August 27, 2008
COMT Gene Variant Makes People More Easily Startled

Are you easily startled? When DNA testing becomes very cheap you'll be able to find out if your COMT gene is to blame. If you can't stop from fixating on bad memories and fixate on bad scary things happening around you blame it on your COMT gene.

Inborn differences may help explain why trauma gives some people bad memories and others the nightmare of post-traumatic stress. Scientists in Germany and the United States have reported evidence linking genes to anxious behavior.


Researchers including Martin Reuter, PhD, of the University of Bonn, Germany, recruited 96 women averaging 22 years old from the Giessen Gene Brain Behavior Project, which investigates biomolecular causes of individual differences in behavior.

The researchers first determined which participants carried which variations (alleles) of the COMT gene, which encodes an enzyme that breaks down dopamine, weakening its signal. (COMT stands for a catabolic enzyme named catechol-O-methyltransferase.) Scientists call its two alleles Val158 and Met158. Depending on ethnicity, more or less half the population carries one copy of each. The rest of the population is roughly divided between carrying two copies of Val158 and two copies of Met158.

Using a well-validated psychophysiological measure, the researchers next measured the intensity of each participant's startle response by attaching electrodes to the eye muscles that, upon emotional arousal, contract and cause a blink. Participants then viewed pictures that were emotionally pleasant (such as animals or babies), neutral (such as a power outlet or hairdryer), or aversive (such as weapons or injured victims at a crime scene) -- 12 pictures of each type for six seconds each. A loud, 35-millisecond white noise, called a startle probe, sounded at random while they watched. When participants blinked, showing the startle response, a bioamplifier took readings from the electrodes and sent the information to a computer for analysis.

People carrying two copies of the Met158 allele of the COMT gene showed a significantly stronger startle reflex in the unpleasant-picture condition than did carriers of either two copies of Val158 allele or one copy of each. The two-Met carriers also disclosed greater anxiety on a standard personality test.

This finding confirms that specific variations in the gene that regulates dopamine signaling may play a role in negative emotionality. The authors speculated that the Met158 allele may raise levels of circulating dopamine in the brain's limbic system, a set of structures that support (among other things) memory, emotional arousal and attention. The researchers said that more dopamine in the prefrontal cortex could result in an "inflexible attentional focus" on unpleasant stimuli, meaning that Met158 carriers can't tear themselves away from something that's arousing -- even if it's bad.

Unpleasant stimuli are often undesirable conditions in one's environment. One can imagine why the tendency to focus on such things would be adaptive. Some of them would be problems that needed solving or threats that needed combating.

COMT is probably one of many genes that influence anxiety and the startle reflex. For some reason the Met158 allele of COMT is fairly new in human evolution. I wonder what the selective pressures were that led to its emergence.

The Met158 allele was created by a relatively recent mutation and only in the evolution of human beings. Other primate species such as chimpanzees carry only the Val/Val genotype. Co-author Christian Montag, Dipl. Psych., observes that for humans, wariness may have been adaptive. He points out, "It was an advantage to be more anxious in a dangerous environment."

Share |      Randall Parker, 2008 August 27 03:15 AM  Brain Genetics

HellKaiserRyo said at August 27, 2008 7:12 PM:

People with high IQs are more likely to respond to audible clicks. I wonder if this varient is correlated with IQ.

epsilon33 said at August 28, 2008 9:31 AM:

Something that startled early humans significantly often and/or with serious impact on reproduction, but other primates hadn't had to deal with as much? How about their own kind sneaking up on them, or suddenly revealing concealed enmity. That's my guess - the main difference in playing Survivor with humans instead of apes is it's harder to tell who your friends are. Call it the knife in the back reflex.

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