De Marzo A M, Marchi V L, Yang E S, Veeraswamy R, Lin X, Nelson W G
Department of Pathology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, USA.
Cancer Res. 1999 Aug 15;59(16):3855-60.
Somatic changes in CpG dinucleotide methylation occur quite commonly in human cancer cell DNA. Relative to DNA from normal human colonic cells, DNA from human colorectal cancer cells typically displays regional CpG dinucleotide hypermethylation amid global CpG dinucleotide hypomethylation. The role of the maintenance DNA methyltransferase (DNMT1) in the acquisition of such abnormal CpG dinucleotide methylation changes in colorectal cancer cells remains controversial; in one study, 60-200-fold increases in DNMT1 mRNA expression were detected in colorectal polyps and cancers relative to normal colonic tissue [W. S. El-Deiry et al., Proc. Natl. Acad. Sci. USA, 88: 3470-3474, 1991], whereas in another study, only small increases in DNMT1 mRNA expression, commensurate with differences in cell proliferation accompanying colonic tumorigenesis, were observed [P. J. Lee et al., Proc. Natl. Acad. Sci. USA, 93: 10366-10370, 1996]. To definitively ascertain whether abnormal DNMT1 expression might accompany human colorectal carcinogenesis, we subjected a series of normal and neoplastic colonic tissues to immunohistochemical staining using a polyclonal antiserum raised against a DNMT1 polypeptide. A concordance of DNMT1 expression with the expression of PCNA and other cell proliferation markers, such as Ki-67 and DNA topoisomerase IIalpha, was observed in normal colonic epithelial cells and in cells comprising other normal epithelia and lymphoid tissues. The polypeptide p21, which has been reported to undermine DNMT1 binding to proliferating cell nuclear antigen at DNA replication sites, was not expressed by normal colonic cells containing DNMT1 and other cell proliferation markers. In adenomatous polyps, although DNMT1 expression coincided with the expression of other cell proliferation markers, many DNMT1-expressing cells also expressed p21. The fidelity of DNMT1 expression was further undermined in colorectal carcinomas, in which a striking heterogeneity in DNMT1 expression, with some carcinoma cells containing very high DNMT1 levels and others containing very low DNMT1 levels, was observed. These results indicate that human colorectal carcinogenesis is accompanied by a progressive dysregulation of DNMT1 expression and suggest that abnormalities in DNMT1 expression may contribute to the abnormal CpG dinucleotide methylation changes characteristic of human colorectal carcinoma cell DNA.
CpG二核苷酸甲基化的体细胞变化在人类癌细胞DNA中相当常见。相对于来自正常人类结肠细胞的DNA，来自人类结肠直肠癌细胞的DNA通常在整体CpG二核苷酸低甲基化的背景下表现出区域性CpG二核苷酸高甲基化。维持性DNA甲基转移酶（DNMT1）在结肠直肠癌细胞中获得这种异常的CpG二核苷酸甲基化变化过程中的作用仍存在争议；在一项研究中，相对于正常结肠组织，在结肠息肉和癌症中检测到DNMT1 mRNA表达增加了60 - 200倍[W. S. 埃尔-戴里等人，《美国国家科学院院刊》，88: 3470 - 3474，1991]，而在另一项研究中，仅观察到DNMT1 mRNA表达有小幅增加，这与结肠肿瘤发生过程中伴随的细胞增殖差异相符[P. J. 李等人，《美国国家科学院院刊》，93: 10366 - 10370，1996]。为了明确确定异常的DNMT1表达是否可能伴随人类结肠直肠癌发生，我们使用针对DNMT1多肽产生的多克隆抗血清，对一系列正常和肿瘤性结肠组织进行免疫组织化学染色。在正常结肠上皮细胞以及构成其他正常上皮和淋巴组织的细胞中，观察到DNMT1表达与增殖细胞核抗原（PCNA）以及其他细胞增殖标志物（如Ki - 67和DNA拓扑异构酶IIα）的表达一致。据报道，多肽p21会破坏DNMT1在DNA复制位点与增殖细胞核抗原的结合，含有DNMT1和其他细胞增殖标志物的正常结肠细胞不表达p21。在腺瘤性息肉中，虽然DNMT1表达与其他细胞增殖标志物的表达一致，但许多表达DNMT1的细胞也表达p21。在结肠直肠癌中，DNMT1表达的稳定性进一步受到破坏，在这些肿瘤中观察到DNMT1表达存在显著异质性，一些癌细胞含有非常高的DNMT1水平，而另一些癌细胞含有非常低的DNMT1水平。这些结果表明，人类结肠直肠癌发生伴随着DNMT1表达的逐渐失调，并提示DNMT1表达异常可能导致人类结肠直肠癌细胞DNA特有的异常CpG二核苷酸甲基化变化。
