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Acquired Aplastic Anemia
C H A P T E R
49 Acquired Aplastic Anemia Robert A. Brodsky and Richard J. Jones Division of Hematology, Department of Medicine and The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Baltimore, MD, United States
O U T L I N E Historic Background
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Immunosuppressive Therapy
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Genetic Features
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Eltrombopag
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Clinical, Pathologic, and Epidemiologic Features
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High-Dose Cyclophosphamide Without Bone Marrow Transplantation
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Human Leukocyte Antigen Haploidentical Bone Marrow Transplant With Posttransplant Cyclophosphamide
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Aplastic Anemia and Clonality
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Concluding Remarks—Future Prospects
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References
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Autoimmune Features and Pathogenic Mechanisms
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Environmental Features
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Animal Models
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Therapy for Aplastic Anemia Bone Marrow Transplantation Bone Marrow Transplantation From Unrelated Donors
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Aplastic anemia manifests with pancytopenia and a hypocellular bone marrow (Brodsky and Jones, 2005). The disease may be acquired or inherited. Most cases of acquired aplastic anemia result from autoimmune destruction of hematopoietic stem/progenitors and respond to immunosuppressive therapies. The inherited forms of aplastic anemia are less common and usually present within the first decade of life (Tsangaris et al., 2011). The inherited bone marrow failure may be due to a variety of genetic mutations such as DNA repair defects (Fanconi anemia), telomerase defects [dyskeratosis congenita (DKC)], ribosomopathies (Shwachman Diamond syndrome), or cMPL mutations (amegakaryocytic thrombocytopenia). Immunosuppressive therapy is not helpful for the most inherited forms of bone marrow failure. This chapter will predominantly focus on acquired aplastic anemia; however, it is important to be aware of these less common inherited forms of aplastic anemia since they can sometimes be hard to distinguish from the acquired form of the disease.
HISTORIC BACKGROUND The earliest case description of aplastic anemia in 1888 was by Dr. Paul Ehrlich (Ehrlich, 1888). He described a young woman who died following an abrupt illness characterized by severe anemia, bleeding, high fever, and a markedly hypocellular bone marrow. Until the early 1970s, most patients with severe aplastic anemia (SAA) died within a year of diagnosis. The advent of allogeneic (allo) bone marrow transplantation (BMT) and The Autoimmune Diseases, 6th. DOI: https://doi.org/10.1016/B978-0-12-812102-3.00049-X
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Copyright © 2020 Elsevier Inc. All rights reserved.
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immunosuppressive therapy markedly improved the outcome for these patients and prompted vigorous clinical and laboratory investigation. These studies have generated an important insight into hematopoietic stem cell biology, immunology, and autoimmunity. Today the majority of patients will survive this potentially fatal autoimmune disorder.
GENETIC FEATURES Distinctive genetic abnormalities are more common with congenital bone marrow failure syndromes since most acquired aplastic anemia is autoimmune. Congenital aplastic anemia tends to present in the first decade of life and is often, but not always, associated with other physical anomalies. Fanconi anemia, the most common form of congenital bone marrow failure, predisposes to cancer and is frequently associated with other congenital abnormalities (e.g., short stature, upper limb anomalies, hypogonadism, cafe´-au-lait spots, etc.) (Bagby, 2003). DKC is another congenital bone marrow failure disorder that can be either X-linked recessive, autosomal dominant, or autosomal recessive (Dokal and Vulliamy, 2003). The X-linked recessive form results from mutations in a gene known as DKC1 whose gene product, dyskerin is important for stabilizing telomerase. The resulting telomerase deficiency leads to short telomeres, bone marrow failure, and premature aging. The autosomal dominant form of DKC results from human telomerase RNA component (hTERC) gene mutations, the RNA component of telomerase. This chapter will focus on the acquired form of aplastic anemia. Genetic abnormalities are less well characterized in acquired aplastic anemia; however, there appears to be an underlying genetic predisposition to acquired aplastic anemia, as evidenced by the overrepresentation of human leukocyte antigen (HLA) DR2 subtypes (Nimer et al., 1994).
CLINICAL, PATHOLOGIC, AND EPIDEMIOLOGIC FEATURES Aplastic anemia manifests as pancytopenia in conjunction with a hypocellular bone marrow. The disease may present abruptly (over days) or insidiously, over weeks to months. The most common clinical manifestations reflect the low blood counts and include dyspnea on exertion, fatigue, easy bruising, petechia, epistaxis, gingival bleeding, heavy menses, headaches, and fever. A complete blood count, leukocyte differential, reticulocyte count, and a bone marrow aspirate and biopsy are essential for diagnosis. Peripheral blood flow cytometry to detect glycosylphosphatidylinositol (GPI) anchor deficient blood cells (Brodsky et al., 2000; Borowitz et al., 2010) as well as cytogenetics and fluorescent in situ hybridization (FISH) analysis should be performed on the bone marrow aspirate. Up to 70% of patients with acquire aplastic anemia have a detectable paroxysmal nocturnal hemoglobinuria (PNH) clone which essentially rules out inherited forms of aplastic anemia (Dezern et al., 2014). Cytogenetic or FISH abnormalities are suggestive of a hypoplastic form of myelodysplasia. Patients under the age of 40 years old should be screened for Fanconi anemia using the clastogenic agents diepoxybutane or mitomycin C, and telomere lengths should be obtained on patients with a family history of bone marrow failure, premature graying, pulmonary fibrosis, or other stigmata of DKC. A hypocellular bone marrow is required for the diagnosis of aplastic anemia. However, some patients will have residual pockets of ongoing hematopoiesis; thus an adequate biopsy (1 2 cm in length) is essential for establishing the diagnosis. Dyserythropoiesis is not uncommon in aplastic anemia, especially in cases with coincidental small-to-moderate PNH populations; however, a small percentage of myeloid blasts, or dysplastic features in the myeloid or megakaryocyte lineages, is more typical of hypoplastic myelodysplastic syndromes (MDS). CD34 is expressed on early hematopoietic progenitors and the number of CD341 cells has also been used to help discriminate between aplastic anemia and hypoplastic myelodysplastic syndrome (hMDS). In aplastic anemia the percentage of cells expressing CD34 is usually less than 0.1%; in hMDS the CD34 count is either normal (0.5% 1.0%) or elevated (Matsui et al., 2006). As with other autoimmune diseases, there is a wide spectrum of disease severity in aplastic anemia. The prognosis in aplastic anemia is proportional to degree of peripheral blood cytopenias. Accordingly, aplastic anemia is classified as nonsevere, severe, and very severe based largely upon the degree of neutropenia (Table 49.1). SAA is defined as bone marrow cellularity of less than 25% and markedly decreased values of at least two of three hematopoietic lineages (neutrophil count ,500/μL, platelet count ,20,000/μL, and absolute reticulocyte count of ,60,000/μL). Very SAA satisfies the above criteria except the neutrophil count is ,200/μL, while non-SAA is characterized by a hypocellular bone marrow but with cytopenias that do not meet the criteria for severe disease.
VII. ORGAN SPECIFIC AUTOIMMUNE DISEASES
AUTOIMMUNE FEATURES AND PATHOGENIC MECHANISMS
TABLE 49.1 Peripheral blood counts
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Aplastic Anemia: Diagnosis and Definitions Nonsevere aplastic anemia (not meeting criteria for severe disease)
Severe aplastic anemia (any 2 of 3)
Very-severe aplastic anemia (meets criteria for severe disease and absolute neutrophils ,200)
absolute neutrophils
, 500/μL
, 200/μL
platelets
, 20,000/μL
reticulocyte count
, 1.0% corrected or ,60,000/μL
Bone marrow cellularity ,25%.
The 2-year mortality rate with supportive care alone for patients with SAA exceeds 50% (Camitta et al., 1979), with invasive fungal infections and overwhelming bacterial sepsis being the most frequent causes of death. Non-SAA is seldom life-threatening and in many instances requires no therapy. Although some cases of nonSAA will progress, many will remain stable for years, and some may spontaneously improve. Aplastic anemia has been associated with drugs, benzene exposure, insecticides, viruses, and other agents. However, over 80% of cases are classified as idiopathic. The disease most commonly affects children and young adults but may occur at any age. Precise estimates of the incidence of aplastic anemia are difficulty due the rarity of the disease and imprecision in establishing the diagnosis. The best estimates of incidence are case control studies that report an incidence of two cases/million inhabitants in Europe (Kaufman et al., 1991) and Israel (Modan et al., 1975), but the incidence may be two to threefold higher in Southeast Asia (Issaragrisil et al., 1997; Szklo et al., 1985). A population-based case control study of aplastic anemia in Thailand found that drugs, the most commonly implicated etiology, explain only 5% of newly diagnosed cases (Issaragrisil et al., 1997). An intriguing association exists between seronegative hepatitis and aplastic anemia. The hepatitis-aplastic anemia syndrome accounts for 3% 5% of newly diagnosed cases of aplastic anemia. The disease predominantly affects young males, with a precipitous onset of severe pancytopenia occurring within 2 3 months after the onset of hepatitis (Brown et al., 1997). Moreover, aplastic anemia has been reported to occur in up to 30% of patients following orthotopic liver transplantation for seronegative hepatitis (Tzakis et al., 1988; Cattral et al., 1994). The aplastic anemia in the hepatitis/aplastic anemia syndrome is also thought to be autoimmune since most cases respond to immunosuppressive therapy (Savage et al., 2007; Locasciulli et al., 2010).
AUTOIMMUNE FEATURES AND PATHOGENIC MECHANISMS Aplastic anemia was originally thought to result from a quantitative deficiency of hematopoietic stem cells precipitated by a direct toxic effect on stem cells. However, the attempts to treat aplastic anemia by simple transfusion of bone marrow from an identical twin failed to reconstitute hematopoiesis in most patients. Retransplant of many of these patients following a high-dose cyclophosphamide preparative regimen was successful, suggesting that the pathophysiology of aplastic anemia was more complicated (Champlin et al., 1984; Hinterberger et al., 1997). In the late 1960s Mathe et al. (1970) were among the first to postulate an immune basis for aplastic anemia. They performed BMT in patients with aplastic anemia using partially mismatched donors after administering antilymphocyte globulin as an immunosuppressive conditioning regimen. Although the patients failed to engraft, the investigators witnessed autologous recovery of hematopoiesis in some patients. This suggested that functional hematopoietic stem cells exist in aplastic anemia patients and that the immune system was somehow suppressing the growth and differentiation of hematopoietic stem cells. The response to immunosuppressive therapy was the first clear evidence that aplastic anemia was truly an autoimmune disease. The first laboratory experiments implicating an autoimmune pathophysiology were coculture experiments showing that T lymphocytes from aplastic anemia patients inhibited hematopoietic colony formation in vitro (Hoffman et al., 1977; Nissen et al., 1980). Since then, it has been shown that the immune destruction of hematopoietic stem cells in aplastic anemia is mediated by cytotoxic T cells and involves inhibitory Th1 cytokines and the Fas-dependent cell death pathway. The cytotoxic T cells are usually more conspicuous in the bone marrow than in the peripheral blood (Zoumbos et al., 1985; Selleri et al., 1994; Melenhorst et al., 1997) and overproduce interferon-γ and tumor necrosis factor (TNF) (Nakao, 1997; Nistico and Young, 1994). TNF and interferon-γ are
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direct inhibitors of hematopoiesis and appear to upregulate Fas expression on CD341 cells (Maciejewski et al., 1995). Immortalized CD41 and CD81 T-cell clones from some aplastic anemia patients have been shown to secrete Th1 cytokines and are capable of lysing autologous CD34 cells (Nakao et al., 1997; Zeng et al., 2001). Evidence for a humoral autoimmune response in aplastic anemia has also been reported (Hirano et al., 2000; Feng et al., 2004). Studies examining T-cell diversity using complementarity-determining region (CDR3) spectratyping have further implicated the role of the immune system in aplastic anemia. Several groups have now found limited heterogeneity of the T-cell receptor β-chain (BV) in aplastic anemia, suggesting that there is oligoclonal or even clonal expansion of T cells in response to a specific antigen (Melenhorst et al., 1997; Zeng et al., 2001; Manz et al., 1997).
ENVIRONMENTAL FEATURES The medical literature is replete with reports of environmental exposures, most notably benzene and radiation, causing aplastic anemia. However, rigorous epidemiologic studies supporting an association between environmental toxins and aplastic anemia are lacking. A major confounder is that benzene, radiation, and other toxins also predispose to MDS and leukemia. Older literature was unlikely to have been able to distinguish different types of marrow failure, such as aplastic anemia, MDS, and hypoplastic leukemia, leading to an overestimation of the association between benzene and aplastic anemia. While the magnitude of the risk remains uncertain, benzene is probably not a major risk factor for aplastic anemia in countries with modern standards of industrial hygiene. A large case control study in Thailand employing modern diagnostic and epidemiologic methods found that individuals of lower economic status and younger age are at greater risk than their counterparts in other countries following exposure to solvents, glues, and hepatitis A (likely a surrogate marker). Grain farmers were also found to have a higher risk of developing aplastic anemia (relative risk 5 2.7) regardless of whether they use insecticides (Issaragrisil et al., 1997). These same investigators noted marked differences in the incidence between northern and southern rural regions of Thailand and among Bangkok suburbs implicating potential environmental factors in causing the disease (Issaragrisil et al., 1999).
ANIMAL MODELS Animal models of bone marrow failure exist, but none of these models fully replicate the human disease acquired aplastic anemia (Chen, 2005). Busulfan, benzene, and irradiation have all been used to establish the models of marrow failure. All three of these agents lead to pancytopenia, and a hypocellular marrow but the marrow failure is due to stem cell injury and damage to the microenvironment rather than autoimmunemediated suppression of hematopoiesis. More recently, infusion of lymphocytes from congenic mice was used to model immune-mediated marrow failure; this model induces a hypocellular bone marrow and severe pancytopenia but is not truly autoimmune (Chen et al., 2004).
THERAPY FOR APLASTIC ANEMIA Definitive therapy for aplastic anemia includes BMT or immunosuppressive therapy. A variety of immunosuppressive agents have been studied, but antithymocyte globulin (ATG) and cyclosporine A (CSA) are the most commonly employed (Bacigalupo, 2017). Supportive care with blood transfusions and antibiotics is commonly required. Administration of hematopoietic growth factors has not been shown to improve survival in aplastic anemia.
Bone Marrow Transplantation BMT is the treatment of choice for young patients who have an HLA-matched sibling donor. Cyclophosphamide (50 mg/kg/day 3 4 days) with or without ATG is most commonly used for conditioning before BMT. This regimen is nonmyeloablative; however, the immunosuppression is sufficient to allow engraftment in most cases (Storb et al., 1997; Kahl et al., 2005). Alternative regimens using fludarabine,
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IMMUNOSUPPRESSIVE THERAPY
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FIGURE 49.1 Survival after allogeneic bone marrow transplantation for severe aplastic anemia. Data for HLA-identical siblings and related matched/mismatched transplants is from the IBMTR. Data for unrelated donor transplants is from the EBMT registry, Fred Hutchinson Cancer Research Center, IBMTR, and the IMUST study group. Survival curves are not adjusted for varying patient, disease, and transplant regimen characteristics. HLA, Human leukocyte antigen; EBMT, European Bone Marrow Transplant; IBMTR, International Bone Marrow Transplant Registry; IMUST, International Marrow Unrelated Search and Transplant Study. Source: Reproduced with permission from the National Bone Marrow Donor Program.
cyclophosphamide, and ATG are increasingly being used (Bacigalupo et al., 2010). Survival rates following matched sibling allo BMT have steadily improved since the 1970s largely because of improved supportive care, improved HLA typing, and better graft-versus-host disease (GVHD) prophylaxis (Bacigalupo, 1999). Late BMTrelated complications such as chronic GVHD occur in up to one-third of patients, with many of these patients requiring long-term therapy for their GVHD (Storb et al., 2001). Patient age and the type of allograft (HLAmatched sibling, unrelated, or mismatched donors) are the most important factors influencing outcome. In patients under 30 years of age the cure rate after HLA-matched sibling BMT ranges from 70% to 90% (Ades et al., 2004; Horowitz, 2000). However, the risk of GVHD steadily increases with age, leading to reduced survival.
Bone Marrow Transplantation From Unrelated Donors BMT from HLA-matched unrelated donors is usually reserved for patients who fail to respond to one or more courses of immunosuppressive therapy. The risk for transplant-related mortality and GVHD is almost twice that of BMT from matched sibling donors (Bacigalupo, 2017). The European Group for Blood and Marrow Transplantation has been conditioning SAA patients for BMT with fludarabine, cyclophosphamide, and ATG 6 total body irradiation. The European Society for Blood and Marrow Transplantation (EBMT) has reported survival rates as high as 75% using this conditioning regimen (Bacigalupo et al., 2010). Survival was the best in children and in patients who undergo BMT within 2 years of diagnosis (Fig. 49.1).
IMMUNOSUPPRESSIVE THERAPY ATG is produced by immunizing animals (horse or rabbit) against human thymocytes and kills human T cells through its cytolytic activity. Both horse (hATG) and rabbit (rATG) are approved for the use in the United States; hATG appears to be superior as first-line treatment (Scheinberg et al., 2011). Cyclosporine A (CSA) suppresses T-cell function by inhibiting the expression of nuclear regulatory proteins. Both single-agent ATG and singleagent CSA can induce remissions in acquired aplastic anemia; however, the combination ATG/CSA leads to a higher response rate and a greater likelihood of achieving transfusion independence (Frickhofen et al., 1991; Marsh et al., 1999). A randomized controlled demonstrated that hATG/CSA is superior to rATG/CSA (Scheinberg et al., 2011). The combination of ATG/CSA leads to 5-year survival rates comparable to BMT, but most of these patients are not cured of their disease. Response rates to hATG/CSA range between 60% and 80%, but in contrast to BMT, most patients do not acquire normal blood counts (Frickhofen et al., 1991; Rosenfeld et al., 2003). Another limitation of this approach is that many patients relapse, become dependent on cyclosporine, or develop secondary clonal disease such as PNH or MDS (Rosenfeld et al., 2003; Bacigalupo et al., 2000; Scheinberg et al., 2006). These late events often lead to substantial morbidity and mortality. The National Institutes of Health treated 122 patients (median age, 35 years) with the combination of ATG/CSA and methylprednisolone over a period of 8 years (Rosenfeld et al., 2003). The response rate was 58% and actuarial survival at 7 years was 55%; 13% of patients died within 3 months of treatment, most from fungal infections (Fig. 49.2). The relapse rate for responders was 40% and 13 patients developed MDS. In an attempt to improve response rate and survival, and to decrease the relapse rate and secondary MDS that occurs after hATG/CSA, the NIH added mycophenolate (1 g twice daily for 18 months) to the standard hATG/CSA
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FIGURE 49.2 (A) Survival probability for 122 patients with severe aplastic anemia following treatment with antithymocyte globulin and cyclosporine. (B) Probability of relapse in 74 patients with aplastic anemia classified as responders at 3 months after treatment with antithymocyte globulin and cyclosporine. (C) Proportion of patients experiencing clonal evolution. Source: Reproduced from Rosenfeld, S., Follmann, D., Nunez, O., Young, N.S., 2003. Antithymocyte globulin and cyclosporine for severe aplastic anemia: association between hematologic response and longterm outcome. JAMA 289 (9), 1130 1135 with permission from JAMA.
regimen. This three drug regimen resulted in a 62% response rate, but 37% of the responders relapsed (most while taking mycophenolate) and 9% progressed to either MDS or leukemia; thus the addition of mycophenolate did not improve response or survival (Scheinberg et al., 2006). Alemtuzumab is a highly immunosuppressive monoclonal antibody that binds to cell surface CD52, which is expressed primarily on B and T cells and monocytes. Alemtuzumab has activity in treating SAA, but response rates in therapy naive patients are less than 30% (Scheinberg et al., 2012).
ELTROMBOPAG Eltrombopag is a small molecule agonist of the c-mpl (TpoR) receptor, which is the physiological target of the hormone thrombopoietin, and is the only new drug approved for the treatment of SAA in the past 30 years. Eltrombopag may be used in an attempt to improve the cytopenias in patients at the refractory state (Desmond et al., 2015). Up to 20% of patients with refractory SAA become transfusion-independent within 3 months; however, similar to immunosuppressive therapy, there is a relatively high likelihood of relapse and secondary clonal disease (Desmond et al., 2014; Marsh and Kulasekararaj, 2013; Olnes et al., 2012). Eltrombopag is now
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HUMAN LEUKOCYTE ANTIGEN HAPLOIDENTICAL BONE MARROW TRANSPLANT WITH POSTTRANSPLANT CYCLOPHOSPHAMIDE
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being studied as an adjuvant to ATG/CSA in newly diagnosed patients in the hopes that it will improve response rates and prevent relapse.
HIGH-DOSE CYCLOPHOSPHAMIDE WITHOUT BONE MARROW TRANSPLANTATION The first successful human allo BMT, reported in 1972 by Thomas et al. (1972) in a patient with aplastic anemia, employed high-dose cyclophosphamide, and this remains (often in conjunction with ATG) the most commonly employed conditioning regimen for aplastic anemia (Storb et al., 2001). Complete reconstitution of autologous hematopoiesis occurs in 10% 15% of patients undergoing allo BMT for aplastic anemia (Thomas et al., 1976; Sensenbrenner et al., 1977; Gmur et al., 1979). The EBMT reported that 10% of SAA patients experience autologous reconstitution following BMT using a cyclophosphamide 1 ATG conditioning regimen. Interestingly, 10-year survival (84%) in patients with autologous recovery was better than in patients who engrafted (74%) (Piccin et al., 2010). The unique pharmacology of cyclophosphamide explains the autologous hematopoietic recovery (Emadi et al., 2009). Cyclophosphamide is a prodrug that is converted to 4-hydroxycyclophosphamide and its tautomer aldophosphamide in the liver. These compounds diffuse into the cell and are converted to the active compound phosphoramide mustard, or they are inactivated by aldehyde dehydrogenase to form the inert carboxyphosphamide. Lymphocytes have low levels of aldehyde dehydrogenase and are rapidly killed by high doses of cyclophosphamide; hematopoietic stem cells possess high levels of aldehyde dehydrogenase and are resistant to cyclophosphamide (Hilton, 1984; Jones et al., 1995). Thus, high-dose cyclophosphamide is highly immunosuppressive, but not myeloablative, allowing endogenous hematopoietic stem cells to reconstitute hematopoiesis. With this background, high-dose cyclophosphamide without BMT was used successfully in aplastic anemia patients who lacked appropriate donor (Brodsky et al., 1996a; Tisdale et al., 2000; Jaime-Prez et al., 2001; Brodsky et al., 2010). The largest and most mature study with high-dose cyclophosphamide is from Johns Hopkins (Brodsky et al., 2010; Gamper et al., 2016). These investigators treated 67 SAA patients with high-dose cyclophosphamide; 44 patients were treatment-naı¨ve and 23 were refractory to one or more previous immunosuppressive regimens. At 10 years, the overall actuarial survival, response rate, and event-free survival were 88%, 71%, and 58%, respectively, for the 44 treatment-naı¨ve patients. Patients with refractory SAA fared less well; at 10 years, the overall actuarial survival, response, and event-free survival rates were 62%, 48%, and 27%, respectively. For the treatment-naı¨ve patients, the median time to a neutrophil count of 0.5 3 109/L was 60 (range, 28 104) days, and the median time to last platelet and red cell transfusion was 117 and 186 days, respectively. Relapse occurred in just two of the treatment-naı¨ve patients, one of whom was retreated with high-dose cyclophosphamide into a second complete remission. Despite the high response rate and low risk of relapse and secondary clonal disease, the duration of deep aplasia (median 60 days to neutrophil recovery) and risk for invasive fungal infections is a major drawback for this approach (Fig. 49.3).
HUMAN LEUKOCYTE ANTIGEN HAPLOIDENTICAL BONE MARROW TRANSPLANT WITH POSTTRANSPLANT CYCLOPHOSPHAMIDE Investigators at Johns Hopkins, in an effort to expand the donor pool for patients in need of a allo bone marrow transplant, have pioneered the use of high-dose cyclophosphamide 3 and 4 days after bone marrow transplant to improve engraftment and decrease the risk of GVHD after HLA-haploidentical bone marrow grafts (Luznik et al., 2008). These authors recently reported on a prospective trial of this approach in 16 consecutive patients with refractory SAA (DeZern et al., 2017). Between July 2011 and August 2016, 16 patients underwent allo BMT for refractory SAA from 13 haploidentical donors and 3 unrelated donors. All 16 patients engrafted and survived with a median follow-up of 21 (range, 3 64) months at the time of publication. Two patients had grade 1 or 2 skin-only acute GVHD. These same two also had mild chronic GVHD of the skin/mouth requiring systemic steroids. One of these GVHD patients was able to come off all immunosuppressive therapy (IST) by 15 months and the other by 17 months. All other patients stopped IST at 1 year (Fig. 49.4).
VII. ORGAN SPECIFIC AUTOIMMUNE DISEASES
FIGURE 49.3 Survival probability following high-dose cyclophosphamide without BMT in 67 patients with SAA. (A) Overall survival for 44 patients with treatment-naı¨ve SAA (topline) and 23 patients with refractory SAA. (B) Failure-free survival after high-dose cyclophosphamide therapy for 44 patients with treatment-naı¨ve SAA (topline) and 23 patients with refractory SAA. BMT, Bone marrow transplantation; SAA, severe aplastic anemia. Source: Reproduced with permission from Blood 2010 (Brodsky, R. A., Chen, A. R., Dorr, D., Fuchs, E. J., Huff, C. A., Luznik, L., et al., (2010). High dose cyclophosphamide for severe aplastic anemia: long-term follow-up. Blood, 115(11), 2136 2141).
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FIGURE 49.4
Model depicting the pathophysiology of bone marrow failure in acquired aplastic anemia. Autoaggressive lymphocytes lyse CD34 1 bone marrow progenitor cells but seem to spare more immature CD34 1 cells known as high-quality stem cells.
APLASTIC ANEMIA AND CLONALITY The survivors of aplastic anemia are at high risk of clonal progression following immunosuppressive therapy (Socie et al., 1993). PNH and MDS are the most common clonal disorders to evolve from aplastic anemia (de Planque et al., 1989; Tichelli et al., 1988). Even before the widespread use of immunosuppressive therapy, 5% of the patients progressed to clonal hematopoiesis. This suggests that the increase in MDS and PNH following immunosuppressive therapy is not caused by the immunosuppression; rather, the increased survival following immunosuppressive therapy may allow time for these underlying clones to expand (Mukhina et al., 2001; Pu et al., 2011). PNH results from the expansion of an abnormal hematopoietic stem cell that harbors a somatic mutation of the X-linked gene, termed phosphatidylinositol glycan class A (PIG-A) (Brodsky, 2014). The product of the PIG-A gene is required for GPI anchor biosynthesis; consequently, PNH cells are deficient in GPI-anchored proteins. Several GPI-anchored proteins (CD59 and CD55) protect cells from complement-mediated destruction, and their absence explains the hemolytic anemia associated with PNH. It is unclear how the PNH stem cell and its progeny achieve clonal dominance in the setting of aplastic anemia, despite the fact that PNH cells are more vulnerable to complement-mediated destruction; however, it may relate to relative resistance to the autoimmune attack due to intrinsic mutations (Brodsky et al., 1996b; Inoue et al., 2006). Specifically, it has be suggested that PNH cells may be relatively resistant to an autoimmune attack, because they are deficient in GPI-anchored ULBPs that serve as ligands for the NKG2D receptor found on natural killer cells and T cells (Hanaoka et al., 2006; Savage et al., 2009). Alternatively, it has been proposed that “second hit” mutations may also give the PNH clone a growth advantage (Inoue et al., 2006; Babushok et al., 2017). MDS also commonly arises in aplastic anemia patients treated with immunosuppressive therapy. In a retrospective review of children with SAA, 11 of 86 patients who received immunosuppressive therapy developed MDS (Ohara et al., 1997). Up to 15% of adult patients with aplastic anemia will also develop MDS following immunosuppressive therapy with monosomy 7 being the most common chromosomal abnormality (Rosenfeld et al., 2003).
CONCLUDING REMARKS—FUTURE PROSPECTS Aplastic anemia was originally thought to be due to a defect in hematopoietic stem cells or their microenvironment. It is now clear that most cases of acquired aplastic anemia are caused by autoreactive lymphocytes that target bone marrow stem/progenitor cells. With modern therapies, the 5-year survival rate for SAA exceeds 85%. BMT offers the best chance for cure but is not available to all patients. Immunosuppressive therapy remains the standard of care for patients who are not suitable candidates for BMT. Remissions are achieved in up to 75% of patients, but the high rate of relapse and secondary clonal diseases limits the efficacy of immunosuppressive therapy. Even complete responds may relapse or develop MDS 5 10 years after immunosuppressive therapy.
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Currently, the advances in mitigating graft failure and GVHD in the setting of alternative donor BMT appear to be outpacing the development of more effective IST therapies for SAA. In the coming years, there is likely to be great use of unrelated and HLA-mismatched BMT to treat SAA, especially in patients who don’t respond or relapse after immunosuppressive therapy. The development of posttransplant cyclophosphamide (CY) to expand the donor pool and mitigate GVHD appears promising (DeZern et al., 2017).
References Ades, L., Mary, J.Y., Robin, M., et al., 2004. Long-term outcome after bone marrow transplantation for severe aplastic anemia. Blood 103 (7), 2490 2497. Babushok, D.V., Stanley, N., Xie, H.M., Huang, H., Bagg, A., Olson, T.S., et al., 2017. Clonal replacement underlies spontaneous remission in paroxysmal nocturnal haemoglobinuria. Br. J. Haematol. 176 (3), 487 490. Bacigalupo, A., 1999. Bone marrow transplantation for severe aplastic anemia from HLA identical siblings. Haematologica 84 (1), 2 4. Bacigalupo, A., 2017. How I treat acquired aplastic anemia. Blood 129 (11), 1428 1436. Bacigalupo, A., Brand, R., Oneto, R., et al., 2000. Treatment of acquired severe aplastic anemia: bone marrow transplantation compared with immunosuppressive therapy—The European Group for Blood and Marrow Transplantation experience. Semin. Hematol. 37 (1), 69 80. Bacigalupo, A., Socie’, G., Lanino, E., et al., 2010. Fludarabine, cyclophosphamide, antithymocyte globulin, with or without low dose total body irradiation, for alternative donor transplants, in acquired severe aplastic anemia: a retrospective study from the EBMT-SAA working party. Haematologica 95 (6), 976 982. Bagby Jr., G.C., 2003. Genetic basis of Fanconi anemia. Curr. Opin. Hematol. 10 (1), 68 76. Borowitz, M.J., Craig, F.E., DiGiuseppe, J.A., et al., 2010. Guidelines for the diagnosis and monitoring of paroxysmal nocturnal hemoglobinuria and related disorders by flow cytometry. Cytometry B Clin. Cytom. 78 (4), 211 230. Brodsky, R.A., 2014. Paroxysmal nocturnal hemoglobinuria. Blood 124 (18), 2804 2811. Brodsky, R.A., Jones, R.J., 2005. Aplastic anaemia. Lancet 365 (9471), 1647 1656. Brodsky, R.A., Sensenbrenner, L.L., Jones, R.J., 1996a. Complete remission in severe aplastic anemia after high-dose cyclophosphamide without bone marrow transplantation. Blood 87 (2), 491 494. Brodsky, R.A., Vala, M.S., Barber, J.P., Medof, M.E., Jones, R.J., 1996b. Resistance to apoptosis caused by PIG-A gene mutations in paroxysmal nocturnal hemoglobinuria (PNH). Blood 88 (10), 140a. Brodsky, R.A., Mukhina, G.L., Li, S., et al., 2000. Improved detection and characterization of paroxysmal nocturnal hemoglobinuria using fluorescent aerolysin. Am. J. Clin. Pathol. 114 (3), 459 466. Brodsky, R.A., Chen, A.R., Dorr, D., Fuchs, E.J., Huff, C.A., Luznik, L., et al., 2010. High-dose cyclophosphamide for severe aplastic anemia: long-term follow-up. Blood 115 (11), 2136 2141. Brown, K.E., Tisdale, J., Barrett, A.J., Dunbar, C.E., Young, N.S., 1997. Hepatitis-associated aplastic anemia [see comments]. N. Engl. J. Med. 336 (15), 1059 1064. Camitta, B.M., Thomas, E.D., NATHAN, D.G., et al., 1979. A prospective study of androgens and bone marrow transplantation for treatment of severe aplastic anemia. Blood 53, 504 514. Cattral, M.S., Langnas, A.N., Markin, R.S., et al., 1994. Aplastic anemia after liver transplantation for fulminant liver failure. Hepatology 20 (4), 813 818. Champlin, R.E., Feig, S.A., Sparkes, R.S., Galen, R.P., 1984. Bone marrow transplantation from identical twins in the treatment of aplastic anaemia: implication for the pathogenesis of the disease. Br. J. Haematol. 56 (3), 455 463. Chen, J., 2005. Animal models for acquired bone marrow failure syndromes. Clin. Med. Res. 3 (2), 102 108. Chen, J., Lipovsky, K., Ellison, F.M., Calado, R.T., Young, N.S., 2004. Bystander destruction of hematopoietic progenitor and stem cells in a mouse model of infusion-induced bone marrow failure. Blood 104 (6), 1671 1678. DeZern, A.E., Zahurak, M., Symons, H., Cooke, K., Jones, R.J., Brodsky, R.A., 2017. Alternative donor transplantation with high-dose posttransplantation cyclophosphamide for refractory severe aplastic anemia. Biol. Blood Marrow Transplant. 23 (3), 498 504. Desmond, R., Townsley, D.M., Dumitriu, B., et al., 2014. Eltrombopag restores trilineage hematopoiesis in refractory severe aplastic anemia that can be sustained on discontinuation of drug. Blood 123 (12), 1818 1825. Desmond, R., Townsley, D.M., Dunbar, C., Young, N.S., 2015. Eltrombopag in aplastic anemia. Semin. Hematol. 52 (1), 31 37. Dezern, A.E., Symons, H.J., Resar, L.S., Borowitz, M.J., Armanios, M.Y., Brodsky, R.A., 2014. Detection of paroxysmal nocturnal hemoglobinuria clones to exclude inherited bone marrow failure syndromes. Eur. J. Haematol. 92, 467 470. Dokal, I., Vulliamy, T., 2003. Dyskeratosis congenita: its link to telomerase and aplastic anaemia. Blood Rev. 17 (4), 217 225. Ehrlich, P., 1888. Ueber einem Fall von Anamie mit Bemer-kungen uber regenerative Veranderungen des Knochenmarks. Charite-Annalen 13, 301 309. Emadi, A., Jones, R.J., Brodsky, R.A., 2009. Cyclophosphamide and cancer: golden anniversary. Nat. Rev. Clin. Oncol. 6 (11), 638 647. Feng, X., Chuhjo, T., Sugimori, C., et al., 2004. Diazepam-binding inhibitor-related protein 1: a candidate autoantigen in acquired aplastic anemia patients harboring a minor population of paroxysmal nocturnal hemoglobinuria-type cells. Blood 104 (8), 2425 2431. Frickhofen, N., Kaltwasser, J.P., Schrezenmeier, H., et al., 1991. Treatment of aplastic anemia with antilymphocyte globulin and methylprednisolone with or without cyclosporine. N. Engl. J. Med. 324, 1297 1304. Gamper, C.J., Takemoto, C.M., Chen, A.R., et al., 2016. High-dose cyclophosphamide is effective therapy for pediatric severe aplastic anemia. J. Pediatr. Hematol. Oncol. 38 (8), 627 635. Gmur, J., Von Felten, A., Rhyner, K., Frick, P.G., 1979. Autologous hematologic recovery from aplastic anemia following high dose cyclophosphamide and HLA-matched allogeneic bone marrow transplantation. Acta Haematol. 62 (1), 20 24. Hanaoka, N., Kawaguchi, T., Horikawa, K., Nagakura, S., Mitsuya, H., Nakakuma, H., 2006. Immunoselection by natural killer cells of PIGA mutant cells missing stress-inducible ULBP. Blood 107 (3), 1184 1191.
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Hilton, J., 1984. Role of aldehyde dehydrogenase in cyclophosphamide-resistant L1210 leukemia. Cancer Res. 44 (11), 5156 5160. Hinterberger, W., Rowlings, P.A., Hinterberger-Fischer, M., et al., 1997. Results of transplanting bone marrow from genetically identical twins into patients with aplastic anemia [see comments]. Ann. Intern. Med. 126 (2), 116 122. Hirano, N., Kojima, S., von Bergwelt-Baildon, M.S., et al., 2000. Distinct autoantigens in American and Japanese immune-mediated aplastic anemia. Blood 96, 8a. Hoffman, R., Zanjani, E.D., Lutton, J.D., Zalusky, R., Wasserman, L.R., 1977. Suppression of erythroid-colony formation by lymphocytes from patients with aplastic anemia. N. Engl. J. Med. 296 (1), 10 13. Horowitz, M.M., 2000. Current status of allogeneic bone marrow transplantation in acquired aplastic anemia. Semin. Hematol. 37 (1), 30 42. Inoue, N., Izui-Sarumaru, T., Murakami, Y., et al., 2006. Molecular basis of clonal expansion of hematopoiesis in 2 patients with paroxysmal nocturnal hemoglobinuria (PNH). Blood 108 (13), 4232 4236. Inoue, N., Izui-Sarumaru, T., Murakami, Y., et al., 2006. Molecular basis of clonal expansion of hematopoiesis in two patients with paroxysmal nocturnal hemoglobinuria (PNH). Blood 108, 4232 4236. Issaragrisil, S., Kaufman, D.W., Anderson, T., et al., 1997. Low drug attributability of aplastic anemia in Thailand. Blood 89 (11), 4034 4039. Issaragrisil, S., Chansung, K., Kaufman, D.W., Sirijirachai, J., Thamprasit, T., Young, N.S., 1997. Aplastic anemia in rural Thailand: its association with grain farming and agricultural pesticide exposure. Aplastic Anemia Study Group. Am. J. Public Health 87 (9), 1551 1554. Issaragrisil, S., Leaverton, P.E., Chansung, K., et al., 1999. Regional patterns in the incidence of aplastic anemia in Thailand. The Aplastic Anemia Study Group. Am. J. Hematol. 61 (3), 164 168. Jaime-Prez, J.C., Gonzlez-Llano, O., Gmez-Almaguer, D., 2001. High-dose cyclophosphamide in the treatment of severe aplastic anemia in children [6]. Am. J. Hematol. 66 (1), 71. Jones, R.J., Barber, J.P., Vala, M.S., Collector, M.I., Kaufmann, S.H., Ludeman, S.M., et al., 1995. Assessment of aldehyde dehydrogenase in viable cells. Blood 85 (10), 2742 2746. Kahl, C., Leisenring, W., Deeg, H.J., et al., 2005. Cyclophosphamide and antithymocyte globulin as a conditioning regimen for allogeneic marrow transplantation in patients with aplastic anaemia: a long-term follow-up. Br. J. Haematol. 130 (5), 747 751. Kaufman, D.W., Kelly, J.P., Levy, M., Shapiro, S., 1991. The Drug Etiology of Agranulocytosis and Aplastic Anemia (first ed.). Oxford University Press, Inc, New York. Locasciulli, A., Bacigalupo, A., Bruno, B., et al., 2010. Hepatitis-associated aplastic anaemia: epidemiology and treatment results obtained in Europe. A report of The EBMT aplastic anaemia working party. Br. J. Haematol. 149 (6), 890 895. Luznik, L., O’Donnell, P.V., Symons, H.J., et al., 2008. HLA-haploidentical bone marrow transplantation for hematologic malignancies using nonmyeloablative conditioning and high-dose, posttransplantation cyclophosphamide. Biol. Blood Marrow Transplant. 14 (6), 641 650. Maciejewski, J.P., Selleri, C., Sato, T., Anderson, S., Young, N.S., 1995. Increased expression of Fas antigen on bone marrow CD34 1 cells of patients with aplastic anaemia. Br. J. Haematol 91 (1), 245 252. Manz, C.Y., Dietrich, P.Y., Schnuriger, V., Nissen, C., Wodnar-Filipowicz, A., 1997. T-cell receptor beta chain variability in bone marrow and peripheral blood in severe acquired aplastic anemia. Blood Cells Mol. Dis. 23 (1), 110 122. Marsh, J., Schrezenmeier, H., Marin, P., et al., 1999. Prospective randomized multicenter study comparing cyclosporin alone versus the combination of antithymocyte globulin and cyclosporin for treatment of patients with nonsevere aplastic anemia: a report from the European Blood and Marrow Transplant (EBMT) Severe Aplastic Anaemia Working Party. Blood 93 (7), 2191 2195. Marsh, J.C., Kulasekararaj, A.G., 2013. Management of the refractory aplastic anemia patient: what are the options? Blood 122, 3561 3567. Mathe, G., Amiel, J.L., Schwarzenberg, L., et al., 1970. Bone marrow graft in man after conditioning by antilymphocytic serum. Br. Med. J. 2, 131 136. Matsui, W.H., Brodsky, R.A., Smith, B.D., Borowitz, M.J., Jones, R.J., 2006. Quantitative analysis of bone marrow CD34 cells in aplastic anemia and hypoplastic myelodysplastic syndromes. Leukemia 20, 458 462. Melenhorst, J.J., Fibbe, W.E., Struyk, L., van der Elsen, P.J., Willemze, R., Landegent, J.E., 1997. Analysis of T-cell clonality in bone marrow of patients with acquired aplastic anaemia. Br. J. Haematol. 96 (1), 85 91. Modan, B., Segal, S., Shani, M., Sheba, C., 1975. Aplastic anemia in Israel: evaluation of the etiological role of chloramphenicol on a community-wide basis. Am. J. Med. Sci. 270 (3), 441 445. Mukhina, G.L., Buckley, J.T., Barber, J.P., Jones, R.J., Brodsky, R.A., 2001. Multilineage glycosylphosphatidylinositol anchor deficient hematopoiesis in untreated aplastic anemia. Br. J. Haematol. 115, 476 482. Nakao, S., 1997. Immune mechanism of aplastic anemia. [Review] [26 refs]. Int. J. Hematol. 66 (2), 127 134. Nakao, S., Takami, A., Takamatsu, H., et al., 1997. Isolation of a T-cell clone showing HLA-DRBI 0405-restricted cytotoxicity for hematopoietic cells in a patient with aplastic anemia. Blood 89 (10), 3691 3699. Nimer, S.D., Ireland, P., Meshkinpour, A., Frane, M., 1994. An increased HLA DR2 frequency is seen in aplastic anemia patients. Blood 84 (3), 923 927. Nissen, C., Cornu, P., Gratwohl, A., Speck, B., 1980. Peripheral blood cells from patients with aplastic anaemia in partial remission suppress growth of their own bone marrow precursors in culture. Br. J. Haematol. 45 (2), 233 243. Nistico, A., Young, N.S., 1994. gamma-Interferon gene expression in the bone marrow of patients with aplastic anemia. Ann. Intern. Med. 120 (6), 463 469. Ohara, A., Kojima, S., Hamajima, N., et al., 1997. Myelodysplastic syndrome and acute myelogenous leukemia as a late clonal complication in children with acquired aplastic anemia. Blood 90 (3), 1009 1013. Olnes, M.J., Scheinberg, P., Calvo, K.R., et al., 2012. Eltrombopag and improved hematopoiesis in refractory aplastic anemia. N. Engl. J. Med. 367 (1), 11 19. de Planque, M.M., Bacigalupo, A., Wursch, A., et al., 1989. Long-term follow-up of severe aplastic anaemia patients treated with antithymocyte globulin. Br. J. Haematol. 73, 121 126. Piccin, A., McCann, S., Socie´, G., Oneto, R., Bacigalupo, A., Locasciulli, A., et al., 2010. Survival of patients with documented autologous recovery after SCT for severe aplastic anemia: a study by the WPSAA of the EBMT. Bone Marrow Transplant. 45 (6), 1008 1013.
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Pu, J.J., Mukhina, G., Wang, H., Savage, W.J., Brodsky, R.A., 2011. Natural history of paroxysmal nocturnal hemoglobinuria clones in patients presenting as aplastic anemia. Eur. J. Haematol. 87 (1), 37 45. Rosenfeld, S., Follmann, D., Nunez, O., Young, N.S., 2003. Antithymocyte globulin and cyclosporine for severe aplastic anemia: association between hematologic response and long-term outcome. JAMA 289 (9), 1130 1135. Savage, W.J., DeRusso, P.A., Resar, L.M., et al., 2007. Treatment of hepatitis-associated aplastic anemia with high-dose cyclophosphamide. Pediatr. Blood Cancer 49 (7), 947 951. Savage, W.J., Barber, J.P., Mukhina, G.L., Hu, R., Chen, G., Matsui, W., et al., 2009. Glycosylphosphatidylinositol-anchored protein deficiency confers resistance to apoptosis in PNH. Exp. Hematol. 37 (1), 42 51. e41. Scheinberg, P., Nunez, O., Young, N.S., 2006. Retreatment with rabbit anti-thymocyte globulin and ciclosporin for patients with relapsed or refractory severe aplastic anaemia. Br. J. Haematol. 133 (6), 622 627. Scheinberg, P., Nunez, O., Wu, C., Young, N.S., 2006. Treatment of severe aplastic anaemia with combined immunosuppression: antithymocyte globulin, ciclosporin and mycophenolate mofetil. Br. J. Haematol. 133 (6), 606 611. Scheinberg, P., Nunez, O., Weinstein, B., et al., 2011. Horse versus rabbit antithymocyte globulin in acquired aplastic anemia. N. Engl. J. Med. 365 (5), 430 438. Scheinberg, P., Nunez, O., Weinstein, B., Scheinberg, P., Wu, C.O., Young, N.S., 2012. Activity of alemtuzumab monotherapy in treatmentnaive, relapsed, and refractory severe acquired aplastic anemia. Blood 119 (2), 345 354. Selleri, C., Anderson, S., Young, N.S., Maciejewski, J.P., 1994. Interferon-gamma and tumor necrosis factor-α suppress early and late stages of hematopoiesis in vitro and induce programmed cell death. Blood 84, 215a. Sensenbrenner, L.L., Steele, A.A., Santos, G.W., 1977. Recovery of hematologic competence without engraftment following attempted bone marrow transplantation for aplastic anemia: Report of a case with diffusion chamber studies. Exp. Hematol. 5 (1), 51 58. Socie, G., Henry-Amar, M., Bacigalupo, A., et al., 1993. Malignant tumors occurring after treatment of aplastic anemia. N. Engl. J. Med. 329, 1152 1157. Storb, R., Leisenring, W., Anasetti, C., et al., 1997. Long-term follow-up of allogeneic marrow transplants in patients with aplastic anemia conditioned by cyclophosphamide combined with antithymocyte globulin. Blood 89 (10), 3890 3891. Storb, R., Blume, K.G., O’Donnell, M.R., et al., 2001. Cyclophosphamide and antithymocyte globulin to condition patients with aplastic anemia for allogeneic marrow transplantations: the experience in four centers. Biol. Blood Marrow Transplant. 7 (1), 39 44. Szklo, M., Sensenbrenner, L., Markowitz, J., Weida, S., Warm, S., Linet, M., 1985. Incidence of aplastic anemia in metropolitan Baltimore: a population-based study. Blood 66 (1), 115 119. Thomas, E.D., Storb, R., Fefer, A., Slichter, S.J., Bryant, J.I., Buckner, C.D., et al., 1972. Aplastic anaemia treated by marrow transplantation. Lancet 1 (7745), 284 289. Thomas, E.D., Storb, R., Giblett, E.R., Longpre, B., Weiden, P.L., Fefer, A., et al., 1976. Recovery from aplastic anemia following attempted marrow transplantation. Exp. Hematol. 4 (2), 97 102. Tichelli, A., Gratwohl, A., Wursch, A., Nissen, C., Speck, B., 1988. Late haematological complications in severe aplastic anaemia. Br. J. Haematol. 69, 413 418. Tisdale, J.F., Dunn, D.E., Geller, N., Plante, M., Nunez, O., Dunbar, C.E., et al., 2000. High-dose cyclophosphamide in severe aplastic anaemia: a randomised trial. Lancet 356 (9241), 1554 1559. Tsangaris, E., Klaassen, R., Fernandez, C.V., Yanofsky, R., Shereck, E., Champagne, J., et al., 2011. Genetic analysis of inherited bone marrow failure syndromes from one prospective, comprehensive and population-based cohort and identification of novel mutations. J. Med. Genet. 48 (9), 618 628. Tzakis, A.G., Arditi, M., Whittington, P.F., et al., 1988. Aplastic anemia complicating orthotopic liver transplantation for non-A, non-B hepatitis. N. Engl. J. Med. 319 (7), 393 396. Zeng, W., Maciejewski, J.P., Chen, G., Young, N.S., 2001. Limited heterogeneity of T cell receptor BV usage in aplastic anemia. J. Clin. Invest. 108 (5), 765 773. Zoumbos, N.C., Gasco´n, P., Djeu, J.Y., Trost, S.R., Young, N.S., 1985. Circulating activated suppressor T lymphocytes in aplastic anemia. N. Engl. J. Med. 312, 257 265.
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49 Acquired Aplastic Anemia Robert A. Brodsky and Richard J. Jones Division of Hematology, Department of Medicine and The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Baltimore, MD, United States
O U T L I N E Historic Background
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Immunosuppressive Therapy
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Genetic Features
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Eltrombopag
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Clinical, Pathologic, and Epidemiologic Features
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High-Dose Cyclophosphamide Without Bone Marrow Transplantation
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Human Leukocyte Antigen Haploidentical Bone Marrow Transplant With Posttransplant Cyclophosphamide
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Aplastic Anemia and Clonality
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Concluding Remarks—Future Prospects
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References
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Autoimmune Features and Pathogenic Mechanisms
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Environmental Features
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Animal Models
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Therapy for Aplastic Anemia Bone Marrow Transplantation Bone Marrow Transplantation From Unrelated Donors
926 926 927
Aplastic anemia manifests with pancytopenia and a hypocellular bone marrow (Brodsky and Jones, 2005). The disease may be acquired or inherited. Most cases of acquired aplastic anemia result from autoimmune destruction of hematopoietic stem/progenitors and respond to immunosuppressive therapies. The inherited forms of aplastic anemia are less common and usually present within the first decade of life (Tsangaris et al., 2011). The inherited bone marrow failure may be due to a variety of genetic mutations such as DNA repair defects (Fanconi anemia), telomerase defects [dyskeratosis congenita (DKC)], ribosomopathies (Shwachman Diamond syndrome), or cMPL mutations (amegakaryocytic thrombocytopenia). Immunosuppressive therapy is not helpful for the most inherited forms of bone marrow failure. This chapter will predominantly focus on acquired aplastic anemia; however, it is important to be aware of these less common inherited forms of aplastic anemia since they can sometimes be hard to distinguish from the acquired form of the disease.
HISTORIC BACKGROUND The earliest case description of aplastic anemia in 1888 was by Dr. Paul Ehrlich (Ehrlich, 1888). He described a young woman who died following an abrupt illness characterized by severe anemia, bleeding, high fever, and a markedly hypocellular bone marrow. Until the early 1970s, most patients with severe aplastic anemia (SAA) died within a year of diagnosis. The advent of allogeneic (allo) bone marrow transplantation (BMT) and The Autoimmune Diseases, 6th. DOI: https://doi.org/10.1016/B978-0-12-812102-3.00049-X
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Copyright © 2020 Elsevier Inc. All rights reserved.
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immunosuppressive therapy markedly improved the outcome for these patients and prompted vigorous clinical and laboratory investigation. These studies have generated an important insight into hematopoietic stem cell biology, immunology, and autoimmunity. Today the majority of patients will survive this potentially fatal autoimmune disorder.
GENETIC FEATURES Distinctive genetic abnormalities are more common with congenital bone marrow failure syndromes since most acquired aplastic anemia is autoimmune. Congenital aplastic anemia tends to present in the first decade of life and is often, but not always, associated with other physical anomalies. Fanconi anemia, the most common form of congenital bone marrow failure, predisposes to cancer and is frequently associated with other congenital abnormalities (e.g., short stature, upper limb anomalies, hypogonadism, cafe´-au-lait spots, etc.) (Bagby, 2003). DKC is another congenital bone marrow failure disorder that can be either X-linked recessive, autosomal dominant, or autosomal recessive (Dokal and Vulliamy, 2003). The X-linked recessive form results from mutations in a gene known as DKC1 whose gene product, dyskerin is important for stabilizing telomerase. The resulting telomerase deficiency leads to short telomeres, bone marrow failure, and premature aging. The autosomal dominant form of DKC results from human telomerase RNA component (hTERC) gene mutations, the RNA component of telomerase. This chapter will focus on the acquired form of aplastic anemia. Genetic abnormalities are less well characterized in acquired aplastic anemia; however, there appears to be an underlying genetic predisposition to acquired aplastic anemia, as evidenced by the overrepresentation of human leukocyte antigen (HLA) DR2 subtypes (Nimer et al., 1994).
CLINICAL, PATHOLOGIC, AND EPIDEMIOLOGIC FEATURES Aplastic anemia manifests as pancytopenia in conjunction with a hypocellular bone marrow. The disease may present abruptly (over days) or insidiously, over weeks to months. The most common clinical manifestations reflect the low blood counts and include dyspnea on exertion, fatigue, easy bruising, petechia, epistaxis, gingival bleeding, heavy menses, headaches, and fever. A complete blood count, leukocyte differential, reticulocyte count, and a bone marrow aspirate and biopsy are essential for diagnosis. Peripheral blood flow cytometry to detect glycosylphosphatidylinositol (GPI) anchor deficient blood cells (Brodsky et al., 2000; Borowitz et al., 2010) as well as cytogenetics and fluorescent in situ hybridization (FISH) analysis should be performed on the bone marrow aspirate. Up to 70% of patients with acquire aplastic anemia have a detectable paroxysmal nocturnal hemoglobinuria (PNH) clone which essentially rules out inherited forms of aplastic anemia (Dezern et al., 2014). Cytogenetic or FISH abnormalities are suggestive of a hypoplastic form of myelodysplasia. Patients under the age of 40 years old should be screened for Fanconi anemia using the clastogenic agents diepoxybutane or mitomycin C, and telomere lengths should be obtained on patients with a family history of bone marrow failure, premature graying, pulmonary fibrosis, or other stigmata of DKC. A hypocellular bone marrow is required for the diagnosis of aplastic anemia. However, some patients will have residual pockets of ongoing hematopoiesis; thus an adequate biopsy (1 2 cm in length) is essential for establishing the diagnosis. Dyserythropoiesis is not uncommon in aplastic anemia, especially in cases with coincidental small-to-moderate PNH populations; however, a small percentage of myeloid blasts, or dysplastic features in the myeloid or megakaryocyte lineages, is more typical of hypoplastic myelodysplastic syndromes (MDS). CD34 is expressed on early hematopoietic progenitors and the number of CD341 cells has also been used to help discriminate between aplastic anemia and hypoplastic myelodysplastic syndrome (hMDS). In aplastic anemia the percentage of cells expressing CD34 is usually less than 0.1%; in hMDS the CD34 count is either normal (0.5% 1.0%) or elevated (Matsui et al., 2006). As with other autoimmune diseases, there is a wide spectrum of disease severity in aplastic anemia. The prognosis in aplastic anemia is proportional to degree of peripheral blood cytopenias. Accordingly, aplastic anemia is classified as nonsevere, severe, and very severe based largely upon the degree of neutropenia (Table 49.1). SAA is defined as bone marrow cellularity of less than 25% and markedly decreased values of at least two of three hematopoietic lineages (neutrophil count ,500/μL, platelet count ,20,000/μL, and absolute reticulocyte count of ,60,000/μL). Very SAA satisfies the above criteria except the neutrophil count is ,200/μL, while non-SAA is characterized by a hypocellular bone marrow but with cytopenias that do not meet the criteria for severe disease.
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AUTOIMMUNE FEATURES AND PATHOGENIC MECHANISMS
TABLE 49.1 Peripheral blood counts
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Aplastic Anemia: Diagnosis and Definitions Nonsevere aplastic anemia (not meeting criteria for severe disease)
Severe aplastic anemia (any 2 of 3)
Very-severe aplastic anemia (meets criteria for severe disease and absolute neutrophils ,200)
absolute neutrophils
, 500/μL
, 200/μL
platelets
, 20,000/μL
reticulocyte count
, 1.0% corrected or ,60,000/μL
Bone marrow cellularity ,25%.
The 2-year mortality rate with supportive care alone for patients with SAA exceeds 50% (Camitta et al., 1979), with invasive fungal infections and overwhelming bacterial sepsis being the most frequent causes of death. Non-SAA is seldom life-threatening and in many instances requires no therapy. Although some cases of nonSAA will progress, many will remain stable for years, and some may spontaneously improve. Aplastic anemia has been associated with drugs, benzene exposure, insecticides, viruses, and other agents. However, over 80% of cases are classified as idiopathic. The disease most commonly affects children and young adults but may occur at any age. Precise estimates of the incidence of aplastic anemia are difficulty due the rarity of the disease and imprecision in establishing the diagnosis. The best estimates of incidence are case control studies that report an incidence of two cases/million inhabitants in Europe (Kaufman et al., 1991) and Israel (Modan et al., 1975), but the incidence may be two to threefold higher in Southeast Asia (Issaragrisil et al., 1997; Szklo et al., 1985). A population-based case control study of aplastic anemia in Thailand found that drugs, the most commonly implicated etiology, explain only 5% of newly diagnosed cases (Issaragrisil et al., 1997). An intriguing association exists between seronegative hepatitis and aplastic anemia. The hepatitis-aplastic anemia syndrome accounts for 3% 5% of newly diagnosed cases of aplastic anemia. The disease predominantly affects young males, with a precipitous onset of severe pancytopenia occurring within 2 3 months after the onset of hepatitis (Brown et al., 1997). Moreover, aplastic anemia has been reported to occur in up to 30% of patients following orthotopic liver transplantation for seronegative hepatitis (Tzakis et al., 1988; Cattral et al., 1994). The aplastic anemia in the hepatitis/aplastic anemia syndrome is also thought to be autoimmune since most cases respond to immunosuppressive therapy (Savage et al., 2007; Locasciulli et al., 2010).
AUTOIMMUNE FEATURES AND PATHOGENIC MECHANISMS Aplastic anemia was originally thought to result from a quantitative deficiency of hematopoietic stem cells precipitated by a direct toxic effect on stem cells. However, the attempts to treat aplastic anemia by simple transfusion of bone marrow from an identical twin failed to reconstitute hematopoiesis in most patients. Retransplant of many of these patients following a high-dose cyclophosphamide preparative regimen was successful, suggesting that the pathophysiology of aplastic anemia was more complicated (Champlin et al., 1984; Hinterberger et al., 1997). In the late 1960s Mathe et al. (1970) were among the first to postulate an immune basis for aplastic anemia. They performed BMT in patients with aplastic anemia using partially mismatched donors after administering antilymphocyte globulin as an immunosuppressive conditioning regimen. Although the patients failed to engraft, the investigators witnessed autologous recovery of hematopoiesis in some patients. This suggested that functional hematopoietic stem cells exist in aplastic anemia patients and that the immune system was somehow suppressing the growth and differentiation of hematopoietic stem cells. The response to immunosuppressive therapy was the first clear evidence that aplastic anemia was truly an autoimmune disease. The first laboratory experiments implicating an autoimmune pathophysiology were coculture experiments showing that T lymphocytes from aplastic anemia patients inhibited hematopoietic colony formation in vitro (Hoffman et al., 1977; Nissen et al., 1980). Since then, it has been shown that the immune destruction of hematopoietic stem cells in aplastic anemia is mediated by cytotoxic T cells and involves inhibitory Th1 cytokines and the Fas-dependent cell death pathway. The cytotoxic T cells are usually more conspicuous in the bone marrow than in the peripheral blood (Zoumbos et al., 1985; Selleri et al., 1994; Melenhorst et al., 1997) and overproduce interferon-γ and tumor necrosis factor (TNF) (Nakao, 1997; Nistico and Young, 1994). TNF and interferon-γ are
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direct inhibitors of hematopoiesis and appear to upregulate Fas expression on CD341 cells (Maciejewski et al., 1995). Immortalized CD41 and CD81 T-cell clones from some aplastic anemia patients have been shown to secrete Th1 cytokines and are capable of lysing autologous CD34 cells (Nakao et al., 1997; Zeng et al., 2001). Evidence for a humoral autoimmune response in aplastic anemia has also been reported (Hirano et al., 2000; Feng et al., 2004). Studies examining T-cell diversity using complementarity-determining region (CDR3) spectratyping have further implicated the role of the immune system in aplastic anemia. Several groups have now found limited heterogeneity of the T-cell receptor β-chain (BV) in aplastic anemia, suggesting that there is oligoclonal or even clonal expansion of T cells in response to a specific antigen (Melenhorst et al., 1997; Zeng et al., 2001; Manz et al., 1997).
ENVIRONMENTAL FEATURES The medical literature is replete with reports of environmental exposures, most notably benzene and radiation, causing aplastic anemia. However, rigorous epidemiologic studies supporting an association between environmental toxins and aplastic anemia are lacking. A major confounder is that benzene, radiation, and other toxins also predispose to MDS and leukemia. Older literature was unlikely to have been able to distinguish different types of marrow failure, such as aplastic anemia, MDS, and hypoplastic leukemia, leading to an overestimation of the association between benzene and aplastic anemia. While the magnitude of the risk remains uncertain, benzene is probably not a major risk factor for aplastic anemia in countries with modern standards of industrial hygiene. A large case control study in Thailand employing modern diagnostic and epidemiologic methods found that individuals of lower economic status and younger age are at greater risk than their counterparts in other countries following exposure to solvents, glues, and hepatitis A (likely a surrogate marker). Grain farmers were also found to have a higher risk of developing aplastic anemia (relative risk 5 2.7) regardless of whether they use insecticides (Issaragrisil et al., 1997). These same investigators noted marked differences in the incidence between northern and southern rural regions of Thailand and among Bangkok suburbs implicating potential environmental factors in causing the disease (Issaragrisil et al., 1999).
ANIMAL MODELS Animal models of bone marrow failure exist, but none of these models fully replicate the human disease acquired aplastic anemia (Chen, 2005). Busulfan, benzene, and irradiation have all been used to establish the models of marrow failure. All three of these agents lead to pancytopenia, and a hypocellular marrow but the marrow failure is due to stem cell injury and damage to the microenvironment rather than autoimmunemediated suppression of hematopoiesis. More recently, infusion of lymphocytes from congenic mice was used to model immune-mediated marrow failure; this model induces a hypocellular bone marrow and severe pancytopenia but is not truly autoimmune (Chen et al., 2004).
THERAPY FOR APLASTIC ANEMIA Definitive therapy for aplastic anemia includes BMT or immunosuppressive therapy. A variety of immunosuppressive agents have been studied, but antithymocyte globulin (ATG) and cyclosporine A (CSA) are the most commonly employed (Bacigalupo, 2017). Supportive care with blood transfusions and antibiotics is commonly required. Administration of hematopoietic growth factors has not been shown to improve survival in aplastic anemia.
Bone Marrow Transplantation BMT is the treatment of choice for young patients who have an HLA-matched sibling donor. Cyclophosphamide (50 mg/kg/day 3 4 days) with or without ATG is most commonly used for conditioning before BMT. This regimen is nonmyeloablative; however, the immunosuppression is sufficient to allow engraftment in most cases (Storb et al., 1997; Kahl et al., 2005). Alternative regimens using fludarabine,
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FIGURE 49.1 Survival after allogeneic bone marrow transplantation for severe aplastic anemia. Data for HLA-identical siblings and related matched/mismatched transplants is from the IBMTR. Data for unrelated donor transplants is from the EBMT registry, Fred Hutchinson Cancer Research Center, IBMTR, and the IMUST study group. Survival curves are not adjusted for varying patient, disease, and transplant regimen characteristics. HLA, Human leukocyte antigen; EBMT, European Bone Marrow Transplant; IBMTR, International Bone Marrow Transplant Registry; IMUST, International Marrow Unrelated Search and Transplant Study. Source: Reproduced with permission from the National Bone Marrow Donor Program.
cyclophosphamide, and ATG are increasingly being used (Bacigalupo et al., 2010). Survival rates following matched sibling allo BMT have steadily improved since the 1970s largely because of improved supportive care, improved HLA typing, and better graft-versus-host disease (GVHD) prophylaxis (Bacigalupo, 1999). Late BMTrelated complications such as chronic GVHD occur in up to one-third of patients, with many of these patients requiring long-term therapy for their GVHD (Storb et al., 2001). Patient age and the type of allograft (HLAmatched sibling, unrelated, or mismatched donors) are the most important factors influencing outcome. In patients under 30 years of age the cure rate after HLA-matched sibling BMT ranges from 70% to 90% (Ades et al., 2004; Horowitz, 2000). However, the risk of GVHD steadily increases with age, leading to reduced survival.
Bone Marrow Transplantation From Unrelated Donors BMT from HLA-matched unrelated donors is usually reserved for patients who fail to respond to one or more courses of immunosuppressive therapy. The risk for transplant-related mortality and GVHD is almost twice that of BMT from matched sibling donors (Bacigalupo, 2017). The European Group for Blood and Marrow Transplantation has been conditioning SAA patients for BMT with fludarabine, cyclophosphamide, and ATG 6 total body irradiation. The European Society for Blood and Marrow Transplantation (EBMT) has reported survival rates as high as 75% using this conditioning regimen (Bacigalupo et al., 2010). Survival was the best in children and in patients who undergo BMT within 2 years of diagnosis (Fig. 49.1).
IMMUNOSUPPRESSIVE THERAPY ATG is produced by immunizing animals (horse or rabbit) against human thymocytes and kills human T cells through its cytolytic activity. Both horse (hATG) and rabbit (rATG) are approved for the use in the United States; hATG appears to be superior as first-line treatment (Scheinberg et al., 2011). Cyclosporine A (CSA) suppresses T-cell function by inhibiting the expression of nuclear regulatory proteins. Both single-agent ATG and singleagent CSA can induce remissions in acquired aplastic anemia; however, the combination ATG/CSA leads to a higher response rate and a greater likelihood of achieving transfusion independence (Frickhofen et al., 1991; Marsh et al., 1999). A randomized controlled demonstrated that hATG/CSA is superior to rATG/CSA (Scheinberg et al., 2011). The combination of ATG/CSA leads to 5-year survival rates comparable to BMT, but most of these patients are not cured of their disease. Response rates to hATG/CSA range between 60% and 80%, but in contrast to BMT, most patients do not acquire normal blood counts (Frickhofen et al., 1991; Rosenfeld et al., 2003). Another limitation of this approach is that many patients relapse, become dependent on cyclosporine, or develop secondary clonal disease such as PNH or MDS (Rosenfeld et al., 2003; Bacigalupo et al., 2000; Scheinberg et al., 2006). These late events often lead to substantial morbidity and mortality. The National Institutes of Health treated 122 patients (median age, 35 years) with the combination of ATG/CSA and methylprednisolone over a period of 8 years (Rosenfeld et al., 2003). The response rate was 58% and actuarial survival at 7 years was 55%; 13% of patients died within 3 months of treatment, most from fungal infections (Fig. 49.2). The relapse rate for responders was 40% and 13 patients developed MDS. In an attempt to improve response rate and survival, and to decrease the relapse rate and secondary MDS that occurs after hATG/CSA, the NIH added mycophenolate (1 g twice daily for 18 months) to the standard hATG/CSA
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FIGURE 49.2 (A) Survival probability for 122 patients with severe aplastic anemia following treatment with antithymocyte globulin and cyclosporine. (B) Probability of relapse in 74 patients with aplastic anemia classified as responders at 3 months after treatment with antithymocyte globulin and cyclosporine. (C) Proportion of patients experiencing clonal evolution. Source: Reproduced from Rosenfeld, S., Follmann, D., Nunez, O., Young, N.S., 2003. Antithymocyte globulin and cyclosporine for severe aplastic anemia: association between hematologic response and longterm outcome. JAMA 289 (9), 1130 1135 with permission from JAMA.
regimen. This three drug regimen resulted in a 62% response rate, but 37% of the responders relapsed (most while taking mycophenolate) and 9% progressed to either MDS or leukemia; thus the addition of mycophenolate did not improve response or survival (Scheinberg et al., 2006). Alemtuzumab is a highly immunosuppressive monoclonal antibody that binds to cell surface CD52, which is expressed primarily on B and T cells and monocytes. Alemtuzumab has activity in treating SAA, but response rates in therapy naive patients are less than 30% (Scheinberg et al., 2012).
ELTROMBOPAG Eltrombopag is a small molecule agonist of the c-mpl (TpoR) receptor, which is the physiological target of the hormone thrombopoietin, and is the only new drug approved for the treatment of SAA in the past 30 years. Eltrombopag may be used in an attempt to improve the cytopenias in patients at the refractory state (Desmond et al., 2015). Up to 20% of patients with refractory SAA become transfusion-independent within 3 months; however, similar to immunosuppressive therapy, there is a relatively high likelihood of relapse and secondary clonal disease (Desmond et al., 2014; Marsh and Kulasekararaj, 2013; Olnes et al., 2012). Eltrombopag is now
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being studied as an adjuvant to ATG/CSA in newly diagnosed patients in the hopes that it will improve response rates and prevent relapse.
HIGH-DOSE CYCLOPHOSPHAMIDE WITHOUT BONE MARROW TRANSPLANTATION The first successful human allo BMT, reported in 1972 by Thomas et al. (1972) in a patient with aplastic anemia, employed high-dose cyclophosphamide, and this remains (often in conjunction with ATG) the most commonly employed conditioning regimen for aplastic anemia (Storb et al., 2001). Complete reconstitution of autologous hematopoiesis occurs in 10% 15% of patients undergoing allo BMT for aplastic anemia (Thomas et al., 1976; Sensenbrenner et al., 1977; Gmur et al., 1979). The EBMT reported that 10% of SAA patients experience autologous reconstitution following BMT using a cyclophosphamide 1 ATG conditioning regimen. Interestingly, 10-year survival (84%) in patients with autologous recovery was better than in patients who engrafted (74%) (Piccin et al., 2010). The unique pharmacology of cyclophosphamide explains the autologous hematopoietic recovery (Emadi et al., 2009). Cyclophosphamide is a prodrug that is converted to 4-hydroxycyclophosphamide and its tautomer aldophosphamide in the liver. These compounds diffuse into the cell and are converted to the active compound phosphoramide mustard, or they are inactivated by aldehyde dehydrogenase to form the inert carboxyphosphamide. Lymphocytes have low levels of aldehyde dehydrogenase and are rapidly killed by high doses of cyclophosphamide; hematopoietic stem cells possess high levels of aldehyde dehydrogenase and are resistant to cyclophosphamide (Hilton, 1984; Jones et al., 1995). Thus, high-dose cyclophosphamide is highly immunosuppressive, but not myeloablative, allowing endogenous hematopoietic stem cells to reconstitute hematopoiesis. With this background, high-dose cyclophosphamide without BMT was used successfully in aplastic anemia patients who lacked appropriate donor (Brodsky et al., 1996a; Tisdale et al., 2000; Jaime-Prez et al., 2001; Brodsky et al., 2010). The largest and most mature study with high-dose cyclophosphamide is from Johns Hopkins (Brodsky et al., 2010; Gamper et al., 2016). These investigators treated 67 SAA patients with high-dose cyclophosphamide; 44 patients were treatment-naı¨ve and 23 were refractory to one or more previous immunosuppressive regimens. At 10 years, the overall actuarial survival, response rate, and event-free survival were 88%, 71%, and 58%, respectively, for the 44 treatment-naı¨ve patients. Patients with refractory SAA fared less well; at 10 years, the overall actuarial survival, response, and event-free survival rates were 62%, 48%, and 27%, respectively. For the treatment-naı¨ve patients, the median time to a neutrophil count of 0.5 3 109/L was 60 (range, 28 104) days, and the median time to last platelet and red cell transfusion was 117 and 186 days, respectively. Relapse occurred in just two of the treatment-naı¨ve patients, one of whom was retreated with high-dose cyclophosphamide into a second complete remission. Despite the high response rate and low risk of relapse and secondary clonal disease, the duration of deep aplasia (median 60 days to neutrophil recovery) and risk for invasive fungal infections is a major drawback for this approach (Fig. 49.3).
HUMAN LEUKOCYTE ANTIGEN HAPLOIDENTICAL BONE MARROW TRANSPLANT WITH POSTTRANSPLANT CYCLOPHOSPHAMIDE Investigators at Johns Hopkins, in an effort to expand the donor pool for patients in need of a allo bone marrow transplant, have pioneered the use of high-dose cyclophosphamide 3 and 4 days after bone marrow transplant to improve engraftment and decrease the risk of GVHD after HLA-haploidentical bone marrow grafts (Luznik et al., 2008). These authors recently reported on a prospective trial of this approach in 16 consecutive patients with refractory SAA (DeZern et al., 2017). Between July 2011 and August 2016, 16 patients underwent allo BMT for refractory SAA from 13 haploidentical donors and 3 unrelated donors. All 16 patients engrafted and survived with a median follow-up of 21 (range, 3 64) months at the time of publication. Two patients had grade 1 or 2 skin-only acute GVHD. These same two also had mild chronic GVHD of the skin/mouth requiring systemic steroids. One of these GVHD patients was able to come off all immunosuppressive therapy (IST) by 15 months and the other by 17 months. All other patients stopped IST at 1 year (Fig. 49.4).
VII. ORGAN SPECIFIC AUTOIMMUNE DISEASES
FIGURE 49.3 Survival probability following high-dose cyclophosphamide without BMT in 67 patients with SAA. (A) Overall survival for 44 patients with treatment-naı¨ve SAA (topline) and 23 patients with refractory SAA. (B) Failure-free survival after high-dose cyclophosphamide therapy for 44 patients with treatment-naı¨ve SAA (topline) and 23 patients with refractory SAA. BMT, Bone marrow transplantation; SAA, severe aplastic anemia. Source: Reproduced with permission from Blood 2010 (Brodsky, R. A., Chen, A. R., Dorr, D., Fuchs, E. J., Huff, C. A., Luznik, L., et al., (2010). High dose cyclophosphamide for severe aplastic anemia: long-term follow-up. Blood, 115(11), 2136 2141).
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FIGURE 49.4
Model depicting the pathophysiology of bone marrow failure in acquired aplastic anemia. Autoaggressive lymphocytes lyse CD34 1 bone marrow progenitor cells but seem to spare more immature CD34 1 cells known as high-quality stem cells.
APLASTIC ANEMIA AND CLONALITY The survivors of aplastic anemia are at high risk of clonal progression following immunosuppressive therapy (Socie et al., 1993). PNH and MDS are the most common clonal disorders to evolve from aplastic anemia (de Planque et al., 1989; Tichelli et al., 1988). Even before the widespread use of immunosuppressive therapy, 5% of the patients progressed to clonal hematopoiesis. This suggests that the increase in MDS and PNH following immunosuppressive therapy is not caused by the immunosuppression; rather, the increased survival following immunosuppressive therapy may allow time for these underlying clones to expand (Mukhina et al., 2001; Pu et al., 2011). PNH results from the expansion of an abnormal hematopoietic stem cell that harbors a somatic mutation of the X-linked gene, termed phosphatidylinositol glycan class A (PIG-A) (Brodsky, 2014). The product of the PIG-A gene is required for GPI anchor biosynthesis; consequently, PNH cells are deficient in GPI-anchored proteins. Several GPI-anchored proteins (CD59 and CD55) protect cells from complement-mediated destruction, and their absence explains the hemolytic anemia associated with PNH. It is unclear how the PNH stem cell and its progeny achieve clonal dominance in the setting of aplastic anemia, despite the fact that PNH cells are more vulnerable to complement-mediated destruction; however, it may relate to relative resistance to the autoimmune attack due to intrinsic mutations (Brodsky et al., 1996b; Inoue et al., 2006). Specifically, it has be suggested that PNH cells may be relatively resistant to an autoimmune attack, because they are deficient in GPI-anchored ULBPs that serve as ligands for the NKG2D receptor found on natural killer cells and T cells (Hanaoka et al., 2006; Savage et al., 2009). Alternatively, it has been proposed that “second hit” mutations may also give the PNH clone a growth advantage (Inoue et al., 2006; Babushok et al., 2017). MDS also commonly arises in aplastic anemia patients treated with immunosuppressive therapy. In a retrospective review of children with SAA, 11 of 86 patients who received immunosuppressive therapy developed MDS (Ohara et al., 1997). Up to 15% of adult patients with aplastic anemia will also develop MDS following immunosuppressive therapy with monosomy 7 being the most common chromosomal abnormality (Rosenfeld et al., 2003).
CONCLUDING REMARKS—FUTURE PROSPECTS Aplastic anemia was originally thought to be due to a defect in hematopoietic stem cells or their microenvironment. It is now clear that most cases of acquired aplastic anemia are caused by autoreactive lymphocytes that target bone marrow stem/progenitor cells. With modern therapies, the 5-year survival rate for SAA exceeds 85%. BMT offers the best chance for cure but is not available to all patients. Immunosuppressive therapy remains the standard of care for patients who are not suitable candidates for BMT. Remissions are achieved in up to 75% of patients, but the high rate of relapse and secondary clonal diseases limits the efficacy of immunosuppressive therapy. Even complete responds may relapse or develop MDS 5 10 years after immunosuppressive therapy.
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Currently, the advances in mitigating graft failure and GVHD in the setting of alternative donor BMT appear to be outpacing the development of more effective IST therapies for SAA. In the coming years, there is likely to be great use of unrelated and HLA-mismatched BMT to treat SAA, especially in patients who don’t respond or relapse after immunosuppressive therapy. The development of posttransplant cyclophosphamide (CY) to expand the donor pool and mitigate GVHD appears promising (DeZern et al., 2017).
References Ades, L., Mary, J.Y., Robin, M., et al., 2004. Long-term outcome after bone marrow transplantation for severe aplastic anemia. Blood 103 (7), 2490 2497. Babushok, D.V., Stanley, N., Xie, H.M., Huang, H., Bagg, A., Olson, T.S., et al., 2017. Clonal replacement underlies spontaneous remission in paroxysmal nocturnal haemoglobinuria. Br. J. Haematol. 176 (3), 487 490. Bacigalupo, A., 1999. Bone marrow transplantation for severe aplastic anemia from HLA identical siblings. Haematologica 84 (1), 2 4. Bacigalupo, A., 2017. How I treat acquired aplastic anemia. Blood 129 (11), 1428 1436. Bacigalupo, A., Brand, R., Oneto, R., et al., 2000. Treatment of acquired severe aplastic anemia: bone marrow transplantation compared with immunosuppressive therapy—The European Group for Blood and Marrow Transplantation experience. Semin. Hematol. 37 (1), 69 80. Bacigalupo, A., Socie’, G., Lanino, E., et al., 2010. 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