Researchers identify “impulsivity gene” in Finnish criminals

Impulsivity, the tendency to act without thinking, is one of those complex behavioral traits that neuroscientists have been recently been attempting to link to functional genes. No small task- many behavioral traits are thought to be controlled by numerous genes, gene expression pathways, and environmental variables. A recent study in Nature, however, has found a single gene mutation in a Finnish population that may be at least partially responsible for violent impulsive behavior.

The Finnish people have become very intersting to geneticists over recent years. The founder population of Finland is believed to derive from two distinct migration events 4,000 and 2,000 years ago. Until very recently,this small population has lived in relative isolation. Because of their isolation and small population size, the Finns have probably undergone “genetic bottlenecking” , the process of losing genetic diversity due to generations of breeding amongst a relatively homogenous population. One consequence of bottlenecking can be a high frequency of genetic muations that would be extremely rare in a larger, more outbred population.

In their search for genes that may link to impulsive behavior, researchers Bevilacqua et al. focused on a group of individuals with “extreme manifestations of impulsiivity” – violent criminal offenders in a foresnsic psychiatric unit. Using high-throughput DNA sequencing technology to examine regions of the genetic code that may offer clues, they found a previously unknown mutation in the HTR2B gene. This gene is involed in modulating both serotonin and dopamine release in the nucleus accumbens, a brain region involved with impulsive behavior.

Bevilacqua et al.1 find that, in a Finnish subpopulation, a mutation in the serotonin receptor HTR2B is linked to severe impulsivity. In the nucleus accumbens region (green) of the brain, projections of neurons that secrete serotonin (red) interact with those that secrete dopamine (blue). This region has been repeatedly shown to play a crucial part in choice and impulsivity. Mutations in HTR2B, which modulates the release of dopamine and serotonin in the nucleus accumbens, may reduce the release of these neurotransmitters, leading to increased impulsive behaviour. ---Image and caption courtesy of John Kelsoe's article in Nature Magazine, December 2010

Over-expression of HTR2B has already been linked to abnormal behavior. The drug ectasy has been shown to increase serotonin and dopamine levels in the brain by directly activating HTR2B. Bevilacqua et al. report a single base-pair mutation in HTR2B that stops RNA from translating the gene into its normal protein product, rendering it entirely non-functional. Individuals with this mutation are likely to have low unusually low levels of serotonin and dopamine, two important mood-enhancing neurotrasmitters. Incidence of this mutation was found to be three times higher among Finnish criminals than a control population.

Though this particular mutaion is unique among Finns (according to current research), other mutations in HTR2B could exist in different populations. Linking abnormal HTR2B genes to impulsive behavior in other populations would strengthen this gene’s candidacy for being a primary driver of impulsivity.

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