By Sara Reardon COULD suicidal behaviour be a disease in its own right, rather than a behaviour resulting from a mood disorder? Mounting evidence shows striking similarities in the brains of people who are suicidal. These are distinct from what is seen in the brains of people who have mood disorders but who died of natural causes. Suicidal people’s brains have striking similarities, distinct from those with mood disorders Such studies have led to “suicide behaviour disorder” being accepted for the first time, albeit in the appendix, in the newest version of psychiatry’s “bible” – the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) – released this week. The appendix contains topics deemed to have enough weight to require further research for possible full inclusion in future editions. Until the 1980s, people who committed suicide were considered, by definition, to be depressed. That didn’t explain why 10 per cent of suicides have no history of mental illness. The view began to change when autopsies revealed distinctive features in the brains of people who had committed suicide, such as structural changes in areas involved in decision-making – regardless of what disorder they had, and even when they had no mental disorder at all (Brain Research, doi.org/cvrpjk). Although research is complicated by a lack of brain samples or an animal model for suicide, the idea that suicidal behaviour has a distinct biology is gaining ground. An increasing number of studies are yielding insights into some of the changes that may underlie such behaviour. For example, when people with bipolar disorder who have attempted suicide begin taking lithium, they tend to stop such attempts even if the drug has no effect on their other symptoms. This suggests that the drug may be acting on neural pathways that specifically influence suicidal tendencies (Annual Review of Pharmacology and Toxicology, doi.org/dfjv57). Gustavo Turecki of McGill University in Montreal, Canada, says that there are likely to be many environmental factors that trigger changes in the brains of people who are already genetically prone to suicide, ultimately increasing risk of the behaviour. This month, in a study of suicidal brains, his group found 366 genes that had a different set of epigenetic markers – chemical switches that are triggered by environmental stressors and turn genes on and off – to those found in the brains of people who had died of natural causes (American Journal of Psychiatry, doi.org/mf7). The results are complicated by the fact that many of the people who committed suicide had a mental disorder, but Turecki says that suicide rather than mental illness was the only significant predictor for the epigenetic changes. Other studies back up the suggestion that your genes influence suicide risk. For example, a study of adopted people who had committed suicide found that their biological relatives were six times more likely to commit suicide than members of their adopted family (American Journal of Medical Genetics, doi.org/fmsncv). Ultimately, biological markers might allow psychiatrists to better predict which patients are most at risk of suicide. This could have implications for how a doctor chooses to treat that person, says Jan Fawcett of the University of New Mexico in Albuquerque. For instance, they may not prescribe certain antidepressants, as there is evidence that some initially increase suicide risk. David Shaffer of Columbia University in New York says that suicide behaviour disorder is “very much in the spirit” of the Research Domain Criteria system that the US National Institute of Mental Health has proposed as an alternative diagnosis standard to DSM-5. Rather than diagnosing people with a specific disorder, such as depression, the NIMH wants mental illness to be diagnosed and treated based on an individual’s symptoms, and their underlying genetics and neurobiology. Ultimately, says Nader Perroud of the University of Geneva in Switzerland, if suicidal behaviour is considered a disease, it will become possible to conduct more focused research. “We might be able to find a proper treatment for suicidal behaviour.” This article appeared in print under the headline “Suicidal behaviour: a treatable disease?