Condensed chromatin and cell inactivation by single-hit kinetics

Mammalian cells are extremely sensitive to gamma rays at mitosis, the time at which their chromatin is maximally condensed, The radiation-induced kill ing of mitotic cells is well described by single-hit inactivation kinetics. To investigate if radiation hypersensitivity by single-hit inactivation co rrelated with chromatin condensation, Chinese hamster ovary (CHO) K1 (wildt ype) and xrs-5 (radiosensitive mutant) cells were synchronized by mitotic s hake-off procedures and the densities of their chromatin cross sections and their radiosensitivities were measured immediately and 2 h into G(1) phase . The chromatin of G(1)-phase CHO K1 cells was dispersed uniformly througho ut their nuclei, and its average density was at least three times less than in the chromosomes of mitotic CHO K1 cells. The or-inactivation coefficien t of mitotic CHO K1 cells was similar to 2.0 Gy(-1) and decreased similar t o 10-fold when cells entered G(1) phase. The density of chromatin in CHO xr s-5 cell chromosomes at mitosis was greater than in CHO K1 cell chromosomes , and the radiosensitivity of mitotic CHO xrs-5 cells was the greatest with alpha = 5.1 Gy(-1). In G(1) phase, CHO xrs-5 cells were slightly more resi stant to radiation than when in mitosis, but a significant proportion of th eir chromatin was found to remain in condensed form adjacent to the nuclear membrane, These studies indicate that in addition to their known defects i n DNA repair and V(D)J recombination, CHO xrs-5 cells may also be defective in some process associated with the condensation and/or dispersion of chro matin at mitosis. Their radiation hypersensitivity could result, in part, f rom their DNA remaining in compacted form during interphase. The condensati on status of DNA in other mammalian cells could define their intrinsic radi osensitivity by single-hit inactivation, the mechanism of cell killing whic h dominates at the dose fraction size (1.8-2.0 Gy) most commonly used in ra diotherapy. (C) 1999 by Radiation Research Society.