1Department of Cellular and Structural Biology, 2Department of Pathology, The University of Texas Health Science Center at San Antonio, San Antonio, TX 78284, USA, 3Department of Genetics, Southwest Foundation for Biomedical Research, San Antonio, TX 78245, USA and 4South Texas Veterans Health Care System, Audie L. Murphy Hospital, San Antonio, TX 78284, USA
With deployment of the international space station, humans living in a space environment will be exposed to increased levels of cosmic radiation. Because DNA contained in germ line cells directs the formation of offspring, understanding the susceptibility of defined populations of germ cells to radiation induced DNA damage and mutagenesis will be important in understanding and defining appropriate safeguards for preventing radiation-induced germ line mutations that might be detrimental to offspring of persons spending extended time in space. DNA repair pathways are important in ameliorating spontaneous and induced DNA damage, thereby minimizing cytotoxicity and mutagenesis. Recently we demonstrated that mouse male germ cells have high DNA base excision repair activity compared to somatic tissues and that all tested defined spermatogenic cell types have high base excision repair activity. Mutagenesis in mouse male germ cells has traditionally been studied with the specific locus test. Results from the specific locus test have led to the suggestion that spermatids and spermatozoa are more susceptible to ionizing radiation induced mutagenesis than spermatogonia. However, a limitation of the specific locus test is that the effects on specific spermatogenic cell types are inferred from phenotypes observed in offspring. In contrast, transgenic mice carrying a lacI transgene can be used to directly study the effects of mutagens on defined spermatogenic cell types. We have designed a series of experiments using the lacI transgenic mouse line to directly examine the effects of ionizing radiation on mutagenesis in defined populations of spermatogenic cell types. Eight-day-old lacI transgenic mice were treated with 0Gy to 5Gy of ionizing radiation and the testes were removed when the animals were 30-days-old and processed by routine methods for histological examination to identify the maximum dose that failed to disrupt spermatogenesis. Failure in the progression from spermatogonia to spermatocytes was observed in samples obtained from does in excess of 3Gy. Subsequently, male mice were exposed to 2.5Gy of ionizing radiation at different ages post partum to irradiate specific populations of differentiating spermatogenic cells. Spermatogenic cells types arising from the irradiated precursor cells have been collected and their mutation frequencies will be determined. These studies will be the first to directly determine the susceptibility of specific spermatogenic cell types to ionizing radiation.