Dynamic Model of Serotonin Presynapse and Its Application to Suicide Attempt in Patients with Bipolar Disorder

Suicide attempts are prevalent among patients with bipolar disorder (BD). Impaired serotonin (5-HT) system in the pathogenesis of suicide attempt is partially heritable. To quantify the combined effects of multiple genetic variants, we developed a dynamic model of the 5-HT presynapse with functionally integrated individual genetic variants. The model includes five genetic variants in 5-HT system genes (<i>TPH2</i>, <i>SLC6A4</i>, <i>MAOA</i>) and quantitatively assesses their influence on 5-HT synthesis, reuptake, and degradation. The model was validated on 140 unaffected individuals and tested on 101 BD patients. Predicted mean concentrations of 5-HT, 5-HT precursor, and degradation product were compared between BD patients with and without a history of attempted suicide, and unaffected individuals. The model consists of eight differential equations that describe the temporal concentration change of model outputs. Calculated concentrations in unaffected control individuals aligned with published experimentally measured values, validating our model. BD patients with a history of suicide attempt showed lower calculated concentrations of 5-HT degradation product 5-hydroxy-3-indolacetic acid (5-HIAA) compared to unaffected individuals (<i>p</i> = 0.044). Additionally, higher calculated concentrations of free cellular 5-HT (<i>p</i> = 0.048) and stored 5-HT (<i>p</i> = 0.047), with the effect size d = 0.35, were observed when comparing suicide attempters to non-attempters.. Our approach illuminated a complex interplay of genetic variants in 5-HT system genes that contributes to the risk of suicide attempt, with quantitative and personalized outputs unattainable through genetic association studies.