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.[18] An association between the 2R allele of the VNTR region of the gene and an increase in the likelihood of committing serious crime or violence has been found.[18][19][20]
A connection between a version of the monoamine oxidase A gene (3R) and several types of antisocial behavior has been found. Maltreated children with genes causing high levels of MAO-A were less likely to develop antisocial behavior.[21] Low MAO-A activity in combination with abuse experienced during childhood results in an increased risk of aggressive behaviour as an adult,[22] and there is evidence suggesting that men with the low activity MAOA allele are more genetically vulnerable even to punitive discipline as a predictor of antisocial behaviour.[23] High testosterone, maternal tobacco smoking during pregnancy, poor material living standards, dropping out of school, and low IQ also predicts violent behavior in men with the low-activity alleles (which are overwhelmingly the 3R allele).[24][25] Even in the absence of such interaction factors, the 3R allele has a small main effect on aggression and antisocial behavior, according to a large meta-analysis, which found no significant publication bias.[26]
Aggression and the "Warrior gene"
A version of the monoamine oxidase-A gene has been popularly referred to as the warrior gene.[27] Several different versions of the gene are found in different individuals, although a functional gene is present in most humans (with the exception of a few individuals with Brunner syndrome).[28] In the variant, the allele associated with behavioural traits is shorter (30 bases) and may produce less MAO-A enzyme.[29] This gene variation is in a regulatory promoter region about 1000 bases from the start of the region that encodes the MAO-A enzyme.
Studies have found differences in the frequency distribution of variants of the MAOA gene between ethnic groups:[29][30] of the participants, 59% of Black men, 54% of Chinese men, 56% of Maori men, and 34% of Caucasian men carried the 3R allele, while 5.5% of Black men, 0.1% of Caucasian men, and 0.00067% of Asian men carried the 2R allele.[20][29][30][31][32][33][34][35][36][37]
In individuals with the low activity MAOA gene, when faced with social exclusion or ostracism showed higher levels of aggression than individuals with the high activity MAOA gene.[38] 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 MAOA 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.[39]
“Monoamine oxidases (MAOs) are enzymes that are involved in the breakdown of neurotransmitters such as serotonin and dopamine and are, therefore, capable of influencing feelings, mood, and behaviour of individuals”.[40] According to this, if there was a mutation to the gene that is involved in the process of promoting or inhibiting MAO enzymes, it could affect a person’s personality or behaviour and could therefore make them more prone to aggression. A deficiency in the MAOA gene has shown higher levels of aggression in males, which could further stimulate more research into this controversial topic. “A deficiency in monoamine oxidase A (MAO-A) has been shown to be associated with aggressive behaviour in men of a Dutch family”.[41]
Legal implications
In a 2009 criminal trial in the United States, an argument based on a combination of "warrior gene" and history of child abuse was successfully used to avoid a conviction of first-degree murder and the death penalty; however, the convicted murderer was sentenced to 32 years in prison.[42][43] The results showed the effects of the 4-repeat allele of MAOA promoter polymorphism on physical aggressive behavior for women. It seems that there is an interaction between the 3-repeat allele of MAOA promoter polymorphism and emotional abuse experiences on aggressive behavior for women.[42]
Epigenetics
Epigenetics likely plays an important role in the expression of the MAOA gene through methylation in women.[44] Studies have linked methylation of the MAOA gene with nicotine and alcohol dependence in women.[45] A second MAOA VNTR promoter, P2, influences epigenetic methylation and interacts with having experienced child abuse to influence antisocial personality disorder symptoms, only in women.[46] Another study found epigenetic methylation of MAOA in men to be very low and with little variability compared to women, while having higher heritability in men than women.[47] Utoljára szerkesztette: Tetsuo, 2016.05.26. 20:10:59
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