Impaired growth and elevated fas receptor expression in PIGA(+) stem cells in primary paroxysmal nocturnal hemoglobinuria

Insights Into the Emergence of Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal Nocturnal Hemoglobinuria (PNH) is a disease as simple as it is complex. PNH patients develop somatic loss-of-function mutations in phosphatidylinositol N-acetylglucosaminyltransferase subunit A gene (PIGA), required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors. Ubiquitous in eukaryotes, GPI anchors are a group of conserved glycolipid molecules responsible for attaching nearly 150 distinct proteins to the surface of cell membranes. The loss of two GPI-anchored surface proteins, CD55 and CD59, from red blood cells causes unregulated complement activation and hemolysis in classical PNH disease. In PNH patients, PIGA-mutant, GPI (-) hematopoietic cells clonally expand to make up a large portion of patients’ blood production, yet mechanisms leading to clonal expansion of GPI (-) cells remain enigmatic. Historical models of PNH in mice and the more recent PNH model in rhesus macaques showed that GPI (-) cells reconstitute near-normal hematopoiesis but have no intrinsic growth advantage and do not clonally expand over time. Landmark studies identified several potential mechanisms which can promote PNH clonal expansion. However, to what extent these contribute to PNH cell selection in patients continues to be a matter of active debate. Recent advancements in disease models and immunologic technologies, together with the growing understanding of autoimmune marrow failure, offer new opportunities to evaluate the mechanisms of clonal expansion in PNH. Here, we critically review published data on PNH cell biology and clonal expansion and highlight limitations and opportunities to further our understanding of the emergence of PNH clones.

Evolutionary dynamics of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal blood disorder characterized by hemolysis and a high risk of thrombosis, that is due to a deficiency in several cell surface proteins that prevent complement activation. Its origin has been traced to a somatic mutation in the PIG-A gene within hematopoietic stem cells (HSC). However, to date the question of how this mutant clone expands in size to contribute significantly to hematopoiesis remains under debate. One hypothesis posits the existence of a selective advantage of PIG-A mutated cells due to an immune mediated attack on normal HSC, but the evidence supporting this hypothesis is inconclusive. An alternative (and simpler) explanation attributes clonal expansion to neutral drift, in which case selection neither favours nor inhibits expansion of PIG-A mutated HSC. Here we examine the implications of the neutral drift model by numerically evolving a Markov chain for the probabilities of all possible outcomes, and investigate the possible occurrence and evolution, within this framework, of multiple independently arising clones within the HSC pool. Predictions of the model agree well with the known incidence of the disease and average age at diagnosis. Notwithstanding the slight difference in clonal expansion rates between our results and those reported in the literature, our model results lead to a relative stability of clone size when averaging multiple cases, in accord with what has been observed in human trials. The probability of a patient harbouring a second clone in the HSC pool was found to be extremely low (~10-8). Thus our results suggest that in clinical cases of PNH where two independent clones of mutant cells are observed, only one of those is likely to have originated in the HSC pool.

The mechanisms leading to expansion of HSC with mutations in the PIG-A gene that leads to the PNH phenotype remains unclear. Data so far suggests there is no intrinsic fitness advantage of the mutant cells compared to normal cells. Assuming neutral drift within the HSC compartment, we determined from first principles the incidence of the disease in a population, the average clone size in patients, the probability of clonal extinction, the likelihood of several separate clones coexisting in the HSC pool, and the expected expansion rate of a mutant clone. Our results are similar to what is observed in clinical practice. We also find that in such a model the probability of multiple PNH clones arising independently in the HSC pool is exceptionally small. This suggests that in clinical cases where more than one distinct clone is observed, all but one of the clones are likely to have emerged in cells that are downstream of the HSC population. We propose that PNH is perhaps the first disease where neutral drift alone may be responsible for clonal expansion leading to a clinical problem.

The mutational landscape of paroxysmal nocturnal hemoglobinuria revealed: new insights into clonal dominance

Paroxysmal nocturnal hemoglobinuria (PNH) is a disorder of hematopoietic stem cells that has largely been considered a monogenic disorder due to acquisition of mutations in the gene encoding PIGA, which is required for glycosylphosphatidylinositol (GPI) anchor biosynthesis. In this issue of the JCI, Shen et al. discovered that PNH is in fact a complex genetic disorder orchestrated by many genetic alterations in addition to PIGA mutations. Some of these mutations predate the acquisition of PIGA mutations, while others occur later. Surprisingly, this work indicates that PNH has a clonal evolution and architecture strikingly similar to that of other myeloid neoplasms, highlighting a potentially broader mechanism of disease pathogenesis in this disorder.

Mechanism of paroxysmal nocturnal hemoglobinuria clonal dominance: possible roles of different apoptosis and CD8+ lymphocytes in the selection of paroxysmal nocturnal hemoglobinuria clones

BACKGROUND AND OBJECTIVES

Paroxysmal nocturnal hemoglobinuria (PNH), a clonal hematopoietic stem cell disorder, manifests when the PNH clone populates in the hematopoietic compartment. We explored the roles of different apoptosis of GPI+ and GPI- (glycosylphosphatidylinositol) cells and CD8+ lymphocytes in a selection of PNH clones.

PATIENTS AND METHODS

Granulocytes from PNH patients and normal controls were subjected to an apoptosis assay using annexin V. Hematopoietic cell in semisolid media were cultured with or without CD8+ lymphocytes.

RESULTS

In PNH, CD59+ granulocytes exhibited more apoptosis than their CD59- counterparts, after 0 or 4 hours in liquid growth culture system (mean [standard error of mean]: 2.1 (0.5) vs 1.2 (0.2), P=.01 at 0 hour and 3.4 [0.7] vs 1.8 [0.3], P=.03 at 4 hour, respectively). The presence of mononuclear cells (MNCs) rendered a greater difference in apoptosis. The percentages of apoptotic CD59+ granulocytes measured at 4 hours with or without MNC fraction were correlated with the sizes of PNH clones (r=0.633, P=.011; and r=0.648, P=.009; respectively). The autologous CD8+ lymphocytes inhibited CFU-GM and BFU-E colony formation in PNH patients when compared with normal controls (mean [SEM] of percentages of inhibition: 61.7 (10.4) vs 11.9 (2.0), P=.008 for CFU-GM and 26.1 (6.9) vs 4.9 (1.0), P=.037 for BFU-E).

CONCLUSIONS

Increased apoptosis of GPI+ blood cells is likely to be responsible in selection and expansion of PNH clones. MNCs or possibly CD8+ lymphocytes may play a role in this phenomenon.

NKG2D-mediated immunity underlying paroxysmal nocturnal haemoglobinuria and related bone marrow failure syndromes

It is considered that a similar immune mechanism acts in the pathogenesis of bone marrow (BM) failure in paroxysmal nocturnal haemoglobinuria (PNH) and its related disorders, such as aplastic anaemia (AA) and myelodysplastic syndromes (MDS). However, the molecular events in immune-mediated marrow injury have not been elucidated. We recently reported an abnormal expression of stress-inducible NKG2D (natural-killer group 2, member D) ligands, such as ULBP (UL16-binding protein) and MICA/B (major histocompatibility complex class I chain-related molecules A/B), on granulocytes in some PNH patients and the granulocyte killing by autologous lymphocytes in vitro. The present study found that the expression of NKG2D ligands was common to both granulocytes and BM cells of patients with PNH, AA, and MDS, indicating their exposure to some incitement to induce the ligands. The haematopoietic colony formation in vitro by the patients' marrow cells significantly improved when their BM cells were pretreated with antibodies against NKG2D receptor, suggesting that the antibodies rescued haematopoietic cells expressing NKG2D ligands from damage by autologous lymphocytes with NKG2D. Clinical courses of patients with PNH and AA showed a close association of the expression of NKG2D ligands with BM failure and a favourable response to immunosuppressive therapy. We therefore propose that NKG2D-mediated immunity may underlie the BM failure in PNH and its-related marrow diseases.

Glycosylphosphatidylinositol-anchored protein deficiency confers resistance to apoptosis in PNH

OBJECTIVE

Investigate the contribution of PIG-A mutations to clonal expansion in paroxysmal nocturnal hemoglobinuria (PNH).

MATERIALS AND METHODS

Primary CD34+ hematopoietic progenitors from PNH patients were assayed for annexin-V positivity by flow cytometry in a cell-mediated killing assay using autologous effectors from PNH patients or allogeneic effectors from healthy controls. To specifically assess the role of the PIG-A mutation in the development of clonal dominance and address confounders of secondary mutation and differential immune attack that can confound experiments using primary cells, we established an inducible PIG-A CD34+ myeloid cell line, TF-1. Apoptosis resistance was assessed after exposure to allogeneic effectors, NK92 cells (an interleukin-2-dependent cell line with the phenotype and function of activated natural killer [NK] cells), tumor necrosis factor (TNF)-alpha, and gamma-irradiation. Apoptosis was measured by annexin-V staining and caspase 3/7 activity.

RESULTS

In PNH patients, CD34+ hematopoietic progenitors lacking glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-AP(-)) were less susceptible than GPI-AP+ CD34+ precursors to autologous (8% vs 49%; p < 0.05) and allogeneic (28% vs 58%; p < 0.05) cell-mediated killing from the same patients. In the inducible PIG-A model, GPI-AP(-) TF-1 cells exhibited less apoptosis than induced, GPI-AP+ TF-1 cells in response to allogeneic cell-mediated killing, NK92-mediated killing, TNF-alpha, and gamma-irradiation. GPI-AP(-) TF-1 cells maintained resistance to apoptosis when effectors were raised against GPI-AP(-) cells, arguing against a GPI-AP being the target of immune attack in PNH. NK92-mediated killing was partially inhibited with blockade by specific antibodies to the stress-inducible GPI-AP ULBP1 and ULBP2 that activate immune effectors. Clonal competition experiments demonstrate that the mutant clone expands over time under proapoptotic conditions with TNF-alpha.

CONCLUSION

PIG-A mutations contribute to clonal expansion in PNH by conferring a survival advantage to hematopoietic progenitors under proapoptotic stresses.

New insights into paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an uncommon acquired hemolytic anemia that manifests with abdominal pain, esophageal spasm, fatigue, and thrombosis. The hallmark of PNH at the cellular level is a deficiency in cell surface glycosylphosphatidylinositol anchored proteins; this deficiency on erythrocytes leads to intravascular hemolysis. Free hemoglobin from hemolysis leads to circulating nitric oxide depletion and is responsible for many of the clinical manifestations of PNH, including fatigue, erectile dysfunction, esophageal spasm, and thrombosis. The recently FDA approved complement inhibitor eculizumab has been shown to decrease hemolysis, decrease erythrocyte transfusion requirements, and improve quality of life for PNH patients.

Altered lipid raft composition and defective cell death signal transduction in glycosylphosphatidylinositol anchor-deficient PIG-A mutant cells

Paroxysmal nocturnal haemoglobinuria (PNH) is a clonal disorder of haematopoietic stem cells caused by somatic PIGA mutations, resulting in a deficiency in glycosylphosphatidylinositol-anchored proteins (GPI-AP). Because GPI-AP associate with lipid rafts (LR), lack of GPI-AP on PNH cells may result in alterations in LR-dependent signalling. Conversely, PNH cells are a suitable model for investigating LR biology. LR from paired, wild-type GPI(+), and mutant GPI(-) cell lines (K562 and TF1) were isolated and analysed; GPI(-) LR contained important anti-apoptotic proteins, not found in LR from GPI(+) cells. When methyl-beta-cyclodextrin (MbetaCD) was utilized to probe for functional differences between normal and GPI(-) LR, increased levels of phospho-p38 mitogen-activated protein kinase (MAPK), and phospho-p65 nuclear factor NF-kappaB were found in control and GPI(-) cells respectively. Subsequent experiments addressing the inhibition of phosphoinositide-3-kinase (PI3K) suggest that the PI3K/AKT pathway may be responsible for the resistance of K562 GPI(-)cells to negative effects of MbetaCD. In addition, transduction of tumour necrosis factor-alpha (TNF-alpha) signals in a LR-dependent fashion increased induction of p38 MAPK in GPI(+) and increased pro-survival NF-kappaB levels in K562 GPI(-) cells. Therefore, we suggest that the altered LR-dependent signalling in PNH-like cells may induce different responses to pro-inflammatory cytokines from those observed in cells with intact GPI-AP.

Paroxysmal nocturnal hemoglobinuria: stem cells and clonality

Paroxysmal nocturnal hemoglobinuria is a clonal hematopoietic stem cell disease that manifests with intravascular hemolysis, bone marrow failure, thrombosis, and smooth muscle dystonias. The disease can arise de novo or in the setting of acquired aplastic anemia. All PNH patients to date have been shown to harbor PIG-A mutations; the product of this gene is required for the synthesis of glycosylphosphatidylinositol (GPI) anchored proteins. In PNH patients, PIG-A mutations arise from a multipotent hematopoietic stem cell. Interestingly, PIG-A mutations can also be found in the peripheral blood of most healthy controls; however, these mutations arise from progenitor cells rather than multipotent hematopoietic stem cells and do not propagate the disease. The mechanism of whereby PNH stem cells achieve clonal dominance remains unclear. The leading hypotheses to explain clonal outgrowth in PNH are: 1) PNH cells evade immune attack possibly, because of an absent cell surface GPI-AP that is the target of the immune attack; 2) The PIG-A mutation confers an intrinsic resistance to apoptosis that becomes more conspicuous when the marrow is under immune attack; and 3) A second mutation occurs in the PNH clone to give it an intrinsic survival advantage. These hypotheses may not be mutually exclusive, since data in support of all three models have been generated.

A quantitative measurement of the human somatic mutation rate

The mutation rate (mu) is a key biological feature of somatic cells that determines risk for malignant transformation, and it has been exceedingly difficult to measure in human cells. For this purpose, a potential sentinel is the X-linked PIG-A gene, because its inactivation causes lack of glycosylphosphatidylinositol-linked membrane proteins. We previously found that the frequency (f) of PIG-A mutant cells can be measured accurately by flow cytometry, even when f is very low. Here we measure both f and mu by culturing B-lymphoblastoid cell lines and first eliminating preexisting PIG-A mutants by flow sorting. After expansion in culture, the frequency of new mutants is determined by flow cytometry using antibodies specific for glycosylphosphatidylinositol-linked proteins (e.g., CD48, CD55, and CD59). The mutation rate is then calculated by the formula mu = f/d, where d is the number of cell divisions occurring in culture. The mean mu in cells from normal donors was 10.6 x 10(-7) mutations per cell division (range 2.4 to 29.6 x 10(-7)). The mean mu was elevated >30-fold in cells from patients with Fanconi anemia (P < 0.0001), and mu varied widely in ataxia-telangiectasia with a mean 4-fold elevation (P = 0.002). In contrast, mu was not significantly different from normal in cells from patients with Nijmegen breakage syndrome. Differences in mu could not be attributed to variations in plating efficiency. The mutation rate in man can now be measured routinely in B-lymphoblastoid cell lines, and it is elevated in cancer predisposition syndromes. This system should be useful in evaluating cancer risk and in the design of preventive strategies.

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.

Aplastic anaemia

Aplastic anaemia is a rare haemopoietic stem-cell disorder that results in pancytopenia and hypocellular bone marrow. Although most cases are acquired, there are unusual inherited forms. The pathophysiology of acquired aplastic anaemia is immune mediated in most cases; autoreactive lymphocytes mediate the destruction of haemopoietic stem cells. Environmental exposures, such as to drugs, viruses, and toxins, are thought to trigger the aberrant immune response in some patients, but most cases are classified as idiopathic. Similarly to other autoimmune diseases, aplastic anaemia has a varied clinical course; some patients have mild symptoms that necessitate little or no therapy, whereas others present with life-threatening pancytopenia representing a medical emergency. Paroxysmal nocturnal haemoglobinuria and myelodysplastic syndrome commonly arise in patients with aplastic anaemia, showing a pathophysiological link between these disorders. Acquired aplastic anaemia can be effectively treated by allogeneic bone-marrow transplantation, immunosuppression (generally antithymocyte globulin and ciclosporin), and high-dose cyclophosphamide.

High-dose cyclophosphamide as salvage therapy for severe aplastic anemia

OBJECTIVE

The treatment options for patients with aplastic anemia who do not respond to conventional immunosuppression are limited. The aim of this study was to evaluate high-dose cyclophosphamide in patients with refractory severe aplastic anemia (SAA).

MATERIALS AND METHODS

We treated 17 SAA patients with high-dose cyclophosphamide (50 mg/kg/day for 4 consecutive days) who previously did not respond to one or more courses of immunosuppressive therapy. Median age was 31 years (range 6-58); median disease duration was 14 months (range 6-58), and 8 patients met criteria for very severe aplastic anemia (absolute neutrophil count <0.2 x 10(9)/L) at the time of treatment.

RESULTS

At median follow-up of 29 months, 10 patients (59%) are alive. Nine patients (53%) achieved a drug-free remission after high-dose cyclophosphamide; 4 patients achieved a complete remission and 5 patients currently meet criteria for a partial remission but continue to improve. One nonresponder to high-dose cyclophosphamide developed paroxysmal nocturnal hemoglobinuria; another nonresponder developed a myelodysplastic syndrome. In responding patients, median time to 500 neutrophils was 54 days (range 35-119), median time to the last platelet transfusion was 99 days (range 51-751), and median time to the last red cell transfusion was 125 days (range 63-796).

CONCLUSION

High-dose cyclophosphamide shows promise for salvaging patients with refractory SAA.

Frequent HPRT mutations in paroxysmal nocturnal haemoglobinuria reflect T cell clonal expansion, not genomic instability

Paroxysmal nocturnal haemoglobinuria (PNH) results from acquired mutations in the PIG-A gene of an haematopoietic stem cell, leading to defective biosynthesis of glycosylphosphatidylinositol (GPI) anchors and deficient expression of GPI-anchored proteins on the surface of the cell's progeny. Some laboratory and clinical findings have suggested genomic instability to be intrinsic in PNH; this possibility has been supported by mutation analysis of hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene abnormalities. However, the HPRT assay examines lymphocytes in peripheral blood (PB), and T cells may be related to the pathophysiology of PNH. We analysed the molecular and functional features of HPRT mutants in PB mononuclear cells from eleven PNH patients. CD8 T cells predominated in these samples; approximately half of the CD8 cells lacked GPI-anchored protein expression, while only a small proportion of CD4 cells appeared to derive from the PNH clone. The HPRT mutant frequency (Mf) in T lymphocytes from PNH patients was significantly higher than in healthy controls. The majority of the mutant T lymphocyte clones were of CD4 phenotype, and they had phenotypically normal GPI-anchored protein expression. In PNH patients, the majority of HPRT mutant clones were contained within the Vbeta2 T cell receptor (TCR) subfamily, which was oligoclonal by complementarity-determining region three (CDR3) size analysis. Our results are more consistent with detection of uniform populations of expanded T cell clones, which presumably acquired HPRT mutations during antigen-driven cell proliferation, and not due to an increased Mf in PNH. HPRT mutant analysis does not support underlying genomic instability in PNH.

A new aspect of the molecular pathogenesis of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal hematologic disorder which is manifest by complement-mediated hemolysis, venous thrombosis, and bone marrow failure. Complement-mediated hemolysis in PNH is explained by the deficiency of glycosylphosphatidylinositol (GPI)-anchored proteins, CD55 and CD59 on erythrocyte surfaces. All the PNH patients had phosphatidylinositol glycan-class A (PIG-A) gene abnormalities in various cell types, indicating that PIG-A gene mutations cause the defects in GPI-anchored proteins that are essential for the pathogenesis of PNH. In addition, a PIG-A gene abnormality results in a PNH clone. Bone marrow failure causes cytopenias associated with a proliferative decrease of its hematopoietic stem cells and appears to be related to a pre-leukemic state. Although it is unclear how a PNH clone expands in bone marrow, it is considered that the most important hypothesis implicates negative selection of a PNH clone, but it does not explain the changes in the clinical features at the terminal stage of PNH. Recently, it has been suggested that an immune mechanism, in an HLA-restricted manner, plays an important role in the occurrence or selection of a PNH clone and GPI may be a target for cytotoxic-T lymphocytes. Also, it has been indicated that the Wilms' tumor gene (WT1) product is related to a PNH clone, but the significance of WT1 expression is not clear because of the functional diversity of the gene. To elucidate this problem, it is important to know the pathophysiology of bone marrow failure in detail and how bone marrow failure affects hematopoietic stem cells and immune mechanisms in bone marrow failure syndromes.

Cytogenetic abnormalities in paroxysmal nocturnal haemoglobinuria usually occur in haematopoietic cells that are glycosylphosphatidylinositol-anchored protein (GPI-AP) positive

Some patients with paroxysmal nocturnal haemoglobinuria (PNH) have bone marrow findings characteristic of myelodysplastic syndrome. We studied nine PNH patients to determine whether these karyotypic abnormalities were more likely to occur in glycosylphosphatidylinositol-anchored protein (GPI-AP)-negative cells. Abnormal chromosome patterns were evident only in the GPI-AP-positive populations of the PNH clone in 8 of 9 cases studied. Purified GPI-AP-negative CD34 cells gave rise only to cells of normal karyotype, whereas the progeny of the GPI-AP-positive CD34 cells showed the karyotypic abnormality. These findings suggest that environmental factors, but not genetic instability of the GPI-AP-deficient clone, foster development or survival of haematopoietic cells with chromosomal abnormalities.

Hematopoietic stem cells in aplastic anemia

Profound cytopenia involving all blood lineages, a hallmark of aplastic anemia (AA), can result in devastating morbidity and high mortality. Although various etiologies and distinct pathophysiologic mechanisms may be involved, a profound defect in the stem cell compartment is a unifying feature in most patients with AA. As a stem cell disease, AA is very instructive and provides insights into the function and quantity of normal hematopoietic stem cells and their ability to regenerate. Pathophysiologically, understanding of AA may reveal mechanisms as to the evolution of other related bone marrow failure syndromes such as paroxysmal nocturnal hemoglobinuria and myelodysplasia-clonal diseases of hematopoiesis associated with defective stem cells. Conversely, constitutional forms of AA occurring in association with Fanconi anemia and dyskeratosis congenita demonstrate the role of specific genes and pathways in the dysfunction of the stem cells leading to the failure of the stem cell compartment. The acquired mechanisms resulting in depletion of stem cells in AA may involve fundamental pathways such as apoptosis and senescence as well as exhaustion of proliferative capacity or excessive differentiation. Inherent in the paucity of the bone marrow in AA, the study of the stem cells in AA has been very difficult due to their natural rarity and disease-specific contraction of the stem cell pool. Despite these scientific challenges, laboratory studies and systematic clinical observation provide valuable information of significance beyond its specific application to AA.

Differential apoptosis and Fas expression on GPI-negative and GPI-positive stem cells: a mechanism for the evolution of paroxysmal nocturnal haemoglobinuria*

Paroxysmal nocturnal haemoglobinuria (PNH) has a dual pathogenesis. PIG-A mutations generate clones of haemopoietic stem cells (HSC) lacking glycosylphosphatidylinositol (GPI)-anchored proteins and, secondly, these clones expand because of a selective advantage related to bone marrow failure. The first aspect has been elucidated in detail, but the mechanisms leading to clonal expansion are not well understood. We have previously shown that apoptosis and Fas expression in HSC play a role in bone marrow failure during aplastic anaemia. We have now investigated apoptosis in PNH. Ten patients were studied. Apoptosis, measured by flow cytometry, was significantly higher among CD34+ cells from patients compared with healthy controls. Fas expression was also increased. Cells that were stained for CD34, CD59 and apoptosis showed a significantly lower apoptosis in CD34+/CD59- compared with CD34+/CD59+ cells from the same patient. In three patients, staining for CD34, CD59 and Fas revealed lower Fas expression on CD34+/CD59- cells. Differential apoptosis of CD34+/CD59- HSC may be sufficient in itself to explain the expansion of PNH clones in the context of aplastic anaemia. In addition to demonstrating a basic mechanism underlying PNH clonal expansion, these results suggest further hypotheses for the evolution of PNH, based on the direct or indirect resistance of GPI-negative HSC to pro-inflammatory cytokines.

Repopulating defect of mismatch repair-deficient hematopoietic stem cells

Mismatch repair deficiency is associated with carcinogenesis, increased spontaneous and induced mutagenesis, and resistance to methylating agents. In humans, leukemias and lymphomas arise in the background of mismatch repair deficiency, raising the possibility that hematopoiesis is abnormal as well. To address hematopoiesis in MSH2-/- mice, we collected marrow and performed serial transplantations of these cells, alone or mixed with wild-type cells, into lethally irradiated healthy mice. Transplant recipients were observed or treated with the methylating agent, temozolomide (TMZ). Methylating agent tolerance was evident by the competitive survival advantage of MSH2-/- marrow progenitors compared with wild-type cells after each TMZ exposure. However, serial repopulation by MSH2-/- cells was deficient compared with wild-type cells. In recipients of mixed populations, the MSH 2-/- cells were lost from the marrow, and mice receiving MSH2-/- cells plus TMZ could not be reconstituted in the third passage, whereas all wild-type cell recipients survived. No differences in telomere length, cell cycle distribution, or homing were observed, but an increase in microsatellite instability was seen in the MSH2-/- early progenitor colony-forming unit (CFU) and Sca+Kit+lin--derived clones. Thus, mismatch repair deficiency is associated with a hematopoietic repopulation defect and stem cell exhaustion because of accumulation of genomic instability.

Effect of proinflammatory cytokines on PIGA- hematopoiesis

OBJECTIVE

Blood cells from patients with paroxysmal nocturnal hemoglobinuria lack glycosyl phosphatidylinositol (GPI)-linked proteins, due to a somatic mutation in the X-linked PIGA gene. It is believed that clonal expansion of PIGA- blood cells is due to a survival advantage in the hostile marrow environment of aplastic anemia. Here we investigated the effects of inhibitory cytokines in mice genetically engineered to have blood cells deficient in GPI-linked proteins.

MATERIALS AND METHODS

The effect of inhibitory cytokines (tumor necrosis factor-alpha [TNF-alpha], interferon-gamma [IFN-gamma], macrophage inflammatory protein-1 alpha [MIP-1alpha], and transforming growth factor-beta1 [TGF-beta1]) was investigated, using clonogenic assays, competitive repopulation, and in vivo induction of proinflammatory cytokines by double-stranded RNA. The expression of Fas on progenitor cells and its up-regulation by inhibitory cytokines were analyzed by flow cytometry.

RESULTS

TNF-alpha, IFN-gamma, MIP-1alpha, and TGF-beta1 suppressed colony formation in a dose-dependent fashion that was similar for PIGA+ and PIGA- blood bone marrow cells. Competitive repopulation of bone marrow cells cultured in IFN-gamma and TNF-alpha resulted in a comparable ability of PIGA+ and PIGA- hematopoietic stem cells to reconstitute hematopoiesis. Fas expression was minimal on PIGA+ and PIGA- progenitor cells and was up-regulated to the same extent in response to IFN-gamma and TNF-alpha as assessed by Fas antibody-mediated apoptosis. Similarly, in vivo induction of proinflammatory cytokines by double-stranded RNA had no effect on the proportion of circulating PIGA- blood cells.

CONCLUSIONS

These results indicate that PIGA+ and PIGA- hematopoietic progenitor cells respond similarly to inhibitory cytokines, suggesting that other factors are responsible for the clonal expansion of paroxysmal nocturnal hemoglobinuria cells.

Severe telomere shortening in patients with paroxysmal nocturnal hemoglobinuria affects both GPI- and GPI+ hematopoiesis

A most distinctive feature of paroxysmal nocturnal hemoglobinuria (PNH) is that in each patient glycosylphosphatidylinositol-negative (GPI-) and GPI+ hematopoietic stem cells (HSCs) coexist, and both contribute to hematopoiesis. Telomere size correlates inversely with the cell division history of HSCs. In 10 patients with hemolytic PNH the telomeres in sorted GPI- granulocytes were shorter than in sorted GPI+ granulocytes in 4 cases, comparable in 2 cases, and longer in the remaining 4 cases. Furthermore, the telomeres of both GPI- and GPI+ hematopoietic cells were markedly shortened compared with age-matched controls. The short telomeres in the GPI- cells probably reflect the large number of cell divisions required for the progeny of a single cell to contribute a large proportion of hematopoiesis. The short telomeres of the GPI+ cells indicate that the residual hematopoiesis contributed by these cells is not normal. This epigenetic change is an additional feature shared by PNH and aplastic anemia.

Increased expression of anti-apoptosis genes in peripheral blood cells from patients with paroxysmal nocturnal hemoglobinuria

Resistance to apoptosis has been described in neutrophils from patients with PNH and related hematologic disorders (aplastic anemia, myelodysplastic syndrome), but its molecular basis is not understood. Using gene expression analysis, PNH granulocytes had relative overexpression of four anti-apoptosis genes (human A1, hHR23B, Mcl-1, and RhoA) compared to normal controls. These findings were confirmed by RT-PCR analysis and observed in both peripheral blood granulocytes and mononuclear cells of patients with PNH. Anti-apoptosis gene upregulation may confer resistance to apoptosis in PNH and related disorders, and provide a common compensatory mechanism after bone marrow injury that allows survival and growth of remaining hematopoietic stem cells.

Clinical manifestations of paroxysmal nocturnal hemoglobinuria: present state and future problems

The clinical pathology of paroxysmal nocturnal hemoglobinuria (PNH) involves 3 complications: hemolytic anemia, thrombosis, and hematopoietic deficiency. The first 2 are clearly the result of the cellular defect in PNH, the lack of proteins anchored to the membrane by the glycosylphosphatidylinositol anchor. The hemolytic anemia results in syndromes primarily related to the fact that the hemolysis is extracellular. Thrombosis is most significant in veins within the abdomen, although a number of other thrombotic syndromes have been described. The hematopoietic deficiency may be the same as that in aplastic anemia, a closely related disorder, and may not be due to the primary biochemical defect. The relationship to aplastic anemia suggests a nomenclature that emphasizes the predominant clinical manifestations in a patient. This relationship does not explain cases that appear to be related to myelodysplastic syndromes or the transition of some cases of PNH to leukemia. Treatment, except for bone marrow transplantation, remains noncurative and in need of improvement.

Phenotypes and phosphatidylinositol glycan-class A gene abnormalities during cell differentiation and maturation from precursor cells to mature granulocytes in patients with paroxysmal nocturnal hemoglobinuria

To define the phosphatidylinositol glycan-class A (PIG-A) gene abnormality in precursor cells and the changes of expression of glycosylphosphatidylinositol-anchored protein and contribution of paroxysmal nocturnal hemoglobinuria (PNH) clones with PIG-A gene abnormalities among various cell lineages during differentiation and maturation, we investigated CD59 expression on bone marrow CD34(+) cells and peripheral granulocytes from 3 patients with PNH and the PIG-A gene abnormalities in the CD59(-), CD59(+/-), and CD59(+) populations by nucleotide sequence analyses. We also performed clonogeneic assays of CD34(+)CD59(+) and CD34(+)CD59(-) cells from 2 of the patients and examined the PIG-A gene abnormalities in the cultured cells. In case 1, the CD34(+) cells and granulocytes consisted of CD59(-) and CD59(+) populations and CD59(-), CD59(+/-), and CD59(+) populations, respectively. Sequence analyses indicated that mutation 1-2 was in the CD59(+/-) granulocyte population (20 of 20) and the CD34(+)CD59(-) population (2 of 38). In cases 2 and 3, the CD34(+) cells and granulocytes consisted of CD59(+) and CD59(-) cells. Sequence analyses in case 3 showed that mutation 3-2 was not in CD34(+)CD59(-) cells and was present in the CD59(-) granulocyte population. However, PIG-A gene analysis of cultured CD34(+)CD59(-) cells showed that they had the mutation. This analysis also revealed that there were some other mutations, which were not found in CD34(+)CD59(-) cells and CD59(-) or CD59(+/-) granulocytes in vivo, and that sometimes they were distributed specifically among different cell lineages. In conclusion, our findings suggest that PNH clones might contribute qualitatively and quantitatively differentially to specific blood cell lineages during differentiation and maturation of hematopoietic stem cells.

The relationship of aplastic anemia and PNH

Bone marrow failure has been regarded as one of the triad of clinical manifestations of paroxysmal noctumal hemoglobinuria (PNH), and PNH in turn has been described as a late clonal disease evolving in patients recovering from aplastic anemia. Better understanding of the pathophysiology of both diseases and improved tests for cell surface glycosylphosphatidylinositol (GPI)-linked proteins has radically altered this view. Flow cytometry of granulocytes shows evidence of an expanded PNH clone in a large proportion of marrow failure patients at the time of presentation: in our large NIH series, about 1/3 of over 200 aplastic anemia cases and almost 20% of more than 100 myelodysplasia cases. Clonal PNH expansion (rather than bone marrow failure) is strongly linked to the histocompatability antigen HLA.-DR2 in all clinical varieties of the disease, suggesting an immune component to its pathophysiology. An extrinsic mechanism of clonal expansion is also more consistent with knock-out mouse models and culture experiments with primary cells and cell lines, which have failed to demonstrate an intrinsic proliferative advantage for PNH cells. DNA chip analysis of multiple paired normal and PIG-A mutant cell lines and lymphoblastoid cells do not show any consistent differences in levels of gene expression. In aplastic anemia/PNH there is surprisingly limited utilization of the V-beta chain of the T cell receptor, and patients' dominant T cell clones, which are functionally inhibitory of autologous hematopoiesis, use identical CDR3 regions for antigen binding. Phenotypically normal cells from PNH patients proliferate more poorly in culture than do the same patient's PNH cells, and the normal cells are damaged as a result of apoptosis and overexpress Fas. Differences in protein degradation might play a dual role in pathophysiology, as GPI-linked proteins lacking an anchor would be predicted to be processed by the proteasome machinery and displayed in a class I H.A. context, in contrast to the normal pathway of cell surface membrane recycling, lysosomal degradation, and presentation by class II HLA. The strong relationship between a chronic, organ-specific immune destructive process and the expansion of a single mutant stem cell clone remains frustratingly enigmatic but likely to be the result of interesting biologic processes, with mechanisms that potentially can be extended to the role of inflammation in producing premalignant syndromes.

Views on the pathophysiology of aplastic anaemia

Aplastic anaemia seems to be predominantly a defect of the stem cell rather than the stroma, though abnormalities of the microenvironment may co-exist. There is highly suggestive evidence that the stem cell is the target of an immune attack, though the main evidence remains the response to immunosuppression with antilymphocyte globulin and cyclosporin. The stem cell defect remains even after recovery of the peripheral blood counts and the AA marrow is a fertile environment for the emergence of abnormal clones, particularly PNH. However, it has recently become apparent that there is an overlap with the myelodysplastic syndromes and clones of monosomy 7 and trisomy 8 amongst others are not uncommon in aplastic anaemia. Recent work has suggested that the emergence of a clone of monosomy 7 cells carries a poor prognosis, whereas trisomy 8 has a good prognosis particularly in response to cyclosporin. However, the setting in which monosomy 7 arises may affect the phenotypic expression. The immune targeting of stem cells may be associated with increased apoptosis in aplastic anaemia, in part mediated by fas expression, but not exclusively. Understanding the pathophysiology of AA should help to improve and perhaps target therapy.

Superior growth of glycophosphatidy linositol-anchored protein-deficient progenitor cells in vitro is due to the higher apoptotic rate of progenitors with normal phenotype in vivo

OBJECTIVE

Recently, phenotypically normal CD34 cells from the marrow of patients with paroxysmal nocturnal hemoglobinuria (PNH) were reported to show impaired growth and elevated Fas receptor expression as compared to glycophosphatidylinositol-anchored protein (GPI-AP)-deficient CD34 cells and CD34 cells from normal individuals. These results are consistent with the theory that PNH cells have an intrinsic growth advantage, but their superior expansion in vitro could also be the outcome of selective extrinsic pressure in vivo.

MATERIAL AND METHODS

Growth characteristics, competitive features, and susceptibility to apoptosis of sorted normal or GPI-AP-deficient CD34(+) cells derived from PNH patients were assessed in suspension and methylcellulose cultures.

RESULTS

When we directly compared the growth of patients' CD34 cells, separated based on expression of GPI-AP CD55 and CD59, in most of the patients studied, mutant CD34 cells showed higher progeny production and outgrew phenotypically normal CD34 cells derived from PNH patients in mixing experiments. However, their proliferation rate did not exceed that of control CD34 cells. To determine whether deficient growth of phenotypically normal CD34 cells in PNH was secondary to a pre-existing in vivo insult, we determined the fraction of apoptotic cells within fresh normal and PNH CD34 cells. Normal CD34 cells from PNH patients showed a high proportion of apoptotic cells and higher Fas expression, while GPI-AP-deficient and control CD34 cells showed similar, low rates of apoptosis. After correction for pre-existing apoptosis, the proliferation potential of normal and PNH CD34 cells was similar.

CONCLUSIONS

These results strongly suggest that clonal expansion of GPI-AP-deficient progenitor cells from PNH patients is due to their selection in the hostile marrow environment of the patient.

Murine models of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal disorder characterized by chronic intravascular hemolysis, cytopenia, and an increased tendency to thrombosis. All patients with PNH studied so far have a somatic mutation of phosphatidyl inositol glycan complementation group A (PIG-A), an X-linked gene involved initially in the biosynthesis of the glycosyl phosphatidylinositol (GPI) molecule, which serves as an anchor for many cell surface proteins. The mutation occurs in a hematopoietic stem cell, and consequently, all cells derived from the mutated stem cell are devoid of GPI-linked proteins. The absence of GPI-linked proteins explains some clinical symptoms of the disease but not the mechanism that allows the expansion of the mutated clone. By using targeted disruption of the PIG-A gene in mouse embryonic stem cells, some mice models of PNH have been generated. These animals have a discrete proportion of blood cells devoid of GPI-linked proteins, and although not anemic, they have evidence of hemolysis. The clinical course of these animals is benign, and there are no signs of a substantial expansion of the PNH clone, as observed in human patients. The fact that these animals do not develop the disease strongly supports the notion that a mutation of PIG-A is not sufficient per se to cause PNH and that another factor, namely, bone marrow failure, is necessary to allow proliferation and expansion of the PNH clone.

Granulocytes from patients with paroxysmal nocturnal hemoglobinuria and normal individuals have the same sensitivity to spontaneous apoptosis

OBJECTIVE

The aim of this study was to determine whether granulocytes from patients with paroxysmal nocturnal hemoglobinuria (PNH) are more or less intrinsically sensitive to spontaneous apoptosis than granulocytes from healthy individuals. Resistance to apoptosis has been suggested as an explanation for the proliferation or selection of PNH clones.

PATIENTS AND METHODS

Peripheral blood granulocytes from five patients with PNH, five patients with myelodysplastic syndrome (MDS), and five healthy volunteers were cultured in the absence of serum. Spontaneous apoptosis of the granulocytes was assessed every 6 hours by flow cytometry. The expression levels of CD16b, CD95, and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor also were studied by flow cytometry, and caspase-3 activity was measured by fluorometry.

RESULTS

There were no significant differences in the proportion or absolute numbers of apoptotic and apoptotic/dead granulocytes between the cells from PNH patients and healthy individuals, whereas those from MDS patients showed significantly lower frequencies of apoptotic granulocytes compared with normal controls. The proportion of CD16b(-) granulocytes was not significantly different among the three groups during in vitro culture. CD95 and GM-CSF receptor was not significantly increased in cultured granulocytes or noncultured granulocytes from, respectively, patients with PNH and normal controls. Caspase-3 activity significantly decreased in cultured granulocytes from MDS patients, but not in granulocytes from PNH patients.

CONCLUSIONS

Granulocytes from PNH patients did not display a reduced sensitivity to spontaneous apoptosis, suggesting that the apoptosis of blood cells in PNH may not be an important factor in proliferation or selection of PNH clones. These findings are in agreement with the normal lifespan of granulocytes in vivo.

Relationship between aplastic anemia and paroxysmal nocturnal hemoglobinuria

Since aplastic anemia-paroxysmal nocturnal hemoglobinuria syndrome was reported in 1967, the overlap of idiopathic aplastic anemia (AA) and paroxysmal nocturnal hemoglobinuria (PNH) has been well known. The link between the 2 diseases became even more evident when immunosuppressive therapy improved survival of patients with severe AA. More than 10% of patients with AA develop clinically evident PNH. Moreover, flow cytometric analysis demonstrates that the majority of patients with AA have a subclinical percentage of granulocytes with PNH phenotype. Some of them have clearly recognizable PNH clones. Granulocytes with a PNH phenotype are also often found in normal individuals, though at much smaller percentages of cells. This finding suggests that a PNH clone is expanded in AA. consistent with a hypothesis that blood cells from patients with PNH are more resistant to an autoimmune environment. Survival of PNH clones in pathologic bone marrow may account for limited expansion of PNH clones; however, additional genetic change(s) that confers cells with growth phenotype may be required for the full development of PNH.

Monitoring of CD59 expression in paroxysmal nocturnal hemoglobinuria treated with danazol

We describe a 52-year-old man with paroxysmal nocturnal hemoglobinuria (PNH) and a moderate transfusion requirement. Prior to and during sequential therapy with androgen (metenolone), glucocorticoid, and danazol, we evaluated CD59-negative expression (PNH clone) in red blood cells, neutrophils, lymphocyte subsets, and bone marrow (BM) CD34(+) cells. Although androgen and glucocorticoid were not effective for recovery of blood cell counts, the hemoglobin and platelet levels increased immediately after the therapy with danazol and the patient became transfusion independent. However, neither the serum level of LDH nor the percentage of PNH clone in each cell lineage, including BM CD34(+) cells, decreased. The number of nucleated cells in BM increased drastically after the start of danazol. These findings suggest that the efficacy of danazol was not only due to the impediment of hemolysis but also due to stimulation of PNH clone proliferation in BM.

Multilineage glycosylphosphatidylinositol anchor-deficient haematopoiesis in untreated aplastic anaemia

Aplastic anaemia and paroxysmal nocturnal haemoglobinuria (PNH) are closely related disorders. In PNH, haematopoietic stem cells that harbour PIGA mutations give rise to blood elements that are unable to synthesize glycosylphosphatidylinositol (GPI) anchors. Because the GPI anchor is the receptor for the channel-forming protein aerolysin, PNH cells do not bind the toxin and are unaffected by concentrations that lyse normal cells. Exploiting these biological differences, we have developed two novel aerolysin-based assays to detect small populations of PNH cells. CD59 populations as small as 0.004% of total red cells could be detected when cells were pretreated with aerolysin to enrich the PNH population. All PNH patients displayed CD59-deficient erythrocytes, but no myelodysplastic syndrome (MDS) patient or control had detectable PNH cells before or after enrichment in aerolysin. Only one aplastic anaemia patient had detectable PNH red cells before exposure to aerolysin. However, 14 (61%) had detectable PNH cells after enrichment in aerolysin. The inactive fluorescent proaerolysin variant (FLAER) that binds the GPI anchors of a number of proteins on normal cells was used to detect a global GPI anchor deficit on granulocytes. Flow cytometry with FLAER showed that 12 out of 18 (67%) aplastic anaemia patients had FLAER-negative granulocytes, but none of the MDS patients or normal control subjects had GPI anchor-deficient cells. These studies demonstrate that aerolysin-based assays can reveal previously undetectable multilineage PNH cells in patients with untreated aplastic anaemia. Thus, clonality appears to be an early feature of aplastic anaemia.

New insights into paroxysmal nocturnal hemoglobinuria

The characteristic, defining defect in paroxysmal nocturnal hemoglobinuria is the somatic mutation of the PIG-A gene (essential to the biosynthesis of the glycosylphosphatidylinositol moiety that affixes a number of proteins to the cellular surface) in hematopoietic cells. These cells thus lack the proteins usually held in place by this anchor. The absence of these proteins is the most reliable diagnostic criterion of the disease and is responsible for many of the clinical manifestations of PNH. The current hypothesis explaining the disorder suggests that there are two components: (1) hematopoietic stem cells with the characteristic defect are present in the marrow of many if not all normal individuals in very small numbers; (2) some aplastogenic influence suppresses the normal stem cells but does not suppress the defective stem cells, thus allowing the proportion of these cells to increase. Current research attempts to substantiate this hypothesis and design therapy consistent with the hypothesis. Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired stem cell disorder characterized by intravascular hemolysis, hypercoagulability, and relative bone marrow failure [1]. It is characterized by a somatic mutation in the gene encoding the alpha1-6-N-acetylglucosaminyltransferase necessary for the formation of the glycosylphosphatidylinositol (GPI) anchor that binds certain proteins to the membrane surface (Fig. 1) [2,3*]. Whereas many of the manifestations can be accounted for by the absence of these proteins on the cells of the hematopoietic system, it is not entirely clear whether this defect is sufficient to make the disease manifest. In this paper, the author reviews recent clinical observations and relates them to the underlying pathophysiology of the disease.