DNA ( cytosine - 5 ) - methyltransferase 1 - [ Isoform 1 ]

The murine equivalent of the human DNMT1 (DNA [cytosine-5]-methyltransferase 1; Swiss-Prot accession number: P26358) gene was deleted in ES cells via gene targeting [1]. Dnmt1-/cells possessed dramatically decreased genomic methylation and were viable; however, the mutation caused a homozygous lethal phenotype when introduced into the germline (see Table of experimental models for DNMT1) [1]. Cre/loxPmediated deletion of Dnmt1 in mice has given rise to several lines of conditional mutants in the nervous system and the immune system [2,3,4]. Mice carrying a hypomorphic Dnmt1 allele showed reduced DNMT1 expression (10% that of wild-type levels) and substantial genome-wide hypomethylation in all tissues [5]. The hypomorphic mutants developed aggressive T-cell lymphomas, supporting a causal role for DNA hypomethylation in tumor formation [5]. When the Dnmt1 hypomorphic allele was introduced to the ApcMin/+ intestinal murine model, a complete suppression of multiple intestinal metaplasia cancerous polyp formation was observed along with reduced CpG island methylation in the intestine, suggesting that Dnmt1 is a genetic suppressor of intestinal polyp formation [6]. Using the same model, the overall inhibition of intestinal tumorigenesis in hypomethylated ApcMin/+ mice was accompanied by microscopic liver tumors; thus, DNA hypomethylation could suppress late stages of intestinal tumorigenesis, but promote early liver lesions [7]. In the human colon cancer cell line HCT116, genetic deletion of DNMT1 did not lead to loss of genomic methylation or reactivation of tumor suppressor genes [8]; however, it has been recently shown that this model was incorrectly targeted, resulting in a catalytically active truncated protein [10,9]. DNMT1 protein has been knocked down via siRNA or oligonucleotide antisense (MG98, see Function and Localization: In disease) degradation of DNMT1 mRNA in HCT116 cells [11]. The effect of depleting DNMT1 from human cancer cell lines using this approach is the basis for MG98 clinical trials. Moreover, the conditional deletion of DNMT1 leads to mitotic catastrophe and ATM/ATR-mediated cell death of HCT116 cells [12]. In HCT116 cells, the conditional deletion model [12] differs from the siRNA knockdown model [11] in that the siRNA is an incomplete knockdown with a less severe phenotype, while the genetic deletion of DNMT1 leads to cell cycle arrest and cell death.