Investigation of colorectal carcinogenesis for genetic diagnosis

越来越多的证据表明，许多类型的癌变不仅涉及遗传改变，而且涉及表观遗传改变。我们利用微卫星、突变和甲基化分析研究了结直肠癌发生的分子机制。最近，我们发现BRAF突变经常出现在增长性息肉（HP）的一个亚群中，称为无根锯齿状腺瘤（SSA），以及散发性结直肠癌（CRC）中，表现出高水平的微卫星不稳定性（MSI-H），两者都与CpG岛甲基化表型（CIMP）相关。我们首次报道了散发性MSI-H CRC可能起源于SSA而不是腺瘤，并且BRAF突变和DNA甲基化是“锯齿状肿瘤通路”的早期事件，与公认的“腺瘤-癌-序列”不同[Gut 53(8): 1137- 1144,2004]。此外，我们已经报道了启动子高甲基化和BRAF突变将遗传性非息肉病性结肠癌与散发性MSI-H型CRC区分开来[Familiar C…More cancer 3(2): 101-107, 2004]。根据WHO标准，结直肠息肉分为HP、混合型息肉、锯齿状腺瘤、绒毛状腺瘤和常规腺瘤。我们将HP分为微泡型和杯状细胞型HP， SAs分为传统的SA和被称为“SSA”的变异HP，筛选这些前体病变和crc，检测致癌基因KRAS和BRAF的激活突变，肿瘤抑制基因APC的失活突变以及CpG岛的甲基化状态。与公认的多步骤突变积累模型相反，我们发现很少有CRC包含KRAS， BRAF和APC突变，几乎CRC只有其中一个基因突变。最常见的突变组合是APC和KRAS(11.2%)，而APC和BRAF/KRAS的突变极为罕见（1.1%/0%）。在结肠息肉中也观察到类似的突变模式。另一方面，我们证实异常DNA甲基化仅是老年MSI-H癌患者的一种疾病，但与所有结直肠癌亚群相关。这些结果表明，CRC存在多种遗传通路，APC和CIMP阴性突变是腺瘤-癌序列的早期事件，而KRAS/BRAF和CIMP阳性突变倾向于锯齿状肿瘤通路。少

There is increasing evidence that not only genetic but also epigenetic alterations are involved in many types of carcinogenesis. We have been studied molecular mechanisms of colorectal carcinogenesis using microsatellite, mutation and methylation analysis. Recently, we have found that the BRAF mutation was frequently seen in a subset of hyperplastic polyp (HP), termed as sessile serrated adenoma (SSA) and in sporadic colorectal cancer (CRC) showing high level microsatellite instability (MSI-H), both of which were associated with CpG island methylator phenotype (CIMP). We have first reported that sporadic MSI-H CRC may originate in SSA and not adenoma, and BRAF mutation and DNA methylation are early event in this "serrated neoplastic pathway" distinct from the accepted "adenoma-carcinoma-sequence" [Gut 53(8): 1137-1144, 2004]. Moreover, we have reported that promoter hypermethylation and BRAF mutations distinguish hereditary non-polyposis colon cancer from sporadic MSI-H CRC [Familiar C … More ancer 3(2): 101-107, 2004].The colorectal polyps are classified into HP, mixed polyp, serrated adenoma (SA), villous adenoma and conventional adenoma using WHO criteria. Classifying HPs into microvesicular and goblet cell-type HP, and SAs into traditional SA and variant HP described as "SSA", we screened these precursor lesions and CRCs for activating mutation of oncogene KRAS and BRAF, inactivating mutation of tumor suppressor gene APC, and methylation status of CpG island. In contrast to well accepted multi-step model of mutation accumulation, we found that few CRC contained mutations in KRAS, BRAF and APC and almost CRC had mutation in only one of these genes. The most common combination of mutations was APC and KRAS (11.2%), whereas mutations in both APC and BRAF/KRAS were extremely rare (1.1%/0%). Similar mutational pattern was also observed in colon polyps. On the other hand, we confirmed that aberrant DNA methylation is only a disease of older individuals with MSI-H cancer but in associated with all subsets of colorectal cancers. These results suggest that multiple genetic pathways to CRC exist and that mutation in APC and CIMP negative was an early event in adenoma-carcinoma sequence, while mutation in KRAS/BRAF and CIMP-positive predispose to serrated neoplastic pathway. Less