Primary Myelofibrosis (PMF)

Myelofibrosis is a reactive, reversible increase in bone marrow collagen often with extramedullary hematopoiesis (primarily in the spleen). Myelofibrosis may be

• Primary (more common)

• Secondary to a number of hematologic, malignant, and nonmalignant conditions (see table Conditions Associated With Myelofibrosis)

Primary myelofibrosis results from neoplastic transformation of a pluripotent hematopoietic stem cell. The primary myelofibrosis progeny cells stimulate bone marrow fibroblasts (which are not part of the neoplastic transformation) to produce excessive collagen. The peak incidence of primary myelofibrosis is between 50 and 70 years, and it occurs predominantly in males.

Mutations of the Janus kinase 2 (JAK2) gene are present in a high proportion of cases of primary myelofibrosis. JAK2 is a member of the class I type tyrosine kinase family of enzymes and is involved in signal transduction for erythropoietin, thrombopoietin, and granulocyte colony-stimulating factor (G-CSF) receptors among other entities. Mutations of the thrombopoietin receptor gene (MPL) or the calreticulin (CALR) gene also may be the cause of primary myelofibrosis. However, there are rare cases of primary myelofibrosis in which none of these three mutations are present (triple-negative primary myelofibrosis). Some of these patients have other uncommon MPL mutations, which can only be detected by next generation sequencing.

In primary myelofibrosis, nucleated red blood cells (normoblasts) and myelocytes are released into the circulation (leukoerythroblastosis) when there is extramedullary hematopoiesis (ie, non-marrow organs have taken over blood cell production because of marrow fibrosis ). The serum lactate dehydrogenase level is often elevated. Bone marrow failure eventually occurs, with consequent anemia and thrombocytopenia. Rapidly progressive, chemotherapy-incurable acute leukemia develops in about 30% of patients.

Malignant myelofibrosis (sometimes called acute myelofibrosis), is a rare variant of myelofibrosis characterized by pancytopenia, myeloblastosis, and marrow fibrosis that has a more rapidly progressive downhill course and is generally due to a type of acute leukemia called acute megakaryoblastic leukemia.

In many patients, myelofibrosis is asymptomatic. Other patients have anemia, splenomegaly, or, in later stages, general malaise, weight loss, fever, or splenic infarction. Hepatomegaly occurs in some patients. Lymphadenopathy is rare. Severe extramedullary hematopoiesis can disturb the function of organs in which it occurs, including the brain.

• Complete blood count (CBC) and peripheral blood smear

• Bone marrow aspirate and biopsy

• Testing for JAK2, CALR, and MPL mutations

Primary myelofibrosis should be suspected in patients with some combination of splenomegaly, splenic infarction, and unexplained anemia. If the disorder is suspected, CBC should be done and peripheral blood morphology and a bone marrow biopsy should be examined. If myelofibrosis is present on bone marrow aspirate and biopsy (as detected by reticulin staining or trichrome staining, indicating excess collagen and osteosclerosis), other disorders associated with myelofibrosis (see table Conditions Associated With Myelofibrosis) should be excluded by appropriate clinical and laboratory evaluation.

The diagnosis of primary myelofibrosis is confirmed by detecting a mutation in JAK2, CALR, or MPL. In some cases, none of these mutations is expressed (triple-negative myelofibrosis). A broader next-generation sequencing panel may detect gene mutations associated with increased risk of leukemic transformation and may therefore be useful for prognosis.

Anemia is typically present and usually increases over time. Blood cell morphology is variable. Red blood cells (RBCs) are poikilocytic. Reticulocytosis and polychromatophilia may be present; teardrop-shaped RBCs (dacryocytes) are characteristic morphologic features. Nucleated RBCs and neutrophil precursors are typically present in peripheral blood. White blood cell counts are usually increased but are highly variable. In advanced stages, myeloblasts may be present, even in the absence of acute leukemia. Platelet counts initially may be high, normal, or decreased; however, thrombocytopenia tends to supervene as the disorder progresses.

• Symptomatic therapy

• Sometimes pegylated interferon

• Sometimes allogeneic stem cell transplantation

Treatment is directed at symptoms and complications. Some patients can be observed without treatment.

In early primary myelofibrosis, pegylated interferon has been shown to reduce marrow fibrosis and spleen size and can be used for low-risk patients as defined by various prognostic scoring systems (1).

× 10⁶JAK2

For patients with advanced disease, allogeneic stem cell transplantation may be beneficial and is the only potentially curative treatment. Nonmyeloablative allogeneic stem cell transplantation has been successfully used in older patients.

2).

The median survival in primary myelofibrosis is 5 years from onset, but variation is wide; some patients have a rapidly progressing disorder, including development of acute myeloid leukemia, with short survival, but most have a more indolent course. Only allogeneic stem cell transplantation is curative.

Unfavorable prognostic markers include hemoglobin < 10 g/dL (< 100 g/L), a history of transfusions, leukocytosis, and a platelet count < 100,000/mcL (< 1000 × 10⁹/L). Patients in the least favorable risk group usually survive < 1 year, but those with low-risk disease have a median survival of 10 years.

A number of useful risk stratification systems are available to aid prognosis and guide decisions to institute medical therapy or stem cell transplantation. The Dynamic International Prognostic Scoring System (DIPSS) for Primary Myelofibrosis can be used to predict progression or survival as the disease evolves (1). Some scoring systems, such as (genetically inspired prognostic scoring system [2]) or the MIPSS70+ (mutation and karyotype-enhanced international prognostic scoring system [3]), also incorporate cytogenetics and molecular markers. These can be accessed on line for calculation of risk in individual patients (4).

Validated symptom assessment tools (eg, Myelofibrosis Symptom Assessment Form) are also available for patients with myelofibrosis and can be useful to monitor therapeutic responses (5).

Patients with polycythemia vera or essential thrombocythemia with myelofibrosis usually have a much better prognosis than those with primary myelofibrosis, and their outcome can be predicted by using the MYelofibrosis SECondary to PV and ET Prognostic Model (MYSEC-PM—6).