Abdel-Fattah Rana, Glick Adam, Rehman Ishtiaq, Maiberger Patrick, Hennings Henry
Laboratory of Cellular Carcinogenesis and Tumor Promotion, National Cancer Institute, NIH, Bethesda, MD 20892, USA.
Int J Cancer. 2006 Feb 1;118(3):527-31. doi: 10.1002/ijc.21316.
Hypermethylation of CpG sites within the promoter region of the O6-methylguanine-DNA methyltransferase (MGMT) gene occurs frequently in human cancer, preventing both MGMT expression and repair of alkylation damage. To assess the role of MGMT in the development of mouse skin tumors induced by initiation-promotion protocols, methylation of the MGMT promoter was examined in tumor DNA using methylation-specific PCR. To determine whether MGMT promoter methylation was affected by the tumor induction protocol, tumors were initiated by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or 7,12-dimethylbenz[a]anthracene (DMBA) and promoted by 12-O-tetradecanoylphorbol-13-acetate (TPA) or mezerein. Although the MGMT promoter was not methylated in normal skin, promoter methylation was found in 56 of 136 papillomas (41.2%) and in 19 of 37 squamous cell carcinomas (51.4%). When methylation of the MGMT promoter was compared in the 4 treatment groups, hypermethylation was found more frequently in tumors initiated by DMBA and promoted by mezerein, a protocol associated with a high frequency of malignant conversion. Methylation was found in some tumors as early as 5 weeks after initiation, but the methylation frequency increased with time. MGMT promoter methylation reduced MGMT expression as determined by immunohistochemistry. Although MGMT promoter methylation was not generally correlated with ras mutations, the frequency of MGMT methylation was higher in MNNG-initiated, mezerein-promoted papillomas with mutations in Ha-ras compared to papillomas with Ki-ras. Methylation of the MGMT promoter, associated with reduced MGMT expression, is found in nearly half of mouse skin tumors, but varies with both the tumor initiator and tumor promoter, and may be a key step in the progression from papillomas to carcinomas.
O6-甲基鸟嘌呤-DNA甲基转移酶（MGMT）基因启动子区域内的CpG位点高甲基化在人类癌症中频繁发生，可阻止MGMT表达及烷基化损伤的修复。为评估MGMT在启动-促癌方案诱导的小鼠皮肤肿瘤发生过程中的作用，利用甲基化特异性PCR检测肿瘤DNA中MGMT启动子的甲基化情况。为确定MGMT启动子甲基化是否受肿瘤诱导方案的影响，分别用N-甲基-N'-硝基-N-亚硝基胍（MNNG）或7,12-二甲基苯并[a]蒽（DMBA）启动肿瘤，并分别用12-O-十四酰佛波醇-13-乙酸酯（TPA）或芫花酯素进行促癌。尽管正常皮肤中MGMT启动子未发生甲基化，但在136个乳头状瘤中的56个（41.2%）和37个鳞状细胞癌中的19个（51.4%）发现了启动子甲基化。比较4个治疗组中MGMT启动子的甲基化情况时发现，在由DMBA启动并由芫花酯素促癌的肿瘤中，高甲基化更为常见，该方案与高频率的恶性转化相关。在启动后5周时，一些肿瘤中就发现了甲基化，但甲基化频率随时间增加。免疫组化结果显示，MGMT启动子甲基化降低了MGMT的表达。尽管MGMT启动子甲基化通常与ras突变无关，但与携带Ki-ras的乳头状瘤相比，在携带Ha-ras突变的MNNG启动、芫花酯素促癌的乳头状瘤中，MGMT甲基化频率更高。在近一半的小鼠皮肤肿瘤中发现了与MGMT表达降低相关的MGMT启动子甲基化，但其随肿瘤启动剂和促癌剂的不同而变化，可能是从乳头状瘤发展为癌的关键步骤。
