Neutrophil secretory defect in the gray platelet syndrome: A new case

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

The gray platelet syndrome (GPS) is a rare platelet disorder, characterized by impaired alpha-granule biogenesis in megakaryocytes and platelets due to NBEAL2 mutations. Typical clinical features include macrothrombocytopenia, bleeding and elevated vitamin B12 levels, while bone marrow fibrosis and splenomegaly may develop during disease progression. Recently, the involvement of other blood lineages has been highlighted, revealing the role of NBEAL2 outside the megakaryocyte-platelet axis. Low leukocyte counts, decreased neutrophil granulation and impaired neutrophil extracellular trap formation represent prominent findings in GPS patients, reflecting deranged innate immunity and associated with an increased susceptibility to infection. In addition, low numbers and impaired degranulation of NK cells have been demonstrated in animal models. Autoimmune diseases involving different organs and a spectrum of autoantibodies are present in a substantial proportion of GPS patients, expanding the syndromic spectrum of this disorder and pointing to dysregulation of the adaptive immune response. Low-grade inflammation, as evidenced by elevation of liver-derived acute-phase reactants, is another previously unrecognized feature of GPS which may contribute to disease manifestations. This review will focus on the mechanisms underlying the pathogenesis of blood cell abnormalities in human GPS patients and NBEAL2-null animal models, providing insight into the effects of NBEAL2 in hemostasis, inflammation and immunity.

Neutrophil specific granule and NETosis defects in gray platelet syndrome

Gray platelet syndrome (GPS) is an autosomal recessive bleeding disorder characterized by a lack of α-granules in platelets and progressive myelofibrosis. Rare loss-of-function variants in neurobeachin-like 2 (NBEAL2), a member of the family of beige and Chédiak-Higashi (BEACH) genes, are causal of GPS. It is suggested that BEACH domain containing proteins are involved in fusion, fission, and trafficking of vesicles and granules. Studies in knockout mice suggest that NBEAL2 may control the formation and retention of granules in neutrophils. We found that neutrophils obtained from the peripheral blood from 13 patients with GPS have a normal distribution of azurophilic granules but show a deficiency of specific granules (SGs), as confirmed by immunoelectron microscopy and mass spectrometry proteomics analyses. CD34+ hematopoietic stem cells (HSCs) from patients with GPS differentiated into mature neutrophils also lacked NBEAL2 expression but showed similar SG protein expression as control cells. This is indicative of normal granulopoiesis in GPS and identifies NBEAL2 as a potentially important regulator of granule release. Patient neutrophil functions, including production of reactive oxygen species, chemotaxis, and killing of bacteria and fungi, were intact. NETosis was absent in circulating GPS neutrophils. Lack of NETosis is suggested to be independent of NBEAL2 expression but associated with SG defects instead, as indicated by comparison with HSC-derived neutrophils. Since patients with GPS do not excessively suffer from infections, the consequence of the reduced SG content and lack of NETosis for innate immunity remains to be explored.

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

Gray platelet syndrome (GPS) is a rare recessive disorder caused by biallelic variants in NBEAL2 and characterized by bleeding symptoms, the absence of platelet α-granules, splenomegaly, and bone marrow (BM) fibrosis. Due to the rarity of GPS, it has been difficult to fully understand the pathogenic processes that lead to these clinical sequelae. To discern the spectrum of pathologic features, we performed a detailed clinical genotypic and phenotypic study of 47 patients with GPS and identified 32 new etiologic variants in NBEAL2. The GPS patient cohort exhibited known phenotypes, including macrothrombocytopenia, BM fibrosis, megakaryocyte emperipolesis of neutrophils, splenomegaly, and elevated serum vitamin B12 levels. Novel clinical phenotypes were also observed, including 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 show that, in addition to the well-described platelet defects in GPS, there are immune defects. The abnormal immune cells may be the drivers of systemic abnormalities such as autoimmune disease.

A Case of Chronic Thrombocytopenia in a 17-Year-Old Female

Storage pool deficiency (SPD) is a group of rare platelet disorders that result from deficiencies in α-granules, δ-granules, or both. One type of α-SPD is gray platelet syndrome (GPS), caused by mutations in the neurobeachin-like 2 (NBEAL2) gene that results in a bleeding diathesis, thrombocytopenia, splenomegaly, and progressive myelofibrosis. Due to the lack of α-granules, platelets have a gray and degranulated appearance by light microscopy. However, definitive diagnosis of GPS requires confirmation of α-granule deficiency by electron microscopy. Treatment is nonspecific, with the conservative utilization of platelet transfusions being the most important form of therapy. We present a case of a 17-year-old female with a past medical history of thrombocytopenia, first identified at the age of five. Her clinical symptomatology included chronic fatigue, gingival bleeding, bruising, menorrhagia, and leg pain. This report will discuss both the clinical and the pathophysiologic aspects of this rare platelet disorder.

Nbeal2 Deficiency Increases Organ Damage but Does Not Affect Host Defense During Gram-Negative Pneumonia-Derived Sepsis

Objective- Nbeal2-/- mice, a model of human gray platelet syndrome, have reduced neutrophil granularity and impaired host defense against systemic Staphylococcus aureus infection. We here aimed to study the role of Nbeal2 deficiency in both leukocytes and platelets during gram-negative pneumonia and sepsis. Approach and Results- We studied the role of Nbeal2 in platelets and leukocytes during murine pneumonia and sepsis by Klebsiella pneumoniae. Apart from platelet α-granule deficiency and reduced neutrophil granularity, also monocyte granularity was reduced in Nbeal2-/- mice, whereas plasma levels of MPO (myeloperoxidase), elastase, NGAL (neutrophil gelatinase-associated lipocalin), and MMP-9 (matrix metalloproteinase 9), and leukocyte CD11b expression were increased. Nbeal2-/- leukocytes showed unaltered in vitro antibacterial response and phagocytosis capacity against Klebsiella, and unchanged reactive nitrogen species and cytokine production. Also during Klebsiella pneumonia and sepsis, Nbeal2-/- mice had similar bacterial growth in lung and distant body sites, with enhanced leukocyte migration to the bronchoalveolar space. Despite similar infection-induced inflammation, organ damage was increased in Nbeal2-/- mice, which was also seen during endotoxemia. Platelet-specific Nbeal2 deficiency did not influence leukocyte functions, indicating that Nbeal2 directly modifies leukocytes. Transfusion of Nbeal2-/- but not of Nbeal2+/+ platelets into thrombocytopenic mice was associated with bleeding in the lung but similar host defense, pointing at a role for platelet α-granules in maintaining vascular integrity but not host defense during Klebsiella pneumosepsis. Conclusions- These data show that Nbeal2 deficiency-resulting in gray platelet syndrome-affects platelets, neutrophils, and monocytes, with intact host defense but increased organ damage during gram-negative pneumosepsis.

NBEAL2 is required for neutrophil and NK cell function and pathogen defense

Mutations in the human NBEAL2 gene cause gray platelet syndrome (GPS), a bleeding diathesis characterized by a lack of α granules in platelets. The functions of the NBEAL2 protein have not been explored outside platelet biology, but there are reports of increased frequency of infection and abnormal neutrophil morphology in patients with GPS. We therefore investigated the role of NBEAL2 in immunity by analyzing the phenotype of Nbeal2-deficient mice. We found profound abnormalities in the Nbeal2-deficient immune system, particularly in the function of neutrophils and NK cells. Phenotyping of Nbeal2-deficient neutrophils showed a severe reduction in granule contents across all granule subsets. Despite this, Nbeal2-deficient neutrophils had an enhanced phagocyte respiratory burst relative to Nbeal2-expressing neutrophils. This respiratory burst was associated with increased expression of cytosolic components of the NADPH oxidase complex. Nbeal2-deficient NK cells were also dysfunctional and showed reduced degranulation. These abnormalities were associated with increased susceptibility to both bacterial (Staphylococcus aureus) and viral (murine CMV) infection in vivo. These results define an essential role for NBEAL2 in mammalian immunity.

Dawning of the age of genomics for platelet granule disorders: improving insight, diagnosis and management

Inherited disorders of platelet granules are clinically heterogeneous and their prevalence is underestimated because most patients do not undergo a complete diagnostic work-up. The lack of a genetic diagnosis limits the ability to tailor management, screen family members, aid with family planning, predict clinical progression and detect serious consequences, such as myelofibrosis, lung fibrosis and malignancy, in a timely manner. This is set to change with the introduction of high throughput sequencing (HTS) as a routine clinical diagnostic test. HTS diagnostic tests are now available, affordable and allow parallel screening of DNA samples for variants in all of the 80 known bleeding, thrombotic and platelet genes. Increased genetic diagnosis and curation of variants is, in turn, improving our understanding of the pathobiology and clinical course of inherited platelet disorders. Our understanding of the genetic causes of platelet granule disorders and the regulation of granule biogenesis is a work in progress and has been significantly enhanced by recent genomic discoveries from high-powered genome-wide association studies and genome sequencing projects. In the era of whole genome and epigenome sequencing, new strategies are required to integrate multiple sources of big data in the search for elusive, novel genes underlying granule disorders.

Gray Platelet Syndrome in a Somalian family

The Gray Platelet Syndrome (GPS) is a rare inherited, hypogranular platelet disorder characterized by virtual absence of alpha granules in bone marrow megakaryocytes and circulating plates. Usually only one member of a family is affected, but families with two or more affected individuals have been reported from France, Australia, and Mexico, and, recently, the United States. The current study has evaluated the first family from East Africa with two affected members, a brother and sister. Neither child has had significant bleeding problems. Electron microscopic studies revealed almost complete absence of alpha granules from their platelets. Instead their platelets were filled with vacuoles similar in size to the missing granules. Dense bodies were normal in number in the girl's platelets, but markedly increased in her brother's cells. Tannic acid staining revealed that very few of the vacuoles were connected to channels of the open canalicular system. As a result, contents of the residual alpha granule vacuoles must leak out of the organelles and diffuse through megakaryocyte and platelet cytoplasm to the outside. The route of escape may differ from other hypogranular platelet syndromes, such as alpha-delta platelet storage pool deficiency.