Ordinarily it pays to be circumspect when a study links a toxic chemical to human disease, especially when that link rests solely on animal experiments. But a study released this evening in the Proceedings of the National Academy of Sciences caught my eye. It was led by neuroscientist Michael Merzenich of the University of California, San Francisco, who has done pioneering research on neuroplasticity, the ability of the brain to change (for good or ill) in response to experience.
He and his colleagues investigated a class of PCBs, or polychlorinated biphenyls, a group of chemicals once used electrical equipment, adhesives, inks and flame retardants but banned in 1977. They fed so-called non-planar PCBs to pregnant and nursing rats. The pups had significant developmental abnormalities. In a departure from many toxicology studies in which the dose exceeds by many-fold what people would ordinarily be exposed to (such as the equivalent of 800 cans of diet soda a day, in a study of artificial sweeteners, for instance), the pups’ blood levels of the PCB were roughly equivalent to those in the blood of breast-fed babies whose mothers were exposed to high levels of PCBs from contaminated fish, soil, water and air.
The PCB altered the rats’ auditory cortex, causing its neural circuitry to be disorganized and impairing the neurons’ ability to change in response to sound. This plasticity underlies, for instance, the “tuning” of auditory neurons during early childhood when you learn your native language; it is impaired in kids with specific language impairment, or dyslexia. And it is also messed up in kids with autism: in them, the auditory cortex responds abnormally to sound. “There are chemicals out there, this being just one example, that could profoundly affect development,” said UCSF’s Tal Kenet, who conducted the research while working under Merzenich and is now at Harvard Medical School. “This is a red flag,” Merzenich said. “The impact of this class of chemicals, whose toxicity has been under-appreciated, must be studied in human populations, and fast.”
There’s been a big push to find genes associated with autism. But that approach has one problem. Genes don’t change fast enough to explain the rise in the rates of autism. That means either that we’re looking for the culprit in the wrong places, or that genes are indeed involved but in a more complicated way “namely, that genes causing autism do so only when they’re acted on by something in the environment. Said Merzenich, “I think environmental poisons, including the chemical we’ve examined in this study, are very good candidates” for that environmental trigger.
