Fleisher A S, Esteller M, Wang S, Tamura G, Suzuki H, Yin J, Zou T T, Abraham J M, Kong D, Smolinski K N, Shi Y Q, Rhyu M G, Powell S M, James S P, Wilson K T, Herman J G, Meltzer S J
Department of Medicine, University of Maryland School of Medicine, Baltimore Veterans Affairs Hospital, 21201, USA.
Cancer Res. 1999 Mar 1;59(5):1090-5.
Human gastric carcinoma shows a higher prevalence of microsatellite instability (MSI) than does any other type of sporadic human cancer. The reasons for this high frequency of MSI are not yet known. In contrast to endometrial and colorectal carcinoma, mutations of the DNA mismatch repair (MMR) genes hMLH1 or hMSH2 have not been described in gastric carcinoma. However, hypermethylation of the hMLH1 MMR gene promoter is quite common in MSI-positive endometrial and colorectal cancers. This hypermethylation has been associated with hMLH1 transcriptional blockade, which is reversible with demethylation, suggesting that an epigenetic mechanism underlies hMLH1 gene inactivation and MMR deficiency. Therefore, we studied the prevalence of hMLH1 promoter hypermethylation in a total of 65 gastric tumors: 18 with frequent MSI (MSI-H), 8 with infrequent MSI (MSI-L), and 39 that were MSI negative. We found a striking association between hMLH1 promoter hypermethylation and MSI; of 18 MSI-H tumors, 14 (77.8%) showed hypermethylation, whereas 6 of 8 MSI-L tumors (75%) were hypermethylated at hMLH1. In contrast, only 1 of 39 (2.6%) MSI-negative tumors demonstrated hMLH1 hypermethylation (P<0.0001 for MSI-H or MSI-L versus MSI-negative). Moreover, hypermethylated cancers demonstrated diminished expression of hMLH1 protein by both immunohistochemistry and Western blotting, whereas nonhypermethylated tumors expressed abundant hMLH1 protein. These data indicate that hypermethylation of hMLH1 is strongly associated with MSI in gastric cancers and suggest an epigenetic mechanism by which defective MMR occurs in this group of cancers.
与其他任何类型的散发性人类癌症相比，人类胃癌的微卫星不稳定性（MSI）患病率更高。MSI高频率出现的原因尚不清楚。与子宫内膜癌和结直肠癌不同，胃癌中尚未发现DNA错配修复（MMR）基因hMLH1或hMSH2的突变。然而，hMLH1 MMR基因启动子的高甲基化在MSI阳性的子宫内膜癌和结直肠癌中相当常见。这种高甲基化与hMLH1转录阻滞有关，而去甲基化可使其逆转，这表明表观遗传机制是hMLH1基因失活和MMR缺陷的基础。因此，我们研究了总共65例胃肿瘤中hMLH1启动子高甲基化的患病率：18例为高频MSI（MSI-H），8例为低频MSI（MSI-L），39例为MSI阴性。我们发现hMLH1启动子高甲基化与MSI之间存在显著关联；在18例MSI-H肿瘤中，14例（77.8%）表现出高甲基化，而8例MSI-L肿瘤中有6例（75%）在hMLH1处发生高甲基化。相比之下，39例MSI阴性肿瘤中只有1例（2.6%）表现出hMLH1高甲基化（MSI-H或MSI-L与MSI阴性相比，P<0.0001）。此外，通过免疫组织化学和蛋白质印迹法，高甲基化癌症显示hMLH1蛋白表达减少，而非高甲基化肿瘤表达丰富的hMLH1蛋白。这些数据表明，hMLH1的高甲基化与胃癌中的MSI密切相关，并提示了这组癌症中发生MMR缺陷的表观遗传机制。
