Plutonium targets the p16 gene for inactivation by promoter hypermethylation in human lung adenocarcinoma

Lung cancer from radon or 239 plutonium exposure has been linked to α-particles that damage DNA through large deletions and point mutations. We investigated the involvement of an epigenetic mechanism, gene inactivation by promoter hypermethylation in adenocarcinomas from plutonium-exposed workers at MAYAK, the first Russian nuclear enterprise established to manufacture weapons plutonium. Adenocarcinomas were collected retrospectively from 71 workers and 69 non-worker controls. Lung adenocarcinomas were examined from workers and non-worker controls for methylation of the CDKN2A (p16), O6 -methylguanine-DNA methyltransferase (MGMT), death associated protein kinase (DAP-K), and Ras effector homolog 1 genes (RASSF1A). The prevalence for methylation of the MGMT or DAP-K genes did not differ between workers and controls, while a higher prevalence for methylation of the RASSF1A gene was seen in tumors from controls. In marked contrast, the prevalence for methylation of p16, a key regulator of the cell cycle, was increased significantly ( P = 0.03) in tumors from workers compared with non-worker controls. Stratification of plutonium exposure into tertiles also revealed a striking dose response for methylation of the p16 gene ( P = 0.008). Workers in the plutonium plant where exposure to internal radiation was highest had a 3.5 times (C.I. 1.5, 8.5; P = 0.001) greater risk for p16 methylation in their tumors than controls. This increased probability for methylation approximated the 4-fold increase in relative risk for adenocarcinoma in this group of workers exposed to plutonium. In addition, a trend ( P = 0.08) was seen for an increase in the number of genes methylated (≥2 genes) with plutonium dose. Here we demonstrate that exposure to plutonium may elevate the risk for adenocarcinoma through specifically targeting the p16 gene for inactivation by promoter methylation.