Discover magazine had some stuff on this this month, suggesting that inflamation and metals such as mercury may have a stronger impact on this stuff than people once thought.
I usually am deeply skeptical of such claims, but there may be data emerging on the inflamation and immunity function connection to autisim.
I would think that something environmental must be having an effect. I don't see spontaneous DNS changes rising with no external factors other than genetics.
If it were mercury exposure, I would have been not merely autistic, but a dracula by now, And I am not, at least not yet.