Arsenic alters cytosine methylation patterns of the promoter of the tumor suppressor gene p53 in human lung cells: a model for a mechanism of carcinogenesis.

Arsenic is a potent human carcinogen to which there is significant worldwide exposure through natural contamination of food and drinking water sources. Because arsenic is detoxified via methylation using a methyltransferase (MTase) and S-adenosylmethionine (SAM) as the methyl donor, we hypothesized that a mechanism of carcinogenesis of arsenic could involve alterations of MTase/SAM-dependent DNA methylation of a tumor suppressor gene. We found that exposure of human lung adenocarcinoma A549 cells to sodium arsenite (0.08-2 microM) or sodium arsenate (30-300 microM), but not dimethylarsenic acid (2-2000 microM), produced significant dose-responsive hypermethylation within a 341-base pair fragment of the promoter of p53. This was determined by quantitative PCR/HpaII restriction site analysis to analyze methylation status of two CCGG sites. In experiments with arsenite, DNA sequencing using bisulfite to visualize 5-methylcytosine (5-MeC) over the entire promoter region confirmed data obtained by restriction analysis. Limited data using SssI methylase also suggested that over-methylation of CpG sequences may exist over the entire genome in response to arsenite exposure. We propose that alteration of DNA methylation by arsenic offers a plausible, unified hypothesis for the carcinogenic mechanism of action of arsenic, and we present a model for arsenic carcinogenesis that utilizes perturbations of DNA methylation as the basis for the carcinogenic effects of arsenic.