A Genomic Investigation of Major Depressive Disorder and Antidepressant Response

Major depressive disorder (MDD) is a common and complex disorder with consistent evidence of genetic influence on predisposition. The search for susceptibility genes has proved to be an arduous task with studies having ever increasing sample numbers still not leading to replicable findings. It is generally believed that the failure of common genetic studies to identify replicable genetic associations is a consequence of the multifactorial and heterogeneous nature of MDD. This etiological heterogeneity may also explain significant variability in treatment response. Accordingly, while effective treatments for MDD are available, approximately half of the patients fail to respond to conventional antidepressant treatment. We hypothesized that peripheral gene expression could help us better understand illness heterogeneity and mechanisms of antidepressant response, possibly helping to identify biomarkers. To test this hypothesis, we have prospectively followed, and treated with the antidepressant citalopram for eight weeks, a cohort of medication naive individuals with MDD. RNA and DNA from pre- and post-treatment blood samples of this cohort were used to perform high-throughput pharmacogenomic and genetic studies to identify genes involved in treatment response and in the pathophysiology of MDD. Significant gene expression alterations in immune related genes were observed after citalopram treatment, pointing to a possible mode of action of the treatment response, as well as possible biomarkers for future treatment response. Additionally, we identified copy number variable regions differentiating MDD and controls and having a significant impact on gene expression. These results provide important additional information which can be used to identify the genomic and molecular underpinnings of MDD and antidepressant response.