Gray Platelet Research Articles

Functional Characterization of a Novel GFI1B Mutation Causing Congenital Macrothrombocytopenia

▪Congenital macrothrombocytopenia (CMTP) is a heterogeneous group of disorders characterized by abnormally large platelets and low platelet counts. So far, genes responsible for CMTP have been identified in approximately half of the cases. The genes currently known are MYH9, GP1BA, GP1BB, GP9, ITGA2B, ITGB3, ACTN1 and other less frequent genes. However, in the remaining half, CMTP-causing genes are unknown. To identify genes causing CMTP, we first performed whole exome sequencing and target sequencing in 46 Japanese CMTP families and identified 1 family with a heterozygous mutation in GFI1B gene. Several groups have recently reported GFI1B as a causative gene for CMTP with alpha-granule deficiency and red cell anisopoikilocytosis. Here, we report on the identification of a novel mutation in GFI1B gene and its functional characterization.We performed whole exome sequencing and additional targeted sequencing in genetically undefined 46 CMTP families and identified 1 family with GFI1B variant (Figure), which co-segregated with CMTP. The proband (arrow in the Figure) of this family was a 15 years old boy with a history of easy bruising and platelet count of 70-115x109/L. His mother also had low platelet count, although details of her bleeding complications are unknown. Morphologic analysis of peripheral blood smear from the affected family members showed gray appeared enlarged platelets and red cell anisocytosis. Subsequent sequencing analysis confirmed a heterozygous point mutation (c.G814+1C, p.G272fsX274) at the splice donor site flanking exon 6 of GFI1B. GFI1B contains six zinc fingers (Znfs), of which Znf 3 to 5 are essential for DNA binding. The mutated transcript predicted a truncation of 58 C-terminal amino acids, which resulted in complete deletion of Znf 5. Since previous reports on GFI1B mutations have also described mutations of Znf 5 (p.H294fsX307 and p.Q287X mutations by Stevenson et al and Monteferrario et al, respectively), we performed biological analyses of these 3 patient-derived mutations affecting essential DNA-binding domain.GFI1B is an essential transcriptional factor for megakaryocyte and erythroid development. Therefore, we tested the effects of the GFI1b mutations on the transcriptional repression of firefly luciferase reporter constructs containing GFI1B promoter as a validated GFI1B target. Whereas wild-type GFI1B repressed the expression of reporter gene, the G272fsX274, H294fsX307 and Q287X mutants were unable to repress the expression. To test whether these patient-derived mutations acted in dominant-negative manner, we measured the effect of increasing the amount of mutant GFI1B cDNA, cotransfected into cells with a set amount of wild-type GFI1B cDNA. All 3 mutants inhibited gene repression mediated by wild-type GFI1B in a dose-dependent manner.To investigate biological effects of mutant GFI1B on proplatelet formation from megakaryocytes, we expressed the mutant GFI1B in mouse fetal liver cells (FLCs) by retrovirus-mediated gene transfer. FLCs harvested from embryonic day 13.5 embryos were transfected with pGCDNsamIRES/EGFP vector containing wild-type or the 3 patient-derived mutant GFI1B and were cultured with thrombopoietin. And we observed proplatelet formation of EGFP positive megakaryocytes. The number of proplatelet-producing megakaryocytes was reduced in mutant GFI1B transduced FLCs on day 3 and 4 after transfection, suggesting that mutant GFI1B inhibited the rate of proplatelet production from megakaryocytes. Next, we investigated the size and number of proplatelet tips from megakaryocytes, and found that proplatelet tips produced from G272fsX274, H294fsX307 and Q287X-transduced megakaryocytes were significantly reduced in numbers, but were increased in the size of the tips. This was compatible with decreased platelet count and enlarged platelet seen in GFI1B mutated patients. These findings indicated that patient-derived mutant GFI1B disrupted proplatelet formation of megakaryocyte.In summary, we have identified a novel GFI1B mutation that causes autosomal dominant macrothrombocytopenia with gray platelets and red cell anisocytosis. The mutant GFI1B acted in a dominant-negative manner over wild-type GFI1B. In addition, we first showed that the mutant-transduced FLCs produced abnormally enlarged proplatelet tips, which was reduced in numbers. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Platelet Transmission Electron Microscopy and Flow Cytometry in the Diagnosis of Congenital/Hereditary Qualitative or Quantitative Platelet Disorders

BackgroundCongenital/hereditary qualitative platelet disorders (CQPD) can be classified by platelet count (eg, normal or decreased) and platelet size as well as by functional defect. Based on mean platelet volume (MPV), congenital thrombocytopenias (CT) are classified as macro-thrombocytopenias (eg, Bernard-Soulier syndrome [BSS], MYH-9 [myosin heavy chain 9, non-muscle]-associated thrombocytopenias, gray platelet syndrome [GPS], etc.); microthrombocytopenias (eg, Wiskott-Aldrich syndrome and variants); and those with normal platelet size. CQPDs with normal platelet count include Glanzmann thrombasthenia (GT), dense granule storage pool disorders (DG-SPD) including Hermansky-Pudlak syndrome (HPS), platelet secretion disorders and many others. Diagnosis has relied on platelet counting and sizing, blood smear light microscopy, platelet aggregation (agg) and platelet function analyzer (PFA-100) studies. Platelet transmission electron microscopy (PTEM) and flow cytometry can provide supplementary information and molecular analysis is also evolving. The objective of this study was to determine the roles of platelet transmission electron microscopy (PTEM) and flow cytometry in the diagnosis of CQPD and CT.MethodsIn this retrospective cohort study, after IRB approval, the electronic medical record system was queried for patients (pt) with a confirmed CQPD or CT seen at Mayo Clinic between 2000 and 2015. A detailed chart review was undertaken.Results54 pt (64% female) met our study criteria; median age was 32 years (range 1 day to 81 years). 18/54 (33%) pt had macrothrombocytopenia: BSS 4 (7%); MYH-9 10 (19%); gray platelet syndrome 4 (7%). 36/54 (66%) had normal MPV: Glanzmann thrombasthenia 6 (11%); storage pool disorders including DG-SPD 10 (19%) and HPS 2 (3%), mild alpha granule deficiency 3 (6%) and York platelet syndrome 4 (7%); platelet secretion disorders 4 (7%); ANKRD26 mutation 3 (6%); congenital amegakaryocytic thrombocytopenia 1 (2%); GATA-1 mutation 1 (2%); RUNX-1 mutation 1 (2%); and Jacobsen syndrome 1 (2%). 44 pt (81%) had a positive bleeding history: epistaxis 52%, cutaneous bleeding 57%, gastrointestinal bleeding 21% and menorrhagia 54%; 10 pt (19%) had a negative bleeding history. Results of standard and esoteric platelet testing are summarized in Table 1.Diagnosis in the 10 pt with glycoprotein deficiency (BSS or GT) was established by agg; flow cytometry was confirmatory but not necessary. Of the 23 pt with SPD, 5 had a negative bleeding history; 4 pt had normal agg and PFA-100 and were diagnosed exclusively by PTEM. In this group, the abnormalities on platelet aggregation and PFA were variable and PTEM allowed for a better definition of the specific abnormalities. For MYH-9 patients, PTEM confirmed leukocyte inclusions that had already been identified on light microscopy. PTEM was the only diagnostic modality able to identify the abnormality in York platelet syndrome. In 5/54 (9%) genetic testing was necessary for diagnosis (GATA-1, ANKRD26 and RUNX-1)ConclusionIn this cohort, standard platelet assays established the diagnosis in the large majority of CQPD and CT. PTEM was essential for confirmation in DG-SPD, mild alpha granule deficiency and York platelet syndrome and useful in combination with molecular testing to establish a diagnosis in selected cases. Guidance on selection of patients for such specialized testing requires further study.Table 1Results of standard and esoteric platelet testing for 54 patients with congenital qualitative platelet disorders and/or thrombocytopeniaDisordern (%)Platelet aggregationPFAPTEMFlow cytometryConfirmation by geneticsDoneAbnormalDoneAbnormalDoneAbnormalDoneAbnormalGlanzmann thrombasthenia6 (11)665510330Bernard-Soulier syndrome & variants4 (7)444430440MYH-9 related disorders10 (19)728355425Dense granule deficiency and HPS12 (22)1281281212400Alpha granule deficiency*7 (13)747677441York platelet syndrome4 (7)101144110TXA2 or PG synthesis/receptor defect4 (7)444330300CATM1 (2)0-0-0-0-1ANKRD263 (6)333233333GATA-11 (2)0-1111111RUNX-11 (2)10110-0-1Jacobsen syndrome1 (2)101111101MYH-9: myosin heavy chain 9, non-muscle (MHY-9) associated thrombocytopenia, HPS: Hermansky-Pudlak Syndrome, TXA2: thromboxane A2, PG: prostaglandin CATM: congenital amegakaryocytic thrombocytopenia. * Includes gray platelet syndrome DisclosuresPardanani:Stemline: Research Funding.

Value of Platelet Esoteric Testing in Laboratory Diagnosis of Platelet Disorders: A Single Center Experience

Introduction Patients with inherited and acquired platelet disorders (PD) frequently present with thrombocytopenia and/or mucocutaneous bleeding. Diagnosis of PDs relies on both clinical and laboratory investigation, which encompasses both routine and esoteric platelet testing. The former includes platelet indices, light transmission aggregation (LTA) and platelet function analyzer (PFA-100). The latter includes platelet glycoprotein (GP) assessment by flow cytometry and platelet transmission electron microscopy (PTEM). Several recent studies have demonstrated limited sensitivities of routine platelet testing. It is uncertain if a combination of both routine and esoteric testing can vastly improve the sensitivity. Our goal is to assess the value of platelet esoteric testing and the ultimate sensitivity of laboratory platelet testing in diagnosing PDs. Methods Patients (between year 2012 and 2015) with a known or suspected inherited or acquired platelet disorders were recruited for platelet routine and esoteric testing. Patients' clinical features including ISTH bleeding assessment tool (BAT) scores, platelet count, LTA, PFA-100 ADP (CADP) and epinephrine (CEPI) cartridge closure times, GP expression by flow cytometry and PTEM were performed. All clinical and laboratory variables were collected and statistically analyzed. Results A total of 69 patients (51 females; median age 40 years; range: 8 months to 76 years) were enrolled to this cohort study. By retrospective chart review, 34 patients (49%) had a positive ISTH-BAT score (range 0 to 16). The most common bleeding manifestations (on any degree of severity) were cutaneous bleeding present in 53% of patients (n=37), menorrhagia in 47% of females (n=24), epistaxis in 36% (n=25) and postoperative bleeding in 35% (n=24). Joint, muscle and CNS hemorrhage were the least frequent with 7%, 6% and 1% of patients reporting bleeding in these sites. A total of 22 patients (32%) had thrombocytopenia at the time of their evaluation. Fifty-eight patients (84%) had platelet aggregation studies and of those 15 (26%) had an abnormal study. PFA-100 was performed in 57 patients (83%) of whom 42% (n=24) had prolonged closure times with either ADP or epinephrine. PTEM was performed on all samples. Twenty-seven patients (39%) had abnormal platelet structure including 17 patients who had dense granule deficiency. GP by flow cytometry was performed in 41 patients (59%) and abnormal glycoprotein expression was detected in 6 patients (15%). Of the 55 patients who had all tests performed, 22 patients (40%) had completely normal results. Of the routine tests, only PFA-CEPI or -CADP prolongation is associated with the likelihood of a PTEM abnormality (P<0.005). With the assistance of molecular testing, there were 2 cases of MYH-9 mutation-related platelet disorders, one of RUNX1 mutation-associated thrombocytopenia, one of York platelet syndrome, two of Bernard Soulier variants, one of Quebec syndrome and three of gray platelet syndrome or variants. Bleeding scores showed no statistically significant association with severity of any laboratory abnormalities. Conclusion In this cohort, the sensitivity of routine platelet testing in clinically suspected PDs is about 30-40%. Addition of platelet esoteric testing improves the sensitivity to 60%. However, the degree of platelet laboratory abnormalities do not predict severity of bleeding. The findings of this cohort underscore the importance of clinical assessment and both platelet routine and esoteric testing in investigation of patients with suspected PDs. Disclosures No relevant conflicts of interest to declare.

Platelet geometry sensing spatially regulates α-granule secretion to enable matrix self-deposition

AbstractAlthough the biology of platelet adhesion on subendothelial matrix after vascular injury is well characterized, how the matrix biophysical properties affect platelet physiology is unknown. Here we demonstrate that geometric orientation of the matrix itself regulates platelet α-granule secretion, a key component of platelet activation. Using protein microcontact printing, we show that platelets spread beyond the geometric constraints of fibrinogen or collagen micropatterns with <5-µm features. Interestingly, α-granule exocytosis and deposition of the α-granule contents such as fibrinogen and fibronectin were primarily observed in those areas of platelet extension beyond the matrix protein micropatterns. This enables platelets to “self-deposit” additional matrix, provide more cellular membrane to extend spreading, and reinforce platelet-platelet connections. Mechanistically, this phenomenon is mediated by actin polymerization, Rac1 activation, and αIIbβ3 integrin redistribution and activation, and is attenuated in gray platelet syndrome platelets, which lack α-granules, and Wiskott-Aldrich syndrome platelets, which have cytoskeletal defects. Overall, these studies demonstrate how platelets transduce geometric cues of the underlying matrix geometry into intracellular signals to extend spreading, which endows platelets spatial flexibility when spreading onto small sites of exposed subendothelium.

Megakaryocytic emperipolesis and platelet function abnormalities in five patients with gray platelet syndrome

The gray platelet syndrome (GPS) is a rare congenital platelet disorder characterized by mild to moderate bleeding diathesis, macrothrombocytopenia and lack of azurophilic α-granules in platelets. Some platelet and megakaryocyte (MK) abnormalities have been described, but confirmative studies of the defects in larger patient cohorts have not been undertaken. We studied platelet function and bone marrow (BM) features in five GPS patients with NBEAL2 autosomal recessive mutations from four unrelated families. In 3/3 patients, we observed a defect in platelet responses to protease-activated receptor (PAR)1-activating peptide as the most consistent finding, either isolated or combined to defective responses to other agonists. A reduction of PAR1 receptors with normal expression of major glycoproteins on the platelet surface was also found. Thrombin-induced fibrinogen binding to platelets was severely impaired in 2/2 patients. In 4/4 patients, the BM biopsy showed fibrosis (grade 2–3) and extensive emperipolesis, with many (36–65%) MKs containing 2–4 leukocytes engulfed within the cytoplasm. Reduced immunolabeling for platelet factor 4 together with normal immunolabeling for CD63 in MKs of two patients demonstrated that GPS MKs display an alpha granule-specific defect. Increased immunolabeling for P-selectin and decreased immunolabeling for PAR1, PAR4 and c-MPL were also observed in MKs of two patients. Marked emperipolesis, specific defect of MK alpha-granule content and defect of PAR1-mediated platelet responses are present in all GPS patients that we could study in detail. These results help to further characterize the disease.

Platelet surface-associated activation and secretion-mediated inhibition of coagulation factor XII.

Coagulation factor XII (fXII) is important for arterial thrombosis, but its physiological activation mechanisms are unclear. In this study, we elucidated the role of platelets and platelet-derived material in fXII activation. FXII activation was only observed upon potent platelet stimulation (with thrombin, collagen-related peptide, or calcium ionophore, but not ADP) accompanied by phosphatidylserine exposure and was localised to the platelet surface. Platelets from three patients with grey platelet syndrome did not activate fXII, which suggests that platelet-associated fXII-activating material might be released from α-granules. FXII was preferentially bound by phosphotidylserine-positive platelets and annexin V abrogated platelet-dependent fXII activation; however, artificial phosphotidylserine/phosphatidylcholine microvesicles did not support fXII activation under the conditions herein. Confocal microscopy using DAPI as a poly-phosphate marker did not reveal poly-phosphates associated with an activated platelet surface. Experimental data for fXII activation indicates an auto-inhibition mechanism (k i/k a = 180 molecules/platelet). Unlike surface-associated fXII activation, platelet secretion inhibited activated fXII (fXIIa), particularly due to a released C1-inhibitor. Platelet surface-associated fXIIa formation triggered contact pathway-dependent clotting in recalcified plasma. Computer modelling suggests that fXIIa inactivation was greatly decreased in thrombi under high blood flow due to inhibitor washout. Combined, the surface-associated fXII activation and its inhibition in solution herein may be regarded as a flow-sensitive regulator that can shift the balance between surface-associated clotting and plasma-dependent inhibition, which may explain the role of fXII at high shear and why fXII is important for thrombosis but negligible in haemostasis.

Pseudo gray platelet syndrome: the first case report in Korea

TO THE EDITOR: Currently, the most common blood test in hospitals, complete blood count (CBC), is typically performed by automated hematology analyzers. However, pseudothrombocytopenia caused by pseudo gray platelet syndrome (PGPS) might cause errors in CBC test results. Here we report what we believe to be the first reported case of PGPS in Korea.

“Self-Deposition” of Matrix Proteins from Platelet α-Granules Enable Extended Adhesion and Spreading on Micron/Submicron-Scale Fibrinogen and Collagen Substrates.

Introduction: While the platelet’s role in achieving hemostasis in the context of bleeding is well-characterized, how platelets support and maintain vascular integrity during homeostatic, non-thrombotic conditions remains unclear. Previous studies have shown that single platelets fill micron/submicron-sized “gaps” of small discontinuities in the endothelium. In this context, however, as the overall force of adhesion correlates with the area of exposed subendothelial matrix, how platelets establish sufficient adhesion to resist the dynamic shear forces of the circulation are unknown. In this study, we investigate, using protein microcontact printing, how platelets sense the geometry of the matrix micro/nano-environment and transduce those cues to affect platelet function. Our results reveal that platelets, upon sensing spatially-limited (<25 µm2 area) micro/nanopatterns of fibrinogen or collagen, spatially regulate actin-rich filopodia extension, via Rac1, beyond the geometric constraints of the matrix pattern, redistribute α-granules to enable “self-deposition” of fibrinogen/fibronectin matrix on those regions, and spread onto those newly deposited matrix via integrin αIIbβ3-mediated interactions. Furthermore, this phenomenon is markedly attenuated in Gray Platelet Syndrome platelets, which lack α-granules, and Wiskott-Aldrich Syndrome platelets, which have impaired integrin αIIbβ3-mediated activation, decreased α-granules, and cytoskeletal defects.Materials and Methods: Human blood was collected in ACD and platelets were isolated via two-step centrifugation or gel filtration. Platelets were suspended in Tyrode’s buffer containing 2 mM MgCl2 and exposed to various pharmacologic agents such as eptifibatide, Y27632, NSC23766. Fibrinogen and collagen micro/nanostamps were prepared on glass coverslips after which platelets were incubated onto. A plasma membrane dye, phalloidin, anti-P-selectin, anti-fibronectin and PAC-1, were used to visualize platelet morphology, f-actin, α-granule secretion, matrix depositions, and activated integrin αIIbβ3, respectively.Results and Discussion: Upon contact with the protein microstamps, platelets formed filopodia and lamellipodia and spread to conform to the micropattern boundaries with high fidelity. However, on micropatterned circles (microdots) of less than 25 µm2 area, platelets spread beyond the geometric constraints (Fig.1A). Anti P-selectin staining revealed α-granule secretion at the extended lamellipodia (Fig.1B left), and interestingly, this phenomenon was not detected with GPS platelets. Also we observed direct deposition of fibronectin from the platelets granules via anti-fibronectin staining (Fig.1B center). In addition, immunostaining with PAC-1 revealed a predominance of activated αIIbβ3 on regions of platelets at and beyond the borders of the microdots (Fig.1B right). When exposed to eptifibatide, an αIIbβ3 antagonist, platelet spreading beyond the microdot boundaries was completely eliminated. Various cytoskeleton inhibition experiments provided insight into how actin rearrangement is involved in this processes of platelet extension beyond matrix boundaries. Interestingly, disruption of cytoplasmic stress fibers by inhibiting Rho kinase with Y25632 facilitated platelet extension beyond the microdot boundaries and strong actin polymerization at the margins. On the other hand, platelet extension beyond the microdot boundaries was completely inhibited with NSC23766, a Rac-1 inhibitor (Fig.1C). We also confirmed that WAS platelets had significantly less f-actin, α-granule secretion, and overall extension beyond microdot boundaries. Overall, our results indicate that the matrix geometry, in and of itself, dictates platelet α-granule secretion, αIIbβ3 distribution and activation, and Rac1 activation, all of which work in concert to extend adhesion and spreading on matrix substrates of <25 µm2. In addition, we show that during this process, directionally directed α-granule secretion enables a platelet to “self-deposit” fibrinogen/fibronectin matrix to accommodate increased spreading. This phenomenon likely endows platelets a certain amount of “spatial flexibility” to establish firm adhesion upon contact with small sites of exposed subendothelium and may provide insight into how platelets help support vascular integrity. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Design and Validate of Next-Generation Sequencing Panel for Inherited Platelet Disorders

IntroductionThe inherited platelet disorders (IPD) are a heterogeneous group of rare diseases including quantitative and/or qualitative platelet defects. Classically, patients with IPD are first functionally tested to know the possible defect before sequencing a single or a few genes. Phenotipyc diagnostic of IPD often requires light transmission aggregometry, quantitative analysis of receptors by flow cytometry and fluorescence and electron microscopy. This diagnostic strategy is complex, poorly standardised and time consuming. In addition, the phenotype can seldom guide the singles candidates genes for conventional Sanger squencing. Therefore, many patients remain without a accurate diagnosis of their IPD. Next generation sequencing (NGS) enables the simultaneous analysis of large groups of candidate genes in IPD and may be useful for rapid genetic diagnosis.The aim of this study was to design and validate a NGS panel for IPD.Patients & MethodsWe describe a strategy for rapid genetic diagnosis of IPD with Illumina sequencing of 60 candidates genes previously associated with IPD (table1). The baits were designed to tile 400 kb of gDNA sequence corresponding to the exons and splice sites in all known transcripts of the candidate genes identified. The bait library was tested by enriching the candidate IPD genes from 50 ng DNA obtained and sequencing by Nextera Rapid Custom Enrichment system. Results were analysed by Variant Studio system and Sequencing Analysis Viewer.A total of 21 patients were studied. For the validation process, DNA samples of 9 unrelated patients with IPD and their mutation known were used: two patients with Glanzmann Thrombasthenia (ITGA2B, p.Ala989Thr, p.Val982Met and p.Glu538Stop; ITGB3, p.Leu222Pro and p.Tyr216Cys), one Hermansky-Pudlak Sd. (HPS1, p.Glu204 Stop), another with Bernard-Soulier Sd. (GPIX, p.Phe71Stop), a case of Congenital Amegakaryocytic Thrombocytopenia (MPL, p.Arg102Cys), and 2 patients with Chediak Higashi Sd. (LYST, p.Gly3725Arg and p.Cys258Arg). Once validated, the NGS panel was used for genetic diagnostic of 8 patients with suspected IPD.ResultsEleven mutations, previously identified in another center by conventional sequencing, were detected by our panel NGS (100% success in the validation process). We then tested this strategy for patients with suspected of IPD without diagnosis: I. a 13 years old girl with agenesis of the corpus callosum, facial dysmorphia, renal agenesis and thrombocytopenia was diagnosed of Thrombocytopenia FLNA-related and Periventricular Nodular Heterotopia (PNHV)[mutation in the FLNA was detected (p.Thr1232Ile)]. II. A two years old patient with severe thrombocytopenia and recurrent infections was diagnosed of Wiskott-Aldrich Sd (WAS, p.Arg268Gly fs Stop40). III. A patient with deafness, macrothrombocytopenia, and Döhle bodies was diagnosed by MYH9 deletion (MYH9; p.Asp1925Thr fs Stop23). IV. Six members of a family (2 of them with symptoms of mucocutaneous bleeding, and macrothrombocytopenia), in which an insertion in NBAL2 (p.Gly1142Arg fs Stop49) gene was found. Therefore, Gray Platelet Sd was diagnosed. Moreover, one patient with “aspirin-like syndrome” showed a P2RY12 mutation (p.Val279Met). Finally, mother and son with mild Hemophilia A (F8; p.Gln2208Arg) were detected.ConclusionsThis NGS panel enables a rapid genetic diagnostic of IPD.The use of NGS-based strategy is a feasible tool for the diagnosis of IPD that could be added to the screening of these disorders.Five mutations have not previously been described in the literature.Table 1:Sixty candidates’ genes previously associated with IPD:Inherited Platelet DisordersGenes = 60Cytoskeletal Assembly and Structural ProteinsGP1BA, GP1BB, GP5, A2M, GP9, VWF, ITGA2, ITGA2B, ITGB3, ABCA1, ANO6,FERMT3, ACTN1, MASTLDisorders of agonist platelet receptorsP2RX1, P2RY1, P2RY12, TBXA2R, TBXAS1, ADRA2A, GP6, CD36 o GP4, DTNBP1Disorders signal transductionGNAI3, GNAQ, GNAS, PLA2G7, PLCB2PTS, GGCX, DPAGT1, DHCR24Disorders of platelet granulesNBEAL2, GFI1B, PLAU, HPS1, HPS3, HPS4, HPS5, HPS6, LYST, MLPH, BLOC1S3, BLOC1S6, AP3B1, VIPAS39, VPS33B, RAB27A, MYO5A, USF1Thrombocytopenias and syndromesWAS, MYH9, FLNA, FLI1, STIM1, HOXA11, ANKRD26, MPL, RBM8A RUNX1, GATA1 DisclosuresNo relevant conflicts of interest to declare.

Germ-Line and Somatic Mutations in Familial Myeloproliferative Neoplasms (MPNs). a Pilot Study

Background: Familial MPNs are uncommon disorders that, like sporadic cases, are characterized by clonal hematopoiesis and presence of somatic mutations, e.g. JAK2, CALR, MPL and occasionally TET2. There is little information, however, about germ-line mutations in these families that may explain the low penetrance hereditary predisposition.Methods: We studied five families with MPNs, each with at least 2 affected members. After obtaining an informed consent, clinical data was obtained from the patients’ electronic medical records. Blood and buccal samples were collected from patients and unaffected relatives. Exome sequencing was performed on the blood DNA samples using Agilent SureSelect Human All Exon V5+UTRs exome capture kit followed by massively parallel sequencing with Illumina HiSeq 2000. Sanger sequencing was then done on both the blood and buccal swab DNA samples to validate selected gene variants and to differentiate the nature of those variants (germ line or somatic).Results: The 5 families participating in this study had the following diagnoses: 1. Mother: polycythemia vera (PV); son: essential thrombocythemia (ET), 2. Mother: primary myelofibrosis (MF); daughter: unclassifiable MPN (UMPN), 3. Father: PV; son: PV, 4. Sister: MF; sister: MF, 5. Two aunts: MF; niece; UMPN. Six patients were positive for JAK2, V617F mutation. Blood and buccal samples were collected from 5 patients and 4 relatives. In all 5 families, the pro-band was younger at the time of diagnosis than his/her affected relatives. The clinical course of the MPNs appeared to be similar to the sporadic form.Exome sequencing revealed TET2 mutations in 2 probands. In addition, novel non-synonymous mutations in several candidate genes, KMT2D, KMT2C, NBEAL1, NBEAL2, AHNAK2, RNF213, were identified in the blood samples from the patients but not their unaffected relatives. These include two novel KMT2D mutations in two unrelated families. These 2 mutations were also found in the matching buccal swab samples, indicating that they are germ line mutations.Discussion: KMT2D and KMT2C mutations have been previously identified as somatic mutations in lymphoid malignancies, including non-Hodgkin’s lymphomas (Morin 2011), and as germ line compound heterozygote mutations in infant MLL and ALL (Valentine 2014). About 32% of diffuse large cell lymphoma and 89% of follicular lymphoma have somatic mutations of KMT2D. NBEAL2 germ line mutations are associated with familial gray platelet syndrome, where some patients have myelofibrosis (Gunay-Aygun 2011). To our knowledge, this is the first report describing germ line mutations in familial MPNs. The possible role of these mutations in predisposition to MPN will be discussed. Studies on additional families with MPNs are planned. DisclosuresNo relevant conflicts of interest to declare.

Gray platelet syndrome: proinflammatory megakaryocytes and α-granule loss cause myelofibrosis and confer metastasis resistance in mice

AbstractNBEAL2 encodes a multidomain scaffolding protein with a putative role in granule ontogeny in human platelets. Mutations in NBEAL2 underlie gray platelet syndrome (GPS), a rare inherited bleeding disorder characterized by a lack of α-granules within blood platelets and progressive bone marrow fibrosis. We present here a novel Nbeal2−/− murine model of GPS and demonstrate that the lack of α-granules is due to their loss from platelets/mature megakaryocytes (MKs), and not by initial impaired formation. We show that the lack of Nbeal2 confers a proinflammatory phenotype to the bone marrow MKs, which in combination with the loss of proteins from α-granules drives the development of bone marrow fibrosis. In addition, we demonstrate that α-granule deficiency impairs platelet function beyond their purely hemostatic role and that Nbeal2 deficiency has a protective effect against cancer metastasis.

Identification of platelet function defects by multi-parameter assessment of thrombus formation.

Assays measuring platelet aggregation (thrombus formation) at arterial shear rate mostly use collagen as only platelet-adhesive surface. Here we report a multi-surface and multi-parameter flow assay to characterize thrombus formation in whole blood from healthy subjects and patients with platelet function deficiencies. A systematic comparison is made of 52 adhesive surfaces with components activating the main platelet-adhesive receptors, and of eight output parameters reflecting distinct stages of thrombus formation. Three types of thrombus formation can be identified with a predicted hierarchy of the following receptors: glycoprotein (GP)VI, C-type lectin-like receptor-2 (CLEC-2)>GPIb>α6β1, αIIbβ3>α2β1>CD36, α5β1, αvβ3. Application with patient blood reveals distinct abnormalities in thrombus formation in patients with severe combined immune deficiency, Glanzmann’s thrombasthenia, Hermansky–Pudlak syndrome, May–Hegglin anomaly or grey platelet syndrome. We suggest this test may be useful for the diagnosis of patients with suspected bleeding disorders or a pro-thrombotic tendency.

Amplification of bacteria-induced platelet activation is triggered by FcγRIIA, integrin αIIbβ3, and platelet factor 4

AbstractBacterial adhesion to platelets is mediated via a range of strain-specific bacterial surface proteins that bind to a variety of platelet receptors. It is unclear how these interactions lead to platelet activation. We demonstrate a critical role for the immune receptor FcγRIIA, αIIbβ3, and Src and Syk tyrosine kinases in platelet activation by Staphylococcus aureus, Streptococcus sanguinis, Streptococcus gordonii, Streptococcus oralis, and Streptococcus pneumoniae. FcγRIIA activation is dependent on immunoglobulin G (IgG) and αIIbβ3 engagement. Moreover, feedback agonists adenosine 5′-diphosphate and thromboxane A2 are mandatory for platelet aggregation. Additionally, platelet factor 4 (PF4) binds to bacteria and reduces the lag time for aggregation, and gray platelet syndrome α-granule–deficient platelets do not aggregate to 4 of 5 bacterial strains. We propose that FcγRIIA-mediated activation is a common response mechanism used against a wide range of bacteria, and that release of secondary mediators and PF4 serve as a positive feedback mechanism for activation through an IgG-dependent pathway.

Update on the causes of platelet disorders and functional consequences.

Platelets are derived from megakaryocytes in the bone marrow that create the cellular machinery the platelet needs to participate in the different processes of primary hemostasis including adhesion, activation and clot formation at the site of injury. Defects related to megakaryocyte differentiation, platelet formation, and/or platelet function can result in bleeding. Patients with thrombopathies can present with mucous membrane bleeding but may also present with bleeding following trauma or surgery. In this review, we have classified inherited platelet bleeding disorders (IPD) according to their underlying defective pathway: transcription regulation, TPO signaling, cytoskeletal organization, apoptosis, granule trafficking, and receptor signaling. Platelet function testing has provided insights into the underlying molecular defects that can result in bleeding. A major step forward was made during the last 3 years using new-generation genetic approaches that resulted in the discovery of novel genes such as NBEAL2, RBM8A, ACTN1, and GFI1B for the well-known IPD that cause gray platelet syndrome, thrombocytopenia-absent radius syndrome, and autosomal dominant thrombocytopenias, respectively. In the near future, it is expected that a similar approach will identify many novel genes that cause IPD of unknown etiology, which are common. The future challenge will be to use a functional, systems biology approach to study the genes mutated in IPD and determine their roles in megakaryocyte and platelet biology and pathology.

As a Rare Disease Bernard–Soulier Syndrome in Differential Diagnosis of Immune Thrombocytopenic Purpura: A Case Report

Introduction: Bernard–Soulier syndrome is a rare autosomal recessive disease that causes a deficiency of glycoprotein Ib, the receptor for von Willebrand factor, which is important for clot formation. It is estimated to occur in fewer than 1 per 1 million persons. The differential diagnosis includes von Willebrand disease, immune thrombocytopenic purpura, May–Hegglin anomaly, thrombocytopenia-absent radius syndrome, grey platelet syndrome, and other inherited giant platelet disorders. Case Presentation: A 30-year-old Turkish woman was admitted to the Department of Heamatology for evaluation of thrombocytopenia. Because she had repeated epistaxis during admission, she was assessed to evaluate for haemorrhagic diathesis. She had been diagnosed with immune thrombocytopenic purpura and given steroid therapy at different times. Peripheral blood smear was characterized by neutrophils 75%, lymphocytes 20%, monocytes 5%, giant platelets and platelets forming three to four clusters, a normal red blood cell morphology. A decreased ristocetin-induced platelet aggregation was detected in low and high concentration. On the basis of these findings, Bernard–Soulier syndrome was screened with flow cytometry and genetic mutation. CD41a 86.2%, CD42a 92.9%, CD42b 92.5%, and CD61 87.8% were detected in flow cytometry. Normal platelet GPIb/IX levels by flow cytometry turned down the suspected diagnosis. genetic analysis, we have applied to our patient and her parents. We detected same mutation in patient and her mother. Laboratuar parameters and flow cytometry of mother did not support diagnosis of Bernard–Soulier syndrome. Conclusion: Bernard–Soulier syndrome should be considered before a young patient is diagnosed with immune thrombocytopenia purpura and peripheral blood smear should be examined for giant platelets.

A Dominant-Negative GFI1B Mutation in the Gray Platelet Syndrome

The gray platelet syndrome is a hereditary, usually autosomal recessive bleeding disorder caused by a deficiency of alpha granules in platelets. We detected a nonsense mutation in the gene encoding the transcription factor GFI1B (growth factor independent 1B) that causes autosomal dominant gray platelet syndrome. Both gray platelets and megakaryocytes had abnormal marker expression. In addition, the megakaryocytes had dysplastic features, and they were abnormally distributed in the bone marrow. The GFI1B mutant protein inhibited nonmutant GFI1B transcriptional activity in a dominant-negative manner. Our studies show that GFI1B, in addition to being causally related to the gray platelet syndrome, is key to megakaryocyte and platelet development.

A Dominant-Negative GFI1B Mutation in Gray Platelet Syndrome

Gray platelet syndrome (GPS) is a hereditary, usually autosomal recessive bleeding disorder caused by defective production of α-granules in platelets. GPS patients show reduced numbers of platelets that are larger and have a typical gray appearance under light microscopy, caused by the lack of α-granules. We describe a large family with an autosomal dominant type of GPS characterized by mild to severe bleeding complications. In addition to large gray platelets, other GPS-associated phenomena like myelofibrosis, thrombocytopenia, and low platelet factor 4 expression were observed in affected individuals. Histopathological examination of a BM biopsy from a patient showed a cellular marrow with increased numbers of megakaryocytes that were pleomorphic in size and shape. Megakaryocytes clustered along BM sinuses and showed dysmorphic stretched features.To determine the disease causing mutation we performed linkage analysis and identified a candidate locus on chromosome 9q34 with a LOD score of 3.9. We considered GFI1B (Growth Factor Independence 1B), located within this region, an excellent candidate gene because of its function as a transcriptional repressor in megakaryocyte development. Sequence analysis identified a nonsense mutation in GFI1B exon 6 (c.859C>T, p.Gln287*) that completely co-segregated with the GPS disease in this family. The mutated transcript predicts a 44 amino acid C-terminally truncated protein, GFI1BTr. The truncation is located within zinc finger 5 of GFI1B, deleting all of its four amino acids that directly interact with DNA. Luciferase gene reporter assays showed that GFI1BTr was unable to repress gene expression. Importantly, GFI1BTr inhibited gene repression mediated by wild type GFI1B, indicating that the mutant interferes with wild type GFI1B in a dominant-negative manner. To validate that GFI1BTr adversely affects normal GFI1B, we expressed the mutant in mouse bone marrow cells followed by induction of megakaryocytic differentiation. Compared to control cells, GFI1BTr-positive megakaryocytes showed dysplastic features including hypolobulation of the nuclei, irregular contours and multiple separate nuclei, that were very similar to those observed in patient cells. This indicates that GFI1BTr causes megakaryocytic abnormalities and that it functions in a dominant-negative manner.GFI1B silencing inhibits the development of human megakaryocyte colonies in vitro. We observed that megakaryocyte colony forming cells were significantly more frequent in patient bone marrow compared to controls. In addition, patient-derived megakaryocyte colonies were significantly larger compared to controls. Immunophenotypic analyses of peripheral blood showed no differences in myeloid and erythroid lineages and the platelet markers GP3B, ITGA2B and ITGB3 among affected an non-affected individuals. However, within the ITGA2B/CD41-positive platelet population, 5 of 6 affected members showed a marked decrease in the platelet surface membrane glycoprotein 1b-α (GP1BA/CD42b), compared to unaffected members. In addition, a strong expression of CD34, which is usually confined to immature hematopoietic progenitors, was detected on platelets from all studied affected individuals. Immunostaining of a BM biopsy from a patient showed the presence of ITGB3/CD61 positive megakaryocytes that intensely expressed CD34. Electron microscopy analysis showed megakaryocytes with few, small, irregularly shaped and centrally located α-granules characterized by an extensive peripheral cytoplasm with irregular proplatelets, largely devoid of cell organelles. To test whether these abnormalities were cell intrinsic, we stimulated CD34+ cells from two patients to differentiate along the megakaryocytic lineage in vitro. Megakaryocytic cells showed dysplastic features reminiscent of those observed in the bone marrow aspirates. In addition, increased CD34 and decreased GP1BA/CD42b expression were observed on megakaryocytes, indicating that GFI1BTr-induced abnormalities are intrinsic to the cell. In summary, we have identified GFI1B as a causative gene in autosomal dominant GPS. GFI1BTr acts in a dominant-negative manner over wild type GFI1B and affects the development of megakaryocytes and platelets, demonstrating a pivotal role of GFI1B in governing normal megakaryopoiesis and platelet production. Disclosures:No relevant conflicts of interest to declare.

Expression Patterns Of NBEAL2 In Human Tissues and a Megakaryocytic Cell Line

Gray platelet syndrome (GPS) is a rare autosomal recessive bleeding disorder characterized by thrombocytopenia, large platelets, and deficient alpha granules in platelets and megakaryocytes. The genetic cause of GPS was recently elucidated by our lab and others, and deleterious mutations were found in the gene NBEAL2 in several affected individuals. NBEAL2 stands for Neurobeachin-like 2 and is a large (30kb, 54 exon) gene on human chromosome 3p21.31. NBEAL2 is a member of a family of proteins that contain a BEACH (Beige and Chediak Higashi) domain which is a highly conserved domain that has been associated with protein functions such as vesicular trafficking, membrane dynamics, and receptor signaling. Very little is known about NBEAL2 other than its involvement with GPS. The relative expression of the NBEAL2 transcript, both in tissues and in subcellular compartments, is not known. It has previously been published that NBEAL2 protein coding isoforms are present in a variety of human tissues, but the abundance of NBEAL2 transcript in each tissue is not known. Furthermore, there is currently no available antibody that recognizes the 302 kDa NBEAL2 protein to validate that mRNA presence translates to protein expression. We have examined the relative transcript abundance of NBEAL2 in a human cDNA library and have validated a novel NBEAL2 antibody in a human megakaryocytic cell line (Dami) and human platelets. We obtained a human mRNA tissue library from Invitrogen which was reverse transcribed to cDNA. Tissues analyzed include bladder, brain, cervix, colon, esophagus, heart, kidney, liver, lung, ovary, placenta, prostate, skeletal muscle, small intestine, spleen, testes, thymus, thyroid, trachea, bone marrow, peripheral leukocytes, CD33+, and CD36+. To investigate relative transcript abundance, we used a Taqman qPCR probe designed to identify all six protein coding isoforms, as determined by Ensembl (http://uswest.ensembl.org). Highest NBEAL2 expression was seen in CD33+ cells, which was 54.3 fold higher than the tissue with lowest expression (skeletal muscle). Relatively high expression was also seen in the peripheral leukocytes, bone marrow, lung, esophagus, and cervix. Alternatively, NBEAL2 expression was low in the brain, despite the homology to the Neurobeachin (NBEA) brain specific protein. While high transcript abundance may infer function, relative protein expression is necessary to validate these findings. Therefore, we designed a novel NBEAL2 antibody against a 14 amino acid peptide (SLEPRRPEEAGAEVC) encoded by exon 1 of NBEAL2 that is 100% conserved between human and mouse. Western blot characterization of this antibody showed the expected approximately 300 kDa band in both soluble and insoluble Dami lysates and human platelet rich plasma lysates. Furthermore, the NBEAL2 antibody was also used for immunofluorescence of Dami cells to determine the approximate subcellular localization of the protein. Preliminary results suggest that NBEAL2 is localized to the cytoplasm, a finding that is consistent with a subcellular localization prediction program (Euk-mPLoc 2.0). In conclusion, we have determined the relative abundance of NBEAL2 transcript in several human tissues, and have begun to characterize a novel antibody against NBEAL2 using the human megakaryocytic Dami cell line and human platelets. Ongoing studies with this novel tool in the Nbeal2 knockout mouse model will likely provide new information about this elusive protein. Disclosures:Di Paola:CSL Behring: Consultancy; Pfizer: DSMB, DSMB Other.

Neurobeachin-like 2 (Nbeal2) knockout mouse model of gray platelet syndrome

Neurobeachin-like 2 (Nbeal2) knockout mouse model of gray platelet syndrome

Gray platelet syndrome and defective thrombo-inflammation in Nbeal2-deficient mice.

Platelets are anuclear organelle-rich cell fragments derived from bone marrow megakaryocytes (MKs) that safeguard vascular integrity. The major platelet organelles, α-granules, release proteins that participate in thrombus formation and hemostasis. Proteins stored in α-granules are also thought to play a role in inflammation and wound healing, but their functional significance in vivo is unknown. Mutations in NBEAL2 have been linked to gray platelet syndrome (GPS), a rare bleeding disorder characterized by macrothrombocytopenia, with platelets lacking α-granules. Here we show that Nbeal2-knockout mice display the characteristics of human GPS, with defective α-granule biogenesis in MKs and their absence from platelets. Nbeal2 deficiency did not affect MK differentiation and proplatelet formation in vitro or platelet life span in vivo. Nbeal2-deficient platelets displayed impaired adhesion, aggregation, and coagulant activity ex vivo that translated into defective arterial thrombus formation and protection from thrombo-inflammatory brain infarction following focal cerebral ischemia. In a model of excisional skin wound repair, Nbeal2-deficient mice exhibited impaired development of functional granulation tissue due to severely reduced differentiation of myofibroblasts in the absence of α-granule secretion. This study demonstrates that platelet α-granule constituents are critically required not only for hemostasis but also thrombosis, acute thrombo-inflammatory disease states, and tissue reconstitution after injury.