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Role of Postreplication Repair in Transformation of Human Fibroblasts to Anchorage Independence¹

Curriculum in Toxicology [J. C. B., W. K. K., M. C-S.], Department of Pathology [W. K. K., M. C-S.], and Lineberger Comprehensive Cancer Center [W. K. K., M. C-S.], The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7525

Cellular capacity for postreplication repair (PRR) and sensitivity to transformation to anchorage independence (AI) were quantified in normal foreskin and xeroderma pigmentosum (XP) variant fibroblasts after treatment with UV or benzo(a)pyrene-diol-epoxide I (BPDE-I). PRR is defined here as a collection of pathways that facilitate the replication of DNA damaged by genotoxic agents. It is recognized biochemically as the process by which nascent DNA grows longer than the average distance between two lesions in the DNA template. PRR refers more directly to the elimination of gaps in the daughter-strand DNA by mechanisms which remain to be determined for human cells, but which may include translesion replication and recombination. PRR was measured in diploid human fibroblasts by analysis of the dose kinetics for inhibition of DNA strand growth in carcinogen-treated cells. Logarithmically growing foreskin fibroblasts (NHF1) displayed D₀ values of 4.3 J/m² and 0.14 µM for the inhibition of DNA synthesis in active replicons by UV and BPDE-I, respectively. XP variant cells (CRL1162) exhibited corresponding D₀ values of 1.5 J/m² and 0.16 µM. The increased sensitivity to inhibition of DNA replication by UV in these XP variant fibroblasts (2.9-fold greater than normal) was mirrored by an enhanced frequency of transformation to AI. XP variant fibroblasts (CRL1162) were 3.2 times more sensitive to transformation to AI by UV than were the normal foreskin fibroblasts. As predicted by the PRR studies, both cell types exhibited similar frequencies of AI colonies induced by BPDE-I. Apparent thresholds were observed for induction of AI by UV (normal fibroblasts, 2.7 J/m²; XP variant fibroblasts, 0.3 J/m²) and BPDE-I (both, 0.05 µM). Doses of UV and BPDE-I above these thresholds produced proportional increases in the inhibition of DNA replication in operating replicons and in the induced frequency of anchorage-independent colonies. At doses of UV and BPDE-I that produced the same degree of inhibition of DNA strand growth, BPDE-I induced a greater number of cells capable of anchorage-independent growth than did UV in both normal and XP variant fibroblasts.

¹ This work was supported by Institutional Grant IN-15-2 from the American Cancer Society and USPHS Grant CA42765 from the National Cancer Institute.

² Current address: Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709.

³ To whom requests for reprints should be addressed.

Received 12/17/90. Accepted 3/20/91.

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