Organization of rat neuronal DNA as a function of dose, time after irradiation and age

The organization of DNA and chromatin structure were examined in male Fisher 344 rat cerebellar neurons at various times from < 5 min to 2 years after exposure to ionizing radiation. Immediately after irradiation, the organization of neuronal DNA was altered. First, the DNA superhelical structure was changed due to removal of the topological constraints on the supercoiled DNA loops. Secondly, the accessibility of bulk neuronal DNA to digestion by micrococcal nuclease was increased. This increase in the m. nuclease sensitivity of bulk DNA did not depend on the oxygen concentration during irradiation. Thirdly, the accessibility of the nuclear matrix-associated DNA to digestion by DNase I was decreased. This decrease was most likely caused by masking the DNA with additional nuclear matrix-associated proteins. This increase in protein content was independent of oxygen, but inhibited if irradiations were performed at 4 degree C. The kinetics were consistent with the saturation kinetics observed for DNA repair in cerebellar neurons. Thus, these proteins may be associated with repair of radiation-induced DNA damage. The neuronal DNA/chromatin structure was restored to its unirradiated state by 24 hr after irradiation with biphasic kinetics having half-times similar to those reported for repair of radiation-induced DNA damage. However, the evidence suggested that residual DNA damage occurred in aging rats that had received a relatively high radiation dose at 4 months of age. In those rats, there was: (a) a decrease in the total nuclear protein content with age, (b) an increase in the digestibility of bulk DNA by m. nuclease with age, and (c) a reduction in the amount of nuclear matrix-associated proteins that persisted with age