ARSENIC ALTERS CYTOSINE METHYLATION PATTERNS OF THE PROMOTER OF THE TUMOR-SUPPRESSOR GENE P53 IN HUMAN LUNG-CELLS - A MODEL FOR A MECHANISMOF CARCINOGENESIS

Arsenic is a potent human carcinogen to which there is significant wor ldwide exposure through natural contamination of food and drinking wat er sources. Because arsenic is detoxified via methylation using a meth yltransferase (MTase) and S-adenosylmethionine (SAM) as the methyl don or, we hypothesized that a mechanism of carcinogenesis of arsenic coul d involve alterations of MTase/SAM-dependent DNA methylation of a tumo r suppressor gene. We found that exposure of human lung adenocarcinoma A549 cells to sodium arsenite (0.08-2 mu M) or sodium arsenate (30-30 0 mu M), but not dimethylarsenic acid (2-2000 mu M), produced signific ant dose-responsive hypermethylation within a 341-base pair fragment o f the promoter of p53. This was determined by quantitative PCR/HpaII r estriction site analysis to analyze methylation status of two CCGG sit es, In experiments with arsenite, DNA sequencing using bisulfite to vi sualize 5-methylcytosine (5-MeC) over the entire promoter region confi rmed data obtained by restriction analysis. Limited data using SssI me thylase also suggested that over-methylation of CpG sequences may exis t over the entire genome in response to arsenite exposure. We propose that alteration of DNA methylation by arsenic offers a plausible, unif ied hypothesis for the carcinogenic mechanism of action of arsenic, an d we present a model for arsenic carcinogenesis that utilizes perturba tions of DNA methylation as the basis for the carcinogenic effects of arsenic.