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DNA repair synthesis in cultured human fibroblasts as a bioassay for chemical carcinogens San, Richard Hing-Cheung

Abstract

The suggestion from epidemiological studies that 80 to 90 per cent of all human cancers may have an environmental factor in its etiology, coupled with the wide use of chemicals in a modern society calls for a simple, rapid and economic prescreening programme to identify chemical carcinogens in the environment. Measures can then be taken to prevent or effectively reduce the exposure of human beings to these agents. The standard "rodent painting and feeding" test for carcinogenicity of a chemical compound (endpoint being tumour production) is unsuitable for a large screening programme. The cost and logistics of handling thousands of rats or mice (200 - 500 rodents per chemical) is staggering. Besides, the completion of this test requires a relatively long time (up to 2 years). Most, if not all, chemical carcinogens bind to DNA, Furthermore, almost all DNA-damaging agents, whether physical or chemical, that have been investigated in the proper test system show evidence of a repair effect. This observation raises the possibility of monitoring carcinogen-induced DNA damage and repair as a screening procedure for identifying chemical carcinogens. Previously, the extent of DNA repair (autoradiographic detection of unscheduled ³HTdR incorporation) in hamster and human cells following exposure to strongly, weakly and non-oncogenic isomers and derivatives of 4-nitroquinoline 1-oxide (4NQO) was examined. A good correlation was observed between the oncogenicity of a compound and the level of DNA repair synthesis. In the present study, 64 compounds representing key groups of carcinogens of different molecular structures were examined for the capacity to evoke an unscheduled DNA synthesis in cultured human fibroblasts. This includes 29 directly active proximate or ultimate carcinogens, 15 precarcinogens, that require metabolic activation, 16 non-oncogenic compounds and 4 chemicals of unknown carcinogenicity. All directly acting carcinogens triggered a DNA repair synthesis, whereas no unscheduled ³HTdR incorporation was observed following the application of the 16 non-oncogenic compounds. As a rule, the precarcinogens (without metabolic activation) did not elicit DNA repair synthesis. However, longer exposures and higher concentrations of the precarcinogens 2-acetyiaminofluorene, aflatoxin and sterigmatocystin evoked an unscheduled ³HTdR uptake. The results suggest the suitability of using DNA repair synthesis as endpoint, and cultured human cells as subjects in a prescreening programme for chemical carcinogens. As a probe into possible variations in sensitivity within the human population towards chemical carcinogens, cells from Xeroderma pigmentosum patients (known to be deficient in correcting UV-induced DNA damage) and normal persons were examined for their DNA repair capacity, frequency of chromosome aberrations and clone forming efficiency following exposure to chemical carcinogens. The XP cells show a considerably reduced DNA repair synthesis when exposed to some but not all chemical carcinogens. With chemicals for which the XP cells exhibited a deficiency in DNA repair they also elicited a higher frequency of chromosome aberrations and lower clone forming capacity than in normal persons. The advantages, limitations and possible modifications of the DNA repair bioassay for chemical carcinogens are discussed.

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