Acute Myeloid Leukemia

Other laboratory studies that may aid in diagnosis include the red cell count, mean corpuscular volume, and red cell distribution width (RDW), particularly when the hematocrit or hemoglobin levels are less than 60% or 20 g/dL, respectively. Only three situations cause microcytic erythrocytosis: β thalassemia trait, hypoxic erythrocytosis, and PV. With β thalassemia trait, the RDW is normal, whereas with hypoxic erythrocytosis and PV, the RDW may be elevated due to associated iron deficiency. Today, however, the assay for JAK2 V617F has superseded other tests for establishing the diagnosis of PV. Of course, in patients with associated acid-peptic disease, occult gastrointestinal bleeding may lead to a presentation with hypochromic, microcytic anemia, masking the presence of PV.

A bone marrow aspirate and biopsy provide no specific diagnostic information because these may be normal or indistinguishable from ET or PMF. Similarly, no specific cytogenetic abnormality is associated with the disease, and the absence of a cytogenetic marker does not exclude the diagnosis.

COMPLICATIONS

Many of the clinical complications of PV relate directly to the increase in blood viscosity associated with red cell mass elevation and indirectly to the increased turnover of red cells, leukocytes, and platelets with the attendant increase in uric acid and cytokine production. The latter appears to be responsible for constitutional symptoms. Peptic ulcer disease can also be due to Helicobacter pylori infection, the incidence of which is increased in PV, while the pruritus associated with this disorder may be a consequence of mast cell activation by JAK2 V617F. A sudden increase in spleen size can be associated with painful splenic infarction. Myelofibrosis appears to be part of the natural history of the disease but is a reactive, reversible process that does not itself impede hematopoiesis and by itself has no prognostic significance. In approximately 15% of patients, however, myelofibrosis is accompanied by significant extramedullary hematopoiesis, hepatosplenomegaly, and transfusion-dependent anemia, which are manifestations of stem cell failure. The organomegaly can cause significant mechanical discomfort, portal hypertension, and progressive cachexia. Although the incidence of acute nonlymphocytic leukemia is increased in PV, the incidence of acute leukemia in patients not exposed to chemotherapy or radiation therapy is low. Interestingly, chemotherapy, including hydroxyurea, has been associated with acute leukemia in JAK2 V617F–negative stem cells in some PV patients. Erythromelalgia is a curious syndrome of unknown etiology associated with thrombocytosis, primarily involving the lower extremities and usually manifested by erythema, warmth, and pain of the affected appendage and occasionally digital infarction. It occurs with a variable frequency and is usually responsive to salicylates. Some of the central nervous system symptoms observed in patients with PV, such as ocular migraine, appear to represent a variant of erythromelalgia.

Left uncontrolled, erythrocytosis can lead to thrombosis involving vital organs such as the liver, heart, brain, or lungs. Patients with massive splenomegaly are particularly prone to thrombotic events because the associated increase in plasma volume masks the true extent of the red cell mass elevation measured by the hematocrit or hemoglobin level. A “normal” hematocrit or hemoglobin level in a PV patient with massive splenomegaly should be considered indicative of an elevated red cell mass until proven otherwise.

PRIMARY MYELOFIBROSIS

Chronic PMF (other designations include idiopathic myelofibrosis, agnogenic myeloid metaplasia, or myelofibrosis with myeloid metaplasia) is a clonal disorder of a multipotent hematopoietic progenitor cell of unknown etiology characterized by marrow fibrosis, extramedullary hematopoiesis, and splenomegaly. PMF is the least common chronic MPN, and establishing this diagnosis in the absence of a specific clonal marker is difficult because myelofibrosis and splenomegaly are also features of both PV and CML. Furthermore, myelofibrosis and splenomegaly also occur in a variety of benign and malignant disorders (Table 131-3), many of which are amenable to specific therapies not effective in PMF. In contrast to the other chronic MPNs and so-called acute or malignant myelofibrosis, which can occur at any age, PMF primarily afflicts men in their sixth decade or later.

ETIOLOGY

The etiology of PMF is unknown. Nonrandom chromosome abnormalities such as 9p, 20q–, 13q–, trisomy 8 or 9, or partial trisomy 1q are common, but no cytogenetic abnormality specific to the disease has been identified. JAK2 V617F is present in approximately 50% of PMF patients, and mutations in the thrombopoietin receptor Mpl occur in about 5%. Most of the rest have mutations in the calreticulin gene (CALR) that alter the carboxy-terminal portion of the gene product. The degree of myelofibrosis and the extent of extramedullary hematopoiesis are also not related. Fibrosis in this disorder is associated with overproduction of transforming growth factor β and tissue inhibitors of metalloproteinases, whereas osteosclerosis is associated with overproduction of osteoprotegerin, an osteoclast inhibitor. Marrow angiogenesis occurs due to increased production of vascular endothelial growth factor. Importantly, fibroblasts in PMF are polyclonal and not part of the neoplastic clone.

CLINICAL FEATURES

No signs or symptoms are specific for PMF. Many patients are asymptomatic at presentation, and the disease is usually detected by the discovery of splenic enlargement and/or abnormal blood counts during a routine examination. However, in contrast to its companion MPN, night sweats, fatigue, and weight loss are common presenting complaints. A blood smear will show the characteristic features of extramedullary hematopoiesis: teardrop-shaped red cells, nucleated red cells, myelocytes, and promyelocytes; myeloblasts may also be present (Fig. 131-1). Anemia, usually mild initially, is the rule, whereas the leukocyte and platelet counts are either normal or increased, but either can be depressed. Mild hepatomegaly may accompany the splenomegaly but is unusual in the absence of splenic enlargement; isolated lymphadenopathy should suggest another diagnosis. Both serum lactate dehydrogenase and alkaline phosphatase levels can be elevated. The LAP score can be low, normal, or high. Marrow is usually inaspirable due to the myelofibrosis (Fig. 131-2), and bone x-rays may reveal osteosclerosis. Exuberant extramedullary hematopoiesis can cause ascites; portal, pulmonary, or intracranial hypertension; intestinal or ureteral obstruction; pericardial tamponade; spinal cord compression; or skin nodules. Splenic enlargement can be sufficiently rapid to cause splenic infarction with fever and pleuritic chest pain. Hyperuricemia and secondary gout may ensue.

FIGURE 131-1   Teardrop-shaped red blood cells indicative of membrane damage from passage through the spleen, a nucleated red blood cell, and immature myeloid cells indicative of extramedullary hematopoiesis are noted. This peripheral blood smear is related to any cause of extramedullary hematopoiesis.

FIGURE 131-2   This marrow section shows the marrow cavity replaced by fibrous tissue composed of reticulin fibers and collagen. When this fibrosis is due to a primary hematologic process, it is called myelofibrosis. When the fibrosis is secondary to a tumor or a granulomatous process, it is called myelophthisis.

DIAGNOSIS

While the clinical picture described above is characteristic of PMF, all of the clinical features described can also be observed in PV or CML. Massive splenomegaly commonly masks erythrocytosis in PV, and reports of intraabdominal thrombosis in PMF most likely represent instances of unrecognized PV. In some patients with PMF, erythrocytosis has developed during the course of the disease. Furthermore, because many other disorders have features that overlap with PMF but respond to distinctly different therapies, the diagnosis of PMF is one of exclusion, which requires that the disorders listed in Table 131-3 be ruled out.

The presence of teardrop-shaped red cells, nucleated red cells, myelocytes, and promyelocytes establishes the presence of extramedullary hematopoiesis, while the presence of leukocytosis, thrombocytosis with large and bizarre platelets, and circulating myelocytes suggests the presence of an MPN as opposed to a secondary form of myelofibrosis (Table 131-3). Marrow is usually inaspirable due to increased marrow reticulin, but marrow biopsy will reveal a hypercellular marrow with trilineage hyperplasia and, in particular, increased numbers of megakaryocytes in clusters and with large, dysplastic nuclei. However, there are no characteristic bone marrow morphologic abnormalities that distinguish PMF from the other chronic MPNs. Splenomegaly due to extramedullary hematopoiesis may be sufficiently massive to cause portal hypertension and variceal formation. In some patients, exuberant extramedullary hematopoiesis can dominate the clinical picture. An intriguing feature of PMF is the occurrence of autoimmune abnormalities such as immune complexes, antinuclear antibodies, rheumatoid factor, or a positive Coombs’ test. Whether these represent a host reaction to the disorder or are involved in its pathogenesis is unknown. Cytogenetic analysis of blood is useful both to exclude CML and for prognostic purposes, because complex karyotype abnormalities portend a poor prognosis in PMF. For unknown reasons, the number of circulating CD34+ cells is markedly increased in PMF (>15,000/μL) compared to the other chronic MPNs, unless they too develop myeloid metaplasia.

Importantly, approximately 50% of PMF patients, like patients with its companion myeloproliferative disorders PV and ET, express the JAK2 V617F mutation, often as homozygotes. Such patients are usually older and have higher hematocrits than the patients who are JAK2 V617F–negative, whereas PMF patients expressing an MPL mutation tend to be more anemic and have lower leukocyte counts. Somatic mutations in exon 9 of the calreticulin gene (CALR) have been found in a majority of patients with PMF and ET who lack mutations in either JAK2 or MPL, and their clinical course appears to be more indolent than patients expressing either a JAK2 or an MPL mutation.

COMPLICATIONS

Survival in PMF varies according to specific risk factors at diagnosis (Tables 131-4 and 131-5) but is shorter in most patients than in PV or ET patients. The natural history of PMF is one of increasing marrow failure with transfusion-dependent anemia and increasing organomegaly due to extramedullary hematopoiesis. As with CML, PMF can evolve from a chronic phase to an accelerated phase with constitutional symptoms and increasing marrow failure. About 10% of patients spontaneously transform to an aggressive form of acute leukemia for which therapy is usually ineffective. Additional important prognostic factors for disease acceleration during the course of PMF include the presence of complex cytogenetic abnormalities, thrombocytopenia, and transfusion-dependent anemia. Most recently, mutations in the ASXL1, EZH2, SRSF2, and IDH1/2 genes have been identified as risk factors for early death or transformation to acute leukemia and may prove to be more useful for PMF risk assessment than any clinical scoring system.

ESSENTIAL THROMBOCYTOSIS

Essential thrombocytosis (other designations include essential thrombocythemia, idiopathic thrombocytosis, primary thrombocytosis, and hemorrhagic thrombocythemia) is a clonal disorder of unknown etiology involving a multipotent hematopoietic progenitor cell manifested clinically by overproduction of platelets without a definable cause. ET is an uncommon disorder, with an incidence of 1–2/100,000 and a distinct female predominance. No clonal marker is available to consistently distinguish ET from the more common nonclonal, reactive forms of thrombocytosis (Table 131-6), making its diagnosis difficult. Once considered a disease of the elderly and responsible for significant morbidity due to hemorrhage or thrombosis, with the widespread use of electronic cell counters, it is now clear that ET can occur at any age in adults and often without symptoms or disturbances of hemostasis. There is an unexplained female predominance in contrast to PMF or the reactive forms of thrombocytosis where no sex difference exists. Because no specific clonal marker is available, clinical criteria have been proposed to distinguish ET from the other chronic MPNs, which may also present with thrombocytosis but have differing prognoses and therapies (Table 131-6). These criteria do not establish clonality; therefore, they are truly useful only in identifying disorders such as CML, PV, or myelodysplasia, which can masquerade as ET, as opposed to actually establishing the presence of ET. Furthermore, as with “idiopathic” erythrocytosis, nonclonal benign forms of thrombocytosis exist (such as hereditary overproduction of thrombopoietin) that are not widely recognized because we currently lack adequate diagnostic tools. Approximately 50% of ET patients carry the JAK2 V617F mutation, but its absence does not exclude the disorder.

ETIOLOGY

Megakaryocytopoiesis and platelet production depend on thrombopoietin and its receptor Mpl. As in the case of early erythroid and myeloid progenitor cells, early megakaryocytic progenitors require the presence of interleukin 3 (IL-3) and stem cell factor for optimal proliferation in addition to thrombopoietin. Their subsequent development is also enhanced by the chemokine stromal cell-derived factor 1 (SDF-1). However, megakaryocyte maturation requires thrombopoietin.

Megakaryocytes are unique among hematopoietic progenitor cells because reduplication of their genome is endomitotic rather than mitotic. In the absence of thrombopoietin, endomitotic megakaryocytic reduplication and, by extension, the cytoplasmic development necessary for platelet production are impaired. Like erythropoietin, thrombopoietin is produced in both the liver and the kidneys, and an inverse correlation exists between the platelet count and plasma thrombopoietic activity. Unlike erythropoietin, thrombopoietin is only constitutively produced, and the plasma thrombopoietin level is controlled by the size of its progenitor cell pool. Also, in contrast to erythropoietin, but like its myeloid counterparts, granulocyte and granulocyte-macrophage colony-stimulating factors, thrombopoietin not only enhances the proliferation of its target cells but also enhances the reactivity of their end-stage product, the platelet. In addition to its role in thrombopoiesis, thrombopoietin also enhances the survival of multipotent hematopoietic stem cells and their bone marrow residence.

The clonal nature of ET was established by analysis of glucose-6-phosphate dehydrogenase isoenzyme expression in patients hemizygous for this gene, by analysis of X-linked DNA polymorphisms in informative female patients, and by the expression in patients of nonrandom, though variable, cytogenetic abnormalities. Although thrombocytosis is its principal manifestation, like the other chronic MPNs, a multipotent hematopoietic progenitor cell is involved in ET. Furthermore, a number of families have been described in which ET was inherited, in one instance as an autosomal dominant trait. In addition to ET, PMF and PV have also been observed in some kindreds. Like PMF, most patients who do not have JAK2 mutations have CALR mutations.

CLINICAL FEATURES

Clinically, ET is most often identified incidentally when a platelet count is obtained during the course of a routine medical evaluation. Occasionally, review of previous blood counts will reveal that an elevated platelet count was present but overlooked for many years. No symptoms or signs are specific for ET, but these patients can have hemorrhagic and thrombotic tendencies expressed as easy bruising for the former and microvascular occlusive events for the latter such as erythromelalgia, ocular migraine, or a TIA. Physical examination is generally unremarkable except occasionally for mild splenomegaly. Splenomegaly is indicative of another MPN, in particular PV, PMF, or CML.

Anemia is unusual, but a mild neutrophilic leukocytosis is not. The blood smear is most remarkable for the number of platelets present, some of which may be very large. The large mass of circulating platelets may prevent the accurate measurement of serum potassium due to release of platelet potassium upon blood clotting. This type of hyperkalemia is a laboratory artifact and not associated with electrocardiographic abnormalities. Similarly, arterial oxygen measurements can be inaccurate unless thrombocythemic blood is collected on ice. The prothrombin and partial thromboplastin times are normal, whereas abnormalities of platelet function such as a prolonged bleeding time and impaired platelet aggregation can be present. However, despite much study, no platelet function abnormality is characteristic of ET, and no platelet function test predicts the risk of clinically significant bleeding or thrombosis.

The elevated platelet count may hinder marrow aspiration, but marrow biopsy usually reveals megakaryocyte hypertrophy and hyperplasia, as well as an overall increase in marrow cellularity. If marrow reticulin is increased, another diagnosis should be considered. The absence of stainable iron demands an explanation because iron deficiency alone can cause thrombocytosis, and absent marrow iron in the presence of marrow hypercellularity is a feature of PV.

Nonrandom cytogenetic abnormalities occur in ET but are uncommon, and no specific or consistent abnormality is notable, even those involving chromosomes 3 and 1, where the genes for thrombopoietin and its receptor Mpl, respectively, are located.

DIAGNOSIS

Thrombocytosis is encountered in a broad variety of clinical disorders (Table 131-6), in many of which production of cytokines is increased. The absolute level of the platelet count is not a useful diagnostic aid for distinguishing between benign and clonal causes of thrombocytosis. About 50% of ET patients express the JAK2 V617F mutation. When JAK2 V617F is absent, cytogenetic evaluation is mandatory to determine if the thrombocytosis is due to CML or a myelodysplastic disorder such as the 5q– syndrome. Because the bcr-abl translocation can be present in the absence of the Ph chromosome, and because bcr-abl reverse transcriptase polymerase chain reaction is associated with false-positive results, fluorescence in situ hybridization (FISH) analysis for bcr-abl is the preferred assay in patients with thrombocytosis in whom a cytogenetic study for the Ph chromosome is negative. CALR mutations are present in most patients who do not have JAK2 mutations, but diagnostic tools to detect these mutations are not yet widespread. Anemia and ringed sideroblasts are not features of ET, but they are features of idiopathic refractory sideroblastic anemia, and in some of these patients, the thrombocytosis occurs in association with JAK2 V617F expression. Splenomegaly should suggest the presence of another MPN, and in this setting, a red cell mass determination should be performed because splenomegaly can mask the presence of erythrocytosis. Importantly, what appears to be ET can evolve into PV or PMF after a period of many years, revealing the true nature of the underlying MPN. There is sufficient overlap of the JAK2 V617F neutrophil allele burden between ET and PV that this cannot be used as a distinguishing diagnostic feature; only a red cell mass and plasma volume determination can distinguish PV from ET, and importantly in this regard, 64% of JAK2 V617F–positive ET patients actually were found to have PV when red cell mass and plasma volume determinations were performed.

COMPLICATIONS

Perhaps no other condition in clinical medicine has caused otherwise astute physicians to intervene inappropriately more often than thrombocytosis, particularly if the platelet count is >1 × 10⁶/μL. It is commonly believed that a high platelet count causes intravascular stasis and thrombosis; however, no controlled clinical study has ever established this association, and in patients younger than age 60 years, the incidence of thrombosis was not greater in patients with thrombocytosis than in age-matched controls, and tobacco use appears to be the most important risk factor for thrombosis in ET patients.

To the contrary, very high platelet counts are associated primarily with hemorrhage due to acquired von Willebrand’s disease. This is not meant to imply that an elevated platelet count cannot cause symptoms in an ET patient, but rather that the focus should be on the patient, not the platelet count. For example, some of the most dramatic neurologic problems in ET are migraine-related and respond only to lowering of the platelet count, whereas other symptoms such as erythromelalgia respond simply to platelet cyclooxygenase-1 inhibitors such as aspirin or ibuprofen, without a reduction in platelet number. Still others may represent an interaction between an atherosclerotic vascular system and a high platelet count, and others may have no relationship to the platelet count whatsoever. Recognition that PV can present with thrombocytosis alone as well as the discovery of previously unrecognized causes of hypercoagulability (Chap. 142) make the older literature on the complications of thrombocytosis unreliable.

ET can also evolve into PMF, but whether this is a feature of ET or represents PMF presenting initially with isolated thrombocytosis is unknown.

INCIDENCE

Acute myeloid leukemia (AML) is a neoplastic disease characterized by infiltration of the blood, bone marrow, and other tissues by proliferative, clonal undifferentiated cells of the hematopoietic system. These leukemias comprise a spectrum of malignancies that, untreated, range from rapidly fatal to slowly growing. In 2013, the estimated number of new AML cases in the United States was 14,590. The incidence of AML is ~3.5 per 100,000 people per year, and the age-adjusted incidence is higher in men than in women (4.5 vs 3.1). AML incidence increases with age; it is 1.7 in individuals age <65 years and 15.9 in those age >65 years. The median age at diagnosis is 67 years.

ETIOLOGY

Heredity, radiation, chemical and other occupational exposures, and drugs have been implicated in the development of AML. No direct evidence suggests a viral etiology.

Heredity     Certain syndromes with somatic cell chromosome aneuploidy, such as trisomy 21 noted in Down syndrome, are associated with an increased incidence of AML. Inherited diseases with defective DNA repair, e.g., Fanconi anemia, Bloom syndrome, and ataxia-telangiectasia, are also associated with AML. Congenital neutropenia (Kostmann syndrome) is a disease with mutations in the genes encoding the granulocyte colony-stimulating factor (G-CSF) receptor and, often, neutrophil elastase that may evolve into AML. Germline mutations of CCAAT/enhancer-binding protein α (CEBPA), runt-related transcription factor 1 (RUNX1), and tumor protein p53 (TP53) have also been associated with a higher predisposition to AML in some series.

Radiation     High-dose radiation, like that experienced by survivors of the atomic bombs in Japan or nuclear reactor accidents, increases the risk of myeloid leukemias that peaks 5–7 years after exposure. Therapeutic radiation alone seems to add little risk of AML but can increase the risk in people also exposed to alkylating agents.

Chemical and Other Exposures     Exposure to benzene, a solvent used in the chemical, plastic, rubber, and pharmaceutical industries, is associated with an increased incidence of AML. Smoking and exposure to petroleum products, paint, embalming fluids, ethylene oxide, herbicides, and pesticides have also been associated with an increased risk of AML.

Drugs     Anticancer drugs are the leading cause of therapy-associated AML. Alkylating agent–associated leukemias occur on average 4–6 years after exposure, and affected individuals have aberrations in chromosomes 5 and 7. Topoisomerase II inhibitor–associated leukemias occur 1–3 years after exposure, and affected individuals often have aberrations involving chromosome 11q23. Newer agents for treatment of other hematopoietic malignancies and solid tumors are also under scrutiny for increased risk of AML. Chloramphenicol, phenylbutazone, and, less commonly, chloroquine and methoxypsoralen can result in bone marrow failure that may evolve into AML.

CLASSIFICATION

The current categorization of AML uses the World Health Organization (WHO) classification (Table 132-1), which includes different biologically distinct groups based on clinical features and cytogenetic and molecular abnormalities in addition to morphology. In contrast to the previously used French-American-British (FAB) schema, the WHO classification places limited reliance on cytochemistry. A major difference between the WHO and the FAB systems is the blast cutoff for a diagnosis of AML as opposed to myelodysplastic syndrome (MDS); it is 20% in the WHO classification and 30% in the FAB. However, within the WHO classification, specific chromosomal rearrangements, i.e., t(8;21)(q22;q22), inv(16)(p13.1q22), t(16;16)(p13.1;q22), and t(15;17)(q22;q12), define AML even with <20% blasts.

^afrom SH Swerdlow et al (eds): World Health Organization Classification of Tumours of Haematopoietic and Lymphoid Tissues. Lyon, IARC Press, 2008. ^bDiagnosis is AML regardless of blast count.

Immunophenotype and Relevance to the WHO Classification     The immunophenotype of human leukemia cells can be studied by multiparameter flow cytometry after the cells are labeled with monoclonal antibodies to cell-surface antigens. This can be important for separating AML from acute lymphoblastic leukemia (ALL) and identifying some subtypes of AML. For example, AML with minimal differentiation that is characterized by immature morphology and no lineage-specific cytochemical reactions may be diagnosed by flow-cytometric demonstration of the myeloid-specific antigens cluster designation (CD) 13 and/or 117. Similarly, acute megakaryoblastic leukemia can often be diagnosed only by expression of the platelet-specific antigens CD41 and/or CD61. Although flow cytometry is useful, widely used, and in some cases essential for the diagnosis of AML, it is supportive only in establishing the different subtypes of AML through the WHO classification.

Clinical Features and Relevance to the WHO Classification     The WHO classification also considers clinical features in subdividing AML. For example, it identifies therapy-related AML as a separate entity that develops following prior therapy (e.g., alkylating agents, topoisomerase II inhibitors, ionizing radiation). It also identifies AML with myelodysplasia-related changes based in part on medical history of an antecedent MDS or myelodysplastic/myeloproliferative neoplasm. The clinical features likely contribute to the prognosis of AML and have therefore been included in the classification.

Genetic Findings and Relevance to the WHO Classification     The WHO classification uses clinical, morphologic, and cytogenetic and/or molecular criteria to identify subtypes of AML and forces the clinician to take the appropriate steps to correctly identify the entity and thus tailor treatment(s) accordingly. The WHO classification is indeed the first AML classification that incorporates genetic (chromosomal and molecular) information. In this classification, subtypes of AML are recognized based on the presence or absence of specific recurrent genetic abnormalities. For example, the diagnosis of acute promyelocytic leukemia (APL) is based on the presence of either the t(15;17)(q22;q12) cytogenetic rearrangement or the PML-RARA fusion product of the translocation. A similar approach is taken with regard to core binding factor (CBF) AML that is now designated based on the presence of t(8;21)(q22;q22), inv(16)(p13.1q22), or t(16;16)(p13.1;q22) or the respective fusion products RUNX1-RUNX1T1 and CBFB-MYH11.

The WHO classification incorporates cytogenetics in the AML classification by recognizing a category of AML with recurrent genetic abnormalities and a category of AML with myelodysplasia-related changes (Table 132-1). The latter category is diagnosed not only by morphologic changes, but also in part by selected myelodysplasia-related cytogenetic abnormalities (e.g., complex karyotypes and unbalanced and balanced changes involving, among others, chromosomes 5, 7, and 11). Only one cytogenetic abnormality has been invariably associated with specific morphologic features: t(15;17)(q22;q12) with APL. Other chromosomal abnormalities have been associated primarily with one morphologic/immunophenotypic group, including inv(16)(p13.1q22) with AML with abnormal bone marrow eosinophils; t(8;21)(q22;q22) with slender Auer rods, expression of CD19, and increased normal eosinophils; and t(9;11)(p22;q23), and other translocations involving 11q23, with monocytic features. Recurring chromosomal abnormalities in AML may also be associated with specific clinical characteristics. More commonly associated with younger age are t(8;21) and t(15;17), and with older age, del(5q) and del(7q). Myeloid sarcomas (see below) are associated with t(8;21), and disseminated intravascular coagulation (DIC) is associated with t(15;17).

The WHO classification also incorporates molecular abnormalities by recognizing fusion genes that are products of recurrent cytogenetic aberrations or have been found mutated and may be involved in leukemogenesis. For instance, t(15;17) results in the fusion gene PML-RARA that encodes a chimeric protein, promyelocytic leukemia (Pml)–retinoic acid receptor α (Rarα), which is formed by the fusion of the retinoic acid receptor α (RARA) gene from chromosome 17 and the promyelocytic leukemia (PML) gene from chromosome 15. The RARA gene encodes a member of the nuclear hormone receptor family of transcription factors. After binding retinoic acid, RARA can promote expression of a variety of genes. The 15;17 translocation juxtaposes PML with RARA in a head-to-tail configuration that is under the transcriptional control of PML. Three different breakpoints in the PML gene lead to various fusion protein isoforms. The Pml-Rarα fusion protein tends to suppress gene transcription and blocks differentiation of the cells. Pharmacologic doses of the Rarα ligand, all-trans-retinoic acid (tretinoin), relieve the block and promote hematopoietic cell differentiation (see below). Similar examples of molecular subtypes of the disease included in the category of AML with recurrent genetic abnormalities are those characterized by the leukemogenic fusion genes RUNX1-RUNX1T1, CBFB-MYH11, MLLT3-MLL, and DEK-NUP214, resulting, respectively, from t(8;21), inv(16) or t(16;16), t(9;11), and t(6;9)(p23;q34).

Two new provisional entities defined by the presence of gene mutations, rather than microscopic chromosomal abnormalities, have been added to the category of AML with recurrent genetic abnormalities: AML with mutated nucleophosmin (nucleolar phosphoprotein B23, numatrin) (NPM1) and AML with mutated CEBPA. AML with fms-related tyrosine kinase 3 (FLT3) mutations is not considered a distinct entity, although determining the presence of such mutations is recommended by WHO in patients with cytogenetically normal AML (CN-AML) because the relatively frequent FLT3-internal tandem duplication (ITD) carries a negative prognostic significance and therefore is clinically relevant. FLT3 encodes a tyrosine kinase receptor important in the development of myeloid and lymphoid lineages. Activating mutations of FLT3 are present in ~30% of adult AML patients due to ITDs in the juxtamembrane domain or point mutations of the activating loop of the kinase (called tyrosine kinase domain mutations). Aberrant activation of the FLT3-encoded protein provides increased proliferation and antiapoptotic signals to the myeloid progenitor cell. FLT3-ITD, the more common of the FLT3 mutations, occurs preferentially in patients with CN-AML. The importance of identifying FLT3-ITD at diagnosis relates to the fact that not only is it a useful prognosticator but it also may predict response to specific treatment such as the tyrosine kinase inhibitors that are in clinical investigation.

PROGNOSTIC FACTORS

Several factors have been demonstrated to predict outcome of AML patients treated with chemotherapy, and they can be used for risk stratification and treatment guidance.

Chromosome findings at diagnosis are currently the most important independent prognostic factors. Several studies have categorized patients as having favorable, intermediate, or poor cytogenetic risk based on the presence of structural and/or numerical aberrations. Patients with t(15;17) have a very good prognosis (~85% cured), and those with t(8;21) and inv(16) have a good prognosis (~55% cured), whereas those with no cytogenetic abnormality have an intermediate outcome risk (~40% cured). Patients with a complex karyotype, t(6;9), inv(3), or –7 have a very poor prognosis. Another cytogenetic subgroup, the monosomal karyotype, has been suggested to adversely impact the outcome of AML patients other than those with t(15;17), t(8;21), or inv(16) or t(16;16). The monosomal karyotype subgroup is defined by the presence of at least two autosomal monosomies (loss of chromosomes other than Y or X) or a single autosomal monosomy with additional structural abnormalities.

For patients lacking prognostic cytogenetic abnormalities, such as those with CN-AML, outcome prediction uses mutated or aberrantly expressed genes. NPM1 mutations without concurrent presence of FLT3-ITD, and CEBPA mutations, especially if concurrently present in two different alleles, have been shown to predict favorable outcome, whereas FLT3-ITD predicts poor outcome. Given the proven prognostic importance of NPM1 and CEBPA mutations and FLT3-ITD, molecular assessment of these genes at diagnosis has been incorporated in AML management guidelines by the National Comprehensive Cancer Network (NCCN) and the European LeukemiaNet (ELN). The same markers have also been incorporated in the definitions of the genetic groups of the ELN standardized reporting system, which are based on both cytogenetic and molecular abnormalities and used for comparing clinical features and treatment response among subsets of patients reported in different studies (Table 132-2). More recently, the prognostic impact of the genetic groups recognized by the ELN reporting system has been demonstrated. Thus, these genetic groups may also be used for risk stratification and treatment guidance.

In addition to NPM1 and CEBPA mutations and FLT3-ITD, other molecular aberrations (Table 132-3) may in the future be routinely used for prognostication in AML and incorporated in the WHO classification and the ELN reporting system. Among these prognostic mutated genes are those encoding receptor tyrosine kinases (e.g., v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog [KIT]), transcription factors (i.e., RUNX1 and Wilms tumor 1 [WT1]), and epigenetic modifiers (i.e., additional sex combs like transcriptional regulator 1 [ASXL1], DNA (cytosine-5-)-methyltransferase 3 alpha [DNMT3A], isocitrate dehydrogenase 1 (NADP+), soluble [IDH1] and isocitrate dehydrogenase 2 (NADP+), mitochondrial [IDH2], lysine (K)-specific methyltransferase 2A [KMT2A, also known as MLL], and tet methylcytosine dioxygenase 2 [TET2]). Although KIT mutations are almost exclusively present in CBF AML and impact adversely the outcome, the remaining markers have been reported primarily in CN-AML. These gene mutations have been shown to be associated with outcome in multivariable analyses independently from other prognostic factors. However, for some of them, the prognostic impact (e.g., TET2 mutations) or the type (adverse vs favorable) of prognostic impact (e.g., IDH1, IDH2) has been found in the majority, but not in all, of the reported studies.

An independent prognostic impact remains to be determined for mutated genes that are either associated primarily with unfavorable cytogenetic aberrations (e.g., TP53) or are found with a relatively lower frequency in AML patients like those encoding epigenetic modifiers (e.g., enhancer of zeste 2 polycomb repressive complex 2 subunit [EZH2]), phosphatases (e.g., protein tyrosine phosphatase, non-receptor type 11 (PTPN11]), putative transcription factors (e.g., PHD finger protein 6 [PHF6]), splicing factors (e.g., U2 small nuclear RNA auxiliary factor 1 [U2AF1]), and proteins involved in chromosome segregation and genome stability (e.g., structural maintenance of chromosomes 1A [SMC1A] or structural maintenance of chromosomes 3 [SMC3]). Finally, other mutated genes are recognized as predictors of treatment response to distinct therapies rather than prognosticators; for example, neuroblastoma RAS viral (v-ras) oncogene homolog (NRAS) and Kirsten rat sarcoma viral oncogene homolog (KRAS) predict a better response to high-dose cytarabine in CBF AML.

In addition to gene mutations, deregulation of the expression levels of coding genes and of short noncoding RNAs (microRNAs) have been reported to provide prognostic information (Table 132-3). Overexpression of genes such as brain and acute leukemia, cytoplasmic (BAALC), v-ets avian erythroblastosis virus E26 oncogene homologue (avian) (ERG), meningioma (disrupted in balanced translocation) 1 (MN1), and MDS1 and EVI1 complex locus (MECOM, also known as EVI1) have been found to be predictive for poor outcome, especially in CN-AML. Similarly, deregulated expression levels of microRNAs, naturally occurring noncoding RNAs that have been shown to regulate the expression of proteins involved in hematopoietic differentiation and survival pathways by degradation or translation inhibition of target coding RNAs, have been associated with prognosis in AML. Overexpression of miR-155 and miR-3151 has been found to affect outcome adversely in CN-AML, whereas overexpression of miR-181a predicts a favorable outcome both in CN-AML and cytogenetically abnormal AML.

Because prognostic molecular markers in AML are not mutually exclusive and often occur concurrently (>80% patients have at least two or more prognostic gene mutations), the likelihood that distinct marker combinations may be more informative than single markers is being recognized.

Epigenetic changes (e.g., DNA methylation) and microRNAs are often involved in deregulation of genes involved in hematopoiesis, contribute to leukemogenesis, and are often associated with the previously discussed prognostic gene mutations. These changes not only have been shown to provide biologic insights into leukemogenic mechanisms, but also independent prognostic information. Indeed, it is anticipated that with the enormous progress made in DNA and RNA sequencing technology, additional genetic and epigenetic aberrations will soon be discovered and will contribute to classification and reporting systems and outcome risk determination in AML patients.

In addition to cytogenetics and/or molecular aberrations, several other factors are associated with outcome in AML. Age at diagnosis is one of the most important risk factors. Advancing age is associated with a poorer prognosis not only because of its influence on the ability to survive induction therapy due to coexisting comorbidities, but also because with each successive decade of age, a greater proportion of patients have an intrinsically more resistant disease. A prolonged symptomatic interval with cytopenias preceding diagnosis or a history of antecedent hematologic disorders including myeloproliferative neoplasms is often found in older patients and is a clinical feature associated with a lower complete remission (CR) rate and shorter survival time. The CR rate is lower in patients who have had anemia, leukopenia, and/or thrombocytopenia for >3 months before the diagnosis of AML when compared to those without such a history. Responsiveness to chemotherapy declines as the duration of the antecedent disorder(s) increases. AML developing after treatment with cytotoxic agents for other malignancies is usually difficult to treat successfully. Finally, it is likely that AML in older patients is also associated with poor outcome because of the presence of distinct biologic features that may increase the aggressiveness of the disease and reduce the likelihood of treatment response. The leukemic cells in older patients more commonly express the multidrug resistance 1 (MDR1) efflux pump that conveys resistance to natural product–derived agents such as the anthracyclines that are frequently incorporated into the initial treatment. In addition, older patients less frequently harbor favorable cytogenetic abnormalities [i.e., t(8;21), inv(16), and t(16;16)] and more frequently harbor adverse cytogenetic (e.g., complex and monosomal karyotypes) and/or molecular (e.g., ASXL1, IDH2, RUNX1, TET2) abnormalities.

Other factors independently associated with worse outcome are a low performance status that influences ability to survive induction therapy and thus respond to treatment and a high presenting leukocyte count that in some series is an adverse prognostic factor for attaining a CR. Among patients with hyperleukocytosis (>100,000/μL), early central nervous system bleeding and pulmonary leukostasis contribute to poor outcome with initial therapy.

Achievement of CR is associated with better outcome and longer survival. CR is defined after examination of both blood and bone marrow. The blood neutrophil count must be ≥1000/μL and the platelet count ≥100,000/μL. Hemoglobin concentration is not considered in determining CR. Circulating blasts should be absent. Although rare blasts may be detected in the blood during marrow regeneration, they should disappear on successive studies. The bone marrow should contain <5% blasts, and Auer rods should be absent. Extramedullary leukemia should not be present. Patients who achieve CR after one induction cycle have longer CR durations than those requiring multiple cycles.

CLINICAL PRESENTATION

Symptoms     Patients with AML most often present with nonspecific symptoms that begin gradually or abruptly and are the consequence of anemia, leukocytosis, leukopenia or leukocyte dysfunction, or thrombocytopenia. Nearly half have had symptoms for ≤3 months before the leukemia was diagnosed.

Half of patients mention fatigue as the first symptom, but most complain of fatigue or weakness at the time of diagnosis. Anorexia and weight loss are common. Fever with or without an identifiable infection is the initial symptom in approximately 10% of patients. Signs of abnormal hemostasis (bleeding, easy bruising) are noted first in 5% of patients. On occasion, bone pain, lymphadenopathy, nonspecific cough, headache, or diaphoresis is the presenting symptom.

Rarely patients may present with symptoms from a myeloid sarcoma that is a tumor mass consisting of myeloid blasts occurring at anatomic sites other than bone marrow. Sites involved are most commonly the skin, lymph node, gastrointestinal tract, soft tissue, and testis. This rare presentation, often characterized by chromosome aberrations [e.g., monosomy 7, trisomy 8, MLL rearrangement, inv(16), trisomy 4, t(8;21)], may precede or coincide with AML.

Physical Findings     Fever, splenomegaly, hepatomegaly, lymphadenopathy, sternal tenderness, and evidence of infection and hemorrhage are often found at diagnosis. Significant gastrointestinal bleeding, intrapulmonary hemorrhage, or intracranial hemorrhage occurs most often in APL. Bleeding associated with coagulopathy may also occur in monocytic AML and with extreme degrees of leukocytosis or thrombocytopenia in other morphologic subtypes. Retinal hemorrhages are detected in 15% of patients. Infiltration of the gingivae, skin, soft tissues, or meninges with leukemic blasts at diagnosis is characteristic of the monocytic subtypes and those with 11q23 chromosomal abnormalities.

Hematologic Findings     Anemia is usually present at diagnosis and can be severe. The degree varies considerably, irrespective of other hematologic findings, splenomegaly, or duration of symptoms. The anemia is usually normocytic normochromic. Decreased erythropoiesis often results in a reduced reticulocyte count, and red blood cell (RBC) survival is decreased by accelerated destruction. Active blood loss also contributes to the anemia.

The median presenting leukocyte count is about 15,000/μL. Between 25 and 40% of patients have counts <5000/μL, and 20% have counts >100,000/μL. Fewer than 5% have no detectable leukemic cells in the blood. The morphology of the malignant cell varies in different subsets. In AML, the cytoplasm often contains primary (nonspecific) granules, and the nucleus shows fine, lacy chromatin with one or more nucleoli characteristic of immature cells. Abnormal rod-shaped granules called Auer rods are not uniformly present, but when they are, myeloid lineage is virtually certain (Fig. 132-1). Poor neutrophil function may be noted functionally by impaired phagocytosis and migration and morphologically by abnormal lobulation and deficient granulation.

FIGURE 132-1   Morphology of acute myeloid leukemia (AML) cells. A. Uniform population of primitive myeloblasts with immature chromatin, nucleoli in some cells, and primary cytoplasmic granules. B. Leukemic myeloblast containing an Auer rod. C. Promyelocytic leukemia cells with prominent cytoplasmic primary granules. D. Peroxidase stain shows dark blue color characteristic of peroxidase in granules in AML.

Platelet counts <100,000/μL are found at diagnosis in ~75% of patients, and about 25% have counts <25,000/μL. Both morphologic and functional platelet abnormalities can be observed, including large and bizarre shapes with abnormal granulation and inability of platelets to aggregate or adhere normally to one another.

Pretreatment Evaluation     Once the diagnosis of AML is suspected, a rapid evaluation and initiation of appropriate therapy should follow. In addition to clarifying the subtype of leukemia, initial studies should evaluate the overall functional integrity of the major organ systems, including the cardiovascular, pulmonary, hepatic, and renal systems (Table 132-4). Factors that have prognostic significance, either for achieving CR or for predicting the duration of CR, should also be assessed before initiating treatment, including cytogenetics and molecular markers (see above). Leukemic cells should be obtained from all patients and cryopreserved for future use as new tests and therapeutics become available. All patients should be evaluated for infection.

Abbreviations: AML, acute myeloid leukemia; BUN, blood urea nitrogen; CBC, complete blood count; CMV, cytomegalovirus; CNS, central nervous system; DIC, disseminated intravascular coagulation; HLA, human leukocyte antigen; HSCT, hematopoietic stem cell transplantation; HSV, herpes simplex virus; IV, intravenous; LDH, lactate dehydrogenase; MRI, magnetic resonance imaging; MUGA, multigated acquisition; PA, posteroanterior; RBC, red blood (cell) count.

Most patients are anemic and thrombocytopenic at presentation. Replacement of the appropriate blood components, if necessary, should begin promptly. Because qualitative platelet dysfunction or the presence of an infection may increase the likelihood of bleeding, evidence of hemorrhage justifies the immediate use of platelet transfusion, even if the platelet count is only moderately decreased.

About 50% of patients have a mild to moderate elevation of serum uric acid at presentation. Only 10% have marked elevations, but renal precipitation of uric acid and the nephropathy that may result is a serious but uncommon complication. The initiation of chemotherapy may aggravate hyperuricemia, and patients are usually started immediately on allopurinol and hydration at diagnosis. Rasburicase (recombinant uric oxidase) is also useful for treating uric acid nephropathy and often can normalize the serum uric acid level within hours with a single dose of treatment. The presence of high concentrations of lysozyme, a marker for monocytic differentiation, may be etiologic in renal tubular dysfunction, which could worsen other renal problems that arise during the initial phases of therapy.

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