The Effect of Chromatin Decondensation on DNA Damage and Repair

The effects of chromatin compaction on X-radiation-induced cell killing and the induction and repair of DNA damage were studied in Chinese hamster ovary cells deprived of isoleucine for 24 h (${\rm Ile}^{-}$ cells) and compared to untreated controls. The results show that chromatin is decondensed in ${\rm Ile}^{-}$ cells; i.e., in ${\rm Ile}^{-}$ cells the nuclear area occupied by heterochromatin decreased 30-fold over control cells, both the rate and limit of micrococcal nuclease digestion were greater for ${\rm Ile}^{-}$ cells, and 14.2% more propidium iodide was intercalated into the ${\rm Ile}^{-}$ cell chromatin. The X-ray-induced cytotoxicity did not change in ${\rm Ile}^{-}$ cells versus the control cells ($D_{0}=0.99\ {\rm Gy}$) nor did the X-ray-induced DNA damage. However, the repair of DNA damage produced by 10 Gy proceeded with different kinetics in ${\rm Ile}^{-}$ cells when compared to the controls. The initial rate of DNA damage repair was slower in ${\rm Ile}^{-}$ cells by a factor of 2 compared to controls (the time required to rejoin 50% of the lesions was 6 versus 3 min, respectively). However, after 2 h of repair no DNA damage was detected in either group. Therefore, we conclude that this decondensation of chromatin, per se, does not directly modify the induction or ultimate repair of DNA damage by X radiation or cell clonogenicity and thus does not appear to be a primary factor in cell survival.