11/ We also investigated protein-protein interaction (PPI) networks. mDis3 proteins were significantly more interconnected than mDis1 proteins. This implies pleiotropic proteins may amplify their effects through network connectivity, rippling through molecular pathways.

9/ We next examined the functional properties of variants and their target genes. Both mDis1 and mDis3 variants were enriched in the excitatory neuronal lineage. In this lineage, mDis1 genes were often expressed in a single cell type, while mDis3 genes showed prolonged, multi-cell-type expression.

8/ Next, we wondered if mDis3 variants have more complex regulatory connectivity (e.g., linking to multiple genes). Again, no significant differences were found between mDis3 and mDis1 loci in the number of genes connected to emVars.

5/ MPRA narrowed ~17k variants to ~680 with validated regulatory activity, which we call expression-modulating variants (emVars). We categorized emVars into: (1) mDis3: linked to 3+ disorders. (2) mDis1: linked to 1 disorder. Variants linked to 2 disorders were excluded.

4/ One challenge: ~17k variants in cross-disorder loci showed nominal associations (p < 1e-5), making it hard to pinpoint key variants. To address this, we used MPRA (massively parallel reporter assay) to test the regulatory activity of thousands of variants simultaneously.

3/ In 2019, the PGC cross-disorder group conducted a meta-GWAS of 8 psychiatric disorders, identifying 136 “cross-disorder loci.” Among these, 109 loci were linked to 2+ disorders (pleiotropic loci). But how do pleiotropic loci differ from disorder-specific loci?

2/ Pleiotropy occurs when genetic variants affect multiple traits. It's common in psychiatry, where many disorders show significant genetic correlations.