BDNF and depressive disorder: Since miRNA may regulate central nervoussystem functions such as cognitive function and reward feedback, we hypothesizedthat with the extension of disease course, changes in miRNA-451a, miRNA-34a-5p, andmiRNA-221-3p expression may cause the deterioration of the central nervous system.Studies have shown that reduced BDNF plays an important role in depression andsuicidal behavior (34).