CpG methylation is maintained in human cancer cells lacking DNMT1

Hypermethylation is associated with the silencing of tumour susceptibility genes in several forms of cancer(1,2); however, the mechanisms responsible for this aberrant methylation are poorly understood(3,4). The prototypic DN A methyltransferase, DNMT1, has been widely assumed to be responsible for m ost of the methylation of the human genome, including the abnormal methylat ion found in cancers(5,6). To test this hypothesis, we disrupted the DNMT1 gene through homologous recombination in human colorectal carcinoma cells. Here we show that cells lacking DNMT1 exhibited markedly decreased cellular DNA methyltransferase activity, but there was only a 20% decrease in overa ll genomic methylation. Although juxtacentromeric satellites became signifi cantly demethylated, most of the loci that we analysed, including the tumou r suppressor gene p16(INK4a), remained fully methylated and silenced. These results indicate that DNMT1 has an unsuspected degree of regional specific ity in human cells and that methylating activities other than DNMT1 can mai ntain the methylation of most of the genome.