The oocyte as a transmitter and mediator of genetic damage to offspring

Abstract

Although numerous chemical germ cell mutagens have been identified in research animals, no chemical mutagens have been positively identified in human germ cells. The paucity of effective methods to investigate mutations in human germ cells intensifies the need for germ cell mutation research in animal models. Furthermore, the vast majority of existing germline mutagenesis research has focused exclusively on male germ cells while very little is known about mutagenesis in female germ cells. The λ transgenic medaka (a small fish carrying the cII mutation reporter gene) was used to investigate the various roles of the oocyte in the transmission and repair of mutations from mutagen-treated parents to their offspring. To investigate the direct transmission of genetic damage to offspring via the oocyte, female transgenic medaka were exposed to the chemical mutagen, 1-ethyl-1-nitrosourea, and bred to wild-type males. Eighteen percent of the offspring (14/80) displayed an increased frequency of mutations in the cII gene (cII MF). This frequency of mutant offspring indicated that female germ cells were more sensitive to chemical mutagens than premeiotic male germ cells (18% vs. 4%, X2, p<0.01), which contradicted past research. Further experiments revealed 2% of offspring derived from treated oocytes showed increased cII MF in the unexposed, paternally derived cII gene. These untargeted mutations occurred in regions of DNA never exposed to the mutagen, a hallmark of genomic instability. Untargeted experiments were repeated in transgenic mice to further investigate untargeted mutations, and revealed a similar response in both fish and mammalian animal models, thus indicating a conserved mechanism. The untargeted mutations occurred post-fertilization, under the direction of the early embryo. Finally, the role of the oocyte’s DNA repair enzymes in response to DNA damage in the early embryo was investigated. The frequency of affected offspring derived from oocytes lacking functional p53 was significantly higher than offspring derived from wild-type oocytes (36% vs. 11%, X2, p<0.01). These experiments demonstrated that female germ-cells may be equally sensitive to chemical mutagens as male germ cells. Additionally, mutations observed in the offspring were fixed after fertilization, where DNA damage responses likely depended on the cellular processes of the oocyte.