Department of Medical Genetics, Genome-Scale Biology Research Program, Biomedicum Helsinki, University of Helsinki, Finland.
Carcinogenesis. 2011 Mar;32(3):305-11. doi: 10.1093/carcin/bgq272. Epub 2010 Dec 16.
Frameshift mutations frequently accumulate in microsatellite-unstable colorectal cancers (MSI CRCs) typically leading to downregulation of the target genes due to nonsense-mediated messenger RNA decay. However, frameshift mutations that occur in the 3' end of the coding regions can escape decay, which has largely been ignored in previous works. In this study, we characterized nonsense-mediated decay-escaping frameshift mutations in MSI CRC in an unbiased, genome wide manner. Combining bioinformatic search with expression profiling, we identified genes that were predicted to escape decay after a deletion in a microsatellite repeat. These repeats, located in 258 genes, were initially sequenced in 30 MSI CRC samples. The mitotic checkpoint kinase TTK was found to harbor decay-escaping heterozygous mutations in exon 22 in 59% (105/179) of MSI CRCs, which is notably more than previously reported. Additional novel deletions were found in exon 5, raising the mutation frequency to 66%. The exon 22 of TTK contains an A(9)-G(4)-A(7) locus, in which the most common mutation was a mononucleotide deletion in the A(9) (c.2560delA). When compared with identical non-coding repeats, TTK was found to be mutated significantly more often than expected without selective advantage. Since TTK inhibition is known to induce override of the mitotic spindle assembly checkpoint (SAC), we challenged mutated cancer cells with the microtubule-stabilizing drug paclitaxel. No evidence of checkpoint weakening was observed. As a conclusion, heterozygous TTK mutations occur at a high frequency in MSI CRCs. Unexpectedly, the plausible selective advantage in tumourigenesis does not appear to be related to SAC.
移码突变经常在微卫星不稳定的结直肠癌（MSI CRC）中积累，通常导致靶基因的下调，因为无意义介导的信使 RNA 衰变。然而，发生在编码区 3'端的移码突变可以逃避衰变，这在以前的工作中很大程度上被忽视了。在这项研究中，我们以无偏倚的、全基因组的方式描述了 MSI CRC 中无意义介导的逃避衰变的移码突变。通过生物信息学搜索与表达谱相结合，我们鉴定了在微卫星重复缺失后预测逃避衰变的基因。这些重复位于 258 个基因中，最初在 30 个 MSI CRC 样本中进行了测序。有丝分裂检查点激酶 TTK 在 59%（105/179）的 MSI CRC 中，在第 22 外显子中含有杂合性突变，这明显高于以前的报道。在第 5 外显子中发现了额外的新缺失，使突变频率增加到 66%。TTK 的第 22 外显子含有一个 A(9)-G(4)-A(7) 基因座，其中最常见的突变是 A(9)（c.2560delA）的单核苷酸缺失。与相同的非编码重复相比，TTK 的突变频率明显高于预期而没有选择优势。由于 TTK 抑制已知会诱导有丝分裂纺锤体检查点（SAC）的失控，我们用微管稳定药物紫杉醇挑战突变的癌细胞。没有观察到检查点减弱的证据。总之，杂合 TTK 突变在 MSI CRC 中高频发生。出乎意料的是，在肿瘤发生中可能的选择优势似乎与 SAC 无关。
