Complete inactivation of DNMT1 leads to mitotic catastrophe in human cancer cells.

Studies have shown that DNA (cytosine-5-)-methyltransferase 1 (DNMT1) is the principal enzyme responsible for maintaining CpG methylation and is required for embryonic development and survival of somatic cells in mice. The role of DNMT1 in human cancer cells, however, remains highly controversial. Using homologous recombination, here we have generated a DNMT1 conditional allele in the human colorectal carcinoma cell line HCT116 in which several exons encoding the catalytic domain are flanked by loxP sites. Cre recombinase-mediated disruption of this allele results in hemimethylation of approximately 20% of CpG-CpG dyads in the genome, coupled with activation of the G2/M checkpoint, leading to arrest in the G2 phase of the cell cycle. Although cells gradually escape from this arrest, they show severe mitotic defects and undergo cell death either during mitosis or after arresting in a tetraploid G1 state. Our results thus show that DNMT1 is required for faithfully maintaining DNA methylation patterns in human cancer cells and is essential for their proliferation and survival.