Effect of Brief Maternal Exposure to Bisphenol A on the Fetal Female Germline in a Mouse Model
Background: Environmental contamination by endocrine disrupting chemicals (EDCs) has created serious public health, ecological, and regulatory concerns. Prenatal exposure can affect a wide range of developing organ systems, adversely affecting behavior, metabolism, fertility, and disease risk in the adult. The most serious and puzzling effect of EDC exposure is the transmission of effects to subsequent unexposed generations (transgenerational effects). This requires the induction of epigenetic changes to the germline that are not subject to the normal processes of erasure and resetting in subsequent generations. Understanding when and how the germline is vulnerable to EDCs is an essential first step in devising strategies to prevent and reverse their effects. Methods: We report here results using a well-established mouse model and the most extensively studied EDC, bisphenol A, to assess effects on the female germline and evaluate the underlying epigenetic changes.  Results: Using a quantitative variable (meiotic recombination), we found germline effects across a wide range of doses with a dose-related increase in severity. We identified a period of dramatic changes in chromatin at the onset of meiotic prophase as the vulnerable window of exposure and found a remarkable effect on chromatin that defies meiotic dogma. Oocyte analysis by mass spectrometry and immunofluorescence suggested H4K20me2, a histone posttranslational modification involved in DNA damage repair, was particularly affected. Subsequent RNA-seq analysis revealed a relatively small number of differentially expressed genes, but in addition to genes involved in chromatin dynamics, several with important roles in DNA repair/recombination and centromere stability also were affected. Discussion: Together, our data suggest BPA exposure induces complex changes to the germline that dysregulate chromatin but also affect several critical and interrelated meiotic pathways. Overall design: Maternal exposure to placebo or 500 ng/g body weight of bisphenol A from embryonic day 11-13.5. Ovaries were collected from different mouse fetuses. Oocytes were isolated by fluorescent activated cell sorting for DNA content for each experimental group and timepoint. Oocytes were pooled from multiple individuals of the same experimental group and timepoint. RNA-sequencing was performed and analysis for differentially expressed genes comparing placebo and bisphenol A groups was performed for embryonic day E13.5.