Ratio of Mutant JAK2-V617F to Wild Type Jak2 Determines the MPD Phenotypes in Transgenic Mice.

The reason why the JAK2-V617F mutation is associated with several phenotypic manifestations of human myeloproliferative disorders (MPD), i.e. polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF), is currently unknown. We established an inducible transgenic mouse model for MPD using a bacterial artificial chromosome (BAC) containing the human JAK2-V617F gene under the control of the JAK2 promoter. The sequences encoding the kinase domain were placed in the inverse orientation and flanked with antiparallel loxP sites to make the construct inducible by Cre-recombinase. A transgenic strain (FF1) containing 9 copies of the JAK2-V617F transgene was analyzed in detail. In this strain, Cre activity can lead to activation and/or excision of the multiple transgene copies. Depending on the number of actively rearranged transgene copies, we observed graded levels of expression of the JAK2-V617F mRNA and different MPD phenotypes. Crossing FF1 mice with transgenic mice expressing Cre-recombinase under the control of the hematopoiesis specific Vav promoter (VavCre) led to reduction of the FF1 copy number and low levels of JAK2-V617F expression (approximately 40% of endogenous wild type Jak2). These FF1;VavCre mice developed a phenotype resembling ET with strongly elevated platelet counts and moderate neutrophilia (Figure 1A). In contrast, induction of the JAK2-V617F transgene with the interferon-inducible MxCre resulted in less excision and higher JAK2-V617F transgene expression (approximately equal to wild type Jak2). These MxCre;FF1 mice displayed a PV phenotype with increased hemoglobin, thrombocytosis and neutrophilia (Figure 1B). The highest expression levels of JAK2-V617F were achieved by retroviral transduction (approximately 300% of wild type Jak2). Transplantation of these bone marrow cells into irradiated recipients caused a PV-like phenotype without thrombocytosis. Thus, the phenotype correlated with the ratio of mutant to wild type JAK2 mRNA. In patients with MPD, we found a similar correlation between the ratio of mutant to wild type JAK2 mRNA and the ET, PV and PMF phenotypes. In contrast to our transgenic mice, which display graded levels of JAK2-V617F with wild type JAK2 being present in every cell, each individual blood cell from patients with MPD can only be homozygous or heterozygous for the mutation, or normal. Therefore, the molecular mechanism determining the phenotype in humans may be more complex than in our mouse model and appears to be linked to the transition of the JAK2-V617F mutation to homozygosity.

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