19p13.2

ADCADN,AIM,CXXC9,DNMT,HSN1E,MCMT,m.HsaI

Intestinal adenomas

Human cancers generally show global DNA hypomethylation accompanied by region-specific hypermethylation. A reduction of DNMT1 activity in ApcMin mice due to heterozygosity of the Dnmt1 gene, in conjunction with treatment using the DNA methyltransferase inhibitor 5-aza-deoxycytidine, reduced the average number of intestinal adenomas (Laird PW et al., 1995). On the other hand, genomic hypomethylation in Nf1+/  p53+/  (NPcis) mice due to introduction of a hypomorphic allele of Dnmt 1 (Dnmt1Chip/ ) induced sarcomas at an earlier age compared with NPcis littermates possessing normal levels of DNA methylation (Dnmt1Chip/+) (Eden et al., 2003). The loss of heterozygosity (LOH) rate was increased in hypomethylated cells in Dnmt1Chip/- mice. One of the proposed roles of DNA methylation is suppression of transposable elements in mammalian cells, and chromosomal instability events accompanied by activation of endogenous retroviral elements was also observed in Dnmt1Chip/- mice (Howard et al., 2008).

Precancerous conditions and cancers associated with chronic inflammation and/or persistent infection with viruses

Examination of clinical tissue samples showed that overexpression of DNMT1 is frequent in precancerous conditions and cancers associated with chronic inflammation and/or persistent infection with viruses or other pathogenic microorganisms, such as hepatitis B or C viruses, Epstein-Barr virus (EBV) and human papillomavirus (HPV). DNMT1 mRNA levels were significantly higher even in non-cancerous liver tissues showing chronic hepatitis or cirrhosis, which are considered to be precancerous conditions of HCC, than in normal liver tissues, and were even higher in HCCs. EBV infection in stomach cancers is significantly associated with marked accumulation of DNA hypermethylation in C-type CpG islands that are usually methylated in a cancer-specific (not age-dependent) manner. Induction of latent membrane protein 1 of EBV has been reported to induce DNMT1 overexpression in cultured cancer cells. Cervical intraepithelial neoplasia (CIN) is a precursor lesion for squamous cell carcinoma of the uterine cervix closely associated with HPV infection. DNMT1 protein expression is increased even in low-grade CINs compared to normal squamous epithelium, and further increased in higher-grade CINs and squamous cell carcinomas of the uterine cevix. HPV-16 E7 protein has been reported to associate directly with DNMT1 and stimulate the methyltransferase activity of DNMT1 in vitro.

Urinary bladder cancers

Even non-cancerous urothelia showing no remarkable histological features obtained from patients with urinary bladder cancers can be considered precancerous, because they may be exposed to carcinogens in the urine. It has been debatable whether increased DNMT1 expression in cancers is due to an increase in the proportion of dividing cells or to an acute increase of DNMT1 expression per individual cancer cell. However, the incidence of nuclear DNMT1 immunoreactivity had already increased independently of cell proliferative activity in non-cancerous urothelia showing no marked histological features obtained from patients with urinary bladder cancers, where the PCNA labeling index had not yet increased, compared to that in normal urothelia obtained from patients without urinary bladder cancers. DNMT1 overexpression was not a secondary effect of increased cell proliferative activity but preceded increased cell proliferative activity during multistage urothelial carcinogenesis.

Ductal carcinomas of the pancreas

Ductal carcinomas frequently emerge in pancreases damaged by chronic pancreatitis. Therefore, at least a proportion of peripheral pancreatic duct epithelia with an inflammatory background may be at the precancerous stage. The incidence of nuclear DNMT1 immunoreactivity was significantly elevated in peripheral pancreatic ductal epithelia with an inflammatory background and pancreatic intraepithelial neoplasia (PanIN) as a precancerous lesion than in peripheral pancreatic ductal epithelia without an inflammatory background. The incidence of nuclear DNMT1 immunoreactivity was significantly associated with the degree of PanIN dysplasia. The incidence of nuclear DNMT1 immunoreactivity was significantly higher in invasive ductal carcinomas of the pancreas than in PanINs. The average number of methylated tumor-related genes in microdissected specimens of ductal carcinomas of the pancreas was significantly correlated with the expression level of DNMT1 protein examined immunohistochemically in the precisely microdissected areas. CpG island methylator phenotype (CIMP) is defined as frequent DNA hypermethylation of C-type CpG islands. The levels of DNMT1 expression were significantly higher in CIMP-positive colorectal and stomach cancers than in CIMP-negative colorectal and stomach cancers, but no such association was observed for the expression of DNMT2, DNMT3a or DNMT3b. Thus DNMT1 may be responsible for de novo methylation of CpG islands during multistage carcinogenesis. A theoretical explanation for the role of DNMT1 in de novo DNA methylation in human cancers with dysfunction of p21WAF1, which competes with DNMT1 for binding with PCNA, has been proposed. Moreover, it has recently been suggested that DNMT1 is capable of de novo DNA methylation activity in vivo as well as having a maintenance function: de novo methylation of CpG islands has actually been observed in human fibroblasts overexpressing DNMT1. Therefore, it is feasible that, in cancers, DNMT1 overexpression participates in regional DNA hypermethylation.

The incidence of increased DNMT1 protein expression in HCCs is significantly correlated with poorer tumor differentiation and portal vein involvement. Moreover, the recurrence-free and overall survival rates of patients with HCCs showing increased DNMT1 protein expression are significantly lower than those of patients with HCCs that do not. Increased DNMT1 protein expression in ductal carcinomas of the pancreas is significantly correlated with the extent of cancer invasion to the anterior pancreatic capsule, retroperitoneal tissue and other surrounding organs, and with advanced stage, suggesting that DNMT1 overexpression is associated with aggressiveness of pancreatic cancers. Moreover, patients with ductal carcinomas of the pancreas showing increased DNMT1 protein expression have a poorer prognosis.