We find that the PLA2 BanI and the COX2 rs4648308 polymorphisms influence the risk of developing IFN-α–induced depression. Moreover, the PLA2 BanI polymorphism is also associated with higher severity of somatic symptoms of depression (but not of other symptoms), in patients with IFN-α–induced depression and in a replication sample of patients with major depression unrelated to cytokine treatment. Finally, these genetic effects seem to be mediated by the regulation of EPA and DHA levels.
The fact that PLA2 BanI polymorphism influenced the risk of IFN-α–induced depression is consistent with previous evidence. The G variant in the BanI polymorphism—the same genotype that increases the risk of IFN-α–induced depression in the present study—has been previously reported to be associated with depression (34
). Moreover, the BanI GG genotype has been reported to be associated with higher PLA2 enzyme activity in platelets, suggesting that this polymorphism is functional (45
). In our study, we have evaluated PUFA levels rather than PLA2 enzyme activity, but the results further support the functional significance of this BanI polymorphism. In fact, subjects who had the PLA2 BanI “at risk” GG genotype had lower EPA levels before IFN-α treatment. Because the PLA2 enzyme deacylates EPA from membrane phospholipids (46
), the lower EPA levels in those with the GG genotype could be explained by the higher PLA2 enzyme activity. In turn, the lower EPA levels could explain the higher risk of developing IFN-α–induced depression: in several case-control studies, lower EPA levels are associated with increased risk of depression and with increased severity of depressive symptoms (48
The PLA2 BanI GG genotype was also associated with more somatic symptoms of depression, both among those who developed IFN-α–induced depression and in a replication sample of patients with major depression. These shared genetic effects across the two samples are consistent with previous reports showing that patients with IFN-α–induced depression have similar neuroendocrine and neuroimaging abnormalities as those described in patients with major depression unrelated to cytokine treatment (50
). The lower EPA levels found in our subjects with the PLA2 BanI GG genotype are a possible explanation for this high risk of specifically developing somatic symptoms. In fact, somatic symptoms of depression are similar to symptoms of “sickness behavior” induced by cytokine administration (6
) and are observed in up to 80% of patients with major depression (53
). Indeed, somatic symptoms have been described by Dantzer (54
) as “the outward manifestation of sensitization of the brain cytokine system that is normally activated in response to activation of the innate immune system and mediates the subjective, behavioral, and physiological components of sickness.”
The other main finding of this study is that the COX2 rs4648308 polymorphism is also associated with IFN-α–induced depression. Specifically, patients with the AG genotype had a higher risk of developing depression during IFN-α treatment. In addition, the “at risk” AG genotype was also associated with lower DHA levels before and during IFN-α treatment, implying a functional significance of this polymorphism. Indeed, lower baseline DHA levels were a risk factor for the development of depression independently from the genotype, further supporting the notion that lower baseline DHA levels are indeed a risk factor for the development of depression. This is consistent with studies showing lower DHA levels in patients with major depression (23
Subjects with lower PUFA levels before starting IFN-α could be at an increased risk of developing depression by having lower PUFA levels or failing to mount a compensatory PUFA increase during the immune challenge or both. As shown in , erythrocyte PUFA levels vary during the 24 weeks of IFN-α, with an initial decrease during the first 2 weeks and an increase in the following weeks. A possible explanation for the initial decrease in PUFA levels is that IFN-α increases the activation of PLA2, leading to the release of PUFAs from the erythrocyte membrane phospholipids (57
). This initial decrease in membrane PUFAs might then result in an increase in free-form PUFAs, which in turn could exert an “inhibitory feedback” and thus attenuate the IFN-α–induced PLA2 activity, leading to the increase in the erythrocyte PUFA levels in the later weeks of treatment (59
Higher depressive symptoms at baseline (before starting IFN-α) are considered one of the strongest factors predicting the IFN-α–induced depression (3
). However, for both PLA2 and COX2 genes, there were no differences in the baseline BDI scores between the “at risk” and the “low risk” genotypes. This indicates that the main effect of these “immune” genes is to increase the risk of developing depression specifically in response to IFN-α, probably by modulating PUFA levels during the treatment. This is consistent with our previous findings showing that the effects of IL-6 gene on IFN-α–induced depression are also independent from baseline depression scores (12
We also attempted to replicate, in this Chinese sample, our previously published findings showing an effect of the IL-6 and 5-HTT genes on IFN-α–induced depression in a Caucasian population (12
). Specifically, we have previously found that the IL-6 “G” and the 5-HTTLPR “s” genotypes increase the risk of IFN-α–induced depression, compared with the “CC” and the “ll” genotypes, respectively (12
). However, there were only IL-6 GG subjects in the present Chinese sample, and thus we could not test the “at risk” effects of this allele. The lack of C genotype in a Chinese sample is a novel finding, although it is consistent with recent HapMap data showing a G allele frequency of 100% in Asians (60
). Interestingly, we also did not find an effect of the 5-HTTLPR “s” polymorphisms in the present study. In this regard, it is important to emphasize that the effects of the 5-HTTLPR genotype in our previous study are only present in those with the IL-6 CC genotype (i.e., in those with the lower immune activation genotype) (12
). Therefore, our negative findings in this Chinese sample (all GG) are actually replicating our previous negative findings in Caucasian GG subjects. Interestingly, the “at risk” effects of the 5-HTTLPR genotype have been replicated by a second (14
) but not a third study (15
), thus supporting the notion that the effects of 5-HTTLPR genotype on IFN-α–induced depression might be dependent on other genes and on the inflammatory status. We did not assess, in this or in our previous study, any other SNPs in the 5HTT gene. Interestingly, Lotrich et al.
) also examined a polymorphism within the “l” allele that might result in lower transcription efficiency, functionally comparable with the “s” allele, as well as a variable number of tandem repeats polymorphism in the second intron. They found that the polymorphism within the “l” allele also influences the risk of developing depression, whereas the variable number of tandem repeats polymorphism has no effect (14
To our knowledge, this is the largest published study examining genetic predictors of IFN-α–induced depression, the only one assessing genes as well as biomarkers in the same sample, and the first one examining biological markers of IFN-α–induced depression in Han Chinese people. Moreover, our findings are strengthened by the fact that all subjects were antidepressants-free before the development of IFN-α–induced depression. Nevertheless, there are a few important methodological considerations and study limitations. The selection of the polymorphisms from PLA2 and COX2 genes was not completed systematically, because of the lack of comprehensive bioinformatic tools when this project started in 2005. Moreover, the distribution of the COX2 rs4648308 in the case group violated HWE, although some authors have argued that this is acceptable, because “cases” in case-control studies cannot be considered “general population” (61
). Furthermore, we did not adjust for the number of statistical comparisons. Many genetic studies, especially those where the choice of genes is hypothesis-driven (like ours), do not apply such adjustment. For example, none of the previous studies investigating genetic predictors of IFN-α–induced depression adjusted for the number of statistical comparisons (12
). Finally, only a subset (48%) of the genetic sample was available for the analysis of PUFA levels. Clearly, our findings will need to be replicated independently.
In conclusion, we describe two polymorphisms in genes relevant for PUFA metabolism that increase the risk of IFN-α–induced depression, together with the putative molecular mechanism (lower EPA and DHA levels) explaining these genetic effects. Moreover, we find a regulatory effect of PLA2 gene on somatic symptoms in major depression unrelated to cytokine treatment. Because n-3 PUFAs have been found to be a safe and effective treatment for depression, these findings provide the rationale to conduct a clinical trial to test the prophylactic effects of treatment with n-3 PUFAs for IFN-α–induced depression. Furthermore, these results confirm the role of inflammatory mechanisms in the pathogenesis of major depression.