA中，发现135 bp上游序列的点突变多出3倍。这些数据可能具有重要的机制和临床意义。
