In the largest genome-wide study of bipolar disorder to date, an international team involving UCL researchers has identified 298 regions of the genome containing DNA variations that increase risk for the disorder.
The research published in Nature heralds a more than four-fold increase over the number of genetic links previously identified.
The study-the first large multi-ancestry genomic analysis of the disorder to include data from people of European, East Asian, African American, and Latino ancestries-also identifies a new region associated with an increased risk for the disorder within the East Asian samples. Cross-referencing a range of methods, including fine-mapping and other variant-to-gene-mapping approaches, the team identified 36 genes suspected to be relevant to bipolar disorder.
Bipolar disorder is an often lifelong mood disorder that impairs quality of life and functional ability, and is associated with suicide risk. It affects an estimated 40-50 million people worldwide.
There are two distinct subtypes: bipolar disorder type 1 is characterised by episodes of both mania and depression, while bipolar disorder type 2 includes episodes of hypomania (a less severe form of mania) and depression. Despite the prevalence of bipolar disorder, it can take an average of eight years to get a proper diagnosis, and much remains unknown about the biology of the condition.
To help elucidate bipolar disorder's underlying biology, an international team of scientists from within the Psychiatric Genomics Consortium conducted a genome-wide association study, scanning the DNA of 2.9 million study participants from cohorts worldwide to identify genetic markers that were more common in those with the condition. This involved scanning more than 6.7 million common variations in the DNA sequences among the study participants, more than 158,000 of whom experience bipolar disorder.
Professor Andrew McQuillin (UCL Psychiatry), one of the senior authors of the paper, said: "Existing drug treatments for bipolar disorder provide much-needed support for many people, but they don't work well enough for everyone, so there's an ongoing need for new medications. We hope that by improving our understanding of which genes are linked to the disorder, we can improve our understand of how bipolar disorder works and potentially develop new treatments."
Dr Niamh Mullins (Icahn School of Medicine at Mount Sinai, US), one of the senior authors of the paper, said: "It is well established that bipolar disorder has a substantial genetic basis, so identifying DNA variations that increase risk is of paramount importance to understanding the condition's genetic architecture. In addition to identifying 298 regions of the genome that contain variations that increase risk for bipolar disorder, the 36 key genes we identified as being linked to the condition can now be followed up in a range of experiments to uncover the biological mechanisms through which each relates to the disorder.
"The newly identified genes may also be used in experiments to explore new drug targets and drug development for bipolar disorder."
The study team also found differences in the genetic characteristics of bipolar disorder between clinical (patients recruited from hospital inpatient or outpatient units), community-based (participants in general population biobanks), and self-reporting (participants in online personal health surveys) participants. These genetic differences are likely to be driven by a higher prevalence of bipolar subtype 1 in the clinical samples versus a higher prevalence of bipolar subtype 2 in the self-reporting samples, which highlights the need for researchers to be mindful of the data-gathering strategies used within their studies of the condition.
According to the research team, the genetic signal of bipolar disorder is related to specific brain cell types, including GABAergic interneurons and medium spiny neurons, in the brain's prefrontal cortex and hippocampus. They also found that cells in the intestine and pancreas are involved, although more research is necessary to further understand this biology.
Professor Ole Andreassen (University of Oslo), senior author of the paper, said: "Although this work does not immediately impact patient care, it opens the possibilities for long-term positive impact for patients and their families who are impacted by this widespread disorder. Our research paves the way for the development of improved treatments, earlier interventions, and precision medicine approaches that will support clinicians in their decision-making to enable them to manage the condition in the most effective way for each patient."
The Psychiatric Genomics Consortium (PGC) is an international consortium of scientists dedicated to studying the genetic basis of psychiatric disorders and includes more than 1,700 researchers from over 65 countries.
