GFI1B Mutation Causes a Platelet Function Defect With Reduced Alpha-Granule Content and Abnormal Aggregation Response

GFI1B is a transcription factor important for erythropoiesis and megakaryocyte development, but previously unknown to be associated with human disease. We have identified a family with an autosomal dominant bleeding disorder associated with thrombocytopenia, red cell anisopoikilocytosis and platelet dysfunction. The severity of bleeding is variable with some affected individuals experiencing spontaneous bleeding while other family members only exhibit abnormal bleeding with surgery. All affected individuals had a moderate thrombocytopenia ranging from 72-149 x10⁹/L with an elevated mean platelet volume. Platelet function was perturbed with prolonged collagen/epinephrine closure times on the PFA-100 (294 vs 125 sec, P <0.001). Platelet aggregation studies were abnormal, with an absent collagen response and primary aggregation only with ADP, arachidonic acid and adrenaline. Genetic linkage analysis and massively parallel sequencing were performed on 16 family members to localize the mutation causing the disease phenotype to chromosome 9. A single nucleotide insertion was identified in GFI1B that predicts a frameshift mutation in the fifth zinc finger DNA-binding domain. This mutation alters the transcriptional activity of the protein as determined by luciferase production from the TGFBR3 and GFI1B promoters. Aberrant transcriptional activity was associated with a marked reduction in platelet alpha-granule content as measured by electron microscopy (1.3 vs 3.1 alpha-granules per 1µm², P<0.001) with similar changes in platelet protein expression. Proteomic analysis of platelet lysates demonstrated altered levels of cytoskeletal proteins, including talin, vinculin and myosin light chain 6. GFI1B mutation represents a novel human bleeding disorder and the described phenotype identifies GFI1B as a critical regulator of platelet number and function.