PROJECT SUMMARY The over-arching goal of this project is to address several major challenges to biologic interpretation, functional validation, and clinical translation of genetic association findings for quantitative red blood cell traits and non- malignant blood cell disorders in the post-genomic era. We seek to significantly advance the work that we have done over the current funding cycle to define how genetic variants can impact erythrocyte (red blood cell) traits and to functionally define mechanisms through which these variants act. In Aim 1, we will apply state-of-the-art statistical genomic and computational tools to extremely large human multi-ethnic datasets containing nearly 1 million individuals with red blood cell traits (hemoglobin, hematocrit, RBC count, MCV, MCH, MCHC, red cell distribution width or RDW) and whole genome sequence (WGS) or exome sequence data (the NHLBI TOPMed WGS project, UK Biobank, All of Us, and deCODE) to provide updated analysis, discovery, and interpretation of results, particularly for rare and ancestry-specific genetic variants associated with red blood cell counts and indices. We will include analysis of genomic regions missed by prior GWAS efforts such as large structural variants, variants on the X chromosome, and mitochondrial DNA copy number. Validation of new red blood cell phenotype-associated genomic loci and genetic variants will occur through imputation and replication in independent data sets using TOPMed WGS as imputation reference panel. We will also continue to provide functional annotation, fine-mapping, and prioritization for new and existing red blood cell trait-associated variants and genes, with an emphasis on utilization of newer multi-omic data sets derived from African and Hispanic individuals that are becoming available through the TOPMed project. In Aim 2, we will assess the contribution of polygenic variation (across the entire allele frequency spectrum) as phenotypic modifiers of red cell disorders such as anemia and erythrocytosis, phenotypic expressivity of high-impact Mendelian red cell disorders, and the contribution of red cell polygenicity to other complex disorders such as venous thromboembolic disease, stroke, and COVID-19 disease severity. In Aim 3, building on previous integrative genomic and functional analyses to define relevant hematopoietic cell states and mechanisms, we will develop comprehensive maps of how genetic variation associated with red blood cell traits can impact human hematopoiesis using emerging single-cell epigenomic chromatin accessibility and RNA sequencing atlases to provide predictive assessments of regulatory non-coding variation and function. We will disseminate all genomic, annotation, and functional information derived from Aims 1, 2, and 3 to ensure knowledge dissemination to the clinical and scientific community, for discovery, fine-mapping, and investigation of causal genes that underlie red blood cell traits and hematological disorders.