## 4 Discussion
When we further investigated the relationships between disorders included in the identified cluster, MR analysis supported the existence of a bidirectional association between MDD and insomnia (Table 3). The majority of SNPs found to drive this association were observed to affect the expression of a number of genes in different brain regions (Tables 4 and 5). Our results are in line with the study from [PERSON] and colleagues, who observed a significant bidirectional association between MDD and insomnia using another MR method compared to the ones we used in the present analysis [53]. Some of these loci, such as the ones having as the nearest genes _TMEM161B_, _LRFN5_ or the _RP11-6N13.1_ non-coding RNA, were also found to be associated with both MDD and insomnia in a recent study conducted by [PERSON] and colleagues, using the conjunctional FDR method to identify genetic loci associated with pairs of traits [21]. When we investigated the loci in which SNPs driving the MR association were located using local genetic correlation analysis, we observed a significant positive local genetic correlation between MDD and insomnia for three loci (Table 7). For two of these loci, cardiometabolic traits were not found to exert a significant effect on the association between MDD and insomnia. At the first locus, the SNP driving the MR results was rs77217059. This SNP is located in the _LINC01122_ long non-coding RNA, the biological role of which is still unknown. The other locus that showed a significant correlation between MDD and insomnia, also when adjusting for BMI, was at chromosome 13 and included the two SNPs rs9536381 and rs9563152 among those driving the MR results. Both SNPs are intergenic and the second was found to drive the expression of the _RP11-24H2.3_ long non-coding RNA in the anterior cingulate brain region in GTEx. Specifically, the T-allele of the rs9563152 SNP, which was associated with increased predisposition to both MDD and insomnia (Table 4), is also associated with reduced expression of _RP11-24H2.3_ in the anterior cingulate. While the biological relevance of this and of several long non-coding RNAs is not known, this class of transcripts has been increasingly investigated in relation to their potential role in the pathogenesis of psychiatric and neurological disorders [54; 55]. For the locus on chr3:47588462-50387742 (SNP: rs9831648), genetically-predicted BMI and CRP levels were found to account for a notable proportion of the local \(r_{g}\) between MDD and insomnia, and the association was no longer significant when adjusting for these variables (Table 8). In particular, BMI and CRP levels were able to explain a large part of the genetic variance for insomnia at this locus (47% and 44%, respectively) and the observed association between MDD and insomnia was probably due to a significant correlation between both MDD and insomnia with higher BMI (Table 7). Our results suggest that cardiometabolic traits such as increased BMI or inflammation might mediate the association between MDD and insomnia at some but not all loci associated with increased predisposition to both traits. Interestingly, a recent study including 1894 participants from the English Longitudinal Study of Ageing showed that sleep disturbances at baseline predicted depressive symptoms at eight-year follow-up [56], even when baseline depression was considered, in accordance with previous studies. In addition, high levels of the CRP inflammatory marker mediated the association between sleep disturbance and depressive symptoms in women [56], supporting the existence of a relationship between depression, cardiometabolic and sleep-related traits. While PTSD and ADHD were included in the same cluster using the k-means algorithm, we found limited evidence or no significant association in the MR analyses in which we tested the bidirectional association between insomnia and these traits (Table S3).