The Role Of p53 In JAK2V617F-Induced Myeloproliferative Neoplasms

Ph- myeloproliferative neoplasms (MPNs) are hematopoietic malignancies in which one or more myeloid lineages are abnormally amplified. These diseases represent a group of chronic conditions including polycythemia vera, essential thrombocythemia, and primary myelofibrosis. MPNs mainly affect older people and have an average onset age of 55 years. Complications associated with MPNs include development of acute leukemia as well as thrombosis, hemorrhage, and myeloid metaplasia. JAK2V617F is found in the majority of patients with MPNs, and an overwhelming number of studies have demonstrated its pathogenicity. However, the precise action of JAK2V617F is not well defined, and other molecular defects involved in MPNs remain elusive. In earlier studies, we have generated transgenic mice expressing JAK2V617F in the hematopoietic system and demonstrated that JAK2V617F can cause MPN-like phenotypes in an age- and transgene dose-dependent manner. In this study, we address the involvement of tumor suppressor p53 in the progression of JAK2V617F-induced MPN phenotypes. We crossed our JAK2V617F transgenic mice with p53 knockout mice. Under the p53^+/+ background, our JAK2V617F transgenic mice developed a mild MPN-like phenotypes in 15 weeks. However, under the heterozygous knockout p53^-/+ condition, the mice developed a strong MPN-like phenotype in 8 weeks, manifested in higher blood cell counts, more severe splenomegaly, and earlier onset of myelofibrosis. This suggests that p53 affects the progression of MPNs, and reduced levels of p53 activity may be responsible for the heterogeneous phenotypes observed in MPN patients. Furthermore, when p53 was deleted from both chromosomes, JAK2V617F mice developed acute erythroleukemia, megakaryoblastic leukemia, or myeloid leukemia and died in 20 weeks with greatly enlarged spleen (>10 times the normal size) and liver (twice the normal size) infiltrated with leukemic cells. This indicates that loss of p53 in addition to JAK2V617F causes leukemic transformation. This is consistent with the earlier findings that p53 mutations exist in JAK2V617F-positive leukemia cell lines and JAK2V617F-positive MPNs patients who developed acute myeloid leukemia. Our study also suggests that targeting JAK2V617F and p53 simultaneously may provide effective treatment for MPNs. We employed nutlin-3 that disrupts the MDM2-p53 interaction thereby reducing degradation of p53. In vitro cell culture studies demonstrated that JAK2 inhibitors and nutlin-3 synergistically inhibited the growth of hematopoietic progenitor cells from JAK2V617F transgenic mice and MPN patients. Finally, from one of the p53^-/- JAK2V617F transgenic mice, we derived an erythroleukemia cell line designated J53Z1. J53Z1 cells are CD71^high and Ter119^low and are dependent on erythropoietin for survival. They were effectively inhibited by known JAK2 inhibitors and should be useful in cell-based assays for screening of JAK2 inhibitors. Altogether, our study demonstrates that the level of p53 dictates the progression of JAK2V617F-induced MPNs and targeting p53 and JAK2V617F simultaneously may provide effective treatment for JAK2V617F-positive MNPs.