Primary Myelofibrosis and the Myeloproliferative Neoplasms: Title and subTitle BreakThe Role of Individual Variation

Mrs W is a 79-year-old woman with a history of myelofibrosis who is treated routinely in the Johns Hopkins hematology clinic. Her disease history dates to 1982, when at age 51 years she was found to have an enlarged liver and spleen on routine examination. A liver biopsy was performed, revealing islands of hematopoietic elements. She was referred to our hematology clinic and denied constitutional symptoms such as fever, weight loss, night sweats, or fatigue. Despite a normal hemoglobin level of 13.4 g/dL and a normal white blood cell and platelet count, her peripheral blood smear was remarkable for teardrop-shaped red blood cells, rare nucleated red blood cells, and early myeloid forms. A bone marrow biopsy showed hypercellularity, a marked increase in bone density and formation (osteosclerosis), megakaryocytic hyperplasia, and collagen fibrosis (Figure 1). These findings were all consistent with what was then referred to as myelofibrosis with myeloid metaplasia and what is now called primary myelofibrosis.

From her diagnosis at age 52 years through her 50s, Mrs W's course was stable, with mild splenomegaly as the sole clinical manifestation of her bone marrow disease. She worked full time and raised her family without limitations. After age 60 years, her disease course was marked by the development of progressive splenomegaly, culminating in a splenectomy in 1997 at age 65 years. Histologic examination of her 5-kg (normal, <200 g) spleen revealed severe congestion, extramedullary hematopoiesis (hematopoiesis occurring at a site other than bone marrow), and atypical megakaryocytes. After splenectomy, her platelet count increased to 2000 × 10³/μL, and hydroxyurea was initiated. One month after the splenectomy, the patient experienced arterial embolic events to both her left foot and right index finger. A patent foramen ovale was surgically closed and no further arterial events occurred. At age 71 years, her hemoglobin level increased to 15.5 g/dL, and subsequently she has required intermittent phlebotomy procedures to maintain her hemoglobin level below 13 g/dL. Her course was complicated by an unprovoked deep venous thrombosis of the right lower extremity at age 73 years.

During Mrs W's 28-year course, there have been major breakthroughs in the understanding of the molecular pathogenesis of the myeloproliferative neoplasms that have not only prompted reclassification of the diseases but also advanced diagnostic and prognostic capabilities and have broadened therapeutic options. Mrs W's unique course presents an opportunity to examine the clinical spectrum of disease that is characteristic of the myeloproliferative neoplasms. In this review, we will focus on features of primary myelofibrosis that are relevant to Mrs W's case.

The classic myeloproliferative neoplasms, which encompass essential thrombocytosis, polycythemia vera, and primary myelofibrosis, share many clinical and biological features. They are acquired clonal hematopoietic stem cell disorders characterized by an overproduction of mature blood elements, bone marrow hypercellularity, extramedullary hematopoiesis, a tendency toward thrombotic or hemorrhagic events, and, rarely, transformation to myelofibrosis or leukemia. More recently, these diseases have been reclassified as neoplasms, along with chronic myeloid leukemia, chronic neutrophilic leukemia, chronic eosinophilic leukemia, mastocytosis, and unclassified myeloproliferative neoplasms.¹ However, despite their classification as neoplastic disorders, essential thrombocytosis, polycythemia vera, and primary myelofibrosis often have a survival measured in decades, even in the absence of treatment.

Primary myelofibrosis is a new name, updating previous labels such as chronic idiopathic myelofibrosis, agnogenic myeloid metaplasia, and myelofibrosis with myeloid metaplasia, among others.² Primary myelofibrosis is the rarest of the classic myeloproliferative neoplasms, with an incidence rate of approximately 0.2 cases per 100 000 persons documented in a recent analysis of the Surveillance, Epidemiology, and End Results and North American Association of Central Cancer Registries databases.³ Primary myelofibrosis patients tend to be older than those with essential thrombocytosis and polycythemia vera because the median age at diagnosis is often in the seventh decade,⁴ and in contrast to essential thrombocytosis, in which women outnumber men, there is a male predominance in primary myelofibrosis.^{5 - 6} Although toxins (radiation and benzene) confer an increased risk of primary myelofibrosis, most cases are idiopathic, as are most cases of essential thrombocytosis and polycythemia vera.⁷ Nearly 25% of patients with primary myelofibrosis have a history of antecedent essential thrombocytosis or polycythemia vera; in these settings, post–essential thrombocythemia myelofibrosis and post–polycythemia vera myelofibrosis are the proposed classifications.⁸ When asymptomatic and not formally diagnosed, an antecedent myeloproliferative neoplasm can be inferred only from review of records demonstrating long-standing thrombocytosis or erythrocytosis.

Quiz Ref IDMany of the classic presenting features of primary myelofibrosis are illustrated in Mrs W's case, including splenomegaly, teardrop-shaped red blood cells, and leukoerythroblastic changes (appearance of nucleated red blood cells and immature granulocytes) on the blood smear and a hypercellular bone marrow with osteosclerosis and collagen fibrosis. In 1982, these features were sufficient to meet criteria for diagnosis, as long as the marrow fibrosis was not due to another cause.^{1 ,9} The myeloproliferative neoplasms can mimic one another. Although Mrs W presented with features of extramedullary hematopoiesis, primary myelofibrosis can also present with elevated platelet counts and mimic essential thrombocytosis, but findings on the blood smear and bone marrow tend to distinguish these entities. According to the World Health Organization (WHO), a diagnosis of essential thrombocytosis requires the absence of peripheral blood leukoerythroblastosis, significant marrow hypercellularity, reticulin fibrosis, and collagen fibrosis.¹ The WHO major criteria for a diagnosis of primary myelofibrosis require 3 of the following: atypical megakaryocytes in the setting of collagen or reticulin fibrosis, exclusion of other myeloid neoplasms such as chronic myelogenous leukemia or polycythemia vera, presence of a clonal marker such as JAK2 V617F, and exclusion of secondary causes of marrow fibrosis. Minor criteria for primary myelofibrosis require 2 of the following: elevated lactate dehydrogenase, leukoerythroblastosis, anemia, and splenomegaly.¹ An absolute erythrocytosis would distinguish primary myelofibrosis from polycythemia vera.

Quiz Ref IDThe absence of symptoms in Mrs W's case is atypical because fatigue complicates nearly 85% of cases, whereas night sweats, pruritus, bone pain, and weight loss are also often present.¹⁰ Quiz Ref IDAs described, Mrs W experienced 2 well-recognized complications that required specific treatment: thrombosis and extramedullary hematopoiesis manifesting as massive splenomegaly. Thrombotic events can be either arterial or venous and may complicate 10% of cases during a 5-year period.¹¹ Although thrombosis is the most common cause of morbidity in essential thrombocytosis and polycythemia vera, the decreased thrombosis rate in primary myelofibrosis may be partially confounded by reduced survival and competing causes of morbidity, including hemorrhage, infection, and transformation to leukemia.¹¹ Studies addressing risk factors for thrombosis have suggested contributions from cardiovascular comorbidities, leukocytosis, and the presence of the JAK2 V617F mutation, which will be further discussed.^{11 - 13} Splenomegaly is present in nearly all patients, is often massive, and can lead to early satiety, pain (sometimes due to splenic infarction), and cytopenia because of splenic sequestration. These features often merit therapy.¹⁴

Of the classic myeloproliferative neoplasms, primary myelofibrosis has the most unfavorable natural history. In addition to morbidity from previously described complications, leukemic transformation results in the death of 15% of myelofibrosis patients within a 5-year period, a fact Mrs W recalls well from her own research into this disease.⁷ However, survival is not uniformly poor, and investigators have established adverse risk factors for survival. Factors associated with longer survival have been lack of symptoms, hemoglobin levels of 10 g/dL or more, platelet counts of 100 × 10³/μL or more, and absence of splenomegaly.⁹ Among these, at diagnosis Mrs W had a hemoglobin level of 13.4 g/dL and only mild splenomegaly. Prognostic models have been developed; the simplest, the Lille score, includes anemia (hemoglobin level <10 g/dL), leukopenia (<4 × 10³/μL), or leukocytosis (>30 × 10³/μL) at diagnosis as adverse indicators.¹⁴ The median survival was only 13 months in the presence of 2 of these risk factors.¹⁵ A more recent model from the International Working Group for Myelofibrosis Research and Treatment, developed in 1054 patients, identified age older than 65 years, presence of constitutional symptoms, hemoglobin level less than 10 g/dL, leukocyte count greater than 25 × 10³/μL, and circulating blast cells at 1% or greater as predictors of shortened survival.⁵ At diagnosis, Mrs W had none of these features and therefore could expect a median survival of 11 years.⁵ As will be discussed, the discovery of the JAK2 V617F mutation has advanced the understanding of the myeloproliferative neoplasms; however, its prognostic importance for primary myelofibrosis patients is not yet clear. A recent report suggests that individuals with the lowest quartile of the mutational burden (the ratio of mutant to wild-type JAK2) have a “myelodepletive” phenotype characterized by bone marrow failure (anemia, leukopenia) and a poorer prognosis compared with individuals with higher proportions of mutant allele.¹⁶ Mrs W has a JAK2 V617F mutational burden in the average range for primary myelofibrosis patients.

In 2005, 23 years into Mrs W's disease, the discovery of the acquired hematopoietic stem cell mutation JAK2 V617F in essential thrombocytosis, polycythemia vera, and primary myelofibrosis substantiated the claim made by Dameshek¹⁷ of a shared pathogenesis for these disorders.^{18 - 21} The JAK2 gene is a member of a large family of tyrosine kinases involved in cytokine receptor signaling. Although the JAK2 gene functions across many different tissues, in the context of hematopoiesis, JAK2 is integral to intracellular signal transduction after the activation of receptors for erythropoietin, thrombopoietin, granulocyte-colony stimulating factor, and granulocyte-macrophage colony stimulating factor. The signal transduction of these cytokines and their receptors is crucial for the coordinated proliferation and differentiation of the erythroid, megakaryocytic, and granulocytic lineages from pluripotent hematopoietic stem cells.²² The JAK2 mutation results in the substitution of valine for phenylalanine at position 617 in the JAK2 protein, where a regulatory domain resides. As a result of this substitution, there is abeyance of a negative regulatory effect on JAK2, allowing for its constitutive activation, independent of ligand.²³ When JAK2 is mutated, intracellular signaling through this normal pathway becomes more robust, accounting for the proliferative component in the myeloproliferative neoplasms (Figure 2). The JAK2 V617F mutation is found in more than 95% of polycythemia vera patients and 60% of patients with essential thrombocytosis and primary myelofibrosis. Clinical testing became available in 2005, and Mrs W's test results were positive for the JAK2 V617F mutation.

Despite their shared genetic lesion, there is considerable phenotypic variability among the 3 disorders, raising the question of how a single mutation could result in 3 distinct disorders. One explanation for the discrepancy between genotype and phenotype in the myeloproliferative neoplasms is a gene dosage effect of JAK2 V617F. JAK2 V617F is an acquired hematopoietic stem cell mutation, yet not all stem cell progenitors within an individual carry the mutation, suggesting that in many patients, hematopoiesis remains polyclonal.^{24 - 26} In addition, many patients with the JAK2 V617F mutation acquire 2 copies through an aberrant mitotic recombination event. Thus, because of the variability in the number of cells that carry the mutation and the number of JAK2 V617F alleles harbored in each cell, there is marked variability in the JAK2 V617F gene dosage, and this is easily measured with polymerase chain reaction. From these measurements, variability within and between phenotypes has become clear; in murine and human studies, the JAK2 V617F allele burden is lowest in essential thrombocytosis compared with that of polycythemia vera and primary myelofibrosis.^{27 - 28} Additionally, increasing allele burdens have been inversely associated with the platelet count and directly associated with reticulocytosis, leukocytosis, and splenomegaly.^{26 - 29}

Variation in JAK2 V617F mutational burden alone, however, cannot explain the variability of disease phenotypes within the myeloproliferative neoplasms. Host genetic background has been shown to play a significant role in the acquisition of the JAK2 mutation itself.^{30 - 32} Additional acquired genetic lesions have been identified in myeloproliferative neoplasm patients, which may modify phenotype.³³ Sex is also an independent modifier of the myeloproliferative neoplasms, influencing JAK2 V617F allele burden within the disease phenotypes and the magnitude of change in the JAK2 V617F allele burden through the course of the disease duration, with women having lower mutational burdens than men.³⁴

This variability in phenotype despite a shared genetic lesion is not typical of other myeloid disorders such as chronic myelogenous leukemia, in which the BCR-ABL oncogene drives an aberrant signal transduction pathway that associates with a very well-defined natural history largely independent of host genetic factors. In this sense, the JAK2 V617F–associated myeloproliferative neoplasms are more similar to the sickle hemoglobinopathies, in which gene dosage and host factors introduce marked variability within disease phenotype, complications, and survival.^{35 - 36}

Although the discovery of JAK2 V617F has contributed to a better understanding of many features of the myeloproliferative disorders, its role in the pathogenesis of bone marrow fibrosis, prominent in Mrs W's disease, has not been established. Fibrosis in primary myelofibrosis has been considered to be a reactive process of normal fibroblasts responding to aberrant cytokines released by developing megakaryocytes. In this regard, activating mutations in the thrombopoietin receptor (Mpl) have been reported in primary myelofibrosis patients, illustrating the importance of the thrombopoietin-receptor axis in primary myelofibrosis pathogenesis.³⁷ Quiz Ref IDMoreover, as in the case of other fibrotic diseases involving the lung, liver, or kidney,³⁸ an excess of transforming growth factor β (TGF-β) elaborated from abnormally proliferating megakaryocytes in primary myelofibrosis bone marrow may contribute. In a thrombopoietin-induced murine myelofibrosis model, TGF-β–null mice failed to develop fibrosis or osteosclerosis, whereas wild-type mice developed severe marrow fibrosis and 4- to 6-fold increases in TGF-β levels in the plasma and extracellular fluid of the spleen.³⁹ This result supports the contribution of TGF-β to myelofibrosis and also suggests that pharmacologic inhibition may provide a therapeutic benefit, especially because fibrosis is a reactive and reversible phenomenon.^{40 - 41} It is likely that other cytokines or growth factors, including platelet-derived growth factor receptor α or basic fibroblast growth factor, or even impaired monocyte function, also contribute to the pathogenesis of fibrosis.⁴²

Unless a patient is a candidate for hematopoietic stem cell transplantation, the treatment strategies are largely palliative or supportive. In Mrs W's case, her morbidity was largely due to massive splenomegaly and thrombosis, which will serve as a focus for this section. The development of massive splenomegaly with early satiety, pain, and cytopenia often merits therapy, and the approaches are either medical or surgical. Medical therapies may include hydroxyurea, oral alkylators, immunomodulatory agents, and interferons, but toxicity and poor efficacy and tolerability can complicate their use.¹⁴ In Mrs W's case, surgery was used, but this requires careful patient selection. Optimal patients are those with minimal comorbidity, an adequate life expectancy, and previous failure of at least 1 medical therapy. The complication rate is near 30% and the fatality rate is 7%.¹⁴ Quiz Ref IDPerioperative complications are frequently attributed to thrombosis and hemorrhage, and careful attention to the postprocedural platelet count is essential.¹⁴ In Mrs W's case, her platelet count increased to 2000 × 10³/μL, and hydroxyurea was initiated but thrombosis ensued. Aspirin was later added to her regimen.

Although thrombosis is a known complication, there is no specific treatment aside from antiplatelet or anticoagulant agents, depending on whether the event is venous or arterial. However, addressing modifiable risk factors is important, especially because cardiovascular risk factors and leukocytosis (and the JAK2 V617F mutation) may increase the risk of thromobosis.^{11 - 13} In Mrs W's case, vascular risk factors, including hypertension and hyperlipidemia, have been managed with aspirin, antihypertensive agents, and statins, and her leukocytosis has been managed with hydroxyurea. Although antecedent essential thrombocytosis or polycythemia vera is not unusual in myelofibrosis, the development of absolute erythrocytosis in a patient with documented primary myelofibrosis is unusual. Nearly 20 years after her diagnosis, Mrs W developed erythrocytosis and has required intermittent phlebotomy, a measure that may decrease her thrombosis risk.

Primary myelofibrosis is an excellent example of the contribution of unique host modifiers to phenotypic diversity in the presence of a shared genetic lesion. Mrs W's medical course began with an obscure rare disorder with limited therapies. In the ensuing 28 years, her disease has been molecularly defined, reclassified, and renamed. Since 1999, disease-specific advocacy in the form of the Myeloproliferative Disorders Foundation has supported education and clinical research, and disease-specific molecularly targeted clinical trials are currently under way. Although the JAK2 inhibitors may not alter the natural history of primary myelofibrosis by affecting anemia, thrombosis rates, or transformations to leukemia, these agents show promise in improving quality of life and reducing spleen size.⁴³ Future investigations of disease pathogenesis will contribute to the development of targeted therapies, and it is hoped that they will allow patients to defy an otherwise poor prognosis.