A study of unscheduled DNA synthesis induced by X-rays in the germ cells of male mice

Abstract

• In vivo DNA repair occurring in meiotic and postmeiotic germ-cell stages of male mice after X-ray treatment has been studied. Making use of the fact that no scheduled DNA synthesis occurs in the germ cells after the last S period in primary spermatocytes, and administering testicular injections of [ 3H]dt, we measured DNA repair after X-ray exposure by the unscheduled incorporation of [ 3H]dT (UDS) into germ cells. • An exposure—response study showed that UDS induced in spermatids is an increasing function of X-ray exposure, at least up to 1000 R. A significant level of UDS was measured in the spermatids at an exposure level of 200 R, which is considerably lower than what has generally been required to measure UDS in mammalian cells. • When testicular injections of [ 3H]dT were administered immediately before X-ray teatment, the amount of unscheduled incorporation of [ 3H]dT into spermatid DNA was maximized. By 4 h following X-ray exposure, no UDS could be detected in the spermatids. This is in marked contrast to the UDS induced in spermatids by alkylating agents such as MMS, EMS, PMS, and IMS, which can still be detected at least 3 days after chemical treatment. In addition, the total amount of UDS measured in spermatids after X-ray treatment is much less than that observed after exposures to these alkylating agents which produce equivalent genetic effects. This may result from relatively fewer repairable DNA lesions being induced by X-rays and from differences in the sizes of the repaired regions. UDS induced after X-ray treatment was detected in the same germ-cell stages (early meiosis through midspermatids) that undergo UDS when exposed to EMS. However, unlike EMS, which produces about the same level of UDS in all the germ-cell stages capable of repair, X-rays appear to produce a higher level of UDS in the earlier meiotic stages. • From genetic data obtained by other workers using X-rays we have observed that dominant-lethal frequencies are at least twice as high in germ-cell stages undergoing UDS than in later stages where no UDS is detected. Furthermore, specific-locus mutation frequencies for germ-cell stages that undergo UDS after a 300-R X-ray exposure are no lower than in later stages where no UDS is observed. Although UDS has been found to occur in germ-cell stages of male mice from early meiosis to midspermatids after X-ray treatment, there is no evidence that this UDS results in a decrease of genetic damage in the germ cells in which it occurs.