Authors:
Hunt PA, Koehler KE, Susiarjo M, Hodges CA, Ilagan A, Voigt RC, Thomas S, Thomas BF, Hassold TJ.

Title:
Bisphenol A exposure causes meiotic aneuploidy in the female mouse.

Source:
Current Biology. 13(7):546-53; 2003.

Summary:
Oocyte chromosomal abnormalities are a leading cause of miscarriages, congenital defects and mental retardation in humans. Meiotic abnormalities called 'meiotic nondisjunction' can result from the failure of the chromosome pairs to segregate evenly during meiosis thereby producing oocytes with an abnormal number of chromosomes. The most significant risk factor for increased meiotic aneuploidy is advanced maternal age, however other factors including irradiation, smoking, drinking, oral contraceptives and fertility drugs and environmental pollutants/pesticides have been proposed to play a role.

In studies examining oocyte aneuploidy in transgenic mice populations, a sudden increase in oocyte aneuploidy ('congression failure') was discovered in control mice. These control mice typically exhibited 1-2% congression failure, however in August 1998, congression failure levels suddenly increased to about 40%. As mice strains used for laboratory studies are extremely sensitive to environmental odours, noise, caging materials, diet and other variables, exacting standards are used in animal care facilities. A second independent study with animals housed at the same animal care facility, also produced a high proportion of aneuploidy in oocytes obtained from control mice. As these independent studies produced sudden, spontaneous increases in oocyte aneuploidy during the same time period, the authors quickly investigated culture media components and other common factors that might be the source of the meiotic disturbances. It was discovered that the increased aneuploidy coincided with the inadvertent use of a harsh alkaline detergent (A33; Airkem Professional Products, Ecolab) on caging materials and water bottles. Deterioration of these housing materials, largely comprised of polycarbonate plastic, caused by use of this detergent was suspected to have caused bisphenol A (BPA), an estrogenic compound used in the production of polycarbonate plastics and epoxy resins, to leach from the plastic cages. Removal of the damaged cages was associated with a corresponding decrease in the incidence of oocyte aneuploidy, suggesting that the damaged caging materials may have been the source of the disturbance.

The authors decided to further investigate the effects of BPA exposure on mouse oocyte aneuploidy. To study the effects of the detergent concentration, control animals were housed in new polycarbonate plastic cages treated with dilute (1/64) or full strength A33 detergent. Hunt et al. demonstrated a direct correlation between the degree of meiotic disturbance in oocytes obtained from animals housed in mildly and severely damaged cages (~5-fold and >10-fold increase in congression failure, respectively). Although there was a significant increase in oocyte aneuploidy compared to controls (8-20%), this increase was not as large as the proportion of aneuploidy following the original incident (~40%). As glass water bottles were used in these experiments, the authors concluded that the original polycarbonate plastic water bottles might have also contributed to the source of aneuploidy. This was confirmed by repeating the experiments using treated cages and treated water bottles. In this set of experiments, the level of meiotic aneuploidy was comparable to the levels produced by the original exposure. Additional experiments confirmed that chemical damage alone caused by exposure to the detergent, was sufficient to produce high levels of aneuploidy in the absence of autoclaving (high temperature sterilization). These experiments provided highly circumstantial evidence of an association between BPA exposure and increased aneuploidy.

A final set of experiments was conducted to directly establish the timing and dose of BPA exposure required to produce oocyte aneuploidy. Female mice were exposed to daily oral doses of 20, 40 or 100 ng/g body weight BPA for 6-8 days. Previous analysis of the BPA levels in the water bottles were estimated to be 100 and 360 ng/ml, which was then calculated to produce daily exposure levels in the range of 14-72 ng/g body weight. The treated animals exhibited a dose-dependent increase in aneuploid oocytes (5.8-10.9%), suggesting that low-dose BPA exposure is correlated with meiotic abnormalities in the mouse oocyte.

These experiments support the hypothesis that low-dose BPA exposure is correlated with meiotic abnormalities in the mouse oocyte. The exact mechanism of BPA in aneuploidy in unknown, however some studies of somatic cells exposed in vitro have demonstrated BPA-disturbances on microtubule organization thereby causing aneuploidy. However, it is not known if an estrogen-mediated pathway caused the effect of BPA on the oocytes as this was not tested in the study.

The strengths of this study are numerous; many independent avenues of investigation were pursued to demonstrate a link between BPA and oocyte aneuploidy, the authors showed BPA contamination of drinking water obtained from damaged water bottles at 100 and 360 ng/ml and oral BPA dosing produced increased rates of aneuploidy in treated mice. However, it must be noted that the only set of experiments designed to directly measure BPA exposure (oral dosing regimen) did not produce rates of aneuploidy comparable to the original exposure. The authors suggest that single dose exposures may be more successfully cleared by the liver, whereas chronic exposure to BPA, as in the original incident, may have a longer pharmacokinetic clearance rate. The authors also did not provide direct exposure measurements of the mice following the original incident, or following the experiments designed to test detergent-damaged cage materials. Therefore, while it is biologically plausible that BPA may indeed be associated with oocyte aneuploidy, it is also probable that other components of the polycarbonate plastic were subject to leaching following detergent treatment. Laboratory mice are exquisitely sensitive to many environmental factors, including perfumes and odours. The damage to the plastic caging would be expected to produce many potentially stressful odours and chemicals, any of which may have contributed to the increased incidence of aneuploidy.

This study is important as it demonstrates a link between BPA exposure and oocyte aneuploidy in the mouse. Extrapolations of these results to humans should be interpreted with caution. Further studies are required to: (1) more fully elucidate the association between BPA exposure and meiotic nondisjunction in the mouse oocyte; (2) elucidate the mechanism of action of BPA on oocyte aneuploidy; and (3) determine the potential relevance of BPA exposure to human health.