More than half of those who have one mental condition will get a second or third later in life. Three-quarters or more have four or more.
As a result, patients may find their therapy challenging and may have emotions of misfortune and discouragement. The study’s findings were published in the journal Nature Genetics.
However, a comprehensive new study of 11 major psychiatric diseases sheds fresh light on why comorbidities are the rule rather than the exception when it comes to mental illness.
“Our findings confirm that high comorbidity across some disorders in part reflects overlapping pathways of genetic risk,” said lead author Andrew Grotzinger, an assistant professor in the Department of Psychology and Neuroscience.
According to him, the discovery might eventually pave the road for medicines that target many psychiatric diseases at once, as well as assist modify the way diagnoses are delivered.
“If you had a cold, you wouldn’t want to be diagnosed with coughing disorder, sneezing disorder and aching joints disorder,” Grotzinger said. “This study is a stepping stone toward creating a diagnostic manual that better maps on to what is actually happening biologically.”
Grotzinger and colleagues from the University of Texas at Austin, Vrije Universiteit Amsterdam, and other collaborating institutions conducted the study by analysing publicly available genome-wide association (GWAS) data from hundreds of thousands of people who submitted genetic material to large-scale datasets like the UK Biobank and the Psychiatric Genomics Consortium.
They investigated genes linked to schizophrenia, bipolar disorder, major depressive disorder, anxiety disorder, anorexia nervosa, obsessive-compulsive disorder, Tourette syndrome, post-traumatic stress disorder, problematic alcohol use, ADHD, and autism.
They also examined data collected by wearable movement monitoring devices and survey data recording physical and behavioural features.
The researchers then used fresh statistical genetic approaches to discover common patterns among illnesses.
They discovered that 70% of the genetic signal linked to schizophrenia is also linked to bipolar illness. This conclusion was striking since, according to current diagnostic criteria, physicians will not normally identify an individual with both.
They also discovered that anorexia nervosa and obsessive-compulsive disorder have a strong genetic architecture, and that those who have a genetic propensity to having a smaller body type or a low BMI (body mass index) have a genetic susceptibility to both diseases.
Not unexpectedly, because the two disorders are frequently associated, the study discovered a significant genetic overlap between anxiety disorder and major depressive disorder.
When the researchers analysed accelerometer data, they discovered that conditions that tend to cluster together also seem to share genes that impact how and when we move during the day.
Internalizing diseases, such as anxiety and depression, for example, have a genetic design related with reduced mobility throughout the day. OCD and anorexia appear to connect with genes related with more movement throughout the day, whereas psychotic diseases (schizophrenia and bipolar disorder) tend to correlate with excess movement in the early morning hours.
“When you think about it, it makes sense,” said Grotzinger, noting that depressed individuals often present as fatigued or low energy while those with compulsive disorders can have difficulty sitting still.
In all, the study identifies 152 genetic variants shared across multiple disorders, including those already known to influence certain types of brain cells.
For instance, gene variants that influence excitatory and GABAergic brain neurons — which are involved in critical signaling pathways in the brain — appear to strongly underly the genetic signal that is shared across schizophrenia and bipolar disorder.
While much more needs to be done to determine exactly what the identified genes do, Grotzinger sees the research as a first step toward developing therapies that can address multiple disorders with one treatment.
“People are more likely today to be prescribed multiple medications intended to treat multiple diagnoses and in some instances those medicines can have side effects,” he said. “By identifying what is shared across these issues, we can hopefully come up with ways to target them in a different way that doesn’t require four separate pills or four separate psychotherapy interventions.”
Meantime, just understanding the genetics underlying their disorders may provide comfort to some.
“It’s important for people to know that they didn’t just get a terrible roll of the dice in life — that they are not facing multiple different issues but rather one set of risk factors bleeding into them all.”