The glycosylphosphatidylinositol anchor and paroxysmal nocturnal haemoglobinuria/aplasia model

Thrombosis and meningococcal infection rates in pegcetacoplan-treated patients with paroxysmal nocturnal hemoglobinuria in the clinical trial and postmarketing settings

Background

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired hematologic disease characterized by complement-mediated hemolysis and thrombosis. Complement component 5 (C5) inhibitors have decreased PNH-related thrombosis rates and reduced mortality compared with those of age-matched controls. A small but significantly increased risk of life-threatening Neisseria infections, especially N meningitidis, represents a long-term safety risk of complement inhibition.

Objectives

To evaluate the rates of thrombosis and meningococcal infections in patients with PNH treated with the complement component 3-targeted therapy pegcetacoplan.

Methods

Cumulative patient-year exposure to pegcetacoplan was calculated, and thrombotic events and meningococcal infections were reviewed in 7 clinical trials and in the postmarketing setting. The clinical trial protocols and pegcetacoplan labeling required vaccination against Streptococcus pneumoniae, N meningitidis, and Haemophilus influenzae before pegcetacoplan use; the label allowed for prophylactic antibiotic use if pegcetacoplan must be administered before vaccination.

Results

As of November 13, 2022, 464 patients with PNH had 619.4 patient-years of pegcetacoplan exposure in completed/ongoing clinical trials and the postmarketing setting. Seven thrombotic events were reported: 5 in clinical trials (2 in the same patient) and 2 in the postmarketing setting. The overall thrombosis rate was 1.13 events per 100 patient-years (clinical trials: 1.22 events/100 patient-years in 409.4 years; postmarketing: 0.95 events/100 patient-years in 210.0 years). No infections with meningococcal bacteria were reported.

Conclusion

Event rates for thrombosis were comparable between pegcetacoplan and previously reported rates of C5 inhibitors in patients with PNH, and no cases of meningococcal infection were reported with pegcetacoplan. Continued follow-up is required.

The platelet function defect of paroxysmal nocturnal haemoglobinuria

Paroxysmal nocturnal haemoglobinuria (PNH) is a rare, acquired stem cell disorder, characterised by an abnormal susceptibility of red blood cells to complement induced lysis, resulting in repeated episodes of intravascular haemolysis and haemoglobinuria, thromboembolic events at atypical locations and, to a much lesser extent, bleeding complications. Platelet function is assumed to be abnormal, however, a defect has not yet been characterised and underlying mechanisms remain elusive. To explore these issues, we investigated platelet function in PNH patients using assays for clot formation under low and high shear force (thrombelastography and PFA100 device), adhesion to glass beads in native whole blood (Hellem method), aggregometry using various agonists (Born method), and flow cytometric assays for baseline and agonist-induced surface expression density of alpha-granule (CD62P) and lysosomal granule proteins (CD63), ligand binding to surface receptors (thrombospondin), and expression density of activation-induced neoepitopes of the fibrinogen receptor complex (PAC-1). Platelet PNH clone size determined by CD55 and CD59 labelling was compared to the clone sizes of granulocytes, monocytes, erythrocytes, and reticulocytes. A profound reduction of platelet reactivity was observed in PNH patients for all "global function" assays (clot formation, adhesion, aggregation). Platelet hyporeactivity was confirmed using flow cytometric assays. Whereas baseline levels of flow cytometrically determined platelet activation markers did not differ significantly between controls and PNH patients, agonist-induced values of all markers were distinctly reduced in the PNH group. Moreover, significantly reduced white blood cell counts (3.1/nl vs. 5.9/nl), haemoglobin values (9.5 vs. 14.3/g per dl), and platelet counts (136 vs. 219/nl) delineate profound tricytopenia in PNH patients. The fraction of particular cell types lacking the surface expression of GPI-anchored glycoproteins is referred to as the respective PNH clone; median PNH clone sizes of cells with short life spans (reticulocytes, platelets, granulocytes) was 50-80% of total cell populations compared to 20% of red blood cells. The results of our laboratory investigations show, that in PNH, reduced platelet counts coincide with reduced platelet reactivity. The foremost clinical complication in PNH, however, is venous thromboembolism, very probably induced by an activated and dysregulated plasmatic coagulation system. From these seemingly contradictory findings we infer, that part of the platelet hyporeactivity is probably due to reactive downregulation of platelet function in response to chronic hyperstimulation. The overall result is thought to be an unsteady balance, associated with thromboembolism in a larger proportion of patients, and with bleeding in a smaller proportion.

Paroxysmal nocturnal hemoglobinuria in childhood and adolescence--a retrospective analysis of 18 cases

OBJECTIVE

To assess the clinical and hematological profile of PNH in children.

METHODS

Clinical and laboratory features of children with PNH diagnosed in the past six years at our centre were reviewed. Various investigations done included a complete blood count and peripheral smear examination, plasma hemoglobin, urine hemosiderin, acid ham test, sucrose lysis test, immunophenotyping of erythrocytes by sephadex column gel card and of granulocytes by flow cytometry. There were 18 children with a marked male predominance (M 14: F 4).

RESULTS

Pallor, jaundice, dark urine and bleeding manifestations were the major presenting complaints. One girl suffered an arterial stroke. All children had cytopenia in at least one cell line. Children were treated with danazol, stanazolol, prednisolone and cyclosporin A variously. Overall response rate was 61%. Children with classical PNH performed slightly better with response rates of 66% (6/9) as compared to aplastic anemia-PNH group which has a response rate of 55% (5/9). Amongst various variables only danazol correlated with better response (p=0.029).

CONCLUSION

PNH is an uncommon disease in children and should be included in the differential diagnosis of children presenting with cytopenia.

Recent advances in the diagnosis, monitoring, and management of patients with paroxysmal nocturnal hemoglobinuria

Until recently, there has been no specific therapy for PNH with clinical management mainly supportive in terms of cytopenias and control of thrombotic risk. Currently, the only curative procedure for PNH is bone marrow transplantation although for the majority of patients the associated risks are too great to justify transplantation. The pioneering use of the therapeutic monoclonal antibody eculizumab, which binds to and prevents the activation of the complement protein C5, represents a significant advance in treatment for patients with PNH and is set to become the future standard therapy for hemolytic PNH. In both an initial pilot study and two phase III clinical trials, eculizumab has been shown to dramatically reduce intravascular hemolysis, hemoglobinuria, and transfusion requirements thus improving the quality of life in patients with PNH. As a clinical entity, PNH is synonymous with glycosylphosphatidylinositol (GPI) deficiency, and is an acquired clonal disorder associated with somatic mutations of the X-linked PIGA gene in hematopoietic stem cells. A recent study identified a novel autosomal recessively inherited form of GPI-deficiency involving a mutation in a promotor component of the pig-m gene and characterized by a thrombotic tendency and seizures. In both these developments, flow cytometry played a critical role. In the first instance, in monitoring direct response to a new therapeutic agent; second, in demonstrating the phenotypic/genotypic link in a new form of GPI deficiency.

The Role of Flow Cytometry in the Diagnosis of Paroxysmal Nocturnal Hemoglobinuria in the Clinical Laboratory

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare acquired hematopoietic stem cell disorder closely related to aplastic anemia. Hemolytic anemia and life-threatening thromboses are common features in many patients. Rapid diagnosis is highly desirable and flow cytometry plays a key role in the laboratory investigation of PNH. By demonstrating absence of cell membrane glycosylphosphatidylinositol-anchored proteins from granulocytes or red cells, a definitive diagnosis of PNH can be established. This can have a considerable impact on patient management and outcome. As with all rare diseases, internal and external quality assurance is essential for good laboratory practice and to fulfill the requirements of national laboratory accreditation schemes.

Paroxysmal nocturnal hemoglobinuria in children

Paroxysmal nocturnal hemoglobinuria (PNH), an acquired hematologic disorder characterized by intravascular hemolysis, nocturnal hemoglobinuria, thrombotic events, serious infections, and bone marrow failure, is very rare in children. PNH is caused by a somatic mutation of the phosphatidylinositol glycan (GPI) complementation class A (PIGA) gene, followed by a survival advantage of the PNH clone, which results in a deficiency of GPI-anchored proteins on hematopoietic cells. Currently, immunophenotypic GPI-linked anchor protein analysis has replaced the acid Ham and sucrose lysis test, as it provides a reliable diagnostic tool for this disease. The presence of PNH clones should be considered in every child with an acquired bone marrow failure syndrome, for example (hypoplastic) myelodysplastic syndrome and aplastic anemia, and/or unexpected serious thrombosis. Treatment of PNH in children is dependent on the clinical presentation. In cases of severe bone marrow failure, stem cell transplantation should be seriously considered as a therapeutic option even if no matched sibling donor is available. This article reviews the reported cases of PNH in children using the recently published guidelines for classification, diagnostics, and treatment.

Computational approach for identification and characterization of GPI-anchored peptides in proteomics experiments

Genes that encode glycosylphosphatidylinositol anchored proteins (GPI-APs) constitute an estimated 1-2% of eukaryote genomes. Current computational methods for the prediction of GPI-APs are sensitive and specific; however, the analysis of the processing site (omega- or omega-site) of GPI-APs is still challenging. Only 10% of the proteins that are annotated as GPI-APs have the omega-site experimentally verified. We describe an integrated computational and experimental proteomics approach for the identification and characterization of GPI-APs that provides the means to identify GPI-APs and the derived GPI-anchored peptides in LC-MS/MS data sets. The method takes advantage of sequence features of GPI-APs and the known core structure of the GPI-anchor. The first stage of the analysis encompasses LC-MS/MS based protein identification. The second stage involves prediction of the processing sites of the identified GPI-APs and prediction of the corresponding terminal tryptic peptides. The third stage calculates possible GPI structures on the peptides from stage two. The fourth stage calculates the scores by comparing the theoretical spectra of the predicted GPI-peptides against the observed MS/MS spectra. Automated identification of C-terminal GPI-peptides from porcine membrane dipeptidase, folate receptor and CD59 in complex LC-MS/MS data sets demonstrates the sensitivity and specificity of this integrated computational and experimental approach.

Effects of a mosquitocidal toxin on a mammalian epithelial cell line expressing its target receptor

The spread of diseases transmitted by Anopheles and Culex mosquitoes, such as malaria and West Nile fever, is a growing concern for human health. Bacillus sphaericus binary toxin (Bin) is one of the few available bioinsecticides able to control populations of these mosquitoes efficiently. We previously showed that Bin binds to Cpm1, an alpha-glucosidase located on the apical side of Culex larval midgut epithelium. We analysed the effects of Bin by expressing a construct encoding Cpm1 in the mammalian epithelial MDCK cell line. Cpm1 is targeted to the apical side of polarized MDCK, where it is anchored by glycosylphosphatidylinositol (GPI) and displays alpha-glucosidase activity. Bin bound to transfected cells and induced a non-specific current presumably related to the opening of pores. The formation of these pores may be related to the location of the toxin/receptor complex in lipid raft microdomains. Finally, Bin promoted the time-dependent appearance of intracytoplasmic vacuoles but did not drive cell lysis. Thus, the dual functionality (enzyme/toxin receptor) of Cpm1 is fully conserved in MDCK cells and Cpm1 is an essential target protein for Bin cytotoxicity in Culex mosquitoes.

Differential gene expression in hematopoietic progenitors from paroxysmal nocturnal hemoglobinuria patients reveals an apoptosis/immune response in 'normal' phenotype cells

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired stem cell disorder characterized clinically by intravascular hemolysis, venous thrombosis, and bone marrow failure. Despite elucidation of the biochemical and molecular defects in PNH, the pathophysiology of clonal expansion of glycosylphosphatidylinositol-anchored protein (GPI-AP)-deficient cells remains unexplained. In pursuit of evidence of differences between GPI-AP-normal and -deficient CD34 cells, we determined gene expression profiles of isolated marrow CD34 cells of each phenotype from PNH patients and healthy donors, using DNA microarrays. Pooled and individual patient samples revealed consistent gene expression patterns relative to normal controls. GPI-AP-normal cells from PNH patients showed upregulation of genes involved in apoptosis and the immune response. Conversely, genes associated with antiapoptotic function and hematopoietic cell proliferation and differentiation were downregulated in these cells. In contrast, the PNH clone of GPI-AP-deficient cells appeared more similar to CD34 cells of healthy individuals. Gene chip data were confirmed by other methods. Similar gene expression patterns were present in PNH that was predominantly hemolytic as in PNH associated with aplastic anemia. Our results implicate an environmental influence on hematopoietic cell proliferation, in which the PNH clone evades immune attack and destruction, while normal cells suffer a stress response followed by programmed cell death.

Childhood paroxysmal nocturnal haemoglobinuria (PNH), a report of 11 cases in the Netherlands

Paroxysmal nocturnal haemoglobinuria (PNH) is characterized by intravascular haemolysis, nocturnal haemoglobinuria, thrombotic events, serious infections and bone marrow failure. This acquired disease, caused by a deficiency of glycosylphosphatidylinositol (GPI) anchored proteins on the haematopoietic cells, is rare in children. We describe 11 Dutch paediatric PNH patients (median age: 12 years, range 9-17 years) diagnosed since 1983, seven cases associated with aplastic anaemia (AA), four with myelodysplastic syndrome (MDS). Presenting symptoms were haemorrhagic diathesis (n = 10), palor/tiredness (n = 8), dark urine (n = 1), fever (n = 1) and serious weight loss (n = 1). Treatment consisted of prednisolone (n = 7), anti-thymocyte globulin (n = 3) and/or androgens (n = 5). Eventually, five patients received a bone marrow transplantation (BMT) (three matched unrelated donors/two matched family donors), of whom four are still alive. PNH, diagnosed by immunophenotypic GPI-linked anchor protein analysis, should be considered in all children with AA or MDS. BMT should be considered as a therapeutic option in every paediatric PNH patient with BM failure.

GPI-defective monocytes from paroxysmal nocturnal hemoglobinuria patients show impaired in vitro dendritic cell differentiation

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal, acquired hematopoietic disorder characterized by a phosphatidylinositol (PI) glycan-A gene mutation, which impairs the synthesis of the glycosyl-PI (GPI) anchor, thus causing the absence of all GPI-linked proteins on the membrane of the clonal-defective cells. The presence of a consistent GPI-defective monocyte compartment is a common feature in PNH patients. To investigate the functional behavior of this population, we analyzed its in vitro differentiation ability toward functional dendritic cells (DCs). Our data indicate that GPI-defective monocytes from PNH patients are unable to undergo full DC differentiation in vitro after granulocyte macrophage-colony stimulating factor and recombinant interleukin (IL)-4 treatment. In this context, the GPI-defective DC population shows mannose receptor expression, high levels of the CD86 molecule, and impaired CD1a up-regulation. The analysis of lipopolysaccharide and CD40-dependent, functional pathways in these DCs revealed a strong decrease in tumor necrosis factor alpha and IL-12 production. Finally, GPI-defective DCs showed a severe impairment in delivering accessory signals for T cell receptor-dependent T cell proliferation.

Natural history of paroxysmal nocturnal haemoglobinuria using modern diagnostic assays

Paroxysmal nocturnal haemoglobinuria (PNH) is an uncommon, acquired disorder of blood cells caused by mutation of the phosphatidylinositol glycan class A (PIG-A) gene. The disease often manifests with haemoglobinuria, peripheral blood cytopenias, and venous thrombosis. The natural history of PNH has been documented in retrospective series; but there has only been one study that correlated the more sensitive and specific flow cytometric assays that have become available in the last decade with severe symptoms associated with PNH. In a retrospective analysis of 49 consecutive patients with PNH evaluated at Johns Hopkins, large PNH clones were associated with an increased risk for thrombosis as well as haemoglobinuria, abdominal pain, oesophageal spasm, and impotence. Of the 14 (29%) patients that developed thrombosis, nine died; six of these from complications related to thromboses. According to logistic regression modelling, for a 10% change in PNH clone size, the odds ratio for risk of thrombosis was estimated to be 1.64. No patient with <61% PNH granulocytes developed a thrombosis, whereas 12 of 22 patients (54.5%) with > or =61% PNH granulocytes manifested with thrombosis. These data not only confirm that the size of the PNH clone correlates with the risk for thrombosis, but they also suggest a correlation of PNH clone size to more symptomatic PNH.

Splenectomy for massive splenic infarction unmasks paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare disorder characterized by pancytopenia, hemolysis, and thrombosis. Abdominal vein thrombosis is a life-threatening manifestation of this disease. We present a patient with complete spleen necrosis due to thrombosis of the splenic vessels. After splenectomy, other causes of thrombophilia were excluded and the diagnosis of PNH was established. The patient was put on anticoagulation but despite the prophylactic international normalized ratio maintained over the last 18 months of follow-up, he had another episode of intrahepatic thrombosis which was treated with tissue plasminogen activator thrombolysis.

Immune-Mediated Hemolytic Anemia

Hemolytic anemia due to immune function is one of the major causes of acquired hemolytic anemia. In recent years, as more is known about the immune system, these entities have become better understood and their treatment improved. In this section, we will discuss three areas in which this progress has been apparent.

In Section I, Dr. Peter Hillmen outlines the recent findings in the pathogenesis of paroxysmal nocturnal hemoglobinuria (PNH), relating the biochemical defect (the lack of glycosylphosphatidylinositol [GPI]-linked proteins on the cell surface) to the clinical manifestations, particularly hemolysis (and its effects) and thrombosis. He discusses the pathogenesis of the disorder in the face of marrow dysfunction insofar as it is known. His major emphasis is on innovative therapies that are designed to decrease the effectiveness of complement activation, since the lack of cellular modulation of this system is the primary cause of the pathology of the disease. He recounts his considerable experience with a humanized monoclonal antibody against C5, which has a remarkable effect in controlling the manifestations of the disease. Other means of controlling the action of complement include replacing the missing modulatory proteins on the cell surface; these studies are not as developed as the former agent.

In Section II, Dr. Alan Schreiber describes the biochemistry, genetics, and function of the Fcγ receptors and their role in the pathobiology of autoimmune hemolytic anemia and idiopathic thrombocytopenic purpura due to IgG antibodies. He outlines the complex varieties of these molecules, showing how they vary in genetic origin and in function. These variations can be related to three-dimensional topography, which is known in some detail. Liganding IgG results in the transduction of a signal through the tyrosine-based activation motif and Syk signaling. The role of these receptors in the pathogenesis of hematological diseases due to IgG antibodies is outlined and the potential of therapy of these diseases by regulation of these receptors is discussed.

In Section III, Dr. Wendell Rosse discusses the forms of autoimmune hemolytic anemia characterized by antibodies that react preferentially in the cold–cold agglutinin disease and paroxysmal cold hemoglobinuria (PCH). The former is due to IgM antibodies with a common but particular structure that reacts primarily with carbohydrate or carbohydrate-containing antigens, an interaction that is diminished at body temperature. PCH is a less common but probably underdiagnosed illness due to an IgG antibody reacting with a carbohydrate antigen; improved techniques for the diagnosis of PCH are described. Therapy for the two disorders differs somewhat because of the differences in isotype of the antibody. Since the hemolysis in both is primarily due to complement activation, the potential role of its control, as by the monoclonal antibody described by Dr. Hillmen, is discussed.