BLVRB redox mutation defines heme degradation in a metabolic pathway of enhanced thrombopoiesis in humans

Humanblood cell counts are tightly maintained within narrow physiologic ranges, largely controlled by cytokine-integrated signaling and transcriptional circuits that regulate multilineage hematopoietic specification. Known genetic loci influencing blood cell production account for <10%of plateletandred blood cell variability, andthrombopoietin/ cellular myeloproliferative leukemia virus liganding is dispensable for definitive thrombopoiesis, establishing that fundamentally important modifier loci remain unelucidated. In this study, platelet transcriptome sequencing and extended thrombocytosis cohort analyses identified a single loss-of-function mutation (BLVRB^S111L) causally associated with clonal and nonclonal disorders of enhanced platelet production. BLVRB^S111L encompassed within the substrate/cofactor [a/b dinucleotide NAD(P)H] binding fold is a functionally defective redox coupler using flavin and biliverdin (BV) IXb tetrapyrrole(s) and results in exaggerated reactive oxygen species accumulation as a putative metabolic signal leading to differential hematopoietic lineage commitment and enhanced thrombopoiesis. These data define the first physiologically relevant function of BLVRB and implicate its activity and/or heme-regulated BV tetrapyrrole(s) in a unique redox-regulated bioenergetic pathway governing terminal megakaryocytopoiesis; these observations also define a mechanistically restricted drug target retaining potential for enhancing human platelet counts.