Window into genetic errors may explain impacts reported in rodents, people

By Janet Raloff

Web edition: November 15, 2010

Bisphenol A does a real number on the genes responsible for successful reproduction in a 1-millimeter-long soil-dwelling roundworm. And that suggests that this chemical — a building block of certain plastics and of coatings on thermal receipt papers — might pose a similar reproductive risk to people. Indeed, geneticists are finding that this tiny critter can be a remarkably useful “lab rat” — predicting impacts in mammals, including us.

Roughly a third of the genes in this worm — Caenorhabditis elegans —closely resemble those in people, not only chemically but also functionally. And that’s why Patrick Allard and Monica Colaiácovo of Harvard Medical School, in Boston, chose to study reproductive impacts from various classes of pollutants in this animal.

Among them: BPA.

Rodent studies of this pollutant by others “have revealed multiple levels of reproductive impairments,” including problems with cell meiosis, the Harvard scientists note. This type of cell division segregates an organism’s pairs of chromosomes into new sets — ones that will exchange genetic segments before separating so that each can serve as the genetic blueprint in some egg or sperm. Moreover, a 2008 study in Mutation Research by European researchers showed that in the test tube, continuous exposure to BPA while a mouse's egg was maturing induced meiotic abnormalities.

Prior to meiosis, a cell duplicates each strand of chromosomes within its DNA. The duplicates swap segments of genes and then separate. Studies have suggested that high urinary concentrations of BPA in people may be related to miscarriages and offspring bearing defective chromosome arrangements, Allard and Colaiácovo observe. Those could be caused by impaired chromosome separations or bum swaps of strand segments during meiosis.

The Boston pair now report severe sterility in worms getting heavy doses (1 millimolar BPA exposures); the animals laid only 18 percent as many eggs as unexposed roundworms. Heavily exposed mama worms had acquired about 2 parts per million BPA in their tissues. Some of these worms successfully produced eggs. But not young. In their eggs — and those laid by worms getting only half as much BPA — virtually every embryo died.

Details appear in a paper posted early online in the Proceedings of the National Academy of Sciences,

Bad breaks
To probe what was going wrong, Colaiácovo’s team used a variety of antibodies to monitor how the copied strands of DNA separate in the process of egg formation. The strands should break cleanly and evenly. When a problem arises, and it often may, repair genes should step in and fix it. RAD-51 is one such gene that plays an integral role in finding and fixing meiotic errors — both in roundworms and in humans. It was elevated in BPA-exposed worms.

This gene’s elevated activity was but one symptom that in these BPA-exposed animals, meiosis had gone awry.

Specifically, “errors in segregation of the chromosomes occurred,” Colaiácovo says. Sometimes part of a copied strand of chromosome remained stuck to the partner from which it was to separate — a problem that can result in an egg ending up with the wrong number of genes.

The Harvard scientists uncovered signs that the problem traced not so much to the initial development of meiotic problems, but instead to the cells’ inability to repair the genetic havoc BPA had wreaked.

Cells recognize they have problems
Ordinarily, “when you don’t repair chromosome breaks properly, you activate what we call a DNA-damage checkpoint,” Colaiácovo says. This signals a cell to slow or stop meiosis, giving itself more time for automatic repair processes to kick in. And “we saw activation of this DNA-damage checkpoint,” she says — “something we validated through three different approaches.”

If meiosis proceeded without chromosome repair, the genetic material should begin to break into fragments. “And,” she says, “that’s what we saw at least 23 percent of the time: chromosome fragmentation.” Colaiácovo and Allard also witnessed clumping of chromosomes and chromosomes that “instead of being round and well shaped now looked frayed — as if there will little tears throughout their periphery.”

Such problems indicate that deformed and low-quality chromosomes evaded quality-control checkpoints and got into eggs. “So what we’re seeing here,” she said, “is most likely a problem with the repair process itself.”

The meiotic havoc that the Boston scientists describe in their new study was largely eliminated when they co-administered estrogen — a female sex hormone — to roundworms along with the BPA. In a way, then, Colaiácovo concludes, “This rescued the defects that BPA causes.” And that points to BPA's screwing up the reproductive machinery by acting as an anti-estrogen, she says.

Five years ago, Japanese scientists reported finding that among apparently healthy women, miscarriage rates appeared to correlate with chromosome errors. The risk of such genetic errors “correlated with how high the levels of BPA were in the women’s blood and urine,” Colaiácovo says.

“For whatever it’s worth, what we’re seeing in this [roundworm] system is a  link between BPA and problems in chromosomes,” she says — and the chromosomes’ impaired copying, or segregation, during early stages of embryo formation.