Particulate chromate (Cr(VI)) compounds are well-established human lung carcinogens, but their carcinogenic mechanisms are not fully understood. It is known that Cr(VI)-induced lung tumors are characterized by chromosome instability (CIN). Particulate Cr(VI) compounds induce chromosomal aberrations (CA) and DNA double strand breaks (DSB) in human lung cells, but it is unknown if these types of damage lead to CIN .We hypothesize that misrepair of Cr(VI)-induced DSBs leads to CIN and ultimately to carcinogenesis. To help elucidate the critical pathways in the repair of these DSBs, we have utilized a number of Chinese hamster mutant cells defective in DSB repair to see how the absence of repair affects the amount and spectrum of chromosome damage. Here we investigate genes involved in either homologous recombination (HR) or non-homologous end joining (NHEJ) repair. Specifically, we consider RAD51C and D (involved in HR) and Ku80 (involved in NHEJ). We find that the absence of a functional RAD51C gene affects the total amount of induced chromosome aberrations and has a large effect on the spectrum of damage, while absence of a functional RAD51D gene only affects the spectrum of damage. Specifically, in the absence of either gene there is a dramatic shift in the spectrum from predominantly chromatid lesions to chromatid exchanges. In the cells without a functional RAD51D, the number of exchanges occurring at an exposure level of 1 μg/cm2 lead chromate increased from 0 in the parent cells to 17 in the deficient cells. Similarly, for cells without RAD51C function, the number of exchanges at this concentration went from 0 in the parent cells to 40 in the deficient cells. Thus HR protects cells from Cr(VI)-induced chromatid exchanges. By contrast, cells deficient in NHEJ do not exhibit a shift in the spectrum of damage, but these cells do exhibit an increase in the total amount of chromosome damage, manifested as an increase in mostly chromatid lesions with no significant increase in isochromatid lesions and chromatid exchanges. Thus NHEJ protects cells only from Cr(VI)-induced chromosome lesions. These data suggest that misrepair of Cr(VI)-induced DSBs will lead to chromosomal instability and that HR and NHEJ are important protective mechanisms for Cr(VI)-exposed cells. This work was supported by NIEHS grant ES10838 (J.P.W.).

[Proc Amer Assoc Cancer Res, Volume 47, 2006]