Mental disorders, also known as mental illnesses are health condition that involves changes in behavior, emotion, thinking, or combination of all of these. Some of the examples of such disorders include attention deficit hyperactivity disorder (ADHD), autism, bipolar disorder, major depression, substance use disorder, and schizophrenia. Mental disorders can lead to trouble in normal personal behavior, and social functioning of an individual. According to World Health Organization about 25% of the world population will develop at least one mental disorder at some stage of their life. According to the NIMH, currently, there are about 450 million people worldwide living with mental disorder. In the US, about 20% (46.6 million) adults have some kind of mental disorder . The mental disorder was higher among women (22.3%) than men (15.1%). When it was compared among different age groups, mental disorder was observed highest (25.8%) among young adults (18-25 years) as compared to adults aged 26-49 years (22.2%) and adults aged 50 years and above (13.8%).
Earlier, people believed that mental disorders are mainly due to environment. However, recent studies found that mental disorders are due to the combination of both environmental and genetic factors. The environmental factors that contribute to the mental disorder include trauma (e.g. sexual, physical and emotional abuse), emotional harm (e.g. bullying), and substance abuse (e.g. tobacco, alcohol, and illegal drugs). Among genetic factors, epigenetic regulation, genetic polymorphism, and single gene changes are major ones. Since the mental disorders are due to the combination of environmental and genetic factors, the inheritance of this kind of disorder is called multifactorial inheritance.
To elucidate the role of genetic factors on mental disorders, a group of international Scientists conducted a genome-wide association study of five major mental disorders (autism, ADHD, depression, bipolar disorder, and schizophrenia). The study included 33,000 patients who had at least one of the five mental disorders. The study also included a control group of 28,000 individuals who did not show any of those disorders. The study revealed a significant association between genetic variation of two genes CACNA1C and CACNB2 and those mental disorders. These genes are known to regulate the flow of calcium into neurons. The gene CACNA1C has already been linked to depression, bipolar disorder and schizophrenia. Whereas, the gene CACNB2 was associated with all five disorders. Scientists also revealed that the disorder-linked variation of those two genes occurred in a particular regions of chromosome 3 and 10. Despite the research reported a significant association of certain genes with those disorders, it is still too early to use genetic method to predict or diagnose the illness because the inheritance of such illnesses is due to multiple factors including environmental factors. However, this kind of finding encourages scientists to explore more to develop methods to diagnose, predict and treat mental disorders using the genetic and environmental information.