Overexpression of NF-E2 In Vivo Causes Thrombocytosis and Acute Leukemia: A Murine Model of Myeloproliferative Neoplasms.

Abstract 961

The transcription factor nuclear factor erythroid-2 (NF-E2) is expressed in hematopoietic stem cells as well as in myeloid, erythroid and megakaryocytic precursors. NF-E2 deficient mice display marked anemia at birth and die perinatally due to thrombopenia, demonstrating an essential role for NF-E2 in both in erythropoiesis and platelet formation. We have previously shown that NF-E2 is overexpressed in the vast majority of patients with Myeloproliferative Neoplasms (MPNs). However, the effect of augmented transcription factor activity has not been studied in vivo. We therefore engineered two independent transgenic mouse lines expressing human NF-E2 under the control of the vav-Promoter, which has previously been shown to direct transgene expression in hematopoietic stem cells as well as in precursor cells of all lineages. The two founder lines differed in the degree of NF-E2 overexpression displayed. While one line showed moderate overexpression (2 – 5-fold), the other line expressed human NF-E2 between 10 and 100-fold above the murine counterpart. Both lines paralleled observations in PV patients, where a wide range of NF-E2 overexpression was noted (median overexpression, 7-fold; range 2-fold to 40-fold; n = 59). The two founder lines show overlapping but distinct phenotypes. In both strains. moderately overexpressing NF-E2 transgenic mice (2 – 10-fold) invariably develop thrombocytosis with a latency of 14 months. In addition, megakaryocyte colony formation in the bone marrow is drastically increased. In contrast, thrombocytosis is not observed in the markedly overexpressing NF-E2 transgenic mice (above 20-fold). A similar inverse correlation between the degree of NF-E2 overexpression and platelet numbers was observed in MPN patients.

In both strains, Epo-independent colony formation, a pathognomonic feature of polycythemia vera, is significantly increased in NF-E2 transgenic animals. Bone marrow histopathology shows findings characteristically seen in MPNs, including the presence of increased megakaryopoiesis with cytologically abnormal forms, often in clusters. Both NF-E2 transgenic strains display significantly increased mortality. Upon autopsy, between 15 and 20% of mice in both strains present with major gastrointestinal bleeding in conjunction with splenic atrophy. Spleen weight is reduced by over 50% (Transgenic mice: 49 +/-15 mg, wild type littermates 103 +/- 30 mg; p < 0.001, n = 8 each). One third of the remaining mice show moderate to marked splenomegaly (2 – 27 fold increase in spleen weight; mean: 434 mg, range: 124 – 2700 mg; p < 0.001 vs. wt littermates, n = 12). Histopathological examination of all spleens revealed mild to moderately expanded red pulp with increased numbers of iron containing histiocytes. This observation indicates increased red cell destruction and may explain the fact that neither hematocrit nor hemoglobin are elevated in NF-E2 transgenic animals. At 18 months of age, one mouse developed acute leukemia, which is currently being phenotyped. In summary, in a murine model moderate NF-E2 overexpression causes a phenotype resembling Essential Thrombocythemia. In addition, our preliminary data indicate that NF-E2 overexpression may predispose to the development of acute leukemia.