Gray platelet syndrome (GPS) is a rare recessive disorder caused by biallelic variants in NBEAL2 and characterized by bleeding symptoms, the absence of platelet ɑ-granules, splenomegaly and bone marrow (BM) fibrosis. Due to its rarity, it has been difficult to fully understand the pathogenic processes that lead to these clinical sequelae. To discern the spectrum of pathological features, we performed a detailed clinical genotypic and phenotypic study of 47 GPS patients. We identified 32 new etiological variants in NBEAL2. Our GPS patient cohort exhibited known phenotypes, including macrothrombocytopenia, BM fibrosis, megakaryocyte emperipolesis of neutrophils, splenomegaly, and elevated serum vitamin B12 levels. We also observed novel clinical phenotypes; these include reduced leukocyte counts and increased presence of autoimmune disease and positive autoantibodies. There were widespread differences in the transcriptome and proteome of GPS platelets, neutrophils, monocytes, and CD4-lymphocytes. Proteins less abundant in these cells were enriched for constituents of granules, supporting a role for Nbeal2 in the function of these organelles across a wide range of blood cells. Proteomic analysis of GPS plasma showed increased levels of proteins associated with inflammation and immune response. One quarter of plasma proteins increased in GPS are known to be synthesized outside of hematopoietic cells, predominantly in the liver. In summary, our data demonstrate that, in addition to the well-described platelet defects in GPS, there are also immune defects. The abnormal immune cells may be the drivers of systemic abnormalities, such as autoimmune disease.

Gray platelet syndrome (GPS) is a rare recessive disorder caused by variants in NBEAL2 and characterized by bleeding symptoms, the absence of platelet alpha-granules, splenomegaly and bone marrow (BM) fibrosis. Due to its rarity, it has been difficult to fully understand the pathogenic processes that lead to these clinical sequelae. To discern the spectrum of pathological features, we performed a detailed clinical genotypic and phenotypic study of 47 GPS patients. We identified 33 new causal variants in NBEAL2. Our GPS patient cohort exhibited known phenotypes, including macrothrombocytopenia, BM fibrosis, megakaryocyte emperipolesis of neutrophils, splenomegaly, and elevated serum vitamin B12 levels. We also observed novel clinical phenotypes; these include reduced leukocyte counts and increased presence of autoimmune disease and positive autoantibodies. There were widespread differences in the transcriptome and proteome of GPS platelets, neutrophils, monocytes, and CD4-lymphocytes. Proteins less abundant in these cells were enriched for constituents of granules, supporting a role for Nbeal2 in the function of these organelles across a wide range of blood cells. Proteomic analysis of GPS plasma showed increased levels of proteins associated with inflammation and immune response. One quarter of plasma proteins increased in GPS are known to be synthesized outside of hematopoietic cells, predominantly in the liver. In summary, our data demonstrate that, in addition to the well-described platelet defects in GPS, there are also immune defects. The abnormal immune cells may be the drivers of systemic abnormalities, such as autoimmune disease.

Mutations in , the gene encoding the scaffolding protein Nbeal2, are causal of gray platelet syndrome (GPS), a rare recessive bleeding disorder characterized by platelets lacking α-granules and progressive marrow fibrosis. We present here the interactome of Nbeal2 with additional validation by reverse immunoprecipitation of Dock7, Sec16a, and Vac14 as interactors of Nbeal2. We show that GPS-causing mutations in its BEACH domain have profound and possible effects on the interaction with Dock7 and Vac14, respectively. Proximity ligation assays show that these 2 proteins are physically proximal to Nbeal2 in human megakaryocytes. In addition, we demonstrate that Nbeal2 is primarily localized in the cytoplasm and Dock7 on the membrane of or in α-granules. Interestingly, platelets from GPS cases and mice are almost devoid of Dock7, resulting in a profound dysregulation of its signaling pathway, leading to defective actin polymerization, platelet activation, and shape change. This study shows for the first time proteins interacting with Nbeal2 and points to the dysregulation of the canonical signaling pathway of Dock7 as a possible cause of the aberrant formation of platelets in GPS cases and deficient mice.