Introduction

Major depressive disorder (MDD) is a major burden in healthcare; worldwide, 12% of individuals suffer from MDD1,2. MDD is considered to develop as a consequence of environmental influences on genetic predispositions, but a definite pathogenesis of MDD remains obscure3,4. There have been several hypotheses for the pathogenesis of MDD, including alterations in neurotrophins, the neuroendocrine and neuroimmune systems, and molecules involved in brain neurotransmission, including monoamines and glutamate, as well as epigenetic mechanisms4,5,6,7,8,9. Other hypotheses include alterations in immune and inflammatory responses, oxidative stress, mitochondrial dysfunction, and disruption of DNA damage responses, and biomarkers related to these mechanisms have been proposed10,11,12,13,14,15,16,17. Although there are environmental factors known to be related to the development of MDD, such as stressful events in childhood, there is still no reliable biomarker that can explain the development of MDD or differences between MDD patients and healthy individuals.

For pharmacotherapy of MDD, many second-generation antidepressants are used, such as selective serotonin reuptake inhibitors (SSRIs), serotonin and norepinephrine reuptake inhibitors (SNRIs), atypical antidepressants, and serotonin modulators. SSRIs potentiate serotonin (5-HT) by inhibiting its neuronal uptake pump. Some SSRIs also have minor noradrenaline and dopamine reuptake inhibitory properties18. Mirtazapine has a dual mode of action, antagonizing the adrenergic α2-autoreceptors and α2-heteroreceptors as well as by blocking 5-HT2 and 5-HT3 receptors19. At least one-third of patients treated with second-generation antidepressants do not achieve response20,21. Although there is no evident difference in overall efficacy among second-generation antidepressants, individuals vary widely in their response to specific antidepressant treatments20,21,22. To predict responses to antidepressants and to choose the appropriate treatment for each individual, the discovery of biomarkers related to therapeutic efficacy is urgently required. Thus far, there have been few studies to identify biomarkers that could predict therapeutic response in MDD, and there is no absolute predictor to help guide the selection of antidepressants23,24,25,26,27,28,29,30,31.

RNA is the immediate expression product of genes and better reflects the current functional status of the biologic system than does DNA. Recently, a number of gene expression profiling studies aiming to discover genetic markers related to the pathogenesis of MDD and antidepressant treatment response using RNA from postmortem brain tissues and peripheral blood have been published32,33,34,35,36,37,81, were among the top 10 upregulated and downregulated genes in our study. Clinical research results on the relationships between these genes and MDD have not yet been reported, and the nature of alterations in immune and inflammatory responses has not been fully elucidated and might involve complex mechanisms10,58.

The genes involved in immune and inflammatory responses that were among the top 10 downregulated and upregulated genes related to responsiveness to SSRI treatment consisted of BTNL8, CLC, CTSW, FCRL6, GNAQ, HLA-DPB1, IGKC, KIR2DS1, NOD2, USP41, VNN1, and XCL1. The BTNL8, HSPH1, IGKC, KATNBL1, LYVE1, MIR15A, PTCH2, and SCARNA17 genes were also found to be differently expressed between MDD patients and controls. The BTNL8 gene, which plays an essential role in primary immune responses82, was among the top 10 downregulated and upregulated genes in both comparisons. In addition, the FCRL6 gene was more highly expressed in responders than in nonresponders and was downregulated relative to the control group after SSRI treatment only in responders. The FCRL6 gene encodes Fc receptor-like protein 6, which is involved in the interaction between cytotoxic lymphocytes and antigen-presenting cells (APCs) and is a member of the MHC class II receptor family83. Therefore, immune and inflammatory systems, including the BTNL8 and FCRL6 genes, might be involved in responsiveness to SSRIs as well as the underlying derangement in MDD. Previous transcriptomic studies have focused on the differences between MDD patients and healthy controls32,63,65,66,84, and there have been few clinical transcriptomic studies attempting to identify genetic biomarkers associated with antidepressant treatment response in MDD27,28,29,30,31. Our study findings on differences in gene expression according to treatment response and gene expression changes with antidepressant treatment have great implications for understanding and predicting treatment responses to antidepressants in MDD. In particular, our finding that the BTNL8 and FCRL6 genes have relatively large differences in expression in two comparisons (Table 3 and Supplementary Tables 2 and 3) has not been previously reported; accordingly, further validation studies will be required to confirm these findings.

Among the 281 genes that showed expression differences related to responsiveness to mirtazapine treatment, 21 were also differentially expressed between MDD patients and controls. In GO and pathway enrichment analyses of these 21 genes, GO terms involved in coagulation and DNA damage responses were identified. In the investigation of the 21 individual DEGs, the SLA2 and TOPORS genes, which might be involved in immune responses and apoptosis, were identified, as well as the CEP63 gene, which plays a role in the response to DNA damage. Disruption of DNA damage responses in MDD has previously been demonstrated in several studies11,12,13, including targeted gene expression analyses16,17. However, individual genes involved in DNA damage responses have not been previously identified in the context of MDD and antidepressant treatment. Our findings support a role for DNA damage as a result of immune activation in MDD, and the finding that the expression of these genes in responders to mirtazapine was closer to the expression levels of controls than to those of nonresponders implies that these genes and their mechanisms might be relevant to treatment resistance to mirtazapine.

In this study, we performed gene expression profiling and various comparison analyses. However, we should acknowledge the limitations of this study. We did not perform quantitative PCR, which has usually been employed in previous microarray studies, and we did not perform replicate analysis of each specimen. However, we validated our study results in other ways, including analysis of public microarray data, comparison with our cytokine study results55, and through replicates of comparisons that were performed in our study: between MDD patients and controls, between responders and nonresponders, and between baseline and 6 weeks after antidepressant treatment. Another limitation of our study involves the issues of study population and sample size. The lack of GO terms and pathways with statistically significant FDR-corrected p-values could be due to the limited sample size. Although our study population had a high proportion of older patients and females, MDD patients and healthy individuals did not show any clustering of baseline expression according to age and sex. The replication of our findings by future studies is needed to clarify and prove that the genes and mechanisms we identified are associated with MDD and responsiveness to antidepressant treatment. Our study has certain advantages in the discovery of genes related to antidepressant treatment response, which has rarely been a subject of transcriptomic studies, and in the classification of analyses according to type of antidepressant used. Most previous studies performed gene expression in MDD patients compared with healthy controls85. There have been few previous transcriptomic studies in MDD patients aiming to identify biomarkers related to antidepressant responsiveness27,28,29,30,31. Our study is the first to examine the treatment response to mirtazapine and to analyze changes in gene expression using genome-wide microarray techniques in responders compared to non-responders, and it is unique in that it was conducted in a non-Caucasian ethnic group39.

Various genes and mechanisms involved in immune and inflammatory responses, including the CXCL8, BTNL8, and FCRL6 genes, which are commonly related to T lymphocytes, were identified in our study. However, as in most previous studies, we did not confirm whether these differences and changes are the causes or the results of a depressive episode. The mechanism underlying these differences and their relation to disease and treatment response remains unknown because our results are preliminary and have not been verified by functional analysis. MDD is a multifactorial disorder, and a previous study proposed that complex and multiple mechanisms, including the disruption of oxidative stress response, damage to DNA and mitochondria, and neuroprogression including neuronal apoptosis and lowered neuroplasticity, as concomitants and sequelae of the activation of immune and inflammatory systems play roles in the development of MDD11. Various genes involved in mechanisms other than those mentioned above also showed alterations in expression in our study. Our study results support the complexity of the development and treatment response of MDD. The complete interrelationships among these multiple mechanisms cannot be detected with the current statistical analyses; thus, the application and development of further bioinformatic analyses would be required to dissect these complex associations in MDD. In addition, it will be necessary to correlate these findings with other studies, including proteomic and metabolic results, to thoroughly investigate the roles of the identified genes and mechanisms in the development of MDD and antidepressant responses.

In summary, our study identified several interesting genes and mechanisms that might be associated with MDD and treatment responsiveness to antidepressants using RNA microarray analyses of peripheral blood specimens from MDD patients. In this respect, our study provides clinical evidence relevant to previous theories on the development of MDD and can serve as a foundation for future studies on antidepressant treatment response. Our study results support the proposition that the development of MDD and antidepressant responses are associated with a series of events that includes DNA damage and apoptosis stemming from immune and inflammatory activation in MDD. Our study is the first to analyze changes in gene expression using genome-wide microarray techniques in treatment responders compared to non-responders. Our findings contribute to the elucidation of the biological disturbance of MDD and might lead to early therapeutic intervention and personalized medicine for the treatment of MDD with antidepressants. Specifically, genes that are involved in immune and inflammatory responses and their sequelae, including BTNL8, CXCL8, and FCRL6, are candidates for prediction of antidepressant treatment response as well as for diagnosis of MDD.