Hematopoietic pluripotent stem cells have self-renewal capability and give rise to either myeloid or the lymphoid lineage which further differentiates into various mature blood cells such as red blood cells (RBC), lymphocytes, granulocytes, megakaryocytes, and macrophages. The hematopoietic process is determined by the bone marrow environment, growth factors, and transcription factors.
The abnormal proliferation of one or more terminal myeloid cell lines in the peripheral blood gives rise to a heterogeneous group of disorders called myeloproliferative neoplasms. In 1951, William Dameshek coined the term myeloproliferative disorders which are now reformed to myeloproliferative neoplasms (MPNs) by the World Health Organization (WHO). Chronic myeloid leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) are four classic types of myeloproliferative neoplasms. WHO classification also included chronic neutrophilic leukemia (CNL), chronic eosinophilic leukemia (CEL), and MPN, unclassifiable. Out of the classic types of MPNs, CML is BCR-ABL1 positive, but PV, ET, and PMF are BCR-ABL1 negative. Besides, the fourth edition of WHO classification for myeloid and acute leukemia was revised in 2016 due to recent advances in hematology with the identification of molecular markers and prognostic markers, giving a better understanding of the molecular pathogenesis and genetics of the hematological malignancies.
Exact etiology of MPNs is not known; however, patients with a genetic mutation (Janus kinase 2) are associated with a higher risk of developing MPNs. In addition, exposure to a higher level of ionizing radiation and toxins such as benzene has been associated with an increase in the risk for development of MPNs.
Chronic myeloid leukemia: It comprises 0.5% of all new cancer cases in the United States. As per recent SEER (Surveillance, Epidemiology, and End Results) national cancer database from the US, CML is commonly diagnosed in older age groups, ranging from 65 to 74 years, and the median age of diagnosis is 65 years. It is more common in males, with an incidence rate of about 2.4 new cases per 100,000 versus 1.4 new cases per 100,000 in females. About 67.6% of patients have a 5-year survival rate, and the median age of death is about 77 years.
Polycythemia vera: The median age of diagnosis of PV is 60 years. It is more prevalent in males as compared to females, with a male to female ratio of 1.8: 1. The estimated incidence of PV ranges from 0.4 to 2.8 per 100,000 per year.
Essential thrombocythemia: The median age for diagnosis of ET is 60 years. It is more common in females, with a male to female ratio of 1:2. The estimated incidence of ET ranges from 1 to 2.5/100,000/year, and incidence increases with increasing age.
Primary myelofibrosis: The median age at the diagnosis is 67 years. The incidence rate varies from 0.8 to 2.1/100,000/year.
Other MPNs such as CNL, CEL, and unclassified MPN are rare; exact incidence and prevalence are not known due to the rarity of the disorders.
In the last decade, the treatment paradigm has changed with the detection of new driver mutations and a better understanding of the molecular pathogenesis of the MPNs. Mutations are divided into “restricted” which are found only in MPNs and “unrestricted” which are also detected in other myeloid malignancies; these mutations in hematopoietic stem cells (HSC) result in the clonal expansion of all myeloid cells, B cell, and natural killer cells, leading to MPNs.
Cytogenetics of CML is characterized by a reciprocal translocation between long arms of chromosomes 22 and 9, leading to the shortening of the length of the chromosome 22, also known as Philadelphia chromosomes, which is diagnostic of CML. The translocation involves the fusion of breakpoint cluster gene (BCR) on chromosome 22 with oncogene Abelson murine leukemia virus (ABL) on the long arm of chromosome 9 resulting in BCR-ABL fusion gene. The tyrosine kinase activity of the ABL oncogene is further enhanced with the fusion of BCR gene due to an encoding of a chimeric protein from BCR-ABL fusion gene.
In BCR-ABL1 negative MPNs, the restricted driver mutations are JAK2, myeloproliferative leukemia virus proto-oncogene (MPL), CALR, and CSF3R, which encodes for various tyrosine kinases.
Janus kinase (JAK) 2 gene mutation: Janus kinase gene is a type of protein kinase which phosphorylates signal transducer and activator of transcription (STAT) in the JAK-STAT pathway, and activation of this pathway results in the pathogenesis of the MPNs. The JAK2 mutation is found in about 70% of MPNs. It is a somatic mutation which involves substitution of valine to phenylalanine at codon 617 (JAK2 V617F) in the pseudokinase domain. The frequency of JAK2 V617F mutation is 95% in patients with PV, about 50% to 70% in ET, and 40% to 50% in PMF, respectively.
In 5% of PV with JAK2 V617F negative individuals, JAK2 exon 12 mutation is found; however, no such mutation was found in ET and PMF.
MPL proto-oncogene mutation: The MPL gene codes the thrombopoietin (TPO) receptor which regulates megakaryopoiesis through a JAK-STAT pathway. The most common somatic MPL mutations are MPL W515L and MPL W515K which cause spontaneous activation of the JAK-STAT pathway, resulting in abnormal hematopoietic cells proliferation. These mutations are found in up to 6% of ET and up to 10% of PMF as per literature. Furthermore, this mutation will help categorize patients diagnosed with negative JAK2 V617F mutation and Philadelphia chromosome MPN. MPL mutations increase the risk of thrombotic complications as compared to JAK2 V617F mutation patients and are associated with low hemoglobin, low bone marrow (BM) cellularity, high platelets, and high serum erythropoietin levels.
Calreticulin (CALR) gene mutation: The CALR gene is located in chromosome 19 (exon 9) which encodes calreticulin, a calcium-binding endoplasmic reticulum chaperone protein that regulates cellular proliferation, differentiation, and apoptosis; this protein also plays a role in immune system function and wound healing. In 2013, the somatic mutation (usually frameshift mutation) of the CALR gene was discovered in patients with ET and PMF who were negative for JAK2 and MPL mutation. The CALR exon 9 mutations are found in approximately 50% to 75% of patients with ET and PMF.
About 10% to 15% of ET and PMF patients lack all three driver mutations (JAK2 V617F, MPL W515L/K, and CALR exon 9), which are also known as triple negative MPN.
Chronic neutrophilic leukemia (CNL) is one of the myeloproliferative neoplasms which is characterized by peripheral leukocytosis with neutrophilia, hypercellularity of the BM with less than 5% myeloblasts, and normal neutrophil maturation. The JAK2 V617F mutation is common in CNL; however, detection of mutation in colony-stimulating factor 3 receptor (CSF3R) gene is one of the WHO diagnostic criteria for CNL and commonly occurs at a CSF3R T618I location in the gene.
The peripheral blood (PB) and bone marrow (BM) findings of the different diseases are as follows:
Chronic myeloid leukemia (CML): The clinical course of the disease is divided into chronic phase, accelerated phase, and blast crisis; histopathology varies with phases of the CML. Peripheral blood findings are positive for leukocytosis, with slight elevation of eosinophils, basophils, and monocytes. In patients with chronic phase, hypercellular bone marrow, myeloid hyperplasia with left shift in maturation, and myeloblasts up to 10% are present. In accelerated phase, myeloblasts range from 10% to 19%, basophils 20% or higher, additional clonal chromosomal abnormality or other diagnostic criteria should be presented, and in blast crisis, myeloblasts are greater than 20%, which is the transformation to acute myeloid leukemia.
Polycythemia vera (PV): PB in pre-polycythemic and overt polycythemic stage shows normochromic and normocytic RBCs, erythrocytosis with elevated hemoglobin (greater than 16.5 g/dL in male and 16.0 g/dL in female), leukocytosis, and thrombocytosis; however, in concomitant iron deficiency, PB shows a microcytic and hypochromic picture. In the post-polycythemic myelofibrosis stage, PB is characterized by leukoerythroblastosis, with teardrop-shaped RBCs and poikilocytosis. The BM findings in the pre-polycythemic and overt polycythemic stage are significant for hypercellularity and panmyelosis, with a variable proliferation of granulocyte precursors, erythroid cells, and megakaryocytes.
Essential thrombocythemia (ET): PB findings are significant for thrombocytosis (450 X 10^9/L or higher) with platelet anisocytosis. In post-ET myelofibrosis, PB is notable for leukoerythroblastosis, with teardrop-shaped RBCs and poikilocytosis. The BM aspiration and biopsy show mild hypercellularity without any dyserythropoiesis, macrocytosis, and dysgranulopoiesis.
Primary myelofibrosis (PMF): PB shows a leukoerythroblastic picture with immature granulocytes and normoblasts. The BM aspirate is usually a dry tap, but biopsy is characterized by a various degree of fibrosis.
Chronic neutrophilic leukemia (CNL): PB is characterized by leukocytosis (WBC 25 X 10^9/L) with segmended or band neutrophils (80% or higher), few neutrophil precursors (less than 10%), and rare myeloblasts. The BM aspirate and biopsy demonstrates hypercellularity and mature granulocytosis with normal maturation and no dysgranulopoiesis.
Chronic eosinophilic leukemia (CEL): Characterized by peripheral eosinophilia (1,500/uL or higher),, BM with elevated eosinophils, myeloblasts less than 20% and a clonal abnormality.
Clinical manifestation of different myeloproliferative neoplasms are as follows:
Chronic myeloid leukemia (CML): Patient may be asymptomatic with an abnormal blood test, but common clinical manifestations include easy fatiguability, anorexia, weight loss, generalized malaise, sweating, abdominal discomfort or pain, abdominal fullness, early satiety, easy bruising, and bleeding. Physical examination may be positive for pallor, ecchymoses, and hepatosplenomegaly. In addition, priapism and sweet syndrome are rare findings.
Polycythemia vera (PV): Patients with PV have increased red cell mass which contributes to hyperviscosity, hypoxia, and thrombosis. Clinical manifestations of PV include headache, dizziness, visual disturbances, pruritus, malaise, tinnitus, paresthesia, erythromelalgia, abdominal pain, loss of appetite, weight loss, bleeding, and symptoms related to thrombosis. Patients often present with aquagenic pruritus, which is itching/stinging or burning sensation after warm bath or shower. Physical findings may include hepato-splenomegaly, facial plethora, superficial thrombophlebitis, gouty arthritis, conjunctival injection, and hypertension.
Essential thrombocythemia (ET): Clinical symptoms of ET varies from asymptomatic presentation to symptoms due to thrombosis, bleeding, and vasomotor symptoms. Common symptoms include a headache, dizziness, visual changes, paresthesia, fatigue, and easy bruising. Microvascular occlusions involving vessels of distal extremities may cause erythromelalgia and gangrene. A patient may present with stroke symptoms due to thromboembolic complications. Physical examination findings may include splenomegaly, which is more common than hepatomegaly.
Primary myelofibrosis (PMF): The common symptoms include fatigue, night sweats, low-grade fever, early satiety, weight loss, abdominal fullness or discomfort, dysuria, hematuria, gastrointestinal (GI) bleeding, arthralgia, and bony pain. PMF is characterized by extramedullary hematopoiesis and BM fibrosis. Physical examination findings include pallor, petechiae, ecchymoses, splenomegaly, hepatomegaly, lymphadenopathy, pleural effusion, pericardial effusion, ascites, pulmonary edema, seizure, altered mental status, and spinal cord compression.
Chronic neutrophilic leukemia (CNL): Most patients are asymptomatic, but common symptoms on presentation include fatigue, night sweats, loss of appetite, weight loss, easy bruising, and bone pain. A patient may have splenomegaly at diagnosis on physical examination.
The laboratory workup for the evaluation of MPNs includes the following:
Evaluation of MPNs involves comprehensive hematological/laboratory workup, BM biopsy, cytogenetic analysis, and clinical assessment. Owing to recent advances in molecular markers or driver mutations, 2008 WHO classification of myeloid neoplasm was updated in 2016 to incorporate new molecular biomarkers. Diagnostic criteria for MPNs based on revised and updated WHO classification are as follows:
WHO diagnostic criteria for CML (Accelerated and Blast Phase)
The diagnosis of the CML requires any one or more of the following for accelerated phase:
The diagnosis criteria for CML (blast phase) includes any one of the above criteria and blast cells greater than or equal to 20% in peripheral blood or BM or extramedullary involvement of lymph nodes, skin, lung, central nervous system, and bone.
Revised WHO Diagnostic Criteria for Polycythemia Vera
Diagnosis of PV necessitates the presence of either all three major criteria or the first two major criteria and one minor criterion.
Of note, patients with PV who fulfill the diagnostic criteria should also be evaluated for secondary causes of polycythemia.
Revised WHO Diagnostic Criteria for Essential Thrombocythemia
Diagnosis of ET requires either the presence of all four major criteria or the first three major criteria and the minor criterion.
Revised WHO Diagnostic Criteria for Primary Myelofibrosis
The diagnosis of PMF requires the presence of all three major criteria and at least one of the minor criteria. Further division of PMF to overt PMF and prefibrotic PMF is based on a degree of fibrosis.
Revised WHO diagnostic criteria for Chronic Neutrophilic Leukemia
Management or treatment of the different MPNs are as follows:
Chronic myeloid leukemia (CML): Philadelphia chromosome is the BCR-ABL1 fusion gene which results in dysregulated tyrosine kinase activity. Tyrosine kinase inhibitors (TKI) targets tyrosine kinase activity of the BCR-ABL1 fusion gene. CML is divided into three phases: chronic stable phase, accelerated phase, and blast crisis. Patients with chronic stable phase are managed initially with TKIs such as imatinib (first generation TKIs) or nilotinib and dasatinib (second generation TKIs). Patients in accelerated phase and blast crisis are managed with hematopoietic stem cell transplantation (HCT), in addition, HCT can also be considered for young patients with a suitable donor in chronic phase with curative potential.
Polycythemia vera (PV): Patients are divided into high risk and low risk based on age and thrombotic events which also guide the management approach. Patient with age less than or equal to 60 years old with no history of a thrombotic event are classified as low-risk PV, however, patients with greater than 60 years or those who have a history of thrombotic events are considered high-risk patients. Low-risk PV is usually managed with low dose aspirin and therapeutic phlebotomy to maintain hematocrit less than 45%. High-risk PV is managed with therapeutic phlebotomy, low dose aspirin and the use of first-line cytoreductive agents such as hydroxyurea, busulfan, or interferon alfa. For those patients who do not respond to the first-line cytoreductive agents, ruxolitinib (tyrosine kinase inhibitor) as second-line therapy may be beneficial. Patients who develop acquired von Willebrand disease should avoid aspirin due to the increased risk of bleeding. Symptoms such as pruritus and erythromelalgia are generally well controlled with low dose aspirin, but recalcitrant cases may benefit from cytoreductive agents or ruxolitinib.
Essential thrombocythemia (ET): The management of patients with ET is based on vasomotor symptoms, and it also involves prevention of complications such as hemorrhage and thrombotic events. Vasomotor symptoms are usually controlled with low dose aspirin. Patients with high risk (age greater than 60 or thrombotic event at any age with positive JAK2 V617F mutation) or intermediate risk (age greater than 60, negative JAK2 mutation, and no thrombotic event) are treated with a cytoreductive agent (hydroxyurea) and low dose aspirin. Very low or low risk includes patients 60 years or younger, with or without JAK2 mutation and no thrombotic event; these patients are managed based on the symptoms with either low-dose aspirin or clinical monitoring alone.
Primary myelofibrosis (PMF): Allogenic hematopoietic cell transplantation is recommended in patients with the high-risk disease with a potential to cure. Asymptomatic, low-risk patients are managed with observation only; however, a symptomatic, low-risk disease is managed with hydroxyurea.
Chronic neutrophilic leukemia (CNL): Because of the rarity of the disease, there is no specific treatment. Most patients are managed with a cytoreductive agent (hydroxyurea).
With the advent of the tyrosine kinase inhibitors, the prognosis of the CML has significantly improved.
Patients with ET usually have a good prognosis with normal life expectancy.
Triple negative PMF patients have been observed as a worse prognosis. Furthermore, Kampfl et al. reported better survival benefit in patients with CALR exon 9 mutations as compared to JAK2 or MPL mutations.
Patients with PV who transform to MDS or AML have a very poor prognosis. As per one of the retrospective studies, older age (greater than 70 years), leukocytosis, and thrombosis are prognostic factors for worse survival in patients with PV.
Thrombosis and hemorrhagic events are the most common complications in MPNs. Other complications based on diseases are as follows:
Chronic myeloid leukemia (CML): Infection is the most common complication due to the immunocompromised state and blast transformation.
Polycythemia vera (PV): Arterial and venous thrombosis and thromboembolic complications may manifest as transient ischemic stroke (TIA)/stroke and pulmonary embolism, myocardial infarction, and hemorrhage. Some patients with PV may present as acquired von Willebrand disease, and they have an increased tendency for bleeding, particularly if they are taking aspirin. Post-PV myelofibrosis and transformation to AML or MDS are other complications of PV. Erythromelalgia is the microvascular complication of PV which manifest as pallor, erythema, or cyanosis of the hands and feet.
Essential thrombocythemia (ET): Small and large vessel thrombosis, hemorrhage, thromboembolic complications such as stroke, pulmonary embolism, myocardial infarction, pulmonary hypertension, and priapism.
Primary myelofibrosis (PMF): Portal hypertension, GI bleeding, spinal cord compression, respiratory distress, transformation to acute leukemia, thrombotic events, immune deficiency leading to infections, and pulmonary hypertension.