Myelofibrosis (MF) generally refers to a myeloproliferative neoplasm that is induced by mutations affecting the maturation, differentiation, and function of hematopoietic stem cells. These mutations may be inherited, giving rise to familial myelofibrosis, or acquired, as is the case with other, more common variants of primary myelofibrosis. MF may also be the natural progression of a different myeloproliferative disease, like polycythemia vera or essential thrombocytosis. The respective patients are diagnosed with secondary myelofibrosis. Finally, reactive myelofibrosis may occasionally be observed in those with non-myeloproliferative cancers and many other conditions, such as endocrine disorders, autoimmune diseases, and infections.
MF presentation is diverse. About one-third of MF patients remain asymptomatic for prolonged periods of time, while others develop debilitating constitutional symptoms and signs of bone marrow failure . Regarding the former, fatigue, fever, dyspnea, night sweats, and weight loss are most commonly reported. Patients may become cachectic. Additional symptoms are usually related to hematological abnormalities: There is a proliferation of predominantly megakaryocytes and granulocytes in the bone marrow, which may give rise to leukocytosis and thrombocytosis, along with circulatory disorders due to thrombophilia, thrombosis, and disseminated intravascular coagulation .
As the disease progresses, the reactive deposition of fibrous connective tissue increasingly interferes with hematopoiesis, and patients develop cytopenias. Anemia is common in MF patients, may increase fatigue and weakness, induce pallor and cyanosis. Anemic patients may also claim headaches and dizziness. Leukopenia, on the other hand, predisposes to infections. Thrombocytopenia is less frequently observed than thrombocytosis and renders patients prone to bleeding.
In an intent to compensate for bone marrow failure, extramedullary hematopoiesis is stimulated, entailing hepatomegaly, splenomegaly, and possibly bone pain and osteosclerosis. Besides abnormal laboratory results, palpable organomegaly may indeed be the only finding on clinical examination.
The World Health Organization has defined diagnostic criteria for primary MF and distinct myeloproliferative neoplasms that may ensue fibrotic changes of the bone marrow . Due to the diversity of conditions that may be associated with reactive myelofibrosis, there are no common guidelines on this matter. In general, the diagnosis of MF is based on morphologic features of the bone marrow, molecular and genetic findings, along with laboratory data:
It is of particular importance to determine the etiology of MF and to adapt the therapeutic regimen to the individual case. Reactive MF may be induced by treatable diseases and tend to have a much better outcome than cases related to myeloproliferative neoplasms . Therapeutic options are limited for patients with primary MF or MF related to other myeloproliferative neoplasms.
JAK 1/2 inhibitor ruxolitinib has received approval for MF treatment in 2012 and continues to be the only drug licensed to this end . Ruxolitinib offers a significant benefit in relieving symptoms and reducing splenomegaly, but there is an inherent risk of worsening cytopenias that requires close monitoring of hematological parameters and possibly pharmacological countermeasures . Danazol, erythroid-stimulating agents, and immunomodulatory drugs may be administered to control ruxolitinib-induced anemia, but the results remain unsatisfactory .
Several other JAK inhibitors have been or are currently examined in clinical studies, e.g., fedratinib, pacritinib, momelotinib, and itacitinib. While some have been withdrawn from development due to severe side effects, others have proven to be well tolerated, highly effective, and less myelosuppressive than ruxolitinib . New concepts of MF treatment may be of particular interest to those when ruxolitinib has to be discontinued, as this decision is generally linked to a dismal outcome .
To date, hematopoietic stem cell transplantation remains the only curative option. There are comprehensive guidelines concerning patient eligibility, the use of JAK inhibitors before and after the transplantation, conditioning regimens, and donor selection. In general, patients with increased or high-risk disease and age <70 years should be considered potential candidates for stem cell transplantation, while those with intermediate-risk MF and age <65 years should be proposed as candidates only if they present with either refractory, transfusion-dependent anemia, or a percentage of blasts in peripheral blood >2%, or adverse cytogenetics . In those with low-risk disease, transplant-related morbidity and mortality outweigh the possible benefits of stem cell transplantation . The respective definitions are part of the International Prognostic Scoring System and its extensions and can be looked up in the literature.
The Dynamic International Prognostic Scoring System is widely used to define the prognosis of an individual patient and considers a total of eight predictors: age >65 years, hemoglobin level <10 g/dl, leukocytes >25*10^9/l, platelet count <100*10^9/l, circulating blasts >1%, constitutional symptoms, transfusion dependency, and unfavorable karyotype . An even better prognostication may be achieved by additionally considering morphological data, most importantly the grade of bone marrow fibrosis . Median survival times of primary MF patients are estimated at 6 years but may range from months to several years, which emphasizes the need for individual assessments .
Besides progressive marrow failure and complications from thrombosis, hemorrhages, infections, and portal hypertension, leukemic transformation has been identified as a common cause of death and has, in turn, been related to severe thrombocytopenia and unfavorable karyotypes. In this context, an unfavorable karyotype or platelet count <100*10^9/l may imply a 10-year risk of leukemic transformation of up to 31% .
According to the current classification of the World Health Organization, primary MF is a clonal myeloproliferative neoplasm, as are Philadelphia chromosome-positive chronic myeloid leukemia, chronic neutrophilic leukemia, chronic eosinophilic leukemia, polycythemia vera, essential thrombocythemia, and mastocytosis . Secondary MF may develop as a complication of any of these diseases, but is most commonly seen in patients diagnosed with polycythemia vera and essential thrombocythemia.
All these conditions - MF as well as other myeloproliferative neoplasms - have been related to mutations of the JAK2, CALR, and MPL genes. JAK2 mutations are the most common driver mutations in MF patients and are identified in little more than half of all cases, while abnormalities of CALR are determined in less than one-third of the affected individuals. MPL mutations account for less than 10% of MF cases, and a similar share of cases is classified as triple-negative. Other, as-of-yet unknown driver mutations are assumed to trigger MF in these patients . It shall be stressed again that none of these mutations is specific for MF. Just to take the example of JAK2 V617F, which is determined in nearly half of MF cases: This same mutation accounts for a similar percentage of essential thrombocythemia and as much as 90% of cases of polycythemia vera .
MF driver mutations eventually alter cytokine release by hematopoietic cells and their precursors. Notwithstanding, cytokines promoting bone marrow fibrosis may originate from non-myeloid cells or iatrogenic sources, which must then be considered as triggers of reactive MF. Kuter and colleagues have compiled an extensive, yet not complete list of conditions associated with increased bone marrow fibrosis .
The annual incidence of MF has been estimated at 0.1-1 per 100,000 individuals. Most patients present during their sixth or seventh decade of life, and only about 10% of all cases are diagnosed in individuals aged <40 years  . MF in children is increasingly rare but may occur in the setting of germline mutations and familial MF . Regardless of age, males and females are affected equally.
MF is caused by the proliferation of clonal hematopoietic stem cells. Their proliferative advantage over normal cells is conferred by distinct mutations entailing the constitutive activation of the JAK-STAT pathway. This applies to all patients, regardless of whether they carry any of the known driver mutations or are classified as triple-negative . Increased signaling via the JAK-STAT pathway interferes with the regulation of cytokine release and thus contributes to the creation of a proinflammatory and fibrogenic milieu. In detail, vascular endothelial growth factor, transforming growth factor-β, interferon-γ, interleukins 2, 8, and 17, and lipocalin-2 are believed to act in concert to promote the deposition of fibrous connective tissue. At the same time, the population of clonal cells continues to grow, occupying more and more space. Both mechanisms - bone marrow fibrosis and overgrowth of clonal cells - progressively impede polyclonal hematopoiesis and ultimately lead to a state of bone marrow failure and cytopenias .
Owing to persisting knowledge gaps regarding the triggers of stem cell degeneration, no recommendations can be given to prevent the onset of primary MF. Concerning MF following essential thrombocythemia and polycythemia vera, novel therapeutics aimed at the prevention of disease progression to myelofibrosis are the focus of current clinical trials . Things look much better with regard to reactive MF: Here, the adequate management of the underlying condition underlying may indeed delay or prevent bone marrow fibrosis.
MF is often used as a synonym of primary MF but is actually a descriptive term referring to the presence of bone marrow fibrosis and extramedullary hematopoiesis . These may be observed in a wide range of diseases, including malignancies, endocrine disorders, autoimmune diseases, and infections, which may or may not originate in the bone marrow . By far the most common variant of MF is primary MF, and the second most important cause of the disease are hematological malignancies. The latter may follow a more aggressive course during their later phases and transform to overt MF. Finally, there's a third group of MF cases that may be classified as reactive bone marrow fibrosis. A number of heterogeneous disorders fall into this category, all of which are uncommon etiologies of bone marrow fibrosis.
Myelofibrosis (MF) denotes the progressive deposition of fibrous tissue in the bone marrow. This condition may have different causes but is most commonly induced by acquired mutations of hematopoietic stem cells, the triggers of which remain unknown. The respective patients are diagnosed with primary MF and are usually of advanced age. Another share of patients suffers from secondary MF, which may be considered an advanced stage of myeloproliferative neoplasms such as essential thrombocythemia and polycythemia vera. Finally, MF may be caused by autoimmune diseases, inflammation, infection, endocrine imbalances, and many other disorders. These patients account for a very small share of MF cases, though.
Both the overgrowth of mutated cells and progressive bone marrow fibrosis interfere with the formation of new, healthy blood cells. MF patients may thus present with distinct hematological abnormalities, such as increased or decreased counts of red blood cells, white blood cells, and platelets. They may claim fatigue and weakness due to anemia, be predisposed to infections, thrombosis, or bleeding. As the body tries to compensate for bone marrow failure, hematopoiesis is stimulated elsewhere, entailing the enlargement of liver and spleen.
None of these symptoms is specific for MF, and a thorough workup is required to diagnose this disease and to identify possible causes. Both treatment and prognosis largely depend on many factors, e.g., the age of the patient, the presence of comorbidities, the stage of the disease and the precise mutations underlying MF. The median survival of patients diagnosed with primary MF, the most common form of the disease, has been estimated at 6 years but may range from months to several years.