Genes methylated by DNA methyltransferase 3b are similar in mouse intestine and human colon cancer

Human cancer cells frequently have regions of their DNA hypermethylated, which results in transcriptional silencing of affected genes and promotion of tumor formation. However, it is still unknown whether cancerassociated aberrant DNA methylation is targeted to specific genomic regions, whether this methylation also occurs in noncancerous cells, and whether these epigenetic events are maintained in the absence of the initiating cause. Here we have addressed some of these issues by demonstrating that transgenic expression of DNA methyltransferase 3b (Thunt3b) in the mouse colon initiates de novo DNA methylation of genes that are similar to genes that become methylated in human colon cancer. This is consistent with the notion that aberrant methylation in cancer may be attributable to targeting of specific sequences by Dnmt3b rather than to random methylation followed by clonal selection. We also showed that Dtunt3b-induced aberrant DNA methylation was maintained in regenerating tissue, even in the absence of continuous Dtunt3b expression. This supports the concept that transient stressors can cause permanent epigenetic changes in somatic stem cells and that these accumulate over the lifetime of an organism in analogy to DNA mutations.