Bisphenol A impairs the double-strand break repair machinery in the germline and causes chromosome abnormalities

Abstract

Bisphenol A (BPA) is a highly prevalent constituent of plastics that has been associated with diabetes, cardiovascular disease, and an increased risk of miscarriages in humans. In mice, BPA exposure disrupts the process of meiosis; however, analysis of the affected molecular pathways is lagging and has been particularly challenging. Here we show that exposure of the nematode Caenorhabditis elegans to BPA, at internal concentrations consistent with mammalian models, causes increased sterility and embryonic lethality. BPA exposure results in impaired chromosome synapsis and disruption of meiotic double-strand break repair (DSBR) progression. BPA carries an anti-estrogenic activity in the germline and results in germline-specific down-regulation of DSBR genes, thereby impairing maintenance of genomic integrity during meiosis. C. elegans therefore constitutes a model of remarkable relevance to mammals with which to assess how our chemical landscape affects germ cells and meiosis.

Fig. 1.

BPA exposure results in sterility, increased embryonic lethality, reduced gonad size, impaired chromosome synapsis, and chromosome integrity in C. elegans. (A) Mean number of eggs laid by worms exposed either to vehicle (0.1% ethanol) or to the indicated doses of BPA. Error bars represent SEM. The entire brood sizes of 10 singled hermaphrodites were scored for each condition (n = 10); *P < 0.0001. (B) Embryonic lethality observed among the progeny of hermaphrodites exposed either to vehicle or to the indicated doses of BPA. Error bars represent SEM. n = 10; *P < 0.001 and **P < 0.0001. (C) Schematic representation of the progression of nuclei throughout the C. elegans germline. Nuclei enter meiosis at the transition zone (TZ). The oocyte closest to the spermatheca (Sp) is referred to as the −1 oocyte. (D) Low magnification images of DAPI-stained whole-mount gonads from age-matched hermaphrodites exposed to either ethanol or BPA. (Scale bar, 20 μm.) (E) At pachytene, incomplete synapsis and chromosome fragmentation are observed in 10% (n = 84) of nuclei (arrowheads) as revealed by DAPI (blue) and SYP-1 (red) immunostaining.

Fig. 2.

Meiotic DSBR is impaired and germline-specific expression of DSB repair genes is altered following BPA exposure. (A) Late pachytene nuclei from BPA-exposed germlines show elevated levels of RAD-51 (green) foci compared with control. (B) Quantitation of germ cell apoptosis. Germ cell corpses were scored in ethanol- and BPA-exposed p53/cep-1 heterozygous or homozygous worms. n = 20–23 gonad arms scored per genotype. Error bars represent SEM. *P < 0.0001. (C) Immunostaining for ATL-1 (red) and phospho CHK-1 (red) on midpachytene nuclei from dissected gonads of ethanol- and BPA-exposed worms. Between 7 and 15 gonads were analyzed for each condition. (Scale bars, 5 μm.) (D) Expression levels of a panel of genes implicated in DSB repair and DNA damage checkpoints were assayed by quantitative RT-PCR. Error bars represent SEM for three biological replicates each performed in duplicate. *P = 0.017 and **P < 0.01.

Fig. 3.

BPA exerts anti-estrogenic effects on the C. elegans germline and causes severe defects at diakinesis. (A) Six intact bivalents are observed in control (ethanol)- and 17β-estradiol (E2)-exposed −1 oocytes. In contrast, chromosomes have a frayed appearance and aggregates, fragments (red arrowheads), and chromatin bridges (blue arrowheads) are observed in −1 oocytes in BPA and ICI 182780 exposed germlines. (B) Quantification of the chromosome morphology defects observed in bivalents at diakinesis. Worms were examined at 1, 2, or 3 d post-L4. Ethanol served as a control for all experiments except for BPA + ICI where a mixture of ethanol (0.1%) and DMSO (1%) was used. (C) Immunolocalization of SYP-1 (red) and AIR-2 (red) on DAPI-stained (blue) −1 oocytes reveals a delay in bivalent differentiation. (D) Histone H3 phosphorylation (pH3, red) is impaired at diakinesis in BPA-exposed worms. (Scale bars, 5 μm.)

Fig. 4.

BPA exposure results in chromosome segregation defects during early embryonic cell division. Time-lapse analysis is shown of the first embryonic division in ethanol- and BPA-exposed H2B::mCherry; γ-tubulin::GFP embryos (n = 60 per condition). Chromosomes (red) fail to properly align along the metaphase plate in the BPA-exposed embryo. The arrow indicates a chromatin bridge observed in the same embryo during anaphase. (Scale bars, 20 μm.)