Genetics of Addiction
Why do some people become addicted while others don’t? Family studies that include identical twins, fraternal twins, adoptees, and siblings suggest that as much as half of a person’s risk of becoming addicted to nicotine, alcohol, or other drugs depends on his or her genetic makeup. Pinning down the biological basis for this risk is an important avenue of research for scientists trying to solve the problem of drug addiction. There is a wealth of information on the genetics of addiction.
The International HapMap Project has identified 3.1 million single nucleotide polymorphisms (SNPs). In some cases, these tiny differences in DNA sequence may increase susceptibility to, or confer resistance against, disease, (U.S. Department of Energy Genome Programs).
Genes – functional units that make up our DNA – provide the information that directs our bodies’ basic cellular activities. Research on the human genome has shown that the DNA sequences of any two individuals are 99.9% identical. However, that 0.1% variation is profoundly important, contributing to visible differences, like height and hair color, and to invisible differences, such as increased risks for, or protection from, heart attack, stroke, diabetes, and addiction.
Some diseases, like sickle cell anemia or cystic fibrosis, are caused by an error in a single gene. Medical research has been strikingly successful at unraveling the mechanisms of these single-gene disorders. However, most diseases, including addiction, are more complicated: variations in many different genes contribute to an individual’s overall level of risk or resistance.
Linking genes to health: genome-wide association studies
Recent advances in DNA analysis are enabling researchers to untangle complex genetic interactions by examining a person’s entire genome at once. These genome-wide association studies (GWAS) identify subtle variations in DNA sequence called single-nucleotide polymorphisms (SNPs) – places where individuals differ in just a single letter of the genetic code. If a SNP appears more often in individuals with a disease than those without, it is presumed to be located in or near a gene that influences susceptibility to that disease.
GWAS are extremely powerful because they are unbiased and comprehensive: they can implicate a known gene in a disorder, and they can identify genes which may have been overlooked or previously unknown. Building on GWAS results, scientists gather additional evidence from affected families, animal models, and biochemical experiments to verify and understand the link between a gene and risk for a disease.
What role does the environment play in a disease like addiction?
That old saying “nature or nurture?” might be better phrased “nature and nurture,” because research shows that individual health is the result of dynamic interactions between genes and environmental conditions. For example, susceptibility to high blood pressure is influenced by both genetics and lifestyle, including diet, stress, and exercise. Environmental influences, such as exposure to drugs or stress, can alter both gene expression and gene function. In some cases, these effects may persist throughout a person’s life. Research suggests that genes can also influence how a person responds to his or her environment, placing some individuals at higher risk than others.
A recent study highlights the complex interactions between genetics, drug exposure, and age of use in the risk of developing a mental disorder.
The COMT gene produces an enzyme that regulates dopamine, a brain chemical involved in schizophrenia. COMT comes in two forms: “Met” and “Val.” Individuals with one or two copies of the “Val” variant have a higher risk of developing symptoms of psychosis and schizophrenic-type disorders if they used cannabis during adolescence.
The promise of personalized medicine – Genetics of Addiction
The emerging science of pharmacogenomics promises to harness the power of genomic information to improve treatments for addiction. Clinicians often find substantial variability in how individual patients respond to treatment. Part of that variability is due to genetics. Genes influence the numbers and types of receptors in our brains, how quickly our bodies metabolize drugs, and how well we respond to different medications.
Armed with an understanding of genetics, health providers will be better equipped to match patients with the most suitable treatments, adjust medication dosages, and avoid or minimize adverse reactions.
A NIDA-sponsored study of alcohol dependent patients treated with naltrexone found that patients with a specific variant in an opioid receptor gene, Asp40, had a significantly lower rate of relapse (26.1%) than patients with the Asn40 variant (47.9%).
In the future, identifying which mu-opioid receptor gene variant a patient possesses may help predict the most effective choice of medication for alcohol addiction and give us more clues in the Genetics of Addiction.