Monitoring populations for DNA repair deficiency and for cancer susceptibility

The induction of a mutator phenotype has been hypothesized to cause the accumulation of multiple mutations in the development of cancer. Recent evidence suggests that the mutator phenotype is associated with DNA repair deficiencies. We have been using a challenge assay to study exposed populations to test our hypothesis that exposure to environmental toxicants can induce DNA repair deficiency in somatic cells. In this assay, lymphocytes were irradiated in vitro to challenge cells to repair the radiation-induced DNA strand breaks. An increase of chromosome aberrations in the challenged cells from toxicant-exposed populations compared to nonexposed populations is used to indicate abnormal DNA repair response. From studies of cigarette smokers, butadiene-exposed workers, and uranium-exposed residents, the assay showed that these exposed populations had mutagen-induced abnormal DNA repair response. The phenomenon was also demonstrated using experimental animals. Mice were exposed in vivo to two different doses of N-methyl-N'-nitro-N-nitroso-guanidine (MNNG) and their lymphocytes were challenged with one dose of a radiomimetic chemical, bleomycin, in vitro. These challenged lymphocytes showed an MNNG dose-dependent increase of abnormal DNA repair response. In a population that was potentially exposed to teratogens - mothers having children with neural tube defects - lymphocytes from these mothers did not have the abnormal response in our assay. In studies with patients, we reported that lymphocytes from Down's syndrome patients have the abnormal DNA repair response. Lymphocytes from skin cancer-prone patients (epidermodysplasia verruciformis) have normal response to γ-ray challenge but abnormal response to UV-light challenge. These patient studies also indicate that the challenge assay is useful in documenting the radiosensitivity of Down's syndrome and the UV sensitivity in EV patients. In most cases, the challenge assay is more sensitive in detecting biological effects than the standard chromosome aberration assay. Our series of studies indicates that the challenge assay can be used to document biological effects from exposure to mutagens and that the effect is an abnormal DNA repair response. This abnormality can increase the risk for development of cancer. The repair deficiency is currently being validated using a plasmid transfection (host-reactivaton) assay. The need to integrate chromosome aberration and the challenge assays with other relevant assays for better documentation of biological effects and for more precise prediction of health risk will be presented. Our experience in using genetic polymorphism and host-reactivation assays will be discussed.