Sunday, September 30, 2007

Genetics of autism: inheritance from an asymptomatic mother with a spontaneous mutation

This was published in Scientific American news in July, but I've only just come across it. The authors are attempting to describe the phenomenon of familial autism in which neither parent appears to be autistic nor to have a family history of autism ... (emphases mine)
New Theory about Autism Roots: Scientific American 7/24/07

... Wigler's rethinking of autism's cause stems from an exhaustive analysis of risk based on a database of families with more than one autistic child. (The Autism Genetic Resource Exchange, or AGRE, manages the database.) The team determined that most cases of autism arise from novel, spontaneous mutations passed down from one or both parents, resulting in large gaps in a person's genome often encompassing several genes, which are then disrupted or inactivated. (This loss of genetic code—known as copy number variation—results in an offspring receiving only one of the standard two copies of a gene, which could cause an insufficient amount of protein to be produced by those genes.) In most instances, this mutation will result in an autistic child. However, in some cases—more likely in girls than boys—the recipient of this mutation will not produce any symptoms.

"When that child matures and becomes a parent, they have a 50 percent chance of transmitting … [their mutation] … to a child that might not be as lucky as they were, especially if … [its] … a boy," Wigler says. "So, they will be transmitting this with close to a 50 percent frequency—and that is the source of the high-risk families."

Wigler says that the team will continue to update its model as new figures are added to the AGRE database and try to gain new insight into the mechanism that gives girls greater resistance than boys. "To understand that [disparity] at a molecular or genetic level would be very important, because you could theoretically treat kids … you could detect something early and intervene," Wigler says. "I view it as the most important thing to understand."

Maja Bucan, a professor of genetics at the University of Pennsylvania, says that the new autism model is a creative way to interpret the familial data. "It's important to come up with new theories and then just test them once we have more data," she explains. "I don't think we have enough data [yet] to say whether this theory is right or wrong."

According to Wigler, the new model "certainly changes the way you think about autism. The paradigm shift is … something can be genetic without being heritable. The field has ignored the contribution of spontaneous mutation for a whole range of things that matter a lot to society," which, he adds, includes schizophrenia and morbid childhood obesity.
This is in line, as the final comment suggests, with a new belief that many common disorders have a genetic root but not a family history; they arise from spontaneous mutations. The key in this model is that women who are afflicted with the mutation may not present with autism; women have redundant systems that protect against expressing the disorder (phenotype). Their male children, however, can inherit the genes and, lacking the protection females have, develop the disorder.

It this is shown to be true it will have implications for our understanding of schizophrenia, autism, and the evolution of the human brain and mind.

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