Myeloproliferative neoplasms are a diverse group of conditions that are characterized by an excess of terminally differentiated myeloid cells (red cells, white cells, and/or platelets) in the peripheral blood.
The classic four types of myeloproliferative neoplasms are the following:
According to the World Health Organization classification, there are additional entities categorized under myeloproliferative neoplasms:
- chronic neutrophilic leukemia
- chronic eosinophilic leukemia, not otherwise specified
In contrast to the latter category, there is a separately defined category of myeloid/lymphoid neoplasms associated with eosinophilia characterized by specific gene arrangements (see below).
Some additional conditions demonstrate overlap of features of myeloproliferative neoplasms and myelodysplastic syndromes:
- chronic myelomonocytic leukemia
- atypical chronic myeloid leukemia, BCR-ABL1-negative
- juvenile myelomonocytic leukemia
- myelodysplastic/myeloproliferative neoplasm with ring sideroblasts and thrombocytosis
- myelodysplastic/myeloproliferative neoplasm, unclassifiable
Chronic myeloid leukemia is caused by a reciprocal translocation between chromosomes 9 and 22, creating the fusion gene BCR-ABL1. The shortened chromosome 22 containing the fusion gene is called the Philadelphia chromosome.
All patients with polycythemia vera and a majority of patients with primary myelofibrosis and essential thrombocytopenia have a gain-of-function mutation in JAK2, which encodes a tyrosine kinase. The most common mutation in patients with polycythemia vera results in substitution of valine for phenylalanine in a negative regulatory domain (V617F). Most of the remaining minority of patients with primary myelofibrosis and essential thrombocythemia have a mutation in the CALR or MPL genes.
Known gene rearrangements associated with myeloproliferative causes of hypereosinophilia involve PDGFRA, PDGFRB, FGFR1, or PCM1-JAK2.
Patients may present radiologically with thrombosis or bleeding . Unexplained splanchnic vein thrombosis, involving the hepatic veins (Budd-Chiari syndrome) or portal vein, is quite commonly associated with myeloproliferative neoplasms (particularly polycythemia vera and essential thrombocythemia) and should prompt testing for JAK2 V617F .
Osseous changes are particularly seen in primary myelofibrosis secondary to marrow infiltration, manifest on MRI as abnormally low T1 signal, on radiographs as osteosclerosis, and on bone scintigraphy as diffusely increased skeletal uptake (superscan)
Splenomegaly is common but non-specific. As peripheral blood count abnormalities may sometimes be masked by hypersplenism, other secondary radiologic findings may be the only clue to an occult myeloproliferative neoplasm.
The most common solid lesion is extramedullary hematopoiesis, which can appear as masses in the spleen, liver, or paravertebral tissues.