In the present study, we found shared genetic architecture between MDD, BIP-I, and BIP-II and biological rhythms; the observed clinical comorbidities appear to have a root in common genetic variation. Although MDD, BIP-I, and BIP-II are highly correlated, direction and strength of their genetic associations with biological rhythms differ, underlining the importance of examining the subgroups of mood disorders.
The observed negative correlations between MDD and increased activity (overall physical activity and moderate activity) reflect clinical observations that mean activity is decreased in MDD patients [5, 40]. Recent studies using genome-wide data have found that physical activity can decrease the risk for depression [41, 42], and also report negative genetic correlations of MDD with overall physical activity and walking [33]. A recent twin study found a genetic relationship between decreased depression and increased activity [43]. The positive genetic correlation of BIP-I and BIP-II with moderate activity and the negative correlation of BIP-I with sedentary behavior are in line with findings showing a positive genetic correlation of BIP (without specification of subtypes) with moderate activity and walking [33] based on a previous BIP GWAS [44]. A Mendelian Randomization study investigating the causal relationship between physical activity and BIP, found that a increase in physical activity is associated with decreased liability to developing BIP [45]. The observed genetic correlation of BIP-I and BIP-II with increased activity, may reflect the genetic factors underlying the respective manic and hypomanic features [7, 10]. Placing the present findings on BIP-I and BIP-II in the context of the genetic literature, it should be noted that the BIP data used in earlier works is based on GWAS where the large majority of the patients were diagnosed with BIP-I [44]; the present analysis uses a larger dataset and separately examines BIP-II.
We observed negative correlations of MDD, and BIP-II, with relative amplitude (lower relative amplitude indicates a more disrupted circadian rhythm); circadian rhythm disruption plays a prominent role in MDD and BIP disease etiology and treatment. Genetic studies provide evidence for a close relationship between circadian, MDD and BIP genes [for reviews, see 46, 47, 48], with one hypothesis being that mood symptoms (such as depressive or manic symptoms) arise from dysregulation of circadian clock genes [49, 50]. Our findings confirm and provide a potential background for clinical observations of less stable patterns of movement and disrupted circadian rhythms in MDD and BIP [51, 52, 15]. We note that the strongest and most statistically significant correlation in the present analysis was between MDD and relative amplitude, suggesting this relationship as a target for further investigation and characterization.
Symptoms such as insufficient sleep duration and frequent awakenings are often reported in mood disorders [53, 54]. Neither MDD, nor either BIP subtypes were found to be significantly correlated with objectively measured sleep duration, leaving it an open question whether and to what extent common genetic factors might contribute to their relationship. The literature so far is also unclear. For example, a GWAS conducted on sleep duration found a significant genetic correlation of sleep duration and BIP, but not with depressive symptoms [55]; further investigations of these relationships await. We also observed that MDD, BIP-I and BIP-II are significantly positively correlated with daytime sleepiness, the only subjective measure used in this analysis. Our use of larger summary statistics appears to clarify prior results which had shown genetic correlations between daytime sleepiness and depression, but not BIP [31]. Although a consequence of a disrupted circadian rhythm and insufficient sleep can be increased daytime sleepiness (both often reported in MDD and BIP [56, 22]), the present results hint at a more fundamental shared etiology, which bears further examination.
In comparing correlations, the most prominent differences were found between MDD and BIP-I, which showed genetic correlations with opposite directions for all objectively assessed traits besides relative amplitude (although in the same direction, correlation strength differed significantly). These differences may be causally linked to the clinical symptoms observed in MDD (low activity) and BIP-I (high activity). BIP-II showed an intermediate correlational pattern, with the only significant differences in relative amplitude and daytime sleepiness to BIP-I and a significant difference in moderate activity to MDD. Notably, MDD and BIP-II showed closer resemblance in sleep duration, relative amplitude and daytime sleepiness than BIP-I and BIP-II, which suggests a stronger link between genetics of depressed features with these phenotypes. At the same time, BIP-I and BIP-II are more similar to each other when compared to MDD with respect to increased activity phenotypes. It is of interest to note that the strongest similarities between the mood disorders are observed in daytime sleepiness suggesting that daytime sleepiness shares common genetic etiology with all mood disorder types. It should be kept in mind, that daytime sleepiness was the only biological rhythms measure, that was assessed by self-report, which might have contributed to this result; highlighting the value of objectively assessing traits in large scale studies. This similarity, as well as the same direction of effect with respect to relative amplitude, may reflect the common genetic underpinnings, and the quantitative measures which differentiate them from healthy controls. In summary, the discovered similarities and differences appear to be clues to delineating these mood disorders with respect to each other, on a continuum or otherwise.
A limitation of this study is the sample size of some of the used GWASs; the power of BIP-II and relative amplitude summary statistics is lower than the other GWASs, which is of note for the statistical comparison of the genetic correlations. Although still well powered, not all genetic correlations or differences between genetic correlations were significant after corrections for multiple testing (see Fig. 1, Table S1.1-S2.3). With larger samples available, the analysis of causality with methods like MR will become possible as the number of identified significant SNPs associated with newer phenotypes such as physical activity and relative amplitude is expected to increase. Continued efforts are needed to acquire increased sample sizes of under-characterized subtypes and these emerging phenotypes.
The present results show that clinically observed relationships of mood disorders and biological rhythms have a common genetic basis, and indicate that alterations in biological rhythms observed in mood disorder patients are linked to the genetic vulnerability for the specific disorder, and not only the current disorder state or medication status. The causality behind this bears further investigation. Biological rhythms should be given more attention in the study of mood disorders and require greater consideration with respect to assessment and treatment. If shared genetic factors jointly affect biological rhythms and liability to mood disorders, further investigation may allow targeting of these factors in treatment, providing potential avenues of improvement for therapeutic approaches.