Novel manifestations of immune dysregulation and granule defects in gray platelet syndrome

Abstract

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.

A Deep Dive into the Pathology of Gray Platelet Syndrome: New Insights on Immune Dysregulation

“…Although no mutational hotspots have been recognized, missense variants are enriched within the BEACH domain, consistent with the essential role of this region in NBEAL2 function. 8 No genotype-phenotype correlation has been demonstrated, either regarding the type of mutation or its location. 8 In some pedigrees, three different variants have been identified and the pathogenic role of each one of them is not clear.…”

“…8 No genotype-phenotype correlation has been demonstrated, either regarding the type of mutation or its location. 8 In some pedigrees, three different variants have been identified and the pathogenic role of each one of them is not clear. 6,8,10,18 Current gene curation efforts for platelet disorders, such as those carried out recently for Glanzmann thrombasthenia, 20 will contribute to accurate interpretation of variants.…”

“…8 In some pedigrees, three different variants have been identified and the pathogenic role of each one of them is not clear. 6,8,10,18 Current gene curation efforts for platelet disorders, such as those carried out recently for Glanzmann thrombasthenia, 20 will contribute to accurate interpretation of variants. Heterozygous NBEAL2 variants may be found in unaffected carriers, who may display a mild decrease in α-granule content, although they are not thrombocytopenic.…”

“…[5][6][7] NBEAL2 is involved in the biogenesis of megakaryocyte and platelet α-granules, although its role beyond hemostasis, particularly in immunity and inflammation, has been recently highlighted by Sims et al in a landmark international study. 8 This review will focus on the mechanisms underlying abnormalities in the megakaryocyte-platelet axis and the immune system in GPS patients and Nbeal2-null animal models.…”

Advances in understanding the pathogenesis of hereditary macrothrombocytopenia

“…81 Dominant negative mutations in GFI1B cause macrothrombocytopenia and a variable bleeding diathesis caused by a deficiency of platelet alpha (a)-granules and abnormal platelet activation responses including cytoskeletal dysregulation. 81,96,97 NBEAL2 encodes a scaffolding protein, crucial to a-granule biogenesis; biallelic variants lead to GPS 27,28 and it is proposed that macrothrombocytopenia results from defects in the Rac1/Cdc42 pathway and subsequent dysfunctional cytoskeleton reorganisation. 82 Paradoxically, variants in other genes with critical roles in cytoskeletal regulation, such as WAS and ARPC1B, lead to microthrombocytopenia, possibly though an alternative effect on proplatelet formation or through increased rates of peripheral platelet clearance, resulting in small platelets.…”