Live and let (MPN cells) die!

In this issue of Blood, Lu et al show that the combination of interferon-α (IFNα) and nutlin-3 is efficient to selectively impair proliferation and differentiation of malignant hematopoietic progenitor cells (HPCs) in myeloproliferative neoplasms (MPNs) in vitro.¹ 

JAK2V617F was the first mutation identified and resembled the ideal candidate for targeted therapy in MPNs.²  By analogy with chronic myeloid leukemia (CML), one could hope that once an oncogenic event is identified, targeted therapy would be developed, leading eventually to a curative approach. Indeed, such a strategy provided a spectacular advance in the therapy of CML with the development of tyrosine kinase inhibitors targeting the BCR-ABL oncogene. We know now that the situation is more complicated in Philadelphia-negative MPNs, as on the one hand more than 15 different mutations (MPL, TET2, ASXL1, and so on) can be found in those illnesses, and on the other, several subclones harboring mutations in genes involved in different pathways can be found in one patient's hematopoietic cells.³  To date, no clear effect on the mutant allele burden of patients with JAKV617F mutation and no impact on bone marrow fibrosis could be firmly demonstrated in JAK2 inhibitor–treated patients, although some promising preliminary results appeared in early phase trials with some of these compounds.⁴  By contrast, IFNα therapy (and particularly therapy using pegylated-IFNα-2a [peg-IFNα-2a]) resulted in significant reduction of the JAK2-mutant allele burden in addition to clinical and histopathologic responses, leading in selected patients with MPN to apparent eradication of the mutated clone with restoration of normal bone marrow morphology.^5,6  Two limitations, however, may hamper the clinical use of IFNα: adverse reactions, leading to treatment discontinuation in 20% to 30% of patients in the long term; and possible resistance of subclones carrying mutations in genes other than JAK2.⁷  These limitations could be overcome using combination therapy allowing reduction of the dose of IFNα (to improve tolerance) and simultaneous targeting of multiple oncogenic pathways.

p53 is one of the most frequently mutated oncogenes in cancer. Such mutations are very rare in chronic-phase MPN but may be acquired during their evolution to acute leukemia.⁸  In addition to genotoxic mutations, wild-type p53 function can be inactivated by increased degradation after binding to murine double minute 2 (MDM2), a negative regulator of the p53 pathway. Inhibition of MDM2 results in p53 stabilization, activation of cell-cycle arrest, and apoptosis pathways, that is, restoring the ability of cancer cells that had developed mechanisms to escape from programmed cell death to die.⁹  Recently, Nakatake et al reported that JAK2V617F-mutated cells exhibit an increased p53 degradation due to an increased MDM2 protein level.¹⁰  In addition, they found that inhibition of MDM2/p53 binding with nutlin-3, a small molecule inhibitor of this interaction, resulted in specific inhibition of the EPO-independent growth of JAK2-mutated cells, suggesting that MDM2 inhibition could reduce the malignant clone in MPNs.

Based on these findings, and because IFNα has been reported to induce apoptosis in target cells in part through up-regulation of p53 activity, Lu et al tested the smart strategy of combining nutlin-3 with peg-IFNα-2a to study their efficacy against primary MPN cells derived from patients with polycythemia vera (PV) with JAK2V617F mutation. Results of this in vitro study show that this combination of drugs is particularly efficient to selectively target MPN-derived HPCs harboring the JAK2V617F mutation with minimal impact on normal hematopoietic cells. While IFNα has many biologic properties that may account for its efficacy in the therapy of MPNs, the authors also provide evidence for a synergistic effect of the IFNα/nutlin-3 combination in the accumulation of p53, by affecting complementary pathways.

The study by Lu et al provides proof of concept for a potential clinical benefit of the combination of low doses of peg-IFNα-2a with nutlin-3 that has to be confirmed in vivo and, potentially, in clinical trials. One important advantage of this combined therapy highlighted by the study is that very low doses of peg-IFNα-2a could be sufficient to achieve efficacy, suggesting that a higher proportion of patients could benefit from long-term IFNα therapy with better tolerance. The second interesting finding is that this combination seems to have a moderate impact on nonclonal hematopoietic cell proliferation and differentiation that could predict lower hematologic toxicity in treated patients. In addition, although this study was performed mainly in cells carrying the JAK2V617F mutation particularly prone to respond to nutlin-3,¹⁰  the mechanisms involved in the effects of both drugs are independent of the presence of JAK2V617F mutation. Therefore, such combined therapy could be equally efficient in all patients with MPNs regardless of the presence of a specific mutation, as suggested by the results observed by Lu et al in cells derived from a patient with myelofibrosis without JAK2 mutation. Finally, measurements of MDM2 protein levels could become a new biomarker useful in MPN management if shown to be able to select patients likely to respond to nutlin therapy or to be a reliable tool to monitor treatment efficacy on the target cells, as was shown for JAK2V617F mutation in patients treated with peg-IFNα-2a.⁵ 

In addition to reducing the apoptotic response to DNA damage, JAK2V617F mutation has also been shown to induce genetic instability, and to influence gene expression through modifications of chromatin structure, mechanisms that may favor evolution to acute leukemia.^2,3  Combining a nonleukemogenic agent like IFNα with nutlin-3 could also reduce the risk of acute transformation by reducing the genomic instability and the accumulation of secondary oncogenic events, and letting the MPN cells die through apoptosis.