Another interesting thing to consider is the research being done by Dr. James Woods at the University of Washington. If you do a search in pubmed for his work you can see he has done a lot of studies showing abnormal urinary porphyrin profiles for people who are exposed to Mercury above a certain very low level. He has shown in some of his studies that the porphyrin abnormalities correlate well with DMPS chelation challenge tests and thus he proposes porphyrins as a decent non-invasive way of assessing mercury body burden.

Perhaps one of the most interesting papers that he recently did showed that in some non insignificant portion of the propulation, they had much higher porphyrin excretion (of the one that was indicative for mercury damage) and all of these people had the "CPOX4" polymorphism.

Check it out:

The association between a genetic p... and neurobehavioral response in humans.

From what I can tell it seemed to show that the people with that genotype correlated with more symptoms. Oh and BTW, his study population is a bunch of dentists, heh kind of ironic isn't it? Someone needs to replicate his findings, but this certainly does point to some sort of genetic succeptibility (or more than one).