Asynchronously Replicating Eu/Heterochromatic Regions Shape Chromosome Damage.

Abstract

In order to shed more light on the influence of DNA replication on the formation and distribution of chromosome aberrations, breakpoints (BP) produced by UV-C and AluI were assigned either to the early replicating short euchromatic arm (Xpe) or to the late replicating long heterochromatic arm (Xqh) of the Chinese hamster (CHO9) X chromosome. Early (ES) or late (LS) S-phase cells were assessed by pulse incorporating the base analogue 5-bromo-2′-deoxyuridine (BrdU) immediately after UV-C irradiation (30 J/m2) or AluI (20 U) poration followed by BrdU immunodetection with FITC-tagged antibodies in metaphase spreads. Short (30 s) UV-C exposures (1 J/m2/s) induced BP preferentially in Xqh in LS cells and a random distribution of BP along Xpe and Xqh in ES cells. However, BP induced by long (5 min) UV-C exposures (0.1 J/m2/s) clustered according to arm replication time (Xpe during ES and Xqh along LS). Giemsa-stained metaphases showed elevated frequencies of UV-C induced chromatid-type aberrations and gaps, especially in cells exposed to longer UV-C irradiation. BP induced by AluI clustered in Xpe in ES but distributed randomly during LS. In contrast to UV-C, AluI did not produce an increase in the yield of gaps, neither in ES nor in LS cells. Putative mechanisms underlying the observed distributions of chromosome damage in replicating CHO9 cells exposed to UV-C and AluI are discussed.