Researchers have discovered that five major mental disorders may be linked to the same common inherited genetic variations, according to a study published in the journal Nature Genetics.
Scientists from the Cross Disorders Group of the Psychiatric Genomic Consortium (PGC) used genome-wide genotype data in an analysis of people with five psychiatric disorders, alongside controls.
The mental health conditions monitored were:
- Bipolar disorder
- Major depressive disorder
- Autism spectrum disorders
- Attention-deficit hyperactivity disorder (ADHD).
Previous research from the group reported the first link between the disorders, revealing that people with these disorders were more likely to have variation within the same four chromosomal sites.
However, this most recent study has investigated the links in more detail, by using the same genome-wide information and large data sets.
The researchers analyzed genetic variation in thousands of people with each of the five disorders, and compared the genetic codes with those of people who did not have the conditions. The researchers calculated to what extent pairs of disorders were linked to the same genetic variants.
Results of the analysis showed the following overlaps in heritability between particular psychiatric disorders as a result of common genetic variation:
- Schizophrenia and bipolar disorder - 15%
- Bipolar disorder and depression - 10%
- Schizophrenia and depression - 9%
- Schizophrenia and autism - 3%.
Overall, the researchers found that common genetic variation accounted for between 17-28% of risk of all five disorders.
Naomi Wray of the University of Queensland, Australia, says:
"Since our study only looked at common gene variants, the total genetic overlap between the disorders is likely higher.
Shared variants with smaller effects, rare variants, mutations, duplications, deletions, and gene-environment interactions also contribute to these illnesses."
The researchers say that these results, particularly the genetic evidence of the link between schizophrenia and depression, may have important implications for diagnostics and research.
The study was part-funded by the National Institutes of Health (NIH). Bruce Cuthbert, director of the National Institute of Mental Health, says:
"Such evidence quantifying shared genetic risk factors among traditional psychiatric diagnoses will help us move toward classification that will be more faithful to nature."
The researchers note that although the study results "attach numbers" to molecular evidence showing the importance of heritability linked to common genetic variation that causes these five psychiatric disorders, much of the inherited genetic contribution to these disorders is unexplained, as are non-inherited genetic factors.
The study authors provide an example of how common genetic variation accounts for 23% of schizophrenia patients, and evidence from twin and family studies estimate schizophrenia's total heritability at 81%.
Thomas Lehner, chief of the genomics research branch at the National Institute of Mental Health, says:
"It is encouraging that the estimates of genetic contributions to mental disorders trace those from more traditional family and twin studies. The study points to a future of active gene discovery for mental disorders."