Wednesday, September 30, 2015

Disruptive mutations and the genetic architecture of autism

New results on the genetic architecture of autism support Mike Wigler's Unified Theory. See earlier post De Novo Mutations and Autism. Recent increases in the incidence of autism could be mainly due to greater diagnostic awareness. However, the new result that women can be carriers of autism-linked variants without exhibiting the same kinds of symptoms as men might alter the usual analysis of the role of assortative mating. Perhaps women who are carriers are predisposed to marry nerdy (but mostly asymptomatic) males who also carry above average mutational load in autism genes?

I suspect many of the ~200 genes identified in this study will overlap with the ~80 SNPs recently found by SSGAC to be associated with cognitive ability. The principle of continuity suggests that in addition to ultra-rare variants with "devastating" effects, there are many moderately rare variants (also under negative, but weaker, selection due to smaller effect size) affecting the same pathways. These would contribute to variance in cognitive ability within the normal population. More discussion in section 3 of On the Genetic Architecture of Intelligence.
Neuroscience News: Quantitative study identifies 239 genes whose ‘vulnerability’ to devastating de novo mutation makes them priority research targets.

... devastating “ultra-rare” mutations of genes that they classify as “vulnerable” play a causal role in roughly half of all ASD cases. The vulnerable genes to which they refer harbor what they call an LGD, or likely gene-disruption. These LGD mutations can occur “spontaneously” between generations, and when that happens they are found in the affected child but not found in either parent.

Although LGDs can impair the function of key genes, and in this way have a deleterious impact on health, this is not always the case. The study, whose first author is the quantitative biologist Ivan Iossifov, a CSHL assistant professor and on faculty at the New York Genome Center, finds that “autism genes” – i.e., those that, when mutated, may contribute to an ASD diagnosis – tend to have fewer mutations than most genes in the human gene pool.

This seems paradoxical, but only on the surface. Iossifov explains that genes with devastating de novo LGD mutations, when they occur in a child and give rise to autism, usually don’t remain in the gene pool for more than one generation before they are, in evolutionary terms, purged. This is because those born with severe autism rarely reproduce.

The team’s data helps the research community prioritize which genes with LGDs are most likely to play a causal role in ASD. The team pares down a list of about 500 likely causal genes to slightly more than 200 best “candidate” autism genes.

The current study also sheds new light on the transmission to children of LGDs that are carried by parents who harbor them but whose health is nevertheless not severely affected. Such transmission events were observed and documented in the families used in the study, comprising the Simons Simplex Collection (SSC). When parents carry potentially devastating LGD mutations, these are more frequently found in the ASD-affected children than in their unaffected children, and most often come from the mother.

This result supports a theory first published in 2007 by senior author Michael Wigler, a CSHL professor, and Dr. Kenny Ye, a statistician at Albert Einstein College of Medicine. They predicted that unaffected mothers are “carriers” of devastating mutations that are preferentially transmitted to children affected with severe ASD. Females have an as yet unexplained factor that protects them from mutations which, when they occur in males, will be significantly more likely to cause ASD. It is well known that at least four times as many males as females have ASD.

Wigler’s 2007 “unified theory” of sporadic autism causation predicted precisely this effect. “Devastating de novo mutations in autism genes should be under strong negative selection pressure,” he explains. “And that is among the findings of the paper we’re publishing today. Our analysis also revealed that a surprising proportion of rare devastating mutations transmitted by parents occurs in genes expressed in the embryonic brain.” This finding tends to support theories suggesting that at least some of the gene mutations with the power to cause ASD occur in genes that are indispensable for normal brain development.
Here is the paper at PNAS:
Low load for disruptive mutations in autism genes and their biased transmission

We previously computed that genes with de novo (DN) likely gene-disruptive (LGD) mutations in children with autism spectrum disorders (ASD) have high vulnerability: disruptive mutations in many of these genes, the vulnerable autism genes, will have a high likelihood of resulting in ASD. Because individuals with ASD have lower fecundity, such mutations in autism genes would be under strong negative selection pressure. An immediate prediction is that these genes will have a lower LGD load than typical genes in the human gene pool. We confirm this hypothesis in an explicit test by measuring the load of disruptive mutations in whole-exome sequence databases from two cohorts. We use information about mutational load to show that lower and higher intelligence quotients (IQ) affected individuals can be distinguished by the mutational load in their respective gene targets, as well as to help prioritize gene targets by their likelihood of being autism genes. Moreover, we demonstrate that transmission of rare disruptions in genes with a lower LGD load occurs more often to affected offspring; we show transmission originates most often from the mother, and transmission of such variants is seen more often in offspring with lower IQ. A surprising proportion of transmission of these rare events comes from genes expressed in the embryonic brain that show sharply reduced expression shortly after birth.

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