Twin studies of psychiatric disorders such as schizophrenia and autism spectrum disorder have employed epidemiological approaches that determine heritability by comparing the concordance rate between monozygotic twins (MZs) and dizygotic twins. The basis for these studies is that MZs share 100% of their genetic information. Recently, biological studies based on molecular methods are now being increasingly applied to examine the differences between MZs discordance for psychiatric disorders to unravel their possible causes. Although recent advances in next-generation sequencing have increased the accuracy of this line of research, there has been greater emphasis placed on epigenetic changes versus DNA sequence changes as the probable cause of discordant psychiatric disorders in MZs. Since the epigenetic status differs in each tissue type, in addition to the DNA from the peripheral blood, studies using DNA from nerve cells induced from postmortem brains or induced pluripotent stem cells are being carried out. Although it was originally thought that epigenetic changes occurred as a result of environmental factors, and thus were not transmittable, it is now known that such changes might possibly be transmitted between generations. Therefore, the potential possible effects of intestinal flora inside the body are currently being investigated as a cause of discordance in MZs. As a result, twin studies of psychiatric disorders are greatly contributing to the elucidation of genetic and environmental factors in the etiology of psychiatric conditions.
Neuropsychiatric disorders such as autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), schizophrenia, bipolar disorder, and major depressive disorder tend to be classified as distinct entities. However, increasing evidence suggests that there are overlaps among these disorders in terms of their genetic risk factors. For example, chromosomal microdeletions and duplications in 16p11.2 have been reported in individuals with ASD as well as those with schizophrenia and intellectual disability, and common copy number variations have been reported in ASD, schizophrenia, and ADHD. Genome-wide association studies have also revealed common risk variants among ASD, ADHD, bipolar disorder, depression, and schizophrenia. Moreover, next-generation sequencing techniques have revealed overlap in de novo mutations among ASD, schizophrenia, and intellectual disability. Together, these results indicate that
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