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 μM) or sodium arsenate (30–300 μM), but not dimethylarsenic acid (2–2000 μM), produced significant dose-responsive hypermethylation within a 341-base pair fragment of the promoter of p53. This was determined by quantitative PCR Hpa II 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.