DNA polymerases in replication and repair of DNA during carcinogenesis induced by feeding N-acetylaminofluorene.

Abstract

To study the roles of DNA polymerases alpha and beta during replication and repair of damaged DNA, use was made of the fact that during chronic treatment with carcinogens, replication and repair do not necessarily follow the same time sequence. Early cell damage and restorative hyperplasia cause a transient wave of DNA synthesis, while repair replication might be expected to continue throughout the period of treatment with the carcinogen. N-acetylaminofluorene (AAF) was fed in the diet for periods of up to 35 weeks, and at intervals during the feeding period measurements were made of DNA synthesis in vivo, and of DNA polymerases alpha and beta as assayed in vitro after fractionation. The activity of polymerase alpha increased and decreased with the transient early wave of DNA synthesis. Polymerase beta showed an initial rapid increase in activity which peaked before the increase in DNA synthesis, and then decreased. The decrease in activity may be due to the fact that, although AAF continues to be fed in the diet, the foci and nodules which develop no longer metabolise the carcinogen to a form which damages DNA. Thus replication occurs in the nodules while DNA damage and repair occur in the surrounding non-neoplastic liver. With the rapid growth of nodules there is overall an increase in neoplastic tissue, a relative decrease in nonneoplastic tissue, and thus a relative decrease in DNA damage, repair, and induction of polymerase beta. Histological examination showed that by 35 weeks the conversion to neoplasia was virtually complete. These results support the concept that polymerase alpha functions in de novo replication of DNA, and is induced during cell replication, while polymerase beta functions in repair replication, and increases in activity during chronic damage to DNA. Whether it is induced by treatment with carcinogens depends on the duration of treatment, and on other processes (e.g. metabolism of the carcinogen) which take place during the development of malignancy.