Hunt, PA, KE Koehler, M Susiarjo, CA Hodges, A Ilagan, RC Voigt, S Thomas, BF Thomas and TJ Hassold. 2003. Bisphenol A exposure causes meiotic aneuploidy in the female mouse. Current Biology 13: 546-553.

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Small prospective study in people finds similar effects

This research links a common contaminant, bisphenol A, to an error in cell division called aneuploidy that causes spontaneous miscarriages and birth defects in people, including Down Syndrome, and is also associated with a series of cancers. Hunt et al. report that in mice, bisphenol A causes aneuploidy even at extremely low doses.

Click here for a detailed explanation of aneuploidy

Aneuploidy is a condition in which cells have the wrong number of chromosomes. It is thought to result from errors in chromosome alignment during cell division. The microscopic photographs to the left show two examples of chromosomal alignment during cell division. The upper is normal. Chromosomes (stained red) are aligned properly. The lower figure shows alignment in a cell exposed to bisphenol A. Chromosomes are scattered throughout the cell. When cell division is completed by the lower cell, chromosomes are unlikely to be distributed properly between the two daughter cells, resulting in aneuploidy. photos from Hunt et al. 2003

These results open up a dramatic new front on the study of human aneuploidy and point toward possible interventions that may help avoid many human birth defects. Hunt's work provides strong new support for measures to reduce exposures to bisphenol A as quickly as possible.

Patricia Hunt and Terry Hassold are two of the world's leading researchers on aneuploidy, and in using animal experiments to understand the causes and impacts of aneuploidy in people. Their laboratories were engaged in collaborative research testing proposed reasons for why aneuploidy, and thus Down Syndrome, is more common in the babies of older women.

In the midst of these experiments, Hunt witnessed an unexplained, eight-fold surge in aneuploidy . Careful study revealed that a lab technician had used a particularly harsh form of detergent to wash the water bottles and the cages.

Link to companion paper describing the accident that
first suggested bisphenol A causes aneuploidy

Made out of polycarbonate plastic, which itself is manufactured with bisphenol A, the bottles leached bisphenol A into the water and thus into the mice in the experiments. The cages were made out of polyphthalate carbonate cages; they also leached bisphenol A after exposure to the detergent. Hunt describes the details of the accident and the detective work to figure out what had happened in a companion paper. Now in this paper she tests BPA's impact at low levels, instead of the high levels that were produced by the harsh detergent.

What did they do? In the first round of experiments, Hunt et al. examined chromosomal aberrations in the oocytes of mice raised in new polyphthalate carbonate cages exposed to two different dilutions of the detergent that had caused aneuploidy in the accident, and compared them to the oocytes of females raised in cages made from materials not containing bisphenol A. All animals drank from glass water bottles (no bisphenol A).

They chose to measure the rate of a chromosomal aberration called "congression failure." This aberration is readily and efficiently detected by examining prepared oocytes in the microscope, and Hunt's earlier work has strongly indicated it is involved in the causation of aneuploidy. Importantly, congression failure is known to increase in older human oocytes, consistent with the increased frequency of aneuploidy in the oocytes of older women.

What did they find?

Hunt et al. found that oocytes from female mice living in the polyphthalate carbonate cages with the heaviest exposure to the detergent had the highest rate of congression failure.

They repeated the experiment with the polyphthalate carbonate water bottles, and found that drinking water from bottles that had been degraded by detergent also increased the rate of chromosomal aberrations.

They then repeated the experiments again, this time with a type of water bottle made only out of polycarbonate. This was to eliminate the possibility that the effects above were caused by the polyester carbonate instead of the bisphenol A. Mice drinking water out of these bottles also showed a highly significant increase in chromosomal errors.

Measurements showed that bisphenol A was present in water samples exposed to carbonate water bottles. Those bottles with more severe damage leached more bisphenol A.

Having confirmed that the effect was caused by bisphenol A, Hunt et al. then focused on the amount of BPA necessary to cause damage. To do this, they performed a series of experiments providing different oral doses of BPA to juvenile female mice and then determining the frequency of congression failure.

In the first of these experiments, mice were treated with 20, 40 or 100 nanograms/g body weight (parts per billion) for 6-8 days prior to analyzing the effects on their oocytes.

As seen in the graph to the right, the rate of congression failure increased significantly with dose of BPA. An effect was apparent at the lowest dose tested. adapted from Hunt et al., Table 3

In the final round of experiments, Hunt et al. asked what effect the duration of exposure had on oocyte health. To do this, they exposed juvenile female mice to 20 ng/g (the lowest dose, above) for 3, 5 or 7 days and then examined their oocytes. In each of the treatments, the exposed animals had a higher frequency of congression failure, although only in animals exposed for 7 days was the effect statistically significant.

What do they conclude?

Hunt et al. state unequivocally that they do not know with scientific certainty whether the effect of BPA they show on mice occurs in humans. "Nevertheless, the meiotic program is extraordinarily conserved, and the results of our studies in mice are disturbing because brief exposures during the final stage of oocyte growth were sufficient to cause significant increases in meiotic abnormalities."

They are also uncertain about the mechanism by which BPA causes this effect. They suggest it is most likely to be an indirect one, via BPA's estrogenic impact on the cellular environment in which the oocyte is undergoing meiosis, rather than one in which BPA acts directly on chromosomes in meiosis.

Despite these uncertainties and the ongoing debates about what might be safe levels of BPA exposure, Hunt et al. point out that the effects they are seeing in mice are at "exposure levels close to or even below those considered 'safe,' and that recent studies indicate that current human exposure levels are within this range. Studies from Germany and Japan support this conclusion; data from the US are not yet available.

In fact, the levels in human tissue from Germany and Japan were similar to the levels in delivered orally to Hunt's mice; because of the way that BPA moves into the body, the blood stream and the womb, the doses the mouse oocytes experienced were virtually certainly much lower than those measured in people.

Hunt et al. conclude "Clearly, the possibility that BPA exposure increases the likelihood of genetically abnormal offspring is too serious to be dismissed without further study."

What does it mean? With this new research, Hunt et al. open up a dramatic new front for inquiries into the causes of aneuploidy. Their results prove that bisphenol A causes aneuploidy in mice, at extremely low levels.

Aneuploidy is an important cause of birth defects in people. Most aneuploid fetuses, however, never even make it that far along and instead are spontaneously aborted. Hence any insights into the causes of aneuploidy, especially insights that might also suggest steps that can be taken to prevent this chromosomal error, will be extremely welcome.

Hunt et al.'s data join a growing body of literature confirming low level effects of bisphenol A, far beneath the levels currently deemed safe by regulatory authorities. BPA is now implicated in adverse effects on prostate development and prostate tumors, breast tissue development, sperm count, on the rate of sexual maturation and even on adipogenesis (suggesting a role in obesity).

Studies have also revealed why early controversies took place, and specifically why efforts by industry to replicate certain low level results failed (and indeed, that one purported failure was upon close unbiased analysis a confirmation; statistical misrepresentation superficially suggested it was a failure).

Taken together, these results provide significant new support for efforts to begin reducing human exposure to BPA now, without waiting for final resolution of remaining scientific uncertainties. Pathways of human exposure to bisphenol A have not been studied carefully. Uses of BPA derived chemicals to line food cans, seal children's teeth, and manufacture baby bottles and other food-containing consumer products are obvious sources.