Introduction: Schizophrenia, bipolar disorder (BD), and major depressive disorder are three common mental disorders. Although their diagnosis and treatment differ, they partially overlap.
Methods: To explore the similarities and characteristics of these three psychiatric diseases, an intelligence quotient (IQ) assessment was performed to evaluate cognitive deficits. Relative catechol-O-methyltransferase (COMT ) expression in peripheral blood mononuclear cells was examined in all three groups compared with healthy controls (HCs).
Results: The results indicated that patients with any of the three psychiatric diseases presented IQ deficits, and that the first-episode schizophrenia (FES) group had even lower cognitive function than the other two groups. The relative COMT  expression decreased in the FES group and increased in the BD group compared with the HC group. The correlation analysis of COMT  expression level and IQ scores showed a positive correlation between relative COMT  expression and full-scale IQ in the HC group. However, this correlation disappeared in all three psychiatric diseases studied.
Conclusion: In conclusion, this cross-disease strategy provided important clues to explain lower IQ scores and dysregulated COMT  expression among three common mental illnesses.
Keywords: first-episode schizophrenia, FES, bipolar disorder, BD, first-episode major depressive disorder, MDD, catechol-O-methyltransferase, COMT, intelligence quotient, IQ