"Evidence is accumulating to show that the effects of variants of many genes that are common in the population depend on environmental factors. Further, these genetic variants affect each other," said Sheilagh Hodgins of the University of Montreal and its affiliated Institute Universitaire en Sante Mentale de Montreal.
    
"We conducted a study to determine whether juvenile offending was associated with interactions between three common genetic variants and positive and negative experiences," Hodgins said.
    
In the study, 1,337 high school students aged 17 to 18 years in Vastmanland, a Swedish county anonymously completed questionnaires reporting on delinquency, family conflict, experiences of sexual abuse, and the quality of their relationship with their parents.
    
They also provided a sample of saliva from which the researchers extracted DNA.
    
The researchers studied the Monoamine oxidase A (MAOA) gene, a key enzyme in the catabolism of brain neurotransmitters, monoamines, especially serotonin.
    
"About 25 per cent of Caucasian men carry the less active variant of MAOA. Among them, those who experience physical abuse in childhood are more likely than those who are not abused to display serious antisocial behaviour from childhood through adulthood," Hodgins said.
    
"Among females it is the high activity variant of the MAOA gene that interacts with adversity in childhood to increase the likelihood of antisocial behaviour," Hodgins said.
    
The brain-derived neurotrophic factor (BDNF) gene modulates neuronal plasticity.
    
"The low expressing variants of BDNF are carried by approximately 30 per cent of individuals and some previous studies had shown that this variant was associated with aggressive behaviour if carriers were exposed to aggressive peers," Hodgins said.
    
"The third gene we studied was the serotonin transporter 5 HTTLPR. The low activity variant of this gene is carried by approximately 20 percent of individuals.
    
"Among carriers of this low activity variant, those exposed to adversity in childhood are more likely than those who are not to display antisocial and aggressive behaviour," Hodgins said.
    
"We found that the three genetic variants interacted with each other and with family conflict and sexual abuse to increase the likelihood of delinquency, and with a positive parent-child relationship to decrease the risk of delinquency," Hodgins said.
    
The study is published in the International Journal of Neuropsychopharmacology.

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