Dissecting JAK2V617F’s double life with and without MPL Free

In this issue of Blood, Papadopoulos et al delineate myeloproliferative leukemia virus oncogene (MPL)-dependent and -independent phenotypes of JAK2V617F⁺ myeloproliferative neoplasms (MPNs) in vivo.¹ 

BCR::ABL1⁻ classical MPNs are a group of clonal hematologic malignancies characterized by the overproduction of myeloid lineage cells including platelets in essential thrombocythemia (ET), red blood cells in polycythemia vera (PV), and primary myelofibrosis (MF) having bone marrow fibrosis and heterogeneous effects on myeloid cells. These disorders originate from an acquired somatic driver mutation, most commonly the JAK2V617F gain-of-function mutation in hematopoietic stem cells (HSCs), often early in life. The resulting clonal hematopoiesis expands slowly over decades, ultimately manifesting as one of the classic MPN phenotypes, typically in late adulthood. Increasing evidence supports the notion that ET, PV, and MF lie along a biological and clinical continuum, with MF representing the most severe stage of disease evolution.

The natural history of the JAK2V617F-driven MPN continuum is effectively recapitulated in vivo in mouse models. However, the resulting disease phenotype ranging from ET and PV to progression toward MF varies depending on the model design, including whether human or murine JAK2V617F is expressed and whether it is introduced as a transgene or as a knockin under the endogenous promoter.^2,3 These pleiotropic phenotypes are closely linked to variable levels of JAK2 kinase activity across models, mirroring clinical observations: lower mutant allele burdens are typically associated with ET, and higher allele frequencies are more often seen in PV and peak in secondary MF.⁴ Despite these correlations, the precise mechanism by which a single point mutation gives rise to 3 clinically distinct MPN subtypes remains incompletely understood. One prevailing hypothesis suggests that the differential engagement of cytokine receptors by JAK2V617F may shape lineage-specific signaling outputs and ultimately influence disease development and phenotype.

The JAK2V617F mutation is located in the pseudokinase domain, which normally functions as an autoinhibitory regulator of the adjacent kinase domain. The mutation disrupts this regulation, resulting in constitutive or cytokine-hypersensitive activation of JAK2. Activation occurs through interaction with type I cytokine receptors such as the erythropoietin receptor (EPOR), thrombopoietin receptor (MPL), and granulocyte colony-stimulating factor receptor (G-CSFR), promoting their stable dimerization and downstream JAK2/STAT signaling. Notably, the structural plasticity of MPL, which allows it to adopt multiple conformational states, may make it uniquely capable of sustaining signaling from low levels of JAK2V617F more effectively than EPOR.⁵ However, excessive MPL signaling can be deleterious, potentially inducing senescence.⁶ Both of these mechanisms may contribute to the phenotypic divergence observed in JAK2V617F-associated diseases. Since thrombopoietin/MPL signaling is essential for megakaryopoiesis, platelet production, and the maintenance and self-renewal of HSCs, it is critical to better understand its specific role in JAK2V617F-induced MPNs. Although 2 previous studies have explored this question, they relied on a human JAK2V617F transgenic mouse model, which presents clear thrombocytosis but shows exquisitely low PV and fails to progress to MF.^2,3 As a result, the role of MPL in the various features of MPNs, particularly the erythroid lineage, was difficult to evaluate in these models, despite evidence of reduced megakaryopoiesis and some improvement in the overall MPN phenotype.

In this issue of Blood, Papadopoulos et al provide critical insights into the contribution of MPL signaling to JAK2V617F MPNs by leveraging a knockin mouse model expressing Jak2V617F under its endogenous promoter in a Mpl-deficient background. Although the Jak2V617F KI mice faithfully recapitulated the different stages of the human MPN with thrombocytosis, robust PV, and progression to MF, the study clearly distinguishes MPL-dependent from MPL-independent disease features. Consistent with the known role of MPL and previous reports with JAK2V617F × Mpl and Tpo knockout (KO) models,^2,3 the authors observed an abolition of thrombocytosis and a reduction in megakaryocytes and megakaryocytic progenitors in the Mpl KO setting. Furthermore, the expansion of Jak2V617F-mutated HSCs was abolished, and their long-term competitive advantage and repopulation capacity were markedly impaired in the absence of MPL, reinforcing the concept that MPL-expressing HSCs constitute the disease-initiating population.

Importantly, the authors made the unexpected observation that PV, along with the expansion of erythroid progenitors and precursors, occurred independently of MPL, in contrast to a previous report using a different mouse model.⁷ Together with the observed reduction in megakaryocytes, these findings suggest a limited role for JAK2V617F-mutant megakaryocytes in PV development as reported.^7,8 Instead, they support the notion that PV is primarily driven by JAK2V617F expression in erythroid MPN cells. Furthermore, hyperleukocytosis and splenomegaly were only modestly reduced in the absence of MPL, again diverging from earlier studies.^2,3 A mild increase in perivascular fibrosis in the spleen was also noted, accompanied by elevated levels of inflammatory cytokines such as interleukin-1α (IL-1α), IL-1β, IL-6, interferon gamma, and S100A8/A9, all of which appear to be MPL independent. These effects may stem from the persistence of monocytes, basophils, and neutrophils in the Mpl-deficient Jak2V617F model driven by G-CSFR activation induced by JAK2V617F. To assess fibrosis, the authors employed a validated machine learning-based method to assign Continuous Index of Fibrosis scores, which proved instrumental in the quantitative grading of splenic fibrosis and may serve as a valuable tool for future studies. Altogether, these findings support a model in which Jak2V617F-driven MF arises through both MPL-dependent and MPL-independent mechanisms: megakaryocyte hyperplasia in the bone marrow likely drives severe fibrosis via transforming growth factor-β secretion, and inflammatory responses mediated by monocytes and granulocytes may contribute to mild splenic fibrosis in the absence of MPL.⁹ 

In aggregate, these findings underscore the dualistic nature of JAK2V617F signaling in MPN pathogenesis: proliferative outputs in early erythroid and granulocytic progenitors appear to be MPL independent, whereas HSC expansion, long-term dominance, and megakaryopoiesis strictly depend on the thrombopoietin-MPL axis (see figure). This functional distinction clearly delineates MPL-dependent and -independent roles of JAK2V617F, highlighting MPL as an attractive therapeutic target for eradicating disease-initiating HSCs. Emerging strategies to exploit this vulnerability include engineered diabodies (dimeric, bivalent antibody format composed of 2 tandem single chain fragment variables) designed to lock the JAK2V617F-induced MPL dimeric conformation into an inactive state, thereby selectively inhibiting oncogenic signaling.^5,10 

Conflict-of-interest disclosure: The authors declare no competing financial interests.