MPNs – Causes, Symptoms, Diagnosis, Treatment

MPNs (Myeloproliferative neoplasms) are a group of rare blood cancers in which excess red blood cells, white blood cells or platelets are produced in the bone marrow. Myelo refers to the bone marrow, proliferative describes the rapid growth of blood cells and neoplasm describes that growth as abnormal and uncontrolled.

Myeloproliferative Neoplasms/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.[rx] 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.[rx]

Types of Myeloproliferative Neoplasms

The types are

• 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.
• 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.
• 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.
• Chronic neutrophilic leukemia (CNL) – PB is characterized by leukocytosis (WBC 25 X 10^9/L) with segmented 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.

According to WHO diagnostic criteria published in 2016, myeloproliferative neoplasms are diagnosed as follows

• Chronic myeloid leukemia – Chronic myeloid leukemia (CML) has a presence of the hallmark Philadelphia Chromosome (BCR-ABL1) mutation.
• Chronic neutrophilic leukemia – Chronic neutrophilic leukemia (CNL) is characterized by a mutation in the CSF3R gene and an exclusion of other causes of neutrophilia.
• Essential thrombocythemia – Essential thrombocythemia (ET) is diagnosed with a platelet count greater than 450 × 109/L and is associated with the JAK2 V617F mutation in up to 55% of cases^[8] and with an MPL (thrombopoietin receptor) mutation in up to 5% of cases:^[9]. There should be no increase in reticulin fibers and the patient should not meet the criteria for other MPNs, in particular Pre-PMF.
• Polycythemia vera – Polycythemia vera (PV) is associated most often with the JAK2 V617F mutation in greater than 95% of cases, whereas the remainder have a JAK2 exon 12 mutation. High hemoglobin or hematocrit counts are required, as is a bone marrow examination showing “prominent erythroid, granulocytic and megakaryocytic proliferation with pleomorphic, mature megakaryocytes.”
• Prefibrotic/early primary myelofibrosis – Prefibrotic primary myelofibrosis (Pre-PMF) is typically associated with JAK2, CALR, or MPL mutations and shows reticulin fibrosis no greater than grade 1. Anemia, splenomegaly, LDH above the upper limits and leukocytosis are minor criteria.
• Overtly fibrotic myelofibrosis – Like pre-PMF, overt primary myelofibrosis is associated with JAK2, CALR, or MPL mutations. However, a bone marrow biopsy will show reticulin and/or collagen fibrosis with a grade 2 or 3. Anemia, splenomegaly, LDH above the upper limits and leukocytosis are minor criteria.
• MPN-U – Patients with otherwise unexplained thrombosis and with neoplasms that can’t be classified in one of the other categories.

According to Cancer. govt,  There are 6 types of chronic myeloproliferative neoplasms.

The type of myeloproliferative neoplasm is based on whether too many red blood cells, white blood cells, or platelets are being made. Sometimes the body will make too many of more than one type of blood cell, but usually one type of blood cell is affected more than the others are. Chronic myeloproliferative neoplasms include the following 6 types:

These types are described below. Chronic myeloproliferative neoplasms sometimes become acute leukemia, in which too many abnormal white blood cells are made.

Pathophysiology

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.[4]

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.[rx]

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 that 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.[rx] The JAK2 mutation is found in about 70% of MPNs. It is a somatic mutation that involves the 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.[rx]
• 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 cell proliferation.[rx] These mutations are found in up to 6% of ET and up to 10% of PMF as per literature.[rx][rx] 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.[rx][rx]
• 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.[rx] The CALR exon 9 mutations are found in approximately 50% to 75% of patients with ET and PMF.[rx] 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.[rx]

Causes of Myeloproliferative Neoplasms

• The 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.
• MPNs arise when precursor cells (blast cells) of the myeloid lineages in the bone marrow develop somatic mutations which cause them to grow abnormally. There is a similar category of disease for the lymphoid lineage, the lymphoproliferative disorders acute lymphoblastic leukemia, lymphomas, chronic lymphocytic leukemia and multiple myeloma.

Symptoms of Myeloproliferative Neoplasms

The increase in red blood cells can cause the blood to thicken and clot, causing signs and symptoms of polycythemia vera. In early stages, patients may have few symptoms. As the disease progresses, the following symptoms may appear:

• Blood clot
• Fatigue
• Headache
• Excessive sweating
• Blurred vision or blind spots
• Weakness
• Dizziness
• Itchiness, especially after a warm shower or bath
• Redness or a purplish appearance of skin
• Peptic ulcers
• Bloating or a feeling of fullness due to an enlarged spleen
• Congestive heart failure or angina
• Gout/inflammation of joints

Diagnosis of Myeloproliferative Neoplasms

The following tests and procedures may be used:

Evaluation

The laboratory workup for the evaluation of MPNs includes the following

• Complete blood count with differential
• Peripheral blood with microscopic examination
• Comprehensive metabolic panel
• Electrolytes
• Peripheral blood fluorescence in situ hybridization (FISH) or Reverse transcription-polymerase chain reaction (RT-PCR) for BCR-ABL1
• Leukocyte alkaline phosphatase (LAP) score which helps to differentiate between reactive leukocytosis from CML. The LAP score is low in patients with CML and paroxysmal nocturnal hemoglobinuria, however, the score is high in leukemoid reaction.
• Serum uric acid
• Lactate dehydrogenase (LDH)
• Erythropoietin (EPO) level – normal or high in secondary polycythemia, but low or normal in PV.
• Von Willebrand factor – PV patients may develop acquired von Willebrand disease with platelet counts greater than 1 million/microL, and these patients are at increased risk of bleeding, particularly if they are on aspirin.
• Bone marrow aspiration and biopsy
• Cytogenetic analysis of the BM aspirate for Philadelphia Chromosome. FISH for bcr-abl. 
• Peripheral blood molecular testing for JAK2, MPL, CALR, and CSF3R mutations.

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[rx]

WHO diagnostic criteria for CML (Accelerated and Blast Phase)

The diagnosis of the CML requires any one or more of the following for the accelerated phase:

• Persistent or increasing WBC (greater than 10 X 10^9/L), Unresponsive to therapy
• Persistent thrombocytosis (greater than 1000 X 10^9/L), Unresponsive to therapy
• Persistent thrombocytopenia (greater than 100 X 10^9/L), Unrelated to therapy
• Persistent or increasing splenomegaly, Unresponsive to therapy
• Basophils greater than or equal to 20% in peripheral blood
• Blast cells 10% to 19% in peripheral blood or bone marrow
• Additional clonal chromosomal abnormality in Philadelphia positive cells at diagnosis
• Any new clonal chromosomal abnormality in Philadelphia positive cells during therapy

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.

Major criteria

• Hemoglobin greater than 16.5 g/dL in male and hemoglobin greater than 16.0 g/dL in female ORHematocrit greater than 49% in male and hematocrit greater than 48% in female ORIncreased red cell mass greater than 25% above mean normal predicted value
• Bone marrow biopsy with trilineage myeloproliferation (panmyelosis) and hypercellularity for age
• Presence of JAK2 V617F or JAK exon 12 mutation

Minor criterion

• Serum erythropoietin below normal

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.

Major criteria

• Platelet count greater than or equal to 450 X 10^9/L
• Bone marrow biopsy with a proliferation of megakaryocyte lineage with increased numbers of large, mature megakaryocytes. No increase in neutrophil granulopoiesis or erythropoiesis.
• Not meeting WHO criteria for BCR-ABL1 positive CML, PMF, PV, MDS, or other myeloid neoplasms
• Detection of JAK2, CALR, or MPL mutation

Minor criterion

• Presence of clonal marker or absence of evidence for reactive thrombocytosis

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.

Major criteria

• Bone marrow findings of megakaryocyte proliferation with atypia, usually accompanied by reticulin and/or collagen fibrosis
• Not meeting the WHO criteria for BCR-ABL1 positive CML, ET, PV, MDS, or other myeloid neoplasms
• Detection of JAK2, CALR, or MPL mutation or presence of clonal markers in the absence of these mutations

Minor criteria

• Anemia not attributed to another comorbid condition
• Leukocytosis greater than or equal to 11 X 10^9/L
• Palpable splenomegaly
• Elevated LDH
• Leukoerythroblastosis

Revised WHO diagnostic criteria for Chronic Neutrophilic Leukemia

• Peripheral blood with WBC greater than or equal to 25 X 10^9/L, segmented neutrophils plus band forms greater than or equal to 80% of WBCs, neutrophil precursors less than 10%, rare myeloblasts, monocyte count less than 1 X 10^9/L, and no dysgranulopoiesis
• Bone marrow hypercellularity with elevated neutrophil granulocytes, normal neutrophil maturation, and myeloblast less than 5% of nucleated cells
• Not meeting WHO criteria for BCR-ABL1 positive CML, ET, PV, or PMF
• No rearrangement of PDGFRA, PDGFRB, or FGFR1, or PCM1-JAK2
• Detection of CSF3R T618I or other activating CSF3R mutation or in the absence of a CSF3R mutation sustained neutrophilia for at least 3 months, splenomegaly and no identifiable cause of reactive neutrophilia including an absence of a plasma cell neoplasm

Treatment of Myeloproliferative Neoplasms

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 phase accelerated phase and blast phase. Patients with chronic phase are managed initially with TKIs such as imatinib (first-generation TKIs) or nilotinib and dasatinib (second-generation TKIs). Based on ENESTND and DASISION, Nilotionib and Dasatinib respectively were superior to Imatinib in terms of achieving cytogenetic and molecular remission.  Patients in accelerated phase are treated with second-generation TKI and depending upon response may need hematopoietic stem cell transplantation (HCT). However blast phase CML patients undergo treatment with TKI and are evaluated for stem cell transplant.
• 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 the 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) – Patients with Intermediate and high-risk disease with Myelofibrosis benefit from Ruxolitinib. Based on the COMFORT trials Ruxolitinib was helpful in reducing spleen volume and improving symptoms related to Myelofibrosis. Allogeneic hematopoietic cell transplantation is recommended in patients with the high-risk disease with the potential to cure. Asymptomatic, low-risk patients are managed with observation only.
• 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).