This study is the first to report an association between the G > C polymorphism in the IL-6 gene and the 5-HTTLPR insertion/deletion polymorphism in the 5-HTT gene, and protection against the development of depressive symptoms induced by IFN-α treatment. Specifically, carrying two copies of the ‘low IL-6’ C allele or of the ‘high transcription’ L polymorphism was associated with a less marked increase in depressive symptoms. The protective effect of ‘low IL-6’ genotype was large and present irrespective of the 5-HTT polymorphism. In contrast, the effect of the 5-HTT polymorphism (in the whole sample) was small, and subsequent subgroup analyses show that this effect was only evident in patients who also carry the ‘low IL-6’ CC genotype. Neither polymorphism offered any protection against the fatigue developed during treatment.
The finding that the ‘low IL-6’ genotype is associated with fewer depressive symptoms during IFN-α treatment is consistent with evidence that plasma IL-6 positively correlates with depressive symptoms during IFN-α treatment,11
and major depression.14-16
Our data also supports previously published preliminary finding from the American sample, examining the role of the IFN-α receptor gene in the development of depression.50
The authors found that the 5/5 or 5/14 genotype of a GT repeat dinucleotide microsatellite polymorphism was associated with a larger increase in depressive scores during IFN-α treatment. Because this genotype has also been associated with a superior antiviral activity,51
these data further support the notion that higher immunological activity is associated with the development of IFN-α-induced depression.
We found that the LL 5-HTT genotype was associated with a much smaller protective effect than the CC IL-6 genotype; interestingly Kraus and co-workers5
found no effect of this polymorphism on IFN-α-induced depression. Of course, previous studies have demonstrated, in psychiatric populations, that the LL 5-HTT genotype is protective against depression30
and is associated with a better clinical response to antidepressants.36,37
Therefore, it is interesting to speculate that the protective effects of the LL genotype can only be evident under specific biological circumstances—for example, in the presence of low inflammation as discussed below. In contrast with the 5-HTT genotype, a 5HT1A receptor gene polymorphism (C-1019G) has been shown by Kraus and co-workers5
to have a profound effect on IFN-α-induced depression, measured as both occurrence of clinically significant symptoms and as changes in depressive symptoms from baseline.
It is somewhat surprising that in our study neither the IL-6 nor the 5-HTT polymorphism was associated with fatigue. Plasma concentrations of IL-6 have previously been found to correlate with fatigue;13
but our findings do not necessarily contradict such evidence. It is important to note that the absence of an association between this polymorphism and fatigue does not exclude IL-6 from the pathophysiology of fatigue, because factors other than the polymorphism may have induced elevated IL-6. However, the role of IL-6 in fatigue is not certain because it can be difficult to differentiate the role of IL-6 and IL-1 in inducing behavioural changes as they are both typically elevated during immune activation. Our study is consistent with a recent paper that found no association between this IL-6 polymorphism and chronic fatigue syndrome.52
There is a paucity of data regarding the relationship between serotonin transporter and fatigue, and the results have thus far been inconsistent. In one small association study, patients with chronic fatigue syndrome had higher prevalence of the L allele compared to controls,53
but a recent neuroimaging study has shown reduced concentrations of 5-HTT in the brain of patients with chronic fatigue syndrome.54
Even if depression and fatigue are both cytokine-induced ‘sickness behaviours’ and typically occur together, do the differences in their genetic regulation indicate a different pathophysiology? Dimensional analysis of symptoms experienced by patients taking IFN-α for malignant melanoma reveals that fatigue (together with psychomotor slowing and anorexia) is distinct from depressive symptoms (depressed mood, anhedonia and anxiety): fatigue occurs earlier in treatment and responds less to antidepressant treatment, whereas depressive symptoms develop later in the treatment and respond more to antidepressants.55
In our previous prospective cohort studies of patients taking IFN-α for HCV (both in the United States and the United Kingdom), the symptoms follow this same pattern, with increases in fatigue evident earlier than depressive symptoms.2,39
However, not all studies have shown depressive symptoms peaking at the end of the treatment, as in this paper and our previous reports;2,39
indeed, samples where depressive symptoms reach maximum scores already at weeks 12–16 have also been described.56,57
Two biological mechanisms also appear to be associated with developing depression but not fatigue during IFN-α treatment. First, patients who have a larger response of the corticotropin-releasing hormone-mediated stress pathway to an initial dose of IFN-α are significantly more likely to develop major depression during treatment, but not fatigue.58
Second, the decrease of plasma tryptophan and the increase of kynurenine and neopterin during IFN-α treatment (indicating increased IDO activity) correlate with the development of depression, but not fatigue.59
Taken together with the present study, these differences between depression and fatigue in time course, response to treatment and correlations with biological variables, suggest that IFN-α-induced depression and fatigue are distinct and reflect different pathophysiological processes.
Our data also indicate the presence of an interaction between the IL-6 and the 5-HTT genes in the development of depressive symptoms. Although the ‘high IL-6’ carriers were consistently more depressed than ‘low IL-6’ carriers, within the ‘high IL-6’ group there was no protective effect associated with the 5-HTT LL genotype: the protective effect of the LL genotype was only evident within the ‘low IL-6’ CC group. Although we did not measure tryptophan availability, these findings indirectly support previous studies indicating that the effects of this 5-HTT polymorphism are influenced by the availability of tryptophan.60,61
Theoretically, subjects with ‘high IL-6’ genotype could have hyperactive IDO and therefore reduced plasma tryptophan, whereas subjects with the ‘low IL-6’ genotype would have normal plasma tryptophan. As suggested above, the fact that putative protective effect of 5-HTT LL genotype is dependent on other conditions (genetic and immunological) could explain the overall small protective effect in our study as well as the lack of any protective effect in the study by Kraus and co-workers.5
One of our aims in conducting this study was to predict which patients were at an increased risk of developing psychopathological symptoms, with the aspiration that genotyping could be used in the future as a clinical screening tool, to offer targeted prophylactic treatment. Although plasma IL-6 concentrations before starting IFN-α treatment have been found to predict the development of symptoms,12
measuring plasma cytokines at one time point does not provide a reliable cytokine secretion profile62
and is therefore not an ideal screening tool to identify trait vulnerability within a clinical setting. Our findings indicate that those patients with both the CC (IL-6) and LL (5-HTT) polymorphisms are less likely to develop severe symptoms, but this identifies a relative small group of resilient individuals, rather than those at greatest risk who would most benefit from prophylactic treatment. Nonetheless, considering the debate around the use of prophylactic treatment with antidepressants,63,64
and the administration of IFN-α in patients with mental disorders,42,43,65
even identifying a ‘very low risk’ group is important.
A few limitations must be mentioned. This study was comprised of subjects recruited in both the United Kingdom and the United States, and depression was measured using two different scales in the two sites. However, in our analysis, we minimized the difference between sites, by standardizing the scores to effect size and adjusting for baseline measures. Moreover, when the data were analyzed in accordance with country of recruitment, the direction of the findings was similar across the two centres (albeit with lower statistical power, and more consistently for the IL-6 genotype then for the 5-HTT genotype). Of course, the findings might be confounded by the use of antidepressant or other psychotropic medications; reassuringly, there were no differences between the genotype groups in these variables. However, the small number of subjects precluded us from conducting further subgroup analyses, and therefore we cannot exclude differences in the pattern of use of psychotropic medications; for example, the protective genotypes might be associated with early use of antidepressants and hence with lower depression scores as a result of antidepressant treatment. Finally, our findings of an interaction between the IL-6 and the 5-HTT genes should be approached with caution, given the relatively small sample size and the lack of concomitant biological measures, such as plasma and cerebrospinal fluid concentrations of IL-6 or tryptophan. A strength of this study is that both candidate genes were hypothesis driven, and based on previous research that has identified these SNPs as being functional. Moreover, the allelic frequencies in our sample were similar to those in the general population, as previously reported in the literature: IL-6 C allele = 37% in our sample vs 40% in the study of Fishman et al
and 5-HTT L allele = 66% in our sample vs 65% in Hu et al
Therefore, we would suggest that data from this sample may be extrapolated to the general Caucasian population.
In conclusion, our study shows that functional polymorphisms in the IL-6 gene (rs1800795) and, with a smaller effect, the serotonin transporter gene (5-HTTLPR) are protective factors against the development of depressive (but not fatigue) symptoms during IFN-α treatment, confirming the role of the inflammatory response and serotonergic systems in the pathophysiology of IFN-α-induced depression and possibly in the pathophysiology of major depression.