Mutation frequency declines during spermatogenesis in young mice but increases in old mice

Abstract

Five percent of live-born human offspring will have a genetic disorder. Of these, 20% are because of germ-line de novo mutations. Several genetic diseases, such as neurofibromatosis and Duchenne muscular dystrophy, are associated with a high percentage of de novo germ-line mutations. Until recently, a direct analysis of spontaneous mutation frequencies in mammalian germ cells has been prevented by technical limitations. We have measured spontaneous mutation frequencies in a lacI transgene by using enriched populations of specific spermatogenic cell types. Similar to previously published results, we observed a lower mutation frequency for seminiferous tubule cell preparations, which contain all stages of spermatogenesis, relative to somatic tissues. We made the unexpected observation of a decline in mutation frequency during spermatogenesis, such that the mutation frequencies of type B spermatogonia and all subsequent stages of spermatogenesis are lower than the frequency for primitive type A spermatogonia. In addition, spermatogenic cells from old mice have significantly increased mutation frequencies compared with spermatogenic cells from young or middle-aged mice. Finally, the mutation frequency was observed to increase during spermiogenesis in postreplicative cell types when spermatogenic cells were obtained from old mice.From a genetic perspective, germ cells are profoundly different from somatic cells because they carry the genetic information that will direct the development of the next generation, not simply the next daughter cell. Thus, safeguarding the integrity of germ-line DNA might provide evolutionary advantages. Indeed, in mice, mutation frequencies obtained from mixed populations of germ cells are lower than for somatic tissues (1). This was demonstrated by using a transgenic system in which the bacteriophage genome carrying the lacI repressor gene and the ␣lacZ gene from the prokaryotic lac operon was introduced into the mouse genome as a transgene. DNA was recovered from genomic DNA preparations by packaging and used to infect a strain of Escherichia coli carrying a lacZ (␤-galactosidase) gene, but lacking a functional lacI gene. Mutation of the lacI gene renders a blue plaque on agarose containing the chromogenic substrate 5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside (X-gal). In contrast, other studies using a lacI transgenic mouse (2) or a lacZ transgenic mouse (3) did not report a significant difference in mutation frequencies for spermatogenic cells compared with somatic cells.Although provocative, interpretation of the results demonstrating a lower mutation frequency for male germ cells is complicated by the fact that adult seminiferous tubules contain a mixture of spermatogenic cell types encompassing all stages of spermatogenesis. Spermatogonia serve as the stem cells for spermatogenesis and undergo mitotic divisions that give rise to cells that will either retain their identity as spermatogonia to maintain the stem cell population or enter meiosis to become primary spermatocy...

Recovery of a low mutant frequency after ionizing radiation-induced mutagenesis during spermatogenesis

“…Consistent with previous results [27], the spontaneous mutant frequency in type A spermatogonia is significantly greater than that in the later spermatogenic cell types and the reduction in mutant frequency takes place between type A spermatogonia and type B spermatogonia (Table 1). However, after IR treatment the return to the low spontaneous mutant frequency occurred later in spermatogenesis (Figures 2-5).…”

“…However, after IR treatment the return to the low spontaneous mutant frequency occurred later in spermatogenesis (Figures 2-5). Together with our previous study [27], it is clearly shown that spontaneous and IR-induced mutant frequencies can be reduced during spermatogenesis. The mechanism by which mutant frequency is reduced is not yet identified.…”

“…However, to date, IR-induced cell type dependent germline mutant frequencies were deduced largely from the mutant frequency in the offspring of exposed mice [24][25][26]. By directly measuring mutant frequency in spermatogenic cells in Lac I transgenic mice, our laboratory has found that different spermatogenic cell types have different spontaneous mutant frequencies with the highest among the tested cell types being detected in primitive type A spermatogonia [27]. This indicates that different spermatogenic cell types have different capabilities of maintaining their genomic integrity that in turn may affect the response to IR.…”

Direct and delayed X‐ray‐induced DNA damage in male mouse germ cells

“…The phenomenon described here may be part of a general selection mechanism against mutations. Mutation frequency declines during spermatogenic differentiation [Walter et al, 1998;Xu et al, 2008] suggesting that mechanisms have evolved to prevent germ cell transmission of genetic damage. Mutations induced by irradiation of A/B spermatogonia were shown to decline with cell differentiation, probably because of delayed apoptosis [Xu et al, 2008[Xu et al, , 2010.…”

The relationship between aging and carcinogenesis: a critical appraisal

“…In the same study it was shown that the risk of childhood acute lymphoblastic leukemia was significantly higher among children of older mothers and fathers, and significant trends with increasing mothers's and fathers' ages were found. It is worthy nothing that spermatogenic cells from old mice have significantly increased mutation frequencies compared with spermatogenic cells from young or middle-aged mice [73]. New dominant mutation rise exponentially with parental age but not matergnal age in humans [74].…”