Expression of Jak2V617F causes a polycythemia vera-like disease with associated myelofibrosis in a murine bone marrow transplant model

Abstract

An acquired somatic mutation, Jak2V617F, was recently discovered in most patients with polycythemia vera (PV), chronic idiopathic myelofibrosis (CIMF), and essential thrombocythemia (ET). To investigate the role of this mutation in vivo, we transplanted bone marrow (BM) transduced with a retrovirus expressing either Jak2 wild-type (wt) or Jak2V617F into lethally irradiated syngeneic recipient mice. Expression of Jak2V617F, but not Jak2wt, resulted in clinicopathologic features that closely resembled PV in humans. These included striking elevation in hemoglobin level/hematocrit, leukocytosis, megakaryocyte hyperplasia, extramedullary hematopoiesis resulting in splenomegaly, and reticulin fibrosis in the bone marrow. Histopathologic and flow cytometric analyses showed an increase in maturing myeloid lineage progenitors, although megakaryocytes showed decreased polyploidization and staining for acetylcholinesterase. In vitro analysis of primary cells showed constitutive activation of Stat5 and cytokine-independent growth of erythroid colony-forming unit (CFU-E) and erythropoietin hypersensitivity, and Southern blot analysis for retroviral integration indicated that the disease was oligoclonal. Furthermore, we observed strain-specific differences in phenotype, with Balb/c mice demonstrating markedly elevated leukocyte counts, splenomegaly, and reticulin fibrosis compared with C57Bl/6 mice. We conclude that Jak2V617F expression in bone marrow progenitors results in a PV-like syndrome with myelofibrosis and that there are strain-specific modifiers that may in part explain phenotypic pleiotropy of Jak2V617F-associated myeloproliferative disease in humans.

Figure 1.

Splenomegaly and reticulin fibrosis in Jak2V617F mice after BM transplantation. (A) Bar graphs indicate an 11-fold increase in spleen weight in Jak2V617F Balb/c compared with Jak2wt Balb/c animals and an approximately 4-fold increase in spleen weight in Jak2V617F C57Bl/6 compared with Jak2wt C57Bl/6 mice. (B) Moderate to marked diffusely increased reticulin fibrosis in BM of Balb/c mice that had received transplants of Jak2V617F 4.5 weeks after transplantation, which was absent in Jak2wt BM (C).

Figure 2.

Histopathology of Jak2V617F and Jak2wt BM transplant models. Histology of Jak2V617F- and Jak2wt-transduced Balb/c mice showing images of PB (A-B) and pathology in representative sections of BM (C-D), spleen (E-H), and liver (I-L). PB smear (B, 600 ×; Wright-Giemsa stain) of a representative Jak2wt animal displays normal white blood cell counts with an unremarkable differential. In contrast, PB smear (A, 600 ×; Wright-Giemsa stain) of a representative Jak2V617F mutant animal reveals marked leukocytosis consisting predominantly of maturing myeloid elements. Bone marrow images from Jak2wt animals display preserved marrow architecture with normal ratios of myeloid/erythroid elements and unremarkable megakaryocytes (D, 600 ×, H&E stain). In comparison, BM sections from Jak2V617F mutant animals demonstrate marrow elements composed of a prominent population of maturing myeloid cells with mildly to moderately increased numbers of megakaryocytes, including large, atypical forms occurring in occasional clusters (C; 600 ×, H&E stain) and showing emperipolesis of neutrophils in megakaryocyte cytoplasm. Spleen sections from Jak2V617F mice display complete effacement of normal splenic architecture (E; 40 ×, H&E stain) with a marked expansion of red pulp predominantly composed of maturing myeloid elements, frequent atypical megakaryocytes in clusters, and occasional erythroid elements (G; 600 ×, H&E stain) compared with unremarkable Jak2wt spleens (F, H; 40 ×, 600 ×, H&E stain). Liver images from Jak2V617F mice illustrate evidence of extensive extramedullary hematopoiesis in a perivascular and sinusoidal distribution (I; 100 ×, H&E stain) composed of frequent large, atypical megakaryocytes, maturing myeloid cells, and erythroid forms (K; 600 ×, H&E stain) notably absent in livers from Jak2wt animals (J, L; 100 ×, 600 ×, H&E stain).

Figure 3.

Flow cytometric analysis of BM and spleen after transplantation with BM transduced with either Jak2V617F or Jak2wt in Balb/c mice. Dot plots demonstrate an approximately 10-fold increase in the ter119⁺ and the CD71⁺/ter119⁺ population in the spleens of Jak2V617F compared with Jak2wt animals. Most CD71⁺/ter119⁺ cells represented a more immature erythroid population at the level of BFU-Es. Representative plots from 1 of 12 independent experiments for the mutant mice and 1 of 10 experiments for the Jak2wt mice are shown. In the BM of Jak2V617F mice, a prominent population of mature Gr1⁺/Mac1⁺ myeloid cells was detectable that had increased approximately 3-fold compared with the BM of Jak2wt mice and approximately 10-fold in spleens of Jak2V617F compared with spleens of Jak2wt mice. B220⁺ B cells were proportionately decreased in the BM and spleens of Jak2V617F mice compared with Jak2wt mice.

Figure 4.

Increase in survival in Jak2V617F C57Bl/6 compared with Jak2V617F Balb/c mice. (A) Kaplan-Meier survival plot showing death of all Jak2V617F-expressing Balb/c mice between 50 and 95 days marked by the red line (n = 22), of 3 C57Bl/6 mice between 65 and 75 days marked by the blue line (3 of 18), and normal life expectancy for Jak2wt Balb/c and C57Bl/6 mice marked by the green line (n = 10 for each group). (B) Southern blot analysis demonstrating oligoclonal retroviral integration in spleen cells of 5 Jak2V617F mice after transplantation, which was polyclonal in Jak2wt mice and absent in the spleen of an untransduced wild-type animal. (C) Inverse correlation between high endogenous HCTs of 0.65 to 0.75 and low serum EPO levels of 10 pg/mL in 4 Balb/c mice and 1 C57Bl/6 mouse expressing Jak2V617F and normal serum EPO levels of 150 and 250 pg/mL in 1 Balb/c mouse and 1 C57Bl/6 mouse expressing Jak2wt with normal HCTs and appropriately high serum EPO levels in 5-fluorouracil–treated anemic animals.

Figure 5.

Jak2V617F and Stat5 are constitutively activated in vivo. (A) Immunoprecipitation and Western blot analysis in GFP-sorted, Jak2V617F, Jak2wt, and empty vector transfected Ba/F3 cells showing constitutively active Jak2 and Stat5 after 4 hours of starvation in 1% bovine serum albumin in Jak2V617F stable Ba/F3 cells. (B) Phospho-flow analysis showing significant increases in phosphorylated Stat5 in the BM and spleens of Jak2V617F mice compared with Jak2wt mice, as indicated by the significant shift to the left (P < .001).

Figure 6.

Jak2V617F increases the number of erythroid colonies (BFU-E and CFU-E) and total numbers of colonies in a BM transplant model. (A) CFU assays in methylcellulose show an approximately 2-fold increase in the total number of colonies in BM (P < .01) and an approximately 5-fold increase in the spleens of Jak2V617F-compared with Jak2wt-expressing mice (P < .01). Distributions of all types of colonies were comparable with the exception of Jak2wt spleens, in which no GEMMs were detectable (n = 5 Jak2V617F BM; n = 4 Jak2wt BM; n = 8 Jak2V617F spleen; n = 4 Jak2wt spleen). (B) Total numbers of BFU-Es were increased approximately 4-fold in Jak2V617F BM compared with Jak2wt BM mice (P < .01; n = 5 Jak2V617F BM; n = 4 Jak2wt BM, in duplicate) and approximately 9-fold increased in spleens of Jak2V617F compared with Jak2wt mice (P < .01; n = 8 Jak2V617F BM; n = 4 Jak2wt spleens, in duplicate). (C) Growth of CFU-Es in the absence of cytokines and hypersensitivity of CFU-E colonies to EPO in Jak2V617F spleens (P < .01) but not in Jak2wt spleens.

Figure 7.

Loss of polyploid megakaryocytes in Jak2V617F animals. (A, top) Unremarkable 2n, 4n, 8n, 16n, 32n, and 64n peaks in megakaryocytes of Jak2wt in gray. (Middle) Normal 2n and 4n peaks in Jak2V617F but loss or decrease of 8n to 64n peaks in the mutant in red. (Bottom) Overlay. Megakaryocytes were distinguished by size and positive staining for CD41, and polyploidy was distinguished by propidium iodide staining. (B) Megacult assay showing an approximately 2-fold decrease in the BM of Jak2V617F-expressing animals (P < .01).