In Utero Exposure to Benzo[ a ]Pyrene Increases Mutation Burden in the Soma and Sperm of Adult Mice

Abstract

Background:Mosaicism, the presence of genetically distinct cell populations within an organism, has emerged as an important contributor to disease. Mutational events occurring during embryonic development can cause mosaicism in any tissue, but the influence of environmental factors on levels of mosaicism is unclear.Objectives:We investigated whether in utero exposure to the widespread environmental mutagen benzo[a]pyrene (BaP) has an impact on the burden and distribution of mutations in adult mice.Methods:We used the Muta™Mouse transgenic rodent model to quantify and characterize mutations in the offspring of pregnant mice exposed to BaP during postconception days 7 through 16, covering the major period of organogenesis in mice. Next-generation DNA sequencing was then used to determine the spectrum of mutations induced in adult mice that were exposed to BaP during fetal development.Results:Mutation frequency was significantly increased in the bone marrow, liver, brain, and sperm of first filial generation (F1) males. Developing embryos accumulated more mutations and exhibited higher proportions of mosaicism than exposed adults, particularly in the brain. Decreased sperm count and motility revealed additional negative impacts on the reproductive function of F1 males.Conclusion:In utero exposure to environmental mutagens contributes to somatic and germline mosaicism, permanently affecting both the genetic health of the F1 and the population gene pool.Citation:Meier MJ, O’Brien JM, Beal MA, Allan B, Yauk CL, Marchetti F. 2017. In utero exposure to benzo[a]pyrene increases mutation burden in the soma and sperm of adult mice. Environ Health Perspect 125:82–88; http://dx.doi.org/10.1289/EHP211

In Utero Exposure to Benzo[a]pyrene Induces Ovarian Mutations at Doses That Deplete Ovarian Follicles in Mice

“…The mutation spectrum in the lacZ gene was consistent with these being the predominant mutation types. We also observed increased deletions in the ovaries, which is consistent with what was observed in somatic tissues, but not in sperm, from males exposed in utero (Meier et al ). It should be noted that since there were very few follicles in the treated groups, the ovarian mutation spectra and frequency presented here essentially represent the contribution of somatic cells only.…”

“…The presence of B[ a ]P‐induced mutations and the low BMD observed in this study suggest that developing ovaries may be more sensitive to environmental mutagens than other tissues, and because the mutations induced by environmental agents can occur randomly throughout the genome, any cancer driver gene may be affected. Furthermore, as it was observed in males (Meier et al ), the number of clonally expanded mutants was higher in BaP‐exposed animals (Fig. ), consistent with observations that in utero exposure is a sensitive window for the induction of germline and somatic mosaicism.…”

“…In this study, we observed statistically significantly increased mutant frequencies in the lacZ transgene at all doses evaluated in the ovaries/oviducts. Bone marrow mutant frequencies were also elevated at all doses in these females and closely matched those observed in the male siblings in our previous work (Meier et al ). Based on dose‐response modeling of the lacZ mutant frequencies, the ovaries/oviducts accumulated mutations at lower doses than the bone marrow (BMDs of 1.3 mg/(kg day) and 3.6 mg/(kg day), respectively, with nonoverlapping confidence intervals).…”

“…It should be noted that since there were very few follicles in the treated groups, the ovarian mutation spectra and frequency presented here essentially represent the contribution of somatic cells only. Indeed, the mutation spectra in the somatic tissues of control animals and those exposed to BaP in utero are highly consistent between females in this study and males in Meier et al (Meier et al ), suggesting that there are few sex‐related differences in mutation mechanisms.…”

“…The ED 50 for follicle depletion in MutaMouse in the present study is clearly below 10 mg/(kg day) and is consistent with the ED 50 of about 2 mg/(kg day) previously observed in C57BL/6J with the same dosing period (Lim et al ; Luderer et al ). When comparing the effects of in utero exposure to BaP in MutaMouse on the ovaries in the present study to those on the testis, statistically significant ovarian follicle depletion is observed at 10 mg/(kg day) BaP, while no statistically significant effects on testis weights, epididymal sperm counts, or sperm motility were observed in the male siblings at that dose (Meier et al ). Similarly, greater sensitivity of the developing ovary than the developing testis was observed in C57BL/6J mice exposed in utero to BaP during the same developmental window (Nakamura et al ; Lim et al ), as well as in cultured embryonic gonads of C57BL/6J mice (Lim et al ).…”