Cell cycle checkpoints and human carcinogenesis

A hallmark of cancer is deregulated cellular proliferation associated with alterations in genes that regulate the cell cycle such as p53, pRb, p16 and cyclin D1. When cell cycle checkpoint, DNA repair and cell death pathways function properly, cells are at less risk for incurring mutations. Inactivation or attenuation of checkpoint function may increase risk for obtaining additional mutations that lead to cancer. We examined cell cycle checkpoint responses in normal human fibroblasts, ataxia telangiectasia lymphoblasts and lymphoblasts from carcinogen-exposed people. Fibroblasts that express human papillomavirus type 16 E6, which inactivates p53, were defective in GI checkpoint responses following γ-rays but displayed normal S checkpoint response of inhibition of replicon initiation after γ-rays as well as normal post-replication repair after UVC. However, E6 cells failed to recover DNA replication after UVC which was associated with enhanced inactivation of colony formation. Sensitivity to inactivation of colony formation in E6-expressing cells was associated with increased DNA fragmentation and appearance of condensed, misshapen nuclei. Further examination of cell death suggested that, regardless of p53 status, fibroblasts did not undergo apoptosis following UVC. Ataxia telangiectasia cells fail to activate DNA damage checkpoints in G1, G2 and S. ATM heterozygotes show radiation hypersensitivity after γ-rays and may be at increased risk for developing cancer. Therefore, it is important to accurately identify individuals who are ATM heterozygotes or who display radiation hypersensitivity. Flow cytometric and chromatid break assays were tested for their capacities to identify ATM heterozygotes. Both assays displayed similar specificity but the chromatid break assay had greater sensitivity for detection of ATM heterozygotes. When the flow cytometric assay was applied to lymphoblast lines from a cohort of vinyl chloride-exposed individuals, cellular hypersensitivity was seen in 34% of samples. However, hypersensitivity was not correlated with VC-induced mutation at the hprt locus in peripheral lymphocytes. ATM null lymphoblasts displayed hypersensitivity to chloroethylene oxide, a metabolite of vinyl chloride. Hypersensitivity to chloroethylene-oxide was also observed in a normal line. ATM- and p53-dependent cell cycle checkpoints slow or arrest growth and enhance DNA repair in carcinogendamaged cells.