Late clonal complications in severe aplastic anemia

Secondary myelodysplastic syndrome and leukemia in acquired aplastic anemia and paroxysmal nocturnal hemoglobinuria

Acquired aplastic anemia (AA) and paroxysmal nocturnal hemoglobinuria (PNH) are pathogenically related nonmalignant bone marrow failure disorders linked to T-cell-mediated autoimmunity; they are associated with an increased risk of secondary myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Approximately 15% to 20% of AA patients and 2% to 6% of PNH patients go on to develop secondary MDS/AML by 10 years of follow-up. Factors determining an individual patient's risk of malignant transformation remain poorly defined. Recent studies identified nearly ubiquitous clonal hematopoiesis (CH) in AA patients. Similarly, CH with additional, non-PIGA, somatic alterations occurs in the majority of patients with PNH. Factors associated with progression to secondary MDS/AML include longer duration of disease, increased telomere attrition, presence of adverse prognostic mutations, and multiple mutations, particularly when occurring early in the disease course and at a high allelic burden. Here, we will review the prevalence and characteristics of somatic alterations in AA and PNH and will explore their prognostic significance and mechanisms of clonal selection. We will then discuss the available data on post-AA and post-PNH progression to secondary MDS/AML and provide practical guidance for approaching patients with PNH and AA who have CH.

Treatment of Severe Aplastic Anemia with Porcine Anti-Human Lymphocyte Globulin

Aplastic anemia (AA) is a bone marrow failure syndrome characterized by pancytopenia. Decreased numbers of hematopoietic stem cells and impaired bone marrow microenvironment caused by abnormal immune function describe the major pathogenesis of AA. Hematopoietic stem cell transplantation and immunesuppressive therapy are the first-line treatments for AA. Porcine anti-lymphocyte globulin (p-ALG) is a new product developed in China. Several studies have shown that p-ALG exhibited good therapeutic effects in AA.

Comparison of High Sensitivity and Conventional Flow Cytometry for Diagnosing Overt Paroxysmal Nocturnal Hemoglobinuria and Detecting Minor Paroxysmal Nocturnal Hemoglobinuria Clones

Background

High sensitivity flow cytometry (HS-FCM) was recently developed for diagnosing paroxysmal nocturnal hemoglobinuria (PNH). We compared its performance with conventional flow cytometry (C-FCM) for diagnosing overt PNH and detecting minor (0.1–1%) PNH clones in aplastic anemia (AA)/low-grade myelodysplastic syndrome (MDS) patients.

Methods

C-FCM and HS-FCM were performed simultaneously on 41 samples from healthy controls and 23 peripheral blood samples from 15 AA/low-grade MDS and eight PNH patients, using a Navios flow cytometer (Beckman Coulter, Miami, FL, USA). Results were compared.

Results

No healthy control samples had PNH clone size >0.01%. For granulocytes, C-FCM detected a smaller PNH clone size than HS-FCM (mean difference: 0.7–1.7%). In AA/low-grade MDS patients, three samples showed >1% PNH clones with C-FCM but not with HS-FCM. Seven samples showed minor PNH clones by C-FCM, but HS-FCM showed negative results for all these samples. In PNH patients, C-FCM detected a smaller PNH clone size than HS-FCM (mean difference: 1.9–5.0%). For red blood cells, C-FCM detected a greater PNH clone size than HS-FCM (mean difference: 1.5%). In AA/low-grade MDS patients, C-FCM showed >1% PNH clones in six samples, but HS-FCM showed >1% PNH clones in none of the samples. C-FCM detected minor PNH clones in nine samples, but six of them were negative by HS-FCM. In PNH patients, C-FCM detected a greater PNH clone size than HS-FCM (mean difference: 2.5%).

Conclusions

HS-FCM can sensitively detect minor PNH clones and reduce false-positive C-FCM minor PNH clone cases in AA/low-grade MDS patients.

The Pathophysiology of Acquired Aplastic Anemia: Current Concepts Revisited

Idiopathic acquired aplastic anemia is a rare, life-threatening bone marrow failure syndrome characterized by cytopenias and hypocellular bone marrow. The pathophysiology is unknown; the most favored model is of a dysregulated immune system leading to autoreactive T-cell destruction of hematopoietic stem and progenitor cells in a genetically susceptible host. The authors review the literature and propose that the major driver of acquired aplastic anemia is a combination of hematopoietic stem and progenitor cells intrinsic defects and an inappropriately activated immune response in the setting of a viral infection. Alterations in bone marrow microenvironment may also contribute to the disease process.

Outcomes of children, adolescents, and young adults following allogeneic stem cell transplantation for secondary acute myeloid leukemia and myelodysplastic syndromes-The MD Anderson Cancer Center experience

We conducted a retrospective analysis of outcomes for children and young adults with sAML/sMDS who underwent HSCT at our institution. Thirty-two patients (median age 20 years) with sAML (n=24) and sMDS (n=8) received HSCT between 1990 and 2013. The median time from sAML/sMDS diagnosis to HSCT was 4.1 months (range: 1.2-27.2 months). The transplant regimens were primarily busulfan based (n=19). BM was the primary donor source (n=15). Eleven recipients were transplanted with residual disease. At a median follow-up of 62.3 months (range: 0.4-250.9 months), 14 patients had disease recurrence. Acute GVHD, grade III/IV, occurred in three patients. Causes of death were as follows: disease relapse (n=12), infection (n=2), pneumonia (n=1), pulmonary hemorrhage (n=1), acute GVHD (n=1), and graft failure (n=1). A PS of ≥90% at the time of HSCT had a significant impact on PFS (P=.02). Patients achieving pretransplant primary CR (n=8) and those with sMDS and RA (n=6) had prolonged PFS (P=.04). On multivariate analysis, shorter time to transplantation (≤6 months from diagnosis of sAML/sMDS) was associated with superior OS (P=.0018) and PFS (P=.0005).

Technical advances in flow cytometry-based diagnosis and monitoring of paroxysmal nocturnal hemoglobinuria

Objective:

To discuss the implementation of technical advances in laboratory diagnosis and monitoring of paroxysmal nocturnal hemoglobinuria for validation of high-sensitivity flow cytometry protocols.

Methods:

A retrospective study based on analysis of laboratory data from 745 patient samples submitted to flow cytometry for diagnosis and/or monitoring of paroxysmal nocturnal hemoglobinuria.

Results:

Implementation of technical advances reduced test costs and improved flow cytometry resolution for paroxysmal nocturnal hemoglobinuria clone detection.

Conclusion:

High-sensitivity flow cytometry allowed more sensitive determination of paroxysmal nocturnal hemoglobinuria clone type and size, particularly in samples with small clones.

Objetivo:

Discutir as melhorias técnicas no diagnóstico e no acompanhamento laboratorial de hemoglobinúria paroxística noturna para a validação da técnica de citometria de fluxo de alta sensibilidade.

Métodos:

Estudo retrospectivo, que envolveu a análise de dados laboratoriais de 745 pacientes com hipótese diagnóstica e/ou acompanhamento de hemoglobinúria paroxística noturna por citometria de fluxo.

Resultados:

Os avanços técnicos não só reduziram o custo do ensaio, mas também melhoraram a identificação e a resolução da citometria de fluxo para a detecção de clone hemoglobinúria paroxística noturna.

Conclusão:

A citometria de fluxo de alta sensibilidade possibilitou a identificação do tipo e do tamanho de clone de hemoglobinúria paroxística noturna, especialmente em amostras com pequeno clone.

Clonal hematopoiesis in acquired aplastic anemia

Clonal hematopoiesis (CH) in aplastic anemia (AA) has been closely linked to the evolution of late clonal disorders, including paroxysmal nocturnal hemoglobinuria and myelodysplastic syndromes (MDS)/acute myeloid leukemia (AML), which are common complications after successful immunosuppressive therapy (IST). With the advent of high-throughput sequencing of recent years, the molecular aspect of CH in AA has been clarified by comprehensive detection of somatic mutations that drive clonal evolution. Genetic abnormalities are found in ∼50% of patients with AA and, except for PIGA mutations and copy-neutral loss-of-heterozygosity, or uniparental disomy (UPD) in 6p (6pUPD), are most frequently represented by mutations involving genes commonly mutated in myeloid malignancies, including DNMT3A, ASXL1, and BCOR/BCORL1 Mutations exhibit distinct chronological profiles and clinical impacts. BCOR/BCORL1 and PIGA mutations tend to disappear or show stable clone size and predict a better response to IST and a significantly better clinical outcome compared with mutations in DNMT3A, ASXL1, and other genes, which are likely to increase their clone size, are associated with a faster progression to MDS/AML, and predict an unfavorable survival. High frequency of 6pUPD and overrepresentation of PIGA and BCOR/BCORL1 mutations are unique to AA, suggesting the role of autoimmunity in clonal selection. By contrast, DNMT3A and ASXL1 mutations, also commonly seen in CH in the general population, indicate a close link to CH in the aged bone marrow, in terms of the mechanism for selection. Detection and close monitoring of somatic mutations/evolution may help with prediction and diagnosis of clonal evolution of MDS/AML and better management of patients with AA.

Severe aplastic anemia preceding acute monocytic leukemia in an adult with acquired trisomy 21: A case report

The current case report presents a patient with acute monocytic leukemia (AML-M5) occurring 14 years following the successful treatment of severe aplastic anemia (SAA) with immunosuppressants and androgens. The patient was treated with induction chemotherapy, but did not achieve remission. The patient succumbed to central nervous system bleeding 2 weeks following the first cycle of chemotherapy. Chromosomal examination revealed 47,XX,+21[10]/46,XX[1]. To the best of our knowledge the present case is the first to be reported of SAA 14 years preceding AML-M5 with acquired trisomy 21.

First-line allogeneic hematopoietic stem cell transplantation of HLA-matched sibling donors compared with first-line ciclosporin and/or antithymocyte or antilymphocyte globulin for acquired severe aplastic anemia

BACKGROUND

Acquired severe aplastic anemia is a rare and potentially fatal disease, which is characterized by hypocellular bone marrow and pancytopenia. The major signs and symptoms are severe infections, bleeding, and exhaustion. First-line allogeneic hematopoietic stem cell transplantation (HSCT) of a human leukocyte antigen (HLA)-matched sibling donor (MSD) is a treatment for newly diagnosed patients with severe aplastic anemia. First-line treatment with ciclosporin and/or antithymocyte or antilymphocyte globulin (as first-line immunosuppressive therapy) is an alternative to MSD-HSCT and is indicated for patients where no MSD is found.

OBJECTIVES

To evaluate the effectiveness and adverse events of first-line allogeneic hematopoietic stem cell transplantation of HLA-matched sibling donors compared to first-line immunosuppressive therapy including ciclosporin and/or antithymocyte or antilymphocyte globulin in patients with acquired severe aplastic anemia.

SEARCH METHODS

We searched the electronic databases MEDLINE (Ovid), EMBASE (Ovid), and The Cochrane Library CENTRAL (Wiley) for published articles from 1946 to 22 April 2013. Further searches included trial registries, reference lists of recent reviews, and author contacts.

SELECTION CRITERIA

The following prospective study designs were eligible for inclusion: randomized controlled trials (RCTs) and non-randomized controlled trials if the allocation of patients to treatment groups was consistent with 'Mendelian randomization'. We included participants with newly diagnosed severe aplastic anemia who received MSD-HSCT or immunosuppressive therapy without prior HSCT or immunosuppressive therapy, and with a minimum of five participants per treatment group. We did not apply limits on publication year or languages.

DATA COLLECTION AND ANALYSIS

Two review authors abstracted the data on study and patient characteristics and assessed the risk of bias independently. We resolved differences by discussion or by appeal to a third review author. The primary outcome was overall mortality. Secondary outcomes were treatment-related mortality, graft failure, no response to first-line immunosuppressive therapy, graft-versus-host-disease (GVHD), relapse after initial successful treatment, secondary clonal and malignant disease, health-related quality of life, and performance score.

MAIN RESULTS

We identified three trials that met the inclusion criteria. None of these trials was a RCT. 302 participants are included in this review. The three included studies were prospectively conducted and had features consistent with the principle of 'Mendelian randomization' as defined in the present review. All studies had a high risk of bias due to the study design. All studies were conducted more than 10 years ago and may not be applicable to the standard of care of today. Primary and secondary outcome data showed no statistically significant difference between treatment groups. We present results for first-line allogeneic hematopoietic stem cell transplantation of an HLA-matched sibling donor, which we denote as the MSD-HSCT group, versus first-line treatment with ciclosporin and/or antithymocyte or antilymphocyte globulin, which we denote as the immunosuppressive therapy group in the following section.The pooled hazard ratio for overall mortality for the MSD-HSCT group versus the immunosuppressive therapy group was 0.95 (95% confidence interval 0.43 to 2.12, P = 0.90, low quality evidence). Therefore, overall mortality was not statistically significantly different between the groups. Treatment-related mortality ranged from 20% to 42% for the MSD-HSCT group and was not reported for the immunosuppressive therapy group (very low quality evidence). The authors reported graft failure from 3% to 16% for the MSD-HSCT group and GVHD from 26% to 51% (both endpoints not applicable for the immunosuppressive therapy group, very low quality evidence). The authors did not report any data on response and relapse for the MSD-HSCT group. For the immunosuppressive therapy group, the studies reported no response from 15% (not time point stated) to 64% (three months) and relapse in one of eight responders after immunosuppressive therapy at 5.5 years (very low quality evidence). The authors reported secondary clonal disease or malignancies for the MSD-HSCT group versus the immunosuppressive therapy group in 1 of 34 versus 0 of 22 patients in one study and in 0 of 28 versus 4 of 86 patients in the other study (low quality evidence). None of the included studies addressed health-related quality of life. The percentage of the evaluated patients with a Karnofsky performance status score in the range of 71% to 100% was 92% in the MSD-HSCT group and 46% in the immunosuppressive therapy group.

AUTHORS' CONCLUSIONS

There are insufficient and biased data that do not allow any conclusions to be made about the comparative effectiveness of first-line allogeneic hematopoietic stem cell transplantation of an HLA-matched sibling donor and first-line treatment with ciclosporin and/or antithymocyte or antilymphocyte globulin (as first-line immunosuppressive therapy). We are unable to make firm recommendations regarding the choice of intervention for treatment of acquired severe aplastic anemia.

The small population of PIG-A mutant cells in myelodysplastic syndromes do not arise from multipotent hematopoietic stem cells

BACKGROUND

Patients with paroxysmal nocturnal hemoglobinuria harbor clonal glycosylphosphatidylinositol-anchor deficient cells arising from a multipotent hematopoietic stem cell acquiring a PIG-A mutation. Many patients with aplastic anemia and myelodysplastic syndromes also harbor small populations of glycosylphosphatidylinositol-anchor deficient cells. Patients with aplastic anemia often evolve into paroxysmal nocturnal hemoglobinuria; however, myelodysplastic syndromes seldom evolve into paroxysmal nocturnal hemoglobinuria. Here, we investigate the origin and clonality of small glycosylphosphatidylinositol-anchor deficient cell populations in aplastic anemia and myelodysplastic syndromes.

DESIGN AND METHODS

We used peripheral blood flow cytometry to identify glycosylphosphatidylinositol-anchor deficient blood cells, a proaerolysin-resistant colony forming cell assay to select glycosylphosphatidylinositol-anchor deficient progenitor cells, a novel T-lymphocyte enrichment culture assay with proaerolysin selection to expand glycosylphosphatidylinositol-anchor deficient T lymphocytes, and PIG-A gene sequencing assays to identify and analyze PIG-A mutations in patients with aplastic anemia and myelodysplastic syndromes.

RESULTS

Twelve of 15 aplastic anemia patients were found to harbor a small population of glycosylphosphatidylinositol-anchor deficient granulocytes; 11 of them were found to harbor a small population of glycosylphosphatidylinositol-anchor deficient erythrocytes, 10 patients were detected to harbor glycosylphosphatidylinositol-anchor deficient T lymphocytes, and 3 of them were detected only after T-lymphocyte enrichment in proaerolysin selection. PIG-A mutation analyses on 3 patients showed that all of them harbored a matching PIG-A mutation between CFU-GM and enriched T lymphocytes. Two of 26 myelodysplastic syndromes were found to harbor small populations of glycosylphosphatidylinositol-anchor deficient granulocytes and erythrocytes transiently. Bone marrow derived CD34(+) cells from 4 patients grew proaerolysin-resistant colony forming cells bearing PIG-A mutations. No glycosylphosphatidylinositol-anchor deficient T lymphocytes were detected in myelodysplastic syndrome patients.

CONCLUSIONS

In contrast to aplastic anemia and paroxysmal nocturnal hemoglobinuria, where PIG-A mutations arise from multipotent hematopoietic stem cells, glycosylphosphatidylinositol-anchor deficient cells in myelodysplastic syndromes appear to arise from more committed progenitors.

Paroxysmal nocturnal hemoglobinuria presenting with a left intraventricular thrombus in a patient with prior thymoma and aplastic anemia

We report the case of a 37-year old man presenting with a left ventricular cardiac thrombus in the setting of subclinical paroxysmal nocturnal hemoglobinura (PNH) developing two years after immunosuppressive therapy for thymoma-associated aplastic anemia. The literature regarding the interplay between autoimmunity and immunosuppression, aplastic anemia, thymoma and the emergence of PNH is reviewed.

Natural history of paroxysmal nocturnal hemoglobinuria clones in patients presenting as aplastic anemia

OBJECTIVE

 To investigate the natural history of paroxysmal nocturnal hemoglobinuria (PNH) clones in patients with acquired aplastic anemia (AA).

PATIENTS AND METHODS

 Twenty-seven patients with AA and a detectable PNH clone were monitored for a median of 5.7 years (range 1.5-11.5 years). Twenty-two patients received high-dose cyclophosphamide (HiCy) therapy. The erythrocyte and granulocyte PNH clone sizes were measured using flow cytometry and analyzed via CellQuest software. PE-conjugated anti-glycophorin A, anti-CD15, FITC-conjugated anti-CD59, and FLAER staining were used to define glycosylphosphatidylinositol-AP-deficient cells.

RESULTS

We found a linear relationship between PNH clone size and the development of intravascular hemolysis, assessed by lactate dehydrogenase (LDH) values (Pearson correlation coefficient = 0.80, P < 0.001 for erythrocyte PNH clones; and Pearson correlation coefficient = 0.73, P < 0.0001 for granulocyte PNH clones). An erythrocyte PNH size of 3-5% and granulocyte PNH size of 23% were the thresholds to predict hemolysis as measured by an elevated LDH (receiver operating characteristic analyses with AUC = 0.96 for erythrocyte PNH clone sizes and AUC = 0.88 for granulocyte PNH clone sizes). Patients with small (≤15%) initial PNH clone sizes were less likely to develop an elevated LDH (mean ± SD: 236.9 ± 109.9 vs. 423.1 ± 248.8; P = 0.02). Over time, the PNH clone sizes remained stable in 25.9% of patients; 48.1% experienced a rise in the PNH clone size; and 25.9% experienced a decrease.

CONCLUSION

The risk of developing clinically significant PNH after HiCy therapy appears to be low in AA patients with PNH clones, especially for those with small initial PNH clones and for those who respond to HiCy therapy.

Guidelines for the diagnosis and monitoring of paroxysmal nocturnal hemoglobinuria and related disorders by flow cytometry

BACKGROUND

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare hematopoietic stem cell disorder characterized by a somatic mutation in the PIGA gene, leading to a deficiency of proteins linked to the cell membrane via glycophosphatidylinositol (GPI) anchors. While flow cytometry is the method of choice for identifying cells deficient in GPI-linked proteins and is, therefore, necessary for the diagnosis of PNH, to date there has not been an attempt to standardize the methodology used to identify these cells.

METHODS

In this document, we present a consensus effort that describes flow cytometric procedures for detecting PNH cells.

RESULTS

We discuss clinical indications and offer recommendations on data interpretation and reporting but mostly focus on analytical procedures important for analysis. We distinguish between routine analysis (defined as identifying an abnormal population of 1% or more) and high-sensitivity analysis (in which as few as 0.01% PNH cells are detected). Antibody panels and gating strategies necessary for both procedures are presented in detail. We discuss methods for assessing PNH populations in both white blood cells and red blood cells and the relative advantages of measuring each. We present steps needed to validate the more elaborate high-sensitivity techniques, including the need for careful titration of reagents and determination of background rates in normal populations, and discuss technical pitfalls that might affect interpretation.

CONCLUSIONS

This document should both enable laboratories interested in beginning PNH testing to establish a valid procedure and allow experienced laboratories to improve their techniques.

Severe aplastic anemia associated with paroxysmal nocturnal hemoglobinuria and lymphoplasmacytic lymphoma

An unusual case of aplastic anemia presenting in association with lymphoplasmacytic lymphoma and paroxysmal nocturnal hemoglobinuria is discussed. An insult to the hematological stem cell compartment may result in multiple pathological entities, potentially influencing our approach to the treatment of hematological clonal disorders.

Could aplastic anaemia be considered a pre-pre-leukaemic disorder?

Clinical evidence for a link between aplastic anaemia, paroxysmal nocturnal haemoglobinuria (PNH) and hypoplastic leukaemia is provided by studies of clonal disorders, which may be a complication of congenital or acquired aplastic anaemia. Fanconi's anaemia is the most common congenital disorder and leukaemia occurs in at least 10% of cases. In acquired aplastic anaemia, a high incidence of myelodysplastic syndrome (MDS) was noted in patients with aplastic anaemia, seemingly cured of their aplasia by antilymphocyte globulins (ALG). In a recent survey, the 10-year cumulative incidence rates were 9.6% for MDS, 6.6% for acute leukaemia (115-fold higher than in the general population). Biological evidence is provided by bone marrow morphology, as a certain degree of dysmyelopoiesis is not unusual in aplastic anaemia. Cytogenetic analyses in aplastic anaemia are scarce, but data have shown clonal cytogenetic abnormalities at diagnosis in otherwise typical aplastic anaemia. Recently, flow cytometry to assess the glycosyl-phosphatidylinositol (GPI) molecule defect in PNH has demonstrated that a significant proportion of patients with otherwise typical aplastic anaemia have, in fact, a GPI defect due to alterations within the PIG-A gene. Finally, aplastic anaemia patients were recently reported to have molecular evidence of clonal haematopoiesis; this must now be discussed in light of recent clonality studies in normal individuals. The clinical and biological evidence for a link between aplastic anaemia, PNH and hypoplastic leukaemia allows the generation of a model of aplastic anaemia as a possible pre-pre-leukaemic disorder.

Antibodies to glycosylphosphatidyl-inositol anchored proteins (GPI-AP) in antithymocyte and antilymphocyte globulin: possible role for the expansion of GPI-AP deficient cells in aplastic anemia

Antithymocyte globulin (ATG) and antilymphocyte globulin (ALG) are currently used successfully for immunosuppressive treatment of aplastic anemia. In this study we have investigated whether commercial ATG/ALG preparations contain antibodies against glycosylphosphatidyl-inositol anchored proteins (GPI-AP), which could be responsible for emergence of GPI-deficient populations in aplastic anemia after ATG/ALG therapy. We analyzed four commercial ATG/ALG preparations by competitive binding assays using flow cytometry. Quantification was achieved by calculating the concentration of ATG/ALG required to give 50% inhibition of binding the specific fluorochrome-labeled monoclonal antibody (EC50). High concentrations of antibodies against the GPI-anchored protein CD52 were found in all preparations (Lymphoglobulin Genzyme, Thymoglobulin Genzyme, ATGAM. Pharmacia & Upjohn, and ATG-Fresenius S Fresenius). Antibodies against the GPI-anchored protein CD48 are present in significant concentrations except in the preparation ATGAM. CD16 antibodies were found in lower concentrations. We could not detect significant concentrations of antibodies against the GPI-anchored proteins CD157 and CD14. Campath-1H, a monoclonal antibody against the GPI-anchored protein CD52, has been used as immunosuppressive tool for T-cell depletion. CD52 antibodies in ATG/ALG preparations might contribute in the same way to the immunosuppressive effects in treatment of aplastic anemia. It is known that in a substantial proportion of patients with aplastic anemia GPI-deficient cells are present in a low level at diagnosis or emerge after immunosuppressive therapy. GPI-anchored antibodies in ATG/ALG preparations might lead to a relative advantage for pre-existing GPI-deficient cells caused by an escape from the antibody-mediated attack.

Outcome of children with aplastic anemia treated with immunosuppressive therapy

BACKGROUND

Immunosuppressive therapy (IST) is the alternative treatment in children with aplastic anemia (AA) who do not have an HLA-matched sibling. The aim of this study is to evaluate the outcome of children with AA treated with IST.

METHODS

We retrospectively reviewed the hospital records of children with AA from 1984 to 2004, treated at our institution with antithymocyte globulin (ATG), cyclosporine (CS), and short course of prednisone.

RESULT

Forty-two patients were treated with IST (24 boys, 18 girls); of whom 26% received G-CSF. The median age at diagnosis was 8.5 years. Sixty-nine, 19, and 12% were diagnosed with severe, very severe, and moderate AA, respectively. Twenty-one percent had hepatitis-associated AA. Median follow-up time was 53.3 months. Sixty-two percent had complete response; 19% had partial response. Two patients relapsed and received a second course of ATG; both had a partial response. The actuarial 5 years survival rate was 67.5%. Two patients developed myelodysplastic syndrome (MDS); both received long-term G-CSF and had partial response after two courses of IST. Fifteen percent of survivors had significant hypertension which persisted after CS was discontinued.

CONCLUSIONS

This study shows promising response in children with AA treated with IST; however, the outcome was inferior to our institutional results with hematopoietic stem cell transplantation from a sibling donor. Hypertension and MDS are late complications. Longer follow-up, larger cohorts, and prospective studies are warranted to evaluate late complications and risk factors.

Autoimmunity and malignancy in hematology—More than an association

Several associations between hematological malignancies and autoimmunity directed against hematopoietic cells exist. Antibody mediated elimination of mature blood cells such as autoimmune hemolytic anemia (AIHA) and immune thrombocytopenia (ITP) are frequent complications of non-Hodgkin lymphomas, most prominently chronic lymphocytic leukemia. Autoimmunity directed against hematopoietic precursor cells is the hallmark of aplastic anemia, but many features of this disease are shared by two related disorders, paroxysmal nocturnal hemoglobinuria (PNH) and myelodysplastic syndrome (MDS). While the clinical associations between hematological malignancy and autoimmunity have been described many decades ago, only in the last several years have the common pathogenetic mechanisms been elucidated. We summarize the recent progress made in understanding how hematological malignancy gives rise to autoimmunity directed against blood cells and vice versa, and illustrate parallels in the etiology of malignant and autoimmune hematological disorders. Specifically, recent progress in the recognition of the association of lymphoproliferative disorders and autoimmunity against mature blood cells, and common pathogenetic background of aplastic anemia, paroxysmal nocturnal hemoglobinuria, and myelodysplastic syndrome are discussed.

PIG-A mutations in paroxysmal nocturnal hemoglobinuria and in normal hematopoiesis

PIG-A is an X-linked gene that is essential for the first step in the biosynthesis of glycosylphosphatidyl-inositol (GPI) anchors. A rare clonal hematopoietic stem cell disease, paroxysmal nocturnal hemoglobinuria (PNH), is caused by mutations in the PIG-A gene. PNH is an acquired disease that may arise de novo or emanate from aplastic anemia. PNH blood cells have an absence or marked deficiency of all GPI anchored proteins. Interestingly, rare GPI anchor deficient blood and marrow cells that harbor PIG-A mutations can also be found in most healthy controls. This review examines the clinical and biological relevance of PIG-A mutations in PNH, aplastic anemia and healthy controls.

Comparable Outcomes of Matched-Related and Alternative Donor Stem Cell Transplantation for Pediatric Severe Aplastic Anemia

Matched sibling donor (MSD) bone marrow transplantation is the treatment of choice for pediatric patients with severe aplastic anemia (SAA); however, only about 33% of patients will have an HLA-identical sibling. Alternative donor (AD) transplants may be an option for these patients, but such therapies have been associated with greater incidence of graft failure and graft-versus-host disease (GVHD). We retrospectively analyzed 36 pediatric patients who received 38 bone marrow or peripheral blood stem cell transplants (15 MSD and 23 AD) for SAA at our institution from April 1997 to October 2005. Nineteen AD recipients received reduced intensity conditioning with cyclophosphamide, low-dose total body irradiation, and antithymocyte globulin (ATG) or Campath. The 4-year overall survival for MSD recipients was 93% versus 89% for AD recipients treated with reduced intensity conditioning regimens at a median follow-up of 52 months (range, 6-99 months). No patient receiving Campath, compared with 3 of 9 patients receiving ATG, developed extensive, chronic GVHD. We conclude that, for children with SAA, AD transplantation is as effective as MSD transplantation. Further, compared with ATG, preparatory regimens containing Campath may be associated with a lower incidence of extensive, chronic GHVD.

Telomere length dynamics in normal hematopoiesis and in disease states characterized by increased stem cell turnover

Telomeres both reflect and limit the replicative lifespan of normal somatic cells. Immature sub-populations of human CD34+38- hematopoietic stem cell (HSC) can be identified in vitro based on their growth kinetics and telomere length. Fluorescence in situ hybridization and flow cytometry (flow-FISH) has been used to characterize telomere length dynamics as a surrogate marker for HSC turnover in vivo. Investigations in normal steady-state hematopoiesis provided the basis for follow-up studies in model scenarios characterized by increased HSC turnover. Disorders with underlying malignant transformation of HSC (e.g., chronic myeloid leukemia (CML)) can be discriminated from disease states with increased HSC turnover rates secondary to depletion of the stem cell compartment, for example, as in defined bone marrow failure syndromes. In some of these model scenarios, the degree of telomere shortening can be correlated with disease duration, disease stage and severity as well as with response to disease-modifying treatment strategies. Whether increased telomere shortening represents a causal link between HSC turnover, replicative senescence and/or the induction of genetic instability in acquired HSC disorders remains to be shown. However, data from congenital disorders, like dyskeratosis congenita (DKC), suggest that disturbed telomere maintenance may play a role for replicative exhaustion of the HSC pool in vivo.

A cohort study of the nature of paroxysmal nocturnal hemoglobinuria clones and PIG-A mutations in patients with aplastic anemia

BACKGROUND

Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by the clonal expansion of blood cells, which are deficient in glycosylphosphatidylinositol anchored proteins (GPI-APs). As PNH frequently occurs during the clinical course of acquired aplastic anemia (AA), it is likely that a process inducing bone marrow failure in AA is responsible for the selection of GPI-AP deficient blood cells or PNH clone.

OBJECTIVE

To explore the nature and mutation of a PNH clone in AA.

METHODS

We performed regular repeated flow cytometric analyses of CD59 expression on peripheral blood cells from a cohort of 32 patients with AA. Mutation of phosphatidylinositol glycan class A (PIG-A) was also studied.

RESULTS

Fifty-one episodes of occurrences of CD59 negative granulocytes out of a total cohort 167 flow cytometric analyses (31%) were observed in 22 patients (69%). CD59 negative erythrocytes were less apparent than the granulocytes. Repeated occurrences of PNH clones were observed in 16 patients. Most of the emerging PNH clones were transient in nature. They were more frequently detected during episodes of lower white blood cell and platelet counts. Persistence and expansion of the GPI-AP deficient blood cell populations to the level of clinical PNH were seen in only four patients (12.5%). Analysis of PIG-A gene demonstrated eight mutations among the four patients, with two and four independent mutations in two patients.

CONCLUSIONS

Our study indicates that PIG-A mutations of hematopoietic stem cells with resultant PNH clones, are relatively common among AA patients. It also supports the hypothesis of selection of the PNH clone by a process or condition associated with or responsible for bone marrow failure in AA. However, there must be an additional factor favoring expansion or growth of the clone to the level of clinical or florid PNH.

Diagnosis and treatment of children with aplastic anemia

BACKGROUND

Long-term survival rates among children diagnosed with severe aplastic anemia (SAA) are excellent due to the success of human leukocyte antigen (HLA)-identical related hematopoietic stem cell transplantation (HSCT), concurrent advances in immunosuppressive treatment (IST), and improved supportive care. The challenge in making treatment recommendations for children with SAA, therefore, is to balance the apparent chronicity and morbidity following IST, with the potential up-front toxicity and complications of HSCT.

METHODS

This review provides an update on the diagnosis and a risk-based treatment algorithm for children with acquired SAA. Recent experience using alternative donor HSCT and efforts to extend HSCT eligibility through advances in donor matching, de-escalation of conditioning regimens, and potential marrow graft engineering are highlighted. We discuss IST response rates, risks of relapse, and complications including clonal evolution.

CONCLUSIONS

While good treatment options exist for a majority of children diagnosed with SAA, novel non-transplantation treatments for unresponsive and relapsed patients without suitable transplant donors are needed. Further improvements in outcome will ultimately require a more complete understanding of the pathophysiology of aplastic anemia (AA).

Riddle: what do aplastic anemia, acute promyelocytic leukemia, and chronic myeloid leukemia have in common?

Secondary myelodysplastic syndrome (MDS)/acute leukemia frequently evolves from severe aplastic anemia (SAA) following immunosuppressive therapy. Secondary clonal cytogenetic abnormalities have now been reported after noncytotoxic therapy in two additional settings: all trans retinoic acid (ATRA) treatment of acute promyelocytic leukemia (APL) and imatinib for chronic myeloid leukemia (CML). We propose that SAA, APL, CML, and MDS represent different manifestations of generalized insults to the bone marrow. In SAA, the insult to hematopoietic progenitors leads to an immune attack, while in APL, CML, and MDS, it gives rise to the malignant clones. A primary insult to bone marrow could simultaneously lead to several abnormal hematopoietic cell clones, with one dominating and the others present but below the level of detection. Such a 'field leukemogenic effect' would be analogous to the 'field cancerization effect' described in solid tumors. Nonspecific cytotoxic therapies, including antileukemic chemotherapy and allogeneic transplantation, have broad activity that could inhibit both the overt disease and other undetectable coexistent abnormal clones. In contrast, disease-specific targeted therapy such as immunosuppressive therapy in aplastic anemia, ATRA in APL, or imatinib in CML would have no activity against other abnormal clones, allowing them to expand and become detectable as the dominant clone declines.

Pathophysiology and treatment of aplastic anemia

Aplastic anemia (AA) is a rare hematological disease characterized by peripheral blood pancytopenia and a hypocellular bone marrow in which normal hematopoietic tissue is replaced by fatty marrow. There is strong in vitro and in vivo evidence suggesting an immunologic mechanism for hematopoietic suppression in the majority of patients with AA. Interferon-gamma and tumor necrosis factor-alpha are considered as soluble mediators of bone marrow (BM) suppression in AA. The events triggering the aberrant immune response are less clear but some viruses and drug metabolites may lead to autoimmune destruction of hematopoietic cells. Patients with severe AA who are younger than 35 to 45 years and who have an HLA-identical sibling donor have a 60-80% chance of being cured by allogeneic BM transplantation. In patients surviving more than two years, chronic graftversus-host disease is the major cause of morbidity and mortality and a solid-tumor malignancy may develop in a few patients. Patients without HLA-identical BM donors and patients older than 35 to 45 years are candidates for combined immunosuppressive treatment with antithymocyte globulin, methylpredisolone and cyclosporine, leading to hematological responses in 70-80% of patients. One has to consider, however, that a significant proportion of these patients will develop further clonal hematological disorders such as paroxysmal nocturnal hemoglobinuria and myelodysplastic syndrome.

The spectrum of PIG-A gene mutations in aplastic anemia/paroxysmal nocturnal hemoglobinuria (AA/PNH): a high incidence of multiple mutations and evidence of a mutational hot spot

Paroxysmal nocturnal hemoglobinuria (PNH) may arise during long-term follow- up of aplastic anemia (AA), and many AA patients have minor glycosylphosphatidylinositol (GPI) anchor-deficient clones, even at presentation. PIG-A gene mutations in AA/PNH and hemolytic PNH are thought to be similar, but studies on AA/PNH have been limited to individual cases and a few small series. We have studied a large series of AA patients with a GPI anchor-deficient clone (AA/PNH), including patients with minor clones, to determine whether their pattern of PIG-A mutations was identical to the reported spectrum in hemolytic PNH. AA patients with GPI anchor-deficient clones were identified by flow cytometry and minor clones were enriched by immunomagnetic selection. A variety of methods was used to analyze PIG-A mutations, and 57 mutations were identified in 40 patients. The majority were similar to those commonly reported, but insertions in the range of 30 to 88 bp, due to tandem duplication of PIG-A sequences, and deletions of more than 10 bp were also seen. In 3 patients we identified identical 5-bp deletions by conventional methods. This prompted the design of mutation-specific polymerase chain reaction (PCR) primers, which were used to demonstrate the presence of the same mutation in an additional 12 patients, identifying this as a mutational hot spot in the PIG-A gene. Multiple PIG-A mutations have been reported in 10% to 20% of PNH patients. Our results suggest that the large majority of AA/PNH patients have multiple mutations. These data may suggest a process of hypermutation in the PIG-A gene in AA stem cells.

Antithymocyte globulin and cyclosporine for severe aplastic anemia: association between hematologic response and long-term outcome

CONTEXT

In most patients, aplastic anemia results from T-cell-mediated immune destruction of bone marrow. Aplastic anemia can be effectively treated by stem cell transplantation or immunosuppression.

OBJECTIVE

To assess long-term outcomes after immunosuppressive therapy.

DESIGN, SETTING, AND PATIENTS

Cohort of 122 patients (31 were < or =18 years and 91 were >18 years) with severe aplastic anemia, as determined by bone marrow cellularity and blood cell count criteria, were enrolled in a single-arm interventional research protocol from 1991 to 1998 at a federal government research hospital.

INTERVENTIONS

A dose of 40 mg/kg per day of antithymocyte globulin administered for 4 days, 10 to 12 mg/kg per day of cyclosporine for 6 months (adjusted for blood levels), and a short course of corticosteroids (1 mg/d of methylprednisolone for about 2 weeks).

MAIN OUTCOME MEASURES

Survival, improvement of pancytopenia and transfusion-independence, relapse, and evolution to other hematologic diseases.

RESULTS

Response rates were 60% at 3 months after initiation of treatment, 61% at 6 months, and 58% at 1 year. The blood cell counts of patients who responded no longer satisfied severity criteria and they were transfusion-independent. Overall actuarial survival at 7 years was 55%. Survival was associated with early satisfaction of response criteria (86% vs 40% at 5 years; P<.001) and by blood counts at 3 months (reticulocyte count or platelet count of >50 x 10(3)/ microL predicted survival at 5 years of 90% [64/71] vs 42% [12/34] for patients with less robust recovery [P<.001 by log-rank test]). There were no deaths among responders more than 3 years after treatment. Relapse was common, but severe pancytopenia usually did not recur. Relapse did not influence survival. Thirteen patients showed evolution to other hematologic diseases, including monosomy 7.

CONCLUSIONS

Approximately half of patients with severe aplastic anemia treated with antithymocyte globulin and cyclosporine have durable recovery and excellent long-term survival. These outcomes were related to the quality of hematologic recovery.

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.

Management issues in paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) arises in the setting of bone marrow injury. Thus, management decisions must take into account whether symptoms are a consequence of the underlying marrow failure or of the expansion of the clone of the PIG-A mutant hematopoietic cells. The primary clinical manifestations of PNH are intravascular hemolysis and thrombophilia. Currently available options for treatment of the hemolysis of PNH are unsatisfactory, but the recent development of specific inhibitors of complement for use in treating human disease should make possible effective management of this pathology. The fundamental basis of the thrombophilia of PNH has not been elucidated. Currently, empiric anticoagulant therapy is the foundation for treating the thromboembolic complications of PNH. The role of warfarin prophylaxis, however, remains an area of active debate. Pregnancy in a patient with PNH presents special concerns about fetal/maternal well-being because of the high potential for thromboembolic complications. Bone marrow transplantation can be considered curative, but the decision to recommend this treatment must take into account factors related both to PNH and to comorbid conditions. Refining the technology for both gene therapy (by transducing stem cells with a functional PIG-A gene) and autotransplantation (by using stem cells selected for the expression of glycosyl phosphatidylinositol-anchored proteins) remain challenges for the future.

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.

Circulating PIG-A mutant T lymphocytes in healthy adults and patients with bone marrow failure syndromes

OBJECTIVE

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal hematological disorder with acquired PIG-A gene mutations and absent surface expression of proteins utilizing glycosylphosphatidylinositol (GPI) anchors. PNH often follows aplastic anemia, suggesting PIG-A mutant cells have relative dominance over normal hematopoietic cells. Somatic PIG-A mutations could arise after aplasia, or healthy persons could have rare PIG-A mutant cells that expand under selection pressure.

METHODS

We developed an in vitro negative selection method to isolate GPI-deficient T lymphocytes using aerolysin, an Aeromonas toxin that binds GPI anchors and induces cell lysis. Peripheral blood mononuclear cells (PBMC) from normal adults and patients with PNH or other bone marrow failure syndromes were analyzed.

RESULTS

From healthy adults, 166 T lymphocyte clones with deficient GPI-linked surface protein expression (CD55, CD59) were isolated. The mean mutant frequency (M(f)) of aerolysin-resistant clones was 17.8 +/- 13.8 per 10(6) PBMC, range 5.0-59.6 per 10(6) cells. Clones had a Class A complementation defect and distinct PIG-A mutations. Patients with PNH had elevated aerolysin-resistant M(f) values averaging 19 x 10(-2), a 10,000-fold difference. Two patients with Fanconi anemia and two others with mild aplastic anemia had M(f) values less than 15 x 10(-6), but two with recovering aplastic anemia had M(f) values of 20 x 10(-4), representing an intermediate value between normal persons and PNH patients.

CONCLUSION

Identification of PIG-A mutant T lymphocytes in healthy adults suggests PNH could develop following intense negative selection of hematopoiesis, with clonal outgrowth of naturally occurring PIG-A mutant stem cells.

Proliferative capacity of single isolated CD34+ hematopoietic stem/progenitor cells in paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) results from somatic mutations of the X-linked PIG-A (phosphatidylinositol glycan-class A) gene, which occurs on a hematopoietic stem cell level, leading to a proportion of blood cells being deficient in all glycosylphosphatidylinositol (GPI)-anchored surface proteins. Although these GPI-deficient cells can explain many of the clinical symptoms of PNH, the pathogenesis of PNH is still somewhat obscure and many questions remain. To assess the hematopoietic defect involved in PNH, CD34+ CD59+ (normal phenotype hematopoietic stem/progenitor) and CD34+ CD59- (PNH phenotype) cells from PNH patients (n = 16) and CD34+ CD59+ cells from healthy volunteers (n = 10) were sorted as single cells into 96-well flat-bottom culture plates containing culture medium supplemented with stem cell factor, interleukin (IL)-3, erythropoietin, granulocyte-macrophage-colony-stimulating factor (GM-CSF), G-CSF, IL-6, thrombopoietin, and Flt-3 ligand. We found that the single PNH CD34+ CD59- cells had a growth advantage over the single CD34+ CD59+ cells to some extent, but they both had impaired growth abilities compared with CD34+ cells from healthy volunteers.

Telomere length changes in patients with aplastic anaemia

To investigate telomere changes in patients with aplastic anaemia (AA) and clinical factors influencing the telomere dynamics, telomere length (TL) was measured in peripheral blood mononuclear cells using Southern blot analysis of 42 patients with AA and 39 healthy normal controls. Nineteen patients received supportive treatment only, while the remaining 23 patients received immunosuppressive therapy with anti-thymocyte globulin or anti-lymphocyte globulin +/- cyclosporin A. In AA patients, TL was on average 1.41 kb shorter than that of age-matched normal controls (P < 0.001). In patients treated with immunosuppression, the mean TL of non-responders was significantly shorter than that of age-matched normal controls (P < 0.001), while no difference in TL was detected in responders compared with controls. Positive correlation was observed between the extent of telomere shortening, the severity of neutropenia (P = 0.05) and the degree of mean corpuscular volume elevation (P = 0.005) at the time of the study. However, there was no correlation with time elapsed since diagnosis (P = 0.214). These findings suggest that haematopoietic stem cells in patients with AA rapidly lose TL at the onset of the disease. The TL shortening may reflect the severity of impairment of haematopoiesis.

Clinical paroxysmal nocturnal hemoglobinuria is the result of expansion of glycosyl-phosphatidyl-inositol-anchored protein-deficient clone in the condition of Deficient Hematopoiesis

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired, clonal hematopoietic stem cell disorder in which PIG-A, gene essential for the biosynthesis of the glycosyl-phosphatidyl-inositol (GPI) anchor, is somatically mutated. Absence of GPI-linked proteins from the surface of blood cells is characteristic of the PIG-A mutant (PNH) clone and is also accountable fo certain manifestations, such as intravascular hemolysis. It is unclear how the PNH clone expands and comes to dominate hematopoiesis. In this study, CD34+ cells--committed progenitors (colony-forming cells) representing immature hematopoietic stem cells--and reticulocytes representing the differentiated erythroid cells were quantitated in peripheral blood of patients with PNH. Compared with normal controls (n = 29), CD34+ cell levels were significantly lower in PNH patients who did not have preexisting aplastic anemia (AA) (n = 12) (2.47+/-1.23 versus 4.68+/-1.05 x 106/L, mean +/- standard error; P = .022). PNH patients with precedent aplastic anemia (AA+/PNH) showed markedly low CD34+ cell levels compared with normal control subjects (0.6+/-0.29 versus 4.68+/-1.05 x 10(6)/L; P = .0001). In addition, colony-forming cells from PNH patients were significantly decreased compared with those from normal volunteers (erythroid burst-forming units, 2.8+/-1.2 versu 25.6+/-6.2/5 x 10(5) mononuclear cells; P = .0006; and granulocyte/macrophage colony-forming units, 1.2+/-0.5 versus 13.3+/-3.0/ 5 x 10(5) mononuclear cells; P = .0006). These findings occur in both aplastic and hemolytic types of PNH, suggesting hematopoietic failure in PNH. On the contrary, the numbers of reticulocytes and the reticulocyte production index of PNH patients were significantly higher than those of normal persons and comparable to those from patients with autoimmune hemolytic anemia, indicating accelerating erythropoiesis in PNH. The degree of reticulocytosis correlated well with the proportion of CD59- (PNH) reticulocytes. All of the findings suggest that in the condition of deficient hematopoiesis, the PNH clone arising from the mutated hematopoietic stem cell expands and maintains a substantial proportion of the patient's hematopoiesis.

Excessive apoptosis in low risk myelodysplastic syndromes (MDS)

The paradox of peripheral cytopenias despite a normo/hypercellular marrow in MDS has been ascribed to excessive intramedullary hematopoietic cell apoptosis. Programmed cell death (PCD) in early disease might be triggered by the BM microenvironment, mediated either through inhibitory cytokines such as tumor necrosis factor alpha (TNF-alpha) or fas/fas ligand signaling or through a relative deficiency in hematopoietic growth factors. Intrinsic cellular defects giving rise to abnormalities in cell-cell or cell-stromal interaction, cell signaling or cell cycling may also underlie hematopoietic progenitor apoptosis. Alternatively, an early 'hit' in the multistep pathogenesis of MDS may result in a higher proliferative rate of the neoplastic clone. Increased apoptosis may thus represent a homeostatic process to control cell numbers. This paper shall summarize current evidence implicating a role for increased PCD in low risk MDS, outline possible etiologic factors and suggest potential therapeutic mechanisms whereby excessive hematopoietic progenitor cell apoptosis might be circumvented.

N-RAS gene mutation in patients with aplastic anemia and aplastic anemia/ paroxysmal nocturnal hemoglobinuria during evolution to clonal disease

Long-term survivors of aplastic anemia (AA) have a high incidence of clonal disorders, in particular paroxysmal nocturnal hemoglobinuria (PNH), myelodysplastic syndromes (MDS), and acute nonlymphocytic leukemia. To investigate the potential involvement of N-RAS gene mutations in the predisposition to leukemic evolution, a subset of patients at potentially increased risk for clonal disease was selected based on evidence of existing clonal evolution. Nine patients showed a monoclonal pattern of X-chromosome inactivation, 18 demonstrated a PNH clone, and in 3 MDS developed during the course of this study. No mutations were detected during the aplastic phase of disease; 2 of 3 patients with MDS after AA also showed no mutations. However, in 1 patient in whom the disease transformed from AA/PNH to MDS, a mutation of GGT → GAT at N-RAS codon 13 became detectable, whereas the PNH mutation disappeared. The authors conclude that N-RAS mutations are not an early event preceding transformation of AA or AA/PNH to leukemia. In a subset of patients, RAS mutations may occur at the time of evolution to MDS, but preexisting RAS mutations do not explain the propensity of AA to leukemogenesis. Although PNH is also associated with leukemia, this may arise in the non-PNH cells, indicating that PIG-A gene mutation is not per se oncogenic.

Glycosyl phosphatidylinositol (GPI)-anchored molecules and the pathogenesis of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by the expansion of one or more clones of stem cells producing progeny of mature blood cells deficient in the plasma membrane expression of all glycosyl phosphatidylinositol (GPI)-anchored proteins (AP). This is due to somatic mutations in the X-linked gene PIGA, encoding one of the several enzymes required for GPI anchor biosynthesis. More than 20 GPI-APs are variously expressed on hematological cells. GPI-APs may function as enzymes, receptors, complement regulatory proteins or adhesion molecules; they are often involved in signal transduction. The absence of GPI-APs may well explain the main clinical findings of PNH, i.e., hemolysis and thrombosis in the venous system. Other aspects of PNH pathophysiology such as various degrees of bone marrow failure and the dominance of the PNH clone may also be linked to the biology and function of GPI-APs. Results of in vitro and in vivo experiments on embryoid bodies and mice chimeric for nonfunctional Piga have recently demonstrated that Piga inactivation confers no intrinsic advantage to the affected hematopoietic clone under physiological conditions; thus additional factors are required to allow for the expansion of the mutated cells. A close association between PNH and aplastic anemia suggests that immune system mediated bone marrow failure creates and maintains the conditions for the expansion of GPI-AP deficient cells. In this scenario, a PIGA mutation would render GPI-AP deficient cells resistant to the cytotoxic autoimmune attack, enabling them to emerge. Even though the 'survival advantage' hypothesis may explain all the various aspects of this intriguing disease, a formal proof of this theory is still lacking.

Atypical chronic myelogenous leukemia following immunosuppressive therapy for severe aplastic anemia

Late clonal complications of aplastic anemia (AA) such as acute leukemia, myelodysplastic syndromes or paroxysmal nocturnal hemoglobinuria have been recognized for a long time. To our knowledge, chronic myelogenous leukemia (CML) as a late complication of severe aplastic anemia has as yet not been reported. We report here a case of AA treated successfully with antilymphocytic globulin and cyclosporin in whom Ph1 negative, BCR/ABL negative CML developed 8 years after diagnosis of AA. This case of atypical, secondary CML was refractory to treatment with interferon alpha and hydroxyurea.