Addiction genes

• What is a genetic predisposition?

  May carry an addictive gene so more likely to become addicted.
• Goldman et al (2005) :


  Addictions are, 'moderately to highly heritable'. Heritability ranges from +0.39 for hallucinogens to +0.72 for cocaine. (Number of specific genes identified that have been associated with addictive behaviour).
• What is the DRD2 gene?

  D2 dopamine receptor gene. People with A1 variant have fewer dopamine receptors & therefore don't get same pleasure from everyday things people find rewarding. More likely to engage in addictive behaviour to try get more stimulation (pleasure).
• Comings et al (1996a,b) :

  48.7% of smokers & ex-smokers carried A1 variant of DRD2 gene compared with 25.9% of general population. 50.9% gambling addicts carried A1 variant of DRD2 gene compared to 25.9% of non-addicts.
• What is the diathesis-stress model?

  Suggests a disorder like addiction only develops when a genetic predisposition to a particular disorder is triggered by an environmental stressor. Explains why not everyone with certain genes will succumb to an addiction.
• Kaufman et al (2007) :

  Serotonin transporter gene (5HTT) has been linked to development of alcoholism, but not on its own. Environmental factors such as childhood maltreatment interact with the gene to cause addiction.
• What are ADH & ALDH genes?

  Responsible for process of how alcohol is metabolised by body. Normally alcohol is metabolised into acetaldehyde due to enzyme alcohol dehydrogenase (ADH). Another enzyme acetaldehyde dehydrogenase (ALDH) breaks it down further into acetic acid & water.
• Edenberg et al (2006):

  ADH4 variants found to affect alcoholism risk in European countries.
• Higuchi et al (2006):


  Lower activity variants of ADH genes associated with increased risk of alcohol dependence in oriental populations.
• How do some variants of ALDH & ADH genes decrease risk of alcoholism?

  By increasing acetaldehyde levels in body. Causes unpleasant reactions to alcohol, e.g, facial flushing & nausea- reaction found in larger concentrations among East Asian populations & may explain lower alcoholism rates in these cultures.
• Kendler & Prescott (1998) :

  Interviewed approx. 2000 twins & found although environmental & social factors would influence whether person initiates cocaine use, whether they become addicted more dependent on genetic factors. E.g, concordance rate for cocaine initiation in MZ twins was 54% & for DZ- 42%. However dependence MZ- 35% & DZ- 0%.
• Marc Lewis (2015) :

  Suggests saying addiction is disease of brain & body is unhelpful & can damage self-esteem & make them feel helpless. Should consider changes in brain as form of learning rather than disease.
• Ojelade et al (2015) :

  Found that when gene Rsu1 wasn't functioning correctly in fruit flies it would lower their sensitivity to alcohol. Brain imaging then used to test for this in humans & found relationship between this & alcohol dependence. Many different genes could cause addiction & different behaviours could be triggered by different genes.
• Boardman et al (2008) :

  Studied sibling & twin pairs- found although there was strong genetic component in onset of smoking behaviour, rate for daily smoking varied across the different schools in sample. Heritability rate highest in school where most popular students were smokers, suggesting whether genetic predisposition to smoke is expressed is influenced by social factors such as peers.
• Kendler et al (2000) :


  Showed heritability of tobacco use changed over time. Studying Swedish twin pairs born between 1910 & 1958 they found in women born in earlier years of study, very little influence of genetic factors. Those born later, heritability around 60%. Rate for men relatively stable. Changing social attitudes towards women as smokers may have influenced likelihood that genetic predisposition expressed itself.