Effects of repair inhibition in the G 1 phase of clastogen-treated human lymphocytes on the frequencies of chromosome-type and chromatid-type aberrations and sister-chromatid exchanges

Abstract

It has been shown that certain types of DNA lesions induced by an S-dependent clastogen are converted to chromosome-type aberrations when their repair is inhibited in the G 1 phase of the cell cycle. The purpose of the present study was to investigate which kinds of repair inhibitors have the ability to induce chromosome-type aberrations in cells having DNA lesions and which kinds of DNA lesions will be converted to chromosome-type aberrations when their repair is inhibited. For this purpose, human peripheral blood lymphocytes, which were treated with a clastogen in their G 0 phase, were post-treated with one of several kinds of repair inhibitors in the G 1 phase, and resulting frequencies of both chromosome-type and chromatid-type aberrations as well as of sister-chromatid exchanges (SCEs) were compared with those of the control cultures: chromatid-type aberrations and SCEs were adopted as cytogenetic indicators of lesions remaining in S and G 2 phases. Chemicals used for the induction of DNA lesions were 4-nitroquinoline 1-oxide (4NQO), methyl methanesulfonate (MMS) and mitomycin C (MMC); inhibitors used were excess thymidine (dThd), caffeine, hydroxyurea (HU), 5-fluoro-2′-deoxyuridine (FdUrd), 1-β- D-arabinofuranosylcytosine (ara C), 9-β- D-arabinofuranosyladenine (ara A), 1-β- D-arabinofuranosylthymine (ara T) and aphidicolin (APC). Induction of chromosome-type aberrations was observed in cells pretreated with 4NQO or MMS followed by ara C, ara A, ara T or APC, whereas other combinations of a clastogen and an inhibitor did not induce them. Among the inhibitors, ara C alone induced chromosome-type aberrations in cells without pretreatment. Chromatid-type aberrations were increaded only in cells pretreated with MMC and their frequency was enhanced further by post-treatment with ara C. All of the clastogens used in the present experiments induced SCEs. Most inhibitors did not modify the SCE frequencies except for ara C which synergistically increased the frequency in MMC-treated cells. The present study offers further evidence that the lesions responsible for chromosome-type aberrations are those which are repaired quickly, and that they are converted to chromosome-type aberrations when repair by polymerase α is inhibited. The effects of ara C on MMC-induced lesions are considered residual effects of ara C treatment in the S or G 2 phases rather than repair inhibition in the G 1 phase.