Myelofibrosis

Myelofibrosis (MF) is a myeloproliferative neoplasm characterized by clonal proliferation of hematopoietic stem cells resulting in progressive bone marrow fibrosis, extramedullary hematopoiesis, and leukoerythroblastic blood picture. It ranks as the most severe of the classic myeloproliferative neoplasms, with a median survival of 5-7 years from diagnosis, though this varies significantly based on prognostic scoring systems. The disease affects approximately 0.5-1.5 per 100,000 persons annually, with peak incidence in the 6th-7th decades of life, showing slight male predominance. Clinical significance derives from its aggressive natural history, risk of acute myeloid leukemia (AML) transformation (10-20% over 10 years), and profound burden of constitutional symptoms and cytopenias that impair quality of life. Understanding MF is essential for board examinations and clinical practice given its intersection with Philadelphia-negative myeloproliferative disease diagnosis and novel JAK inhibitor therapies.

The pathologic foundation of myelofibrosis involves acquisition of somatic mutations in the JAK2, CALR (calreticulin), or MPL (thrombopoietin receptor) genes in hematopoietic stem cells. JAK2 V617F, the most common mutation (present in ~50% of cases), causes constitutive activation of the JAK2 tyrosine kinase, leading to autonomous cytokine signaling independent of normal thrombopoietin (TPO) binding. This results in aberrant STAT3 and STAT5 phosphorylation with uncontrolled proliferation of megakaryocytic and erythroid precursors. CALR mutations (25-30% of cases) produce truncated calreticulin proteins that aberrantly activate the TPO-signaling pathway through MPL engagement. MPL mutations (3-5% of cases) similarly dysregulate TPO signaling. The cumulative effect is autonomous megakaryopoiesis that bypasses normal homeostatic controls, establishing the clonal dominance that characterizes primary MF.

The proliferating megakaryocytes drive the characteristic bone marrow fibrosis through sustained secretion of transforming growth factor-beta (TGF-β), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and fibroblast growth factor (FGF). These cytokines directly stimulate bone marrow fibroblasts and osteoblasts, promoting excessive deposition of reticulin fibers (Grade 1-3 fibrosis, which may progress to collagen fibrosis Grade 3), osteosclerosis, and architectural distortion of the marrow microenvironment. This fibrotic transformation is reactive rather than clonal—the fibroblasts themselves are not malignant but are perpetually activated by the cytokine milieu. Progressive marrow fibrosis paradoxically leads to ineffective hematopoiesis despite increased cellularity, as the fibrotic architecture disrupts normal cell-cell interactions, impairs nutrient delivery, and increases apoptosis of developing blood cells. The severity of fibrosis correlates with cytokine burden and disease progression.

As bone marrow fibrosis progressively impairs intramedullary hematopoiesis, extramedullary hematopoiesis (EMH) emerges in the spleen, liver, and other organs as a compensatory mechanism. Mobilization of circulating CD34+ progenitor cells and homing to extramedullary sites occurs through altered expression of chemokines (particularly SDF-1/CXCL12) and adhesion molecules. The spleen becomes massively enlarged (splenomegaly present in ~90% at diagnosis), progressively producing the bulk of circulating blood cells but also contributing substantially to the cytokine burden through inflammatory cell infiltration. This extramedullary hematopoiesis, while initially compensatory, becomes pathologic as enlarged organs cause mechanical complications (splenic infarction, portal hypertension) and further exacerbate cytokine-mediated constitutional symptoms. The shift from medullary to extramedullary hematopoiesis is reflected in the leukoerythroblastic peripheral blood picture (immature WBCs and nucleated RBCs) characteristic of the disease.

MF is fundamentally a cytokine-driven inflammatory neoplasm. Beyond TGF-β, PDGF, and VEGF, proliferating myeloid cells and megakaryocytes produce IL-1, IL-6, IL-8, TNF-α, and IFN-γ at substantially elevated levels. These inflammatory mediators underlie the constitutional symptom complex (fever, night sweats, weight loss), drive angiogenesis (telangiectasias, bleeding), contribute to cachexia and hypermetabolism, and promote endothelial dysfunction. IL-6 in particular correlates with disease activity and prognosis; IL-6 elevation drives hepatic acute-phase response and contributes to fatigue and cachexia. The inflammatory milieu also promotes progression toward AML through increased genomic instability and impaired apoptosis of dysplastic progenitors.

Beyond the initiating JAK2/CALR/MPL mutations, myelofibrosis accumulates additional somatic mutations in TP53, ASXL1, EZH2, SRSF2, and IDH1/2 genes. These secondary mutations correlate with disease progression, higher blast burden, and increased AML transformation risk. TP53 mutations and ASXL1 mutations are particularly adverse, indicating impaired DNA damage response and dysregulated chromatin remodeling. The acquisition of additional mutations is driven by the chronic inflammatory microenvironment, increased hematopoietic turnover, and inherent genomic instability of neoplastic stem cells.

Primary myelofibrosis is a Philadelphia-negative myeloproliferative neoplasm arising from clonal acquisition of JAK2, CALR, or MPL mutations in hematopoietic stem cells in the absence of prior myeloproliferative disease or other identifiable cause. This is the most common form (~65% of MF cases) and represents true de novo transformation of the bone marrow architecture. The initiating mutation event is stochastic and age-dependent, explaining the peak incidence in older adults. Patients with primary MF have normal bone marrow fibrosis at baseline, and the progressive fibrosis develops insidiously over months to years as the malignant clone expands and cytokine secretion escalates.

Approximately 5-10% of patients with previously diagnosed polycythemia vera (PV) progress to post-PV MF, typically 10-20 years after initial PV diagnosis. This transformation is characterized by the emergence of megakaryopoiesis-driven marrow fibrosis, loss of erythrocytosis, development of cytopenias, and leukoerythroblastosis. The JAK2 V617F clone responsible for the initial erythrocytosis persists and evolves, acquiring additional mutations (ASXL1, TP53) that drive the phenotypic shift toward megakaryocytic dominance and fibrosis. Post-PV MF carries worse prognosis than primary MF due to increased AML transformation risk and higher mutational burden. Prior phlebotomy burden and cytoreductive therapy (hydroxyurea) have historically been considered potential contributors, though recent evidence suggests the natural evolution of the JAK2 clone is the primary driver.

Approximately 1-5% of essential thrombocythemia (ET) patients progress to post-ET MF, similarly reflecting evolution of the underlying myeloproliferative clone. Post-ET MF occurs in both JAK2 V617F-positive and CALR-positive ET patients, with CALR-positive ET carrying lower transformation risk. The mechanism parallels post-PV MF: the initial ET clone continues to proliferate, acquires additional mutations conferring megakaryocytic dominance and marrow-fibrosis-driving phenotype, and progresses to overt MF. Post-ET MF generally carries intermediate prognosis between primary MF and post-PV MF, though outcomes vary with mutation status and comorbidities.

Secondary myelofibrosis (5-10% of all MF) develops from conditions other than PV/ET, including:

In secondary MF, the underlying disease process or cytokine milieu drives reactive bone marrow fibrosis; these cases lack the characteristic JAK2/CALR/MPL mutations and carry distinct natural histories and treatment implications.

Approximately 70-80% of patients present with profound constitutional symptoms including fever, night sweats, and unintentional weight loss (the "B symptoms"), which are among the most distressing and prognostically significant features. These symptoms are driven by the elevated cytokine burden (particularly IL-6, TNF-α, IL-1β) secreted by the hyperproliferative myeloid cells, megakaryocytes, and the expanded extramedullary hematopoietic compartment. Patients typically describe drenching night sweats requiring nighttime clothing changes, low-grade fevers (37.5-38.5°C) that may be intermittent, and progressive unintentional weight loss averaging 5-15% of baseline weight over months. Fatigue and weakness are nearly universal, severe enough to substantially impair functional capacity and quality of life, and are directly correlated with cytokine levels and disease burden. Pruritus (itching) occurs in ~30% of patients and may be severe and refractory, thought to be mediated by histamine release from proliferating mast cells and basophils or by inflammatory cytokines. These constitutional symptoms cause the most significant morbidity for many patients and are key indications for treatment escalation.

Massive splenomegaly is present in approximately 90% of patients at diagnosis, frequently extending 10-15 cm below the left costal margin and occasionally reaching the midline or crossing into the right upper quadrant. The splenomegaly results from both extramedullary hematopoiesis and leukemic infiltration of the splenic parenchyma. Patients commonly report early satiety (inability to eat normal-sized meals due to splenic mass effect on the stomach), left upper quadrant discomfort or pain (splenic infarction from rapid splenic enlargement and ischemia, or splenic rupture from minor trauma), and occasionally palpable left-sided mass on examination. Splenic infarction is a recognized acute complication presenting with acute left upper quadrant pain, elevated LDH, and imaging confirmation. Hepatomegaly is present in 30-50% of patients due to extramedullary hematopoiesis and contributes to abdominal distension. Portal hypertension can develop secondary to hepatic infiltration and splenic venous congestion, leading to ascites, variceal bleeding, and splenomegaly worsening in a vicious cycle.

Despite the hyperproliferative marrow, patients characteristically develop progressive cytopenias as marrow fibrosis advances and ineffective hematopoiesis worsens. Anemia develops in ~50-70% of patients at diagnosis and intensifies over time; the mechanism is multifactorial (ineffective erythropoiesis, marrow fibrosis, chronic disease anemia, splenic sequestration, gastrointestinal bleeding). Thrombocytopenia occurs in ~50% at diagnosis and may be severe, leading to spontaneous bleeding (petechiae, purpura, epistaxis, gum bleeding), particularly when platelets fall below 50,000/μL, though some patients maintain reasonable platelet counts despite marrow fibrosis. Leukopenia is less common at presentation but can develop with disease progression. The combination of severe anemia, thrombocytopenia, and impaired immune function (from disease and treatment) substantially increases infection risk. Infections are a leading cause of morbidity and mortality in advanced disease.

The sustained elevation of inflammatory cytokines drives a hypermetabolic state manifested by cachexia, muscle wasting, accelerated bone loss, and metabolic derangements. Patients have markedly elevated resting energy expenditure, increased cortisol and catecholamine turnover, insulin resistance, and dyslipidemia. Hyperuricemia is common due to rapid myeloid cell turnover and can precipitate gout or uric acid nephropathy; tumor lysis syndrome may occur with aggressive disease or early in treatment. Elevated lactate dehydrogenase (LDH) reflects both hematopoietic and extramedullary cell turnover and correlates with disease burden. Electrolyte abnormalities including hypocalcemia can occur.

Paradoxically, despite thrombocytopenia in some patients, thromboembolic phenomena (deep vein thrombosis, pulmonary embolism, stroke) occur in approximately 10-15% of MF patients, particularly those with higher platelet counts at diagnosis. The thrombotic risk is driven by platelet dysfunction (despite normal or elevated counts), endothelial activation from inflammatory mediators, JAK2 V617F-associated platelet hyperreactivity, and leukocyte-mediated thromboinflammation. Splenic vein thrombosis is a particularly common complication in MF. Patients with prior thrombotic events have particularly high recurrence risk.

Approximately 10-20% of MF patients develop AML transformation over 10 years of follow-up. This typically presents acutely with rapid worsening of symptoms, increasing blast percentage (by definition ≥20% blasts in blood or marrow), disseminated intravascular coagulation (DIC), severe cytopenias, and rapid hemodynamic deterioration. Some patients develop accelerated phase (10-19% blasts) before frank AML. The risk of blast transformation is stratified by prognostic scoring systems; patients with unfavorable mutations (TP53, complex karyotype) or high ASXL1 allele burden have particularly elevated risk.

Diagnosis of primary myelofibrosis requires integration of morphologic, biochemical, and molecular findings per the 2016 WHO classification. The diagnosis is often suspected from the constellation of cytopenias, massive splenomegaly, constitutional symptoms, and leukoerythroblastic blood picture, but definitive diagnosis requires bone marrow biopsy demonstrating bone marrow fibrosis (Grade ≥1, measured on reticulin staining) plus WHO criteria confirming the myeloproliferative neoplasm. The major criterion is presence of JAK2 V617F, CALR mutation, or MPL mutation; if absent

Anemia-directed therapy: erythropoiesis-stimulating agents when endogenous erythropoietin is low and the patient is not heavily transfusion-dependent; androgens (danazol); immunomodulators (thalidomide/lenalidomide, the latter especially with del(5q)); transfusion support with iron-chelation consideration.