Transcriptional regulation of the human 5-HT1A receptor gene: Implications in major depression and suicide

Abstract

Major depressive disorder (MDD) constitutes the most commonly diagnosed mental illness affecting 16% of the population. Reduction in serotonergic tone is the most widely accepted etiological hypothesis for MDD and antidepressant treatments enhance serotonin (5-hydroxytryptamine, 5-HT) neurotransmission. Negative regulation of serotonergic raphe neurons is mediated by somatodendritic 5-HT1A autoreceptors, which are increased in depressed suicides and become down-regulated before antidepressants take effect. I hypothesized that genetic variations in regulatory regions of this receptor that dictate its expression, could contribute to predisposition to depression and treatment responsiveness. I initially addressed the basal mechanisms of human 5-HT1A receptor gene regulation using transient transfections with luciferase reporter constructs of 5' flanking sequences. A region between -1624 and -1550 by displayed strong repressor activity and contained at least three repressor elements: a consensus RE-1 and two copies of a novel dual repressor element (DRE). By yeast one-hybrid screening we identified a novel calcium-regulated repressor (Freud-1) that binds to DRE to reduce basal 5-HT1A receptor expression in neurons. Using an inhibitor of histone deacetylase (HDAC), we have demonstrated that Freud-1 mediates HDAC-independent repression in neuronal 5-HT1A positive cells, while REST or other DRE binding proteins recruit HDAC-dependant mechanisms to silence the receptor in non-neuronal 5-HT1A-negative cells. I also searched for sequence variations in 5-HT1A regulatory regions that may associate with depression. Further downstream from this region, we have identified a functional C(-1019)G polymorphism in the human 5-HT1A promoter that associates with major depression and completed suicide. The occurrence of the G allele at -1019 by prevents binding and repression by specific transcription factors NUDR and Hes5, identified by yeast one hybrid approach, and results in de-repression of the 5-HT1A receptor gene and hence, may contribute to the predisposition to depression. In conclusion, I have identified important transcriptional regulatory elements and proteins of the 5-HT1A gene implicated in serotonin neurotransmission, and characterized the mechanism of a new functional 5-HT1A promoter polymorphism involved in both suicide and MDD. This study may provide an improved marker for diagnosis and treatment of depression and provide a model for correlation between polymorphisms, gene expression and mental illnesses.