Functional classification of platelet gene variants using CRISPR HDR in CD34⁺ cell-derived megakaryocytes

The interpretation of genetic variants in inherited diseases, such as inherited platelet disorders (IPDs), remains a major clinical challenge, as most are classified as variants of uncertain significance (VUSs). A key barrier to functional evaluation is the lack of accessible, lineage-appropriate assays that reliably reflect native gene regulation and cell-specific biology. To address this gap, we developed CRIMSON HD (CRISPR-edited megakaryocytes [MKs] for surveying platelet variant functions through homology-directed repair [HDR]), a CRISPR-Cas9 HDR-based genome-editing platform applicable to CD34⁺ cell-derived blood lineages and optimized for evaluating platelet-associated variants. Using this system, we modeled known and candidate disease-associated variants in integrin alpha 2b (ITGA2B) and integrin beta 3 (ITGB3), which encode the platelet αIIb/β3 integrin and are causative in Glanzmann thrombasthenia (GT). We introduced precise variants into primary human MKs derived from CD34⁺ hematopoietic stem and progenitor cells, achieving >90% editing efficiency. Edited MKs faithfully recapitulated both expression and functional phenotypes of known type I, II, and III GT variants. CRIMSON HD enabled functional evaluation and reclassification of several GT VUSs, including αIIb Gly201Ala, a population variant now shown to cause near-complete loss of αIIb/β3 expression; αIIb Ala777Asp, which results in intermediate αIIb/β3 expression and impaired agonist-induced integrin binding; and β3 Arg119Gln, previously linked to the loss of anti-HPA1a antibody binding in fetal and neonatal alloimmune thrombocytopenia (FNAIT), now shown to impair integrin surface expression. These findings demonstrate the importance of lineage-specific, physiologically relevant assays for the functional classification of platelet-related variants, providing mechanistic information and clinically meaningful insights for individuals with IPDs.