On the origin of hydroxyurea-induced chromatid aberrations in G2 chromosomes with BrdUrd in only one of the sister chromatids

Abstract

A dose-dependent increase (up to 40%) of metaphases with aberrations of mainly ‘open’ chromatid type aberrations (isochromatid breaks without sister reunion, chromatid breaks, incomplete chromatid translocations) preferentially involving heterochromatin-containing segments was induced by hydroxyurea (HU) in G2-prophase chromosomes of Vicia faba, if one strand of the two DNA sister helices contained BrdUrd. In FPG-stained slides, nearly one half of the chromatid breaks were associated with an SCE, about one third of the remaining chromatid breaks occurred in the light (substituted) and about two thirds in the dark (unsubstituted) chromatid. A model is presented to interpret the origin of the aberrations observed, and the different proportions by which chromatid breaks were located in dark chromatids, light chromatids or at sites of SCEs. It is proposed that the final joining of DNA strands, involved in recombinative bypass of lesions that block DNA replication in the BrdUrd-containing strand, becomes impaired by HU during G2 prophase (disturbance of G2 repair according to Hartley-Asp et al., 1980). This may lead to DNA double-strand breaks which in turn result in ‘open’ chromatid aberrations. The model also includes an interpretation of the biological significance of SCE. SCEs are interpreted to represent the microscopically recognizable fraction of recombinative bypass processes at sites of lesions blocking DNA replication. They are assumed to originate by a combination of D-loop formation at the replication gap, copy choice of the blocked daughter DNA strand and recombination according to the Holliday model.