Researchers identify gene regulating amygdala volume

Fear factor: Healthy people with a smaller-than-average amygdala cannot recognize fearful facial expressions, but easily identify other emotions.

A variant of the gene for fibroblast growth factor 14, or FGF14, may decrease the volume of the amygdala, a brain structure needed to interpret emotions in facial expressions, according to results presented on Sunday at the World Congress of Psychiatric Genetics in Washington, D.C.

Individuals with the variant have trouble identifying fearful facial expressions, but their ability to recognize other emotions is unaffected.

FGF14 is a growth hormone factorthat is expressed in both developing and adult brains, including in the amygdala and areas around it. Mouse models suggest that FGF14 plays a role in neuronal signaling.

“Human growth hormone factors have been associated with autism,” says lead investigator David Glahn, associate professor of psychiatry at Yale University. “If indeed there is an association with growth hormone factors, this particular gene may be contributing to some of the social deficits of people with autism.”

Although amygdala volume is known to heritable, this is the first study to link a specific gene to the phenotype.

Studies have found a smaller amygdala in the brains of people with autism. The amygdala also shows faulty neuronal signaling in individuals with fragile X syndrome, the most common inherited form of autism.

In the new study, Glahn and his colleagues carried out a genome-wide association study for variants that might influence amygdala volume in 605 healthy people who are part of a larger study of brain structure and function. All of the individuals in the new study are adults, with an average age of 48.

The researchers recruited these participants nearly two decades ago as part of the San Antonio Family Heart Study, which focused on a single U.S. census tract that is 90 percent Mexican American. The study now includes more than 3,000 individuals, including 49 extended families. More than 1,350 are participating in a study of the genetics of brain structure and function.

For the amygdala study, the researchers scanned more than one million genetic variants in randomly selected extended families for whom they have collected extensive neuroanatomical and neurocognitive data.

They identified a single nucleotide polymorphism, or SNP, located within intron 1 of the FGF14 gene that is significantly associated with amygdala volume.

Magnetic resonance imaging suggests that the FGF14 variant, rs1336722, accounts for nearly four percent of the variations seen in amygdala volume in individuals who carry it on one allele, and eight percent in those who carry it on both alleles, the researchers say.

To determine whether the variant influences the participants’ perception of emotional expressions, the researchers presented them with images of 40 faces, and asked them to identify whether the emotion expressed was happy, sad, angry, fearful or neutral.

Those with a smaller amygdala were unable to recognize fearful facial expressions, though their ability to recognize the other expressions was unaffected. The effect was stronger in males than in females.

The researchers plan to carry out the experiments in younger participants and to include a screening instrument for traits of autism.