MAO-A: violence through to depression

From BBC News today:

"A genetic analysis of almost 900 offenders in Finland has revealed two genes associated with violent crime. Those with the genes were 13 times more likely to have a history of repeated violent behaviour. The authors of the study, published in the journal Molecular Psychiatry, said at least 5-10% of all violent crime in Finland could be attributed to individuals with these genotypes.
...
"Each criminal was given a profile based on their offences, categorising them into violent or non-violent. The association between genes and previous behaviour was strongest for the 78 who fitted the "extremely violent offender" profile. This group had committed a total of 1,154 murders, manslaughters, attempted homicides or batteries. A replication group of 114 criminals had all committed at least one murder. These all carried a low-activity version of the MAO-A gene, which previous research has dubbed the "warrior gene" because of its link to aggressive behaviour."

Monoamine oxidase A, also known as MAO-A, is an enzyme encoded by the MAO-A gene. It operates in nerve tissue where it degrades neurotransmitters such as dopamine, norepinephrine, and serotonin. This affects mood.

This "warrior gene" thing is somewhat old news. Wikipedia states (in a rather badly-written article):

"In humans, there is a 30-base repeat sequence repeated in one of several different numbers of times in the promoter region of the gene coding for MAO-A. There are 2R (two repeats), 3R, 3.5R, 4R, and 5R variants of the repeat sequence, with the 3R and 4R variants most common in Caucasians. The 3.5R and 4R variants have been found to be more highly active than 3R or 5R, in a study which did not examine the 2R variant. An association between the 2R allele of the Variable Number Tandem Repeat region of the gene and an increase in the likelihood of committing serious crime or violence has been found.

"MAO-A levels in the brain as measured using positron emission tomography are elevated by an average of 34% in patients with major depressive disorder. Genetic association studies examining the relationship between high-activity MAO-A variants and depression have produced mixed results, with some studies linking the high-activity variants to major depression in females, depressed suicide in males, major depression and sleep disturbance in males and major depressive disorder in both males and females.
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"The frequency distribution of variants of the MAO-A gene differs between ethnic groups. 59% of Black men, 54% of Chinese men, 56% of Maori men, and 34% of Caucasian men carry the 3R allele. 5.5% of Black men, 0.1% of Caucasian men, and 0.00067% of Asian men carry the 2R allele.

"A connection between a version of the monoamine oxidase A gene (3R) and several types of antisocial behaviour has been found. MAO-A had no statistically significant main effect on antisocial behaviour. Maltreated children with genes causing high levels of MAO-A were less likely to develop antisocial behaviour. Low MAO-A activity in combination with abuse experienced during childhood results in an increased risk of aggressive behaviour as an adult. High testosterone, maternal tobacco smoking during pregnancy, poor material living standards, dropping out of school, and low IQ can also trigger violent behaviour in men with the low-activity alleles (which are overwhelmingly the 3R allele).

"In individuals with the low activity MAO-A gene, when faced with social exclusion or ostracism showed higher levels of aggression than individuals with the high activity MAO-A gene. Low activity MAO-A could significantly predict aggressive behaviour in a high provocation situation, but was less associated with aggression in a low provocation situation. Individuals with the low activity variant of the MAO-A gene were just as likely as participants with the high activity variant to retaliate when the loss was small. However, they were more likely to retaliate and with greater force when the loss was large."

So to summarise and over-simplify: low levels of MAO-A lead to aggression; high levels lead to depression.

Egocentricity asserted itself at this point and my thoughts turned to my 23andMe genotype data. The 2R (two repeats), 3R, 3.5R, 4R, and 5R variants of the repeat sequence aren't looked at by 23andMe, as they only test for SNPs. However, the gene which codes for MAO-A also has alleles which differ in SNPs, specifically rs6323.

Only one nucleotide to report here for me as it's on the X-chromosome and I'm male. The variant which expresses the most MAO-A is the G-variant, which makes its carrier prone to depression.

I'm T: so a tiny bit more warrior than someone on antidepressants.