Functional genomic dissection of MS risk loci reveals convergence of cis and trans gene regulatory mechanisms in microglia
Neurological diseases (NDs) are a major source of unmet medical need, and translational insights have been hampered by complex underlying pathophysiologies and limitations of experimental models.  Noncoding single nucleotide polymorphisms (SNPs) at hundreds of loci have been linked to ND risk by genome-wide association studies (GWAS), but the causal genes and pathways are largely unknown. Despite the multicellular pathology of complex traits like multiple sclerosis (MS), functional studies that aim to characterize the molecular impact of disease-associated SNPs often investigate all SNPs linked to disease in the same cellular context. Here, we combine a computational approach to predict the pathogenic cell type of individual risk loci with functional CRISPR perturbation studies in iPSC-derived microglia cells (iMGLs). ND SNP enrichment in cell type-specific enhancers is similar between primary and iPSC-derived cells, and mechanistically supported by shared enhancer-promoter interactions. We apply a novel Perturb-seq platform to interrogate MS risk SNPs in iMGL, identifying likely cis-acting causal risk genes at 5 of 9 loci, as well as downstream differentially expressed genes (DEGs). Despite being found in trans to MS risk SNPs, downstream DEGs are substantially enriched for MS heritability. Downstream DEGs from all 5 target genes show significant overlap, converging on genes related to cytokinesis, phagocytosis, and mitochondrial metabolism. We then compared downstream DEGs to gene expression patterns observed in MS patient tissue studies and observed marked similarities, demonstrating that genes dysregulated as the result of GWAS loci perturbation mirrored effects observed in microglia found in MS patient lesions. Collectively, these results demonstrate that cell type aware functional studies can be used to translate ND SNP associations into mechanistic insights and reveal novel convergent biological mechanisms underlying complex traits. Overall design: iPSC-derived microglia (iMGLs) were transduced with Vpx-associated lentivirus containing a Perturb-seq sgRNA library after 7 days in microglia maturation media.  7 days later, transduced cells were isolated by FACS for BFP expression, with gating on the top ~50% mCherry levels, representing dCas9-KRAB ZIM3 transgene expression.  After sorting cells were immediately used to prepare libraries for scRNA-seq