The role of Janus Kinase 2 V617F mutation in extramedullary hematopoiesis of the spleen in neoplastic myeloid disorders

Abstract

Extramedullary hematopoiesis (EMH) in the spleen is a characteristic feature of the chronic myeloproliferative disorders (CMPDs) and various other neoplastic or reactive myeloid conditions. However, the origin of these hematopoietic precursor cells and the molecular mechanisms underlying their development in the spleen is uncertain. The V617F mutation in the Janus Kinase 2 gene (JAK2 V617F ) was recently shown to be frequently and preferentially present in the peripheral blood and bone marrow cells of CMPD patients, and the resulting dysregulation of its downstream targets is important to CMPD pathogenesis. To determine the occurrence and potential role of JAK2 V617F in splenic EMH cells, we studied splenectomy specimens from 47 patients with significant EMH. JAK2 V617F was detected by real-time PCR melting curve analysis in 22 specimens, including 11/17 chronic idiopathic myelofibrosis, 7/7 polycythemia vera, 1/1 essential thrombocythemia, 1/3 CMPD unclassifiable, 1/5 chronic myelomonocytic leukemia, 0/5 chronic myelogenous leukemia, 1/3 myelodysplastic syndrome and 0/6 acute myeloblastic leukemia cases, whereas only the JAK2 wild-type allele was detected in the other 25. Nineteen of 20 cases with adequate bone marrow samples available for molecular examination demonstrated concordant JAK2 genotypes. Laser-capture microdissection was then used to enrich the EMH and non-EMH splenic cell fractions, confirming that the mutant alleles specifically originated from the EMH cells. Furthermore, megakaryocytes in the JAK2 V617F -positive splenectomy specimens expressed higher levels of Bcl-xL, an antiapoptotic protein and downstream target of the JAK2/STAT5 pathway. Thus, JAK2 V617F is frequently present in splenic EMH cells associated with CMPD, but it is rarely identified in splenic EMH cells associated with other myeloid disorders. Our results indicate that the precursor cells leading to extramedullary hematopoietic expansion in CMPD most likely originate from the transformed bone marrow clone. Also, dysregulation of downstream pathways such as Bcl-xL may be important to CMPD disease pathogenesis in the spleen. Modern Pathology (2007) 20, 929-935;

Contrasting dynamic responses in vivo of the Bcl-xL and Bim erythropoietic survival pathways

“…28,29,53 High Bcl-x L was suggested as a cause of Epo-independent erythroid differentiation in polycythemia vera and of the apoptosis resistance of other myeloproliferative syndromes and neoplasms. 28,53,54 Our results, supported by recent reports of SOCS protein inactivation in myeloproliferative disease, 55,56 suggest that impairment of p-Stat5 and Bcl-x L adaptation may contribute to their prolonged activation. Together, these point to the importance of adaptation in the Bcl-x L response as a homeostatic and tumor-suppressive mechanism.…”

The implication of identifying JAK2 V617F in myeloproliferative neoplasms and myelodysplastic syndromes with bone marrow fibrosis

“…For each case, DNA was extracted from stained or unstained peripheral blood smears, stained or unstained bone marrow aspirate smears, or formalin-fixed paraffin-embedded bone marrow clot sections using the DNeasy Tissue Kit (Qiagen, Valencia, CA, USA) as previously described [20, 21]. There was a relatively even distribution of each specimen type within each disease category (Table 1).…”

“…We retrospectively evaluated the JAK2 genotype of 45 fibrotic bone marrow specimens, including 19 cases that were originally diagnosed as “with features indeterminate for MDS versus MPN” using our assay that reliably detects JAK2 V617F in archived and paraffin-embedded materials [20, 21]. Our results demonstrated that the presence or absence of JAK2 V617F may have diagnostic implications for these cases.…”

Splenomegaly in myelofibrosis—new options for therapy and the therapeutic potential of Janus kinase 2 inhibitors

“…It is one of 3 Philadelphia chromosome–negative myeloproliferative neoplasms (MPNs) that share elements of pathogenesis and symptomatology that may be related to dysregulated Janus kinase (JAK) signaling [2-4]. The other two are polycythemia vera (PV) and essential thrombocythemia (ET).…”

“…The relationship between dysregulated JAK signaling and the signs and symptoms of MPNs is well established [2-4]. The understanding of dysregulated JAK-STAT (signal transducer and activator of transcription) activity in MPNs as the basic pathophysiologic abnormality in practically all patients with MPNs has led to the clinical development of several JAK2 inhibitors.…”