By Carrie Chenault, M.D.
Although first documented as a unique class of diseases in the 1950s, the pathophysiology of BCR-ABL-negative chronic myeloproliferative neoplasms has been a mystery until recently, when in 2005 four different groups released groundbreaking information in the discovery of a single point mutation in the Janus 2 kinase (JAK2) gene. Since its discovery, JAK2 V617F has been identified in 95% of cases of polycythemia vera (PV) and 50% of cases of essential thrombocythemia (ET) and primary myelofibrosis (MF). In addition, of the roughly 5% JAK2 V617F-negative PV cases, most have been found to harbor exon 12 JAK2 mutations, and some cases of JAK2 V617F-negative ET and MF have activating mutations in the thrombopoeitin receptor (TpoR/MPL) at codon 515. All of these mutations result in activation of the JAK-STAT pathway (Fig. 1).
JAK2 is a tyrosine kinase in the Janus kinase family and acts as an intermediate between membrane receptors such as the erythropoietin or thrombopoeitin receptor and signaling molecules such as STAT, MAP kinase, PI-3 kinase, and Akt. When a membrane receptor binds with a particular cytokine or growth factor, JAK2 becomes phosphorylated and activated. This results in activation of the downstream signaling pathways. The JAK2 V617F mutation is in the pseudokinase domain of the protein and leads to the constitutive activation of the kinase domain. This results in activation of the downstream STAT, MAP kinase, PI-3 kinase, and Akt signaling pathways and thus in survival, proliferation, and differentiation of myeloid progenitor cells (Fig. 1). Interestingly, the mutation at codon 617 is always the same and results in a valine to phenylalanine substitution, and although other amino acid substitutions at codon 617 also lead to constitutive activation of JAK2, none have been identified in vivo. Also, constitutive activation of JAK2 can be seen in a number of malignancies, including lymphoma and myeloma, but the JAK2 V617F mutation is exclusive to myeloid malignancies. Thus, there appears to be something distinct about this mutation and the resultant activation that has yet to be elucidated.
Other important questions still remain. Mainly, how does one mutation yield three distinctly different disease phenotypes?
Early on in the discovery of the JAK2 V617F mutation, it was noted that some patients were homozygous for the mutation while others were heterozygous. Homozygosity is most common in patients with PV while rare in those with ET or MF, suggesting a possible dosing phenomenon. Subsequent studies have bolstered this theory, showing in mice models that low levels of JAK2 V617F lead to an ET-like phenotype with thrombocytosis and high levels lead to a PV-like phenotype with erythrocytosis and leukocytosis without thrombocytosis. Inherited alleles may also play a part in the pathogenesis of myeloproliferative diseases (MPD). Even though JAK2 V617F is a somatic mutation, 5-10% of patients with an MPD have a first degree relative with an MPD suggesting that there are low-penetrance inherited alleles that may play a part in this presumed sporadic disease (Fig. 2).
When should you order a JAK2 mutational analysis?
PCR analysis for the JAK2 V617F mutation is performed in-house at ProPath and can be run on the peripheral blood or bone marrow of any patient where a chronic myeloproliferative disorder is suspected. One purple top tube (EDTA tube) of either bone marrow or peripheral blood can be sent as a part of a bone marrow evaluation or separately, or you can order a complete hematologic evaluation on any blood or bone marrow specimen allowing the ProPath hematopathologist to add such testing if needed. JAK2 V617F testing is important because it yields vital information required for chronic myeloproliferative disease diagnosis and classification by the current WHO classification scheme.
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