Abstract
Background: The JAK2 V617F mutation is present in almost all patients with polycythemia vera (PV) and to a large proportion in patients with essential thrombocythemia and idiopathic myelofibrosis. Mice transplanted with JAK2 V617F infected bone marrow develop a myeloproliferative disorder (MPD) resembling PV with secondary myelofibrosis. We have shown that mice with JAK2 V617F induced MPD have significantly higher plasma TNF-alpha levels compared to controls (1). We therefore hypothesized that TNF-alpha may be involved in the pathogenesis of murine JAK2 V617F-induced MPD.
Methods: In a side by side experiment 5-FU stimulated murine bone marrow cells from TNF-alpha knock out mice (B6;129S6-Tnftm1Gkl/J, referred to as TNF-alpha null) and wild type F1-hybrid control mice (B6;129S6F1/J, referred to as TNF-alpha WT) were infected with MIGR1-IRES-GFP retroviral vector carrying murine wild type JAK2 (JAK2-WT) or JAK2-V617F or empty vector control (MIGR1). Cells were transplanted into lethally irradiated TNF-alpha null or WT recipients. The mice were monitored by observation and blood counts. At day 90 mice were euthanized for detailed pathologic analysis.
Results: TNF-alpha WT mice transplanted with TNF-alpha WT cells infected with JAK2-V617F (but not JAK2 WT or MIGR1) developed a PV-like MPD characterized by plethora and elevated hematocrit (HCT) levels (Figure 1). Autopsy and histology revealed splenomegaly with extramedullary hematopoiesis, while the bone marrow showed trilineage hyperplasia and severe reticulin fibrosis. In contrast TNF-alpha null or WT mice transplanted with JAK2-V617F infected TNF-alpha null marrow retained normal blood counts and their spleen weights were similar to controls (Figure 1). Spleen histology showed only mild to moderate infiltration with myeloid cells, with preservation of splenic architecture. Bone marrow cellularity was only slightly increased, and reticulin fibrosis was absent except in one mouse with focally moderate fibrosis. As expected, recipients of JAK2 WT or MIGR1 infected marrow showed no signs of MPD.
Conclusion: TNF-alpha is required for induction of murine JAK2-V617F-positive MPD, at least in the B6;129S6 background. The underlying mechanism is under study. One possibility is that suppression of residual normal hematopoiesis by JAK2-V617F-induced TNF-alpha is required for complete penetrance of the MPD phenotype. This would be in analogy to human PV, where erythroblasts from JAK2 V617F-positive PV patients show increased resistance to CD95-induced apoptosis compared to normal erythroblasts, which may give them a proliferative advantage over normal cells (2).
