There's some fresh news in the (quite possibly endless) debate about the vaccine preservative thimerosal. The Institute of Medicine is working on another report, due in several months. Their last report, in 2001, found no evidence to support a link, but didn't dismiss the possibility, either.
I've written about this topic before. My belief, then and now, is that autism and thimerosal are very unlikely to be related. I haven't seen any data that make me lean the other way, and the evidence against a link has continued to pile up. One of my objections to the hypothesis has been that it's hard to rationalize, mechanistically. Mercury compounds are certainly neurological bad news, but autism hasn't generally been noted as a symptom of developmental mercury exposure. (There's a different set of effects, instead.) It's hard to come up with an explanation for why thimerosal's effects would be different and specific, while still partaking of the general toxicity of mercury compounds. (Why no rising epidemic of, say, cerebral palsy?) And there's the matter of the low dose, too.
But now there's a paper in Molecular Psychiatry by a team of researchers (from Northeastern and several other schools) which suggests a mechanism. They're looking at the synthase enzyme that produces the amino acid methionine, which is an important source of methyl groups for other enzymatic systems. DNA methylation is particularly important in gene expression, and many cellular growth factor pathways seem to have a methylation requirement in them as well. They've found that thimerosal is a powerful inhibitor of two particular growth-factor driven methylation reactions, with an IC50 of about 1 nanomolar.
Single-digit nanomolar is the kind of inhibitory constant that we look for in a new drug, too, so it's certainly plausible that that could have effects in vivo (if the pharmacokinetic behavior - blood and tissue levels of the compound - go along.) The paper points out that the ethylmercury blood levels produced by thimerosal-containing vaccines are in the 4 to 30 nM range, which is enough of a multiple of the IC50 to keep the hypothesis going (but see below.) So is this the proof against thimerosal, or not?
Well, on one level, this could answer some of my mechanistic objections. But I still have the same questions as before. This could be a refinement, but not enough by itself to establish a link. We're still in the same place: It's not that I can't imagine that thimerosal could be toxic, it's that I have trouble with it being toxic in just such a way as to produce only autism. Methylation pathways are ubiquitous - how does such a specific phenotype show up from this? (More such scepticism from an immunologist at McGill is here.
The authors do suggest some possible answers. Perhaps some individuals have a less robust methylation system than others, especially in specific brain development pathways, and are thus predisposed to thimerosal-induced damage. That's definitely a hypothesis worth investigating. If that looks like it's the case, I'll have to upgrade my take on the whole idea, subject to the 800-pound-gorilla in the last paragraph below.
And they suggest some limitations to their work. For one thing, they're working on cultured cell lines, which are tumor-derived and may well respond quite differently than primary cells in vivo. They also point out the potential complication that their cells have not fully differentiated into neurons. It's responsible of them to mention these factors.
I also wonder what the hit rate is in this assay - if we run a few thousand natural products or other environmental-exposure compounds through, how many are positive at nanomolar levels? I could also add that the ethylmercury blood levels they quote might not mirror the levels in the brain. That's been a battleground in the whole thimerosal debate, because we don't have definitive mercury pharmacokinetics in the brains of human children (and I sure can't think of an acceptable way to get 'em, either. The methods we use for that kind of data in rodent studies are clearly not going to apply!)
But my biggest objection to a thimerosal/autism link is epidemiological. So far, there seems to have been no change in the autism rate in response to the discontinuation of thimerosal. Data continue to be collected, of course, but there's no apparent connection, even from countries that eliminated the compound before the US did (and thus have a longer baseline.) The real-world numbers trump any amount of biochemical speculation. That goes for my own ideas as well, as my research projects demonstate to me regularly.