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Autism has a strong genetic component: With one identical twin autistic, the other has a 70 percent chance of having it, a risk 10 times that of fraternal twins. Yet great, unsuccessful effort has been spent looking for its genetics. To Wigler, the key lies in spontaneous mutations — novel alterations in the parental germ line of the offspring. Last year he formed a controversial theory for it. It suggests that females, who develop autism with a 1/ 4th frequency with which males do, may carry the genetic profile for it.
Wigler attributes the failure of conventional studies to their studies on families with more than one autistic child to search for differences in one genetic base. These differences could be any alteration in a base called SNPs. Uncovering SNPs shared by affected people would uncover high-risk people. The problem is locating the same target: they have implicated loci on 20 of the 23 human chromosomal pairs.
In his first autistic research, Wigler, with Sebat, tried to determine the role of spontaneous mutations, called copy number variations. Before human genome sequencing, researchers thought an individual always had two copies of a gene. In 2004, the team showed that even in healthy individuals, they could go missing from (or be added to) the genome via genetic rearrangements. Studies on families with only one autistic member showed that up to 10 percent of non-inherited autism cases could be caused by these rearrangements. They found that the structural events were primarily deletions, leaving individuals with only one copy of a particular gene and leading, sometimes, to its functional disruption.
Later, Wigler unveiled a unified genetic theory, which he cobbled together by examining families with multiple autistic individuals and incorporating both hereditary and spontaneous events. Focusing on families with the first two children affected, he found that third-born male children have a 50 percent risk of acquiring the disorder, whereas the risk for third-born girls is closer to 20 percent. From there, Wigler developed a two-tiered hypothesis: The majority fall into the low-risk category, having spontaneous mutation. Contrarily, high-risk families — 25 percent of all, manifest the disease when an unaffected individual, mostly female, carries a sporadic mutation. In case of a male, the chances are roughly half.
Although Wigler’s model is seen as a simpler way to view the genetics of autism, others find it incomplete. Critics note that it does not explain observations of families with an autistic child in which either second- or third-degree relatives are also affected or in which first-degree relatives show mild symptoms. And the model fails to explain why girls do not get autism as frequently as boys. Wigler believes that more data might help prove him. For instance, the girl-boy discrepancy could be explained if the genetic modifiers are sex-specific, an effect that might become apparent if researchers look at cases in which a normal mother has an autistic daughter.