In the Genes: Study shows significant genetic overlap between five major mental illnesses

by: Danling Chen

A recent study conducted by the Psychiatric Genomics Consortium has shown that the five most common mental illnesses share many of the same genetic variations. The study, funded by the National Institute for Mental Health, was carried out by more than 300 scientists at 80 research centers in 20 countries. It provided evidence that people with the disorders are more likely to display a characteristic variation at the same four chromosomal sites, thus suggesting a genetic relation between five of the major mental illnesses.

It has long been established that the five major mental illnesses (bipolar disorder, schizophrenia, autism, ADHD, and depression) feature inherited components. The findings of this study established a common genetic basis for the illnesses. Researchers found that the genetic overlap between the illnesses “is likely higher” as compared to a control. They looked at single-nucleotide polymorphisms (SNPs) as markers for the presence of the mental disorder. (SNPs are DNA sequence variations at a single point in the genome.) There was less SNP variation within the five mental disorder genomes, and greater SNP variation between the mental disorder genomes and the control (non-disorder genomes). This positive SNP association among the disorders suggests a shared genetic basis for several mental disorders, including depression.  

Genetic overlap was moderate for bipolar disorder and depression, and for ADHD and depression, signifying that long-established incidence of comorbidity of depression and other mental illnesses does indeed feature a genetic underpinning. The overlap in heritability was about 10% in bipolar disorder and depression and about 9% in schizophrenia and depression.

The study results also pinpointed a confounding phenomenon. While previous twin and family studies have shown that genetic heritability is 81% for schizophrenia and 37% for depression, evidence from the study placed genetic heritability at only 23% and 21% for the diseases, respectively. This suggests that non-inherited genetic factors or environmental factors may play just as a large a part in manifesting mental illness.

This newfound evidence may have significant implications for how mental illnesses are classified and diagnosed. Currently, mental illnesses are diagnosed and treated solely according to their symptoms—in other words, focusing on the consequence, and not on the cause. Emerging data from genomic research is increasingly illuminating the genetic commonalities shared by most, if not all, mental illnesses. This study may call for a new holistic approach to diagnosing and treating mental illness, especially for disorders that demonstrate a significant genetic contribution, such as depression.

Reference:

Cross-Disorder Group of the Psychiatric Genomics Consortium. "Genetic relationship between five psychiatric disorders estimated from genome-wide SNPs." Nature Genetics 45.9 (2013): 984-994. doi:10.1038/ng.2711. http://www.nature.com/ng/journal/v45/n9/full/ng.2711.html.