Hienonen Tuija, Sammalkorpi Heli, Enholm Susa, Alhopuro Pia, Barber Thomas D, Lehtonen Rainer, Nupponen Nina N, Lehtonen Heli, Salovaara Reijo, Mecklin Jukka-Pekka, Järvinen Heikki, Koistinen Riitta, Arango Diego, Launonen Virpi, Vogelstein Bert, Karhu Auli, Aaltonen Lauri A
Department of Medical Genetics, Biomedicum Helsinki, University of Helsinki, Finland.
Cancer Res. 2005 Jun 1;65(11):4607-13. doi: 10.1158/0008-5472.CAN-05-0165.
DNA mismatch repair (MMR)-deficient cells typically accumulate mutations in short repetitive DNA tracts. This microsatellite instability (MSI) facilitates malignant transformation when affecting genes with growth-related and caretaker functions. To date, several putative MSI target genes have been proposed mainly based on high mutation frequency within their coding regions. However, some intronic repeat mutations have also been suggested to associate with MSI tumorigenesis, indicating the need for additional analyses on noncoding repeats. Here we have analyzed an intronic T9 repeat of semenogelin I (SEMG1) and report mutation frequencies of 51% (75 of 146) and 62% (8 of 13) in MMR-deficient primary colorectal cancers and cell lines, respectively. The putative effect of the SEMG1 mutations was assessed by RNA and protein level analyses, but no differences were detected between colorectal cancer cell lines with different SEMG1 status. Subsequently, the general background mutation frequency of MSI colorectal cancers was assessed by screening for intergenic T9 repeat alterations. One of 10 examined repeats was mutated in 70% (102 of 145) of the colorectal cancers evaluated. The frequencies observed here are notably higher than previously published in noncoding repeats shorter than 10 bp in MMR-deficient primary tumors. Our results indicate that high mutation frequencies, similar or higher than those observed in proposed and approved target genes, can be detected in repeat tracts of MSI tumors without any apparent selection pressure. These data call for urgent and thorough large-scale evaluation of mutation frequencies in neutral short repetitive sequences in MMR-deficient tumors.
DNA错配修复（MMR）缺陷的细胞通常会在短重复DNA片段中积累突变。这种微卫星不稳定性（MSI）在影响具有生长相关和维护功能的基因时会促进恶性转化。迄今为止，主要基于其编码区域内的高突变频率提出了几个假定的MSI靶基因。然而，也有一些内含子重复突变被认为与MSI肿瘤发生有关，这表明需要对非编码重复序列进行额外分析。在这里，我们分析了精液凝素I（SEMG1）的一个内含子T9重复序列，并报告在MMR缺陷的原发性结直肠癌和细胞系中，其突变频率分别为51%（146个中有75个）和62%（13个中有8个）。通过RNA和蛋白质水平分析评估了SEMG1突变的假定效应，但在具有不同SEMG1状态的结肠癌细胞系之间未检测到差异。随后，通过筛选基因间T9重复序列改变来评估MSI结直肠癌的一般背景突变频率。在所评估的145例结直肠癌中，10个检测的重复序列中有1个在70%（102个）中发生了突变。这里观察到的频率明显高于先前发表的MMR缺陷原发性肿瘤中短于10 bp的非编码重复序列的频率。我们的结果表明，在MSI肿瘤的重复片段中可以检测到与在已提出和认可的靶基因中观察到的相似或更高的高突变频率，且没有任何明显的选择压力。这些数据呼吁对MMR缺陷肿瘤中中性短重复序列的突变频率进行紧急和全面的大规模评估。
