Carcinogenesis of drinking water disinfection by-products: Studies of additivity of mixtures and modes of action

Surface water treated with chemical disinfectants for human consumption contains a complex mixture of disinfection by-products (DBPs), including some that are carcinogenic in animals. Cancer risk assessment methodology assumes additivity of carcinogenic effects in the regulation of mixtures. The series of investigations presented here describe the response of a defined mixture of DBPs on several morphological and molecular endpoints compared to the individual chemical responses. The overriding hypothesis tested in this series of studies was that of additivity of chemical responses on toxicological endpoints, including carcinogenesis. A rodent model of hereditary renal cancer was used to evaluate the carcinogenicity of a mixture of DBPs. Tsc2 mutant (Eker) rats develop renal tumors secondary to a germline mutation in a tumor suppressor gene, and are highly susceptible to the effects of renal carcinogens. Eker rats were exposed to low and high doses of potassium bromate (KBrO3), 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), chloroform (CHCl3), and bromodichloromethane (BDCM), or a mixture of all four DBPs. The study endpoints included analysis of renal oxidative DNA damage and preneoplastic and neoplastic lesions in the kidneys, urinary bladder, and colon. Results from the molecular study demonstrated accumulation of oxidative DNA damage in rat renal tissue without the predicted up-regulation of base excision repair. Treatment of Eker rats with a mixture of DBPs caused predominantly antagonistic effects on renal proliferative lesions at low doses in male rats and high doses in female rats, and additive responses at high doses in male rats. Treatment with the high dose of MX caused transitional epithelial hyperplasia and cell proliferation in the rat urinary bladder, and this effect was diminished in the high dose mixture animals. Treatment with the DBPs caused the development of preneoplastic lesions of colon cancer, and the response from the mixture of DBPs was less than additive. The data presented in these studies indicate that the current default risk assessment method of using additivity of cancer endpoints in chemical mixtures is likely public health protective. However, these data also suggest that this conservative approach may overestimate the cancer risk associated with human exposure to low levels of chemicals in a mixture.