PROJECT SUMMARY Complex vascular diseases such as coronary artery disease (CAD), myocardial infarction (MI), and coronary artery calcification (CAC) pose considerable public health burden worldwide and involve both genetic and environmental risk factors over a lifetime. Given the rising prevalence of vascular diseases across human populations, there is an urgent need for new treatments and preventative measures that target the primary disease processes in the vessel wall. Genome-wide association studies (GWAS) have identified hundreds of genetic loci associated with vascular disease risk. Large-scale functional genomic studies have begun to resolve many of the causal genes, variants, and pathways at these loci and demonstrated shared genetic etiologies. However, it still remains a challenge to translate these genetic discoveries into biologically and clinically relevant insights. More than half of the CAD/MI loci are associated independently of classical risk factors and may point to vascular dysfunction. Our group and others have adopted a systems-based approach to prioritize the genes and mechanisms altered by disease risk loci in human coronary artery smooth muscle cells (SMC). SMC normally regulate vascular tone but play critical roles in atherosclerosis as their contractile gene program is hijacked during phenotypic switching to immune cell, fibroblast-like, and osteoblast-like cells. Using multi-omics and quantitative trait locus mapping in human coronary artery SMC and tissues we recently identified candidate causal genes and mechanisms for CAD-related vascular dysfunction. Single-cell analyses of human coronary lesions demonstrated a critical role for CAD-associated transcription factors (e.g. TCF21) in regulating SMC phenotypic switching during atherosclerosis. Using single-cell epigenomic profiling of coronary arteries (n=41) we also identified novel SMC specific transcriptional regulators that are associated with multiple vascular diseases. Integrative fine-mapping analyses prioritized Four-and-a-Half LIM domains 5 (FHL5) as a causal gene for CAD/MI and subclinical vascular diseases. Interestingly, FHL5 overexpression decreased SMC contractility, and increased proliferation and calcification, consistent with the genetic association for CAC. Finally, FHL5 chromatin and transcriptome profiling in SMC support its role as a transcriptional cofactor, by altering SMC contractility and extracellular matrix expression/regulation. These data suggest that elucidating its trans-regulatory pathways may resolve mechanisms of pleiotropic risk across these conditions. Herein, we plan to perform functional genomic studies of FHL5 in both human vascular cells and arteries ex vivo to determine its role in vascular dysfunction, through altered actin cytoskeleton and extracellular matrix regulation, and vasoreactivity. We will further reveal its target binding regions, protein interactomes, and construct multi-omic gene regulatory networks to determine th...