Rejoining of isochromatid breaks induced by heavy ions in G₂-phase normal human fibroblasts

We reported previously that exposure of normal human fibroblasts in G₂ phase of the cell cycle to high-LET radiation produces a much higher frequency of isochromatid breaks than exposure to γ rays. We concluded that an increase in the production of isochromatid breaks is a signature of initial high-LET radiation-induced G₂-phase damage. In this paper, we report the repair kinetics of isochromatid breaks induced by high-LET radiation in normal G₂-phase human fibroblasts. Exponentially growing human fibroblasts (AG1522) were irradiated with γ rays or energetic carbon (290 MeV/nucleon), silicon (490 MeV/nucleon), or iron (200 MeV/nucleon) ions. Prematurely condensed chromosomes were induced by calyculin A after different postirradiation incubation times ranging from 0 to 600 min. Chromosomes were stained with Giemsa, and aberrations were scored in cells at G₂ phase. G₂-phase fragments, the result of the induction of isochromatid breaks, decreased quickly with incubation time. The curve for the kinetics of the rejoining of chromatid-type breaks showed a slight upward curvature with time after exposure to 440 keV/μm iron particles, probably due to isochromatid-isochromatid break rejoining. The formation of chromatid exchanges after exposure to high-LET radiation therefore appears to be underestimated, because isochromatid-isochromatid exchanges cannot be detected. Increased induction of isochromatid breaks and rejoining of isochromatid breaks affect the overall kinetics of chromatid-type break rejoining after exposure to high-LET radiation.