DNA HYPOMETHYLATION LEADS TO ELEVATED MUTATION-RATES

Genome-wide demethylation has been suggested to be a step in carcinoge nesis(1). Evidence for this notion comes from the frequently observed global DNA hypomethylation in tumour cells(2), and from a recent study suggesting that defects in DNA methylation might contribute to the ge nomic instability of some colorecal tumour cell lines(3). DNA hypometh ylation has also been associated with abnormal chromosomal structures, as observed in cells from patients with ICF (Immunodeficiency, Centro meric instability and Facial abnormalities) syndrome(4,5) and in cells treated with the demethylating agent 5-azadeoxycytidine(6). Here we r eport that murine embryonic stem cells nullizygous for the major DNA m ethyltransferase (Dnmt1) gene exhibited significantly elevated mutatio n rates at both the endogenous hypoxanthine phosphoribosyltransferase (Hprt) gene and an integrated viral thymidine kinase (tk) transgene. G ene deletions were the predominant mutations at both loci. The major c ause of the observed tk deletions was either mitotic recombination or chromosomal loss accompanied by duplication of the remaining chromosom e. Our results imply an important role for mammalian DNA methylation i n maintaining genome stability.