DNA REPAIR/REPLICATION IN CHEMICAL CARCINOGENESIS

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The present proposal forms a part of our overall research program oriented towards understanding the molecular mechanisms involved in the carcinogenic process in general and initiation step in particular. The rationale for the experiments detailed is derived from our earlier observations that (i) cell proliferation with attendent replication of carcinogen-damaged DNA, before the repair of certain critical lesions, were key events for initiation; (ii) replication of carcinogen-damaged DNA in vivo induced recombination events and (iii) such replicated DNA is hypomethylated by at least 20-40%. In view of the importance of DNA methylation in gene expression and differentiation, we have entertained the hypothesis that creation of such undermethylated DNA may be one of the mechanisms by which cell proliferation exerts its effect. Accordingly the experiments detailed in this project request make use of 5 azacytidine and 5-azadeoxycytidine agents which are known to inhibit methylation and study their effect on the initiation step of liver carcinogenesis induced by chemicals. In the first series of experiments we will confirm our earlier observations that replication of N-methyl-N-nitrosourea (MNU)-treated liver DNA induces hypomethylation in the replicated hybrid and try to correlate the degree of methylation of the DNA with the extent of initiation induced by MNU. The second series will determine (i) the effect of administration of different doses of 5-azacytidine and 5-aza-deoxycitydine after the carcinogen on the initiation phenomenon; (ii) whether the effect of these analogues is mediated via incorporation into DNA and (iii) whether the initiated hepatocytes by the 5-azacytidine model develop into hepatocellular carcinoma. Preliminary results obtained in this direction are encouraging. An attractive feature of this hypothesis is that it dissolves out the chemical diversity of the carcinogen-induced relevant lesion while emphasizing their similarity in being able to induce hypomethylation in DNA. In addition, the hypothesis is amenable for experimental validation.