XRCC1 Deficiency Sensitizes Human Lung Epithelial Cells to Genotoxicity by Crocidolite Asbestos and Libby Amphibole

BACKGROUND Asbestos induces DNA and chromosomal damage, but the DNA repair pathways protecting human cells against its genotoxicity are largely unknown. Polymorphisms in XRCC1 have been associated with altered susceptibility to asbestos-related diseases. However, it is unclear whether oxidative DNA damage repaired by XRCC1 contributes to asbestos-induced chromosomal damage. OBJECTIVES We sought to examine the importance of XRCC1 in protection against genotoxic effects of crocidolite and Libby amphibole asbestos. METHODS We developed a genetic model of XRCC1 deficiency in human lung epithelial H460 cells and evaluated genotoxic responses to carcinogenic fibers (crocidolite asbestos, Libby amphibole) and nongenotoxic materials (wollastonite, titanium dioxide). RESULTS XRCC1 knockdown sensitized cells to the clastogenic and cytotoxic effects of oxidants [hydrogen peroxide (H₂O₂), bleomycin] but not to the nonoxidant paclitaxel. XRCC1 knockdown strongly enhanced genotoxicity of amphibole fibers as evidenced by elevated formation of clastogenic micronuclei. Crocidolite induced primarily clastogenic micronuclei, whereas Libby amphibole induced both clastogenic and aneugenic micronuclei. Crocidolite and bleomycin were potent inducers of nuclear buds, which were enhanced by XRCC1 deficiency. Libby amphibole and H₂O₂ did not induce nuclear buds, irrespective of XRCC1 status. Crocidolite and Libby amphibole similarly activated the p53 pathway. CONCLUSIONS Oxidative DNA damage repaired by XRCC1 (oxidized bases, single-strand breaks) is a major cause of chromosomal breaks induced by crocidolite and Libby amphibole. Nuclear buds are a novel biomarker of genetic damage induced by exposure to crocidolite asbestos, which we suggest are associated with clustered DNA damage. These results provide mechanistic evidence for the epidemiological association between XRCC1 polymorphisms and susceptibility to asbestos-related disease.