In a small series of patients with myeloproliferative neoplasms (MPNs), a novel JAK2 insertion/deletion mutation, which the investigators termed JAK2ex13InDel, was found to drive the development of chronic eosinophilic leukemia (CEL) and polycythemia vera (PV). The protein product of JAK2ex13InDel bears a mechanistic resemblance to the common JAK2V617F mutant protein that drives many myeloproliferative neoplasms and activates downstream, signaling in the absence of a cytokine receptor ligand, the study authors explained in Blood.
“With such a small number of patients, it is hard to draw definitive conclusions, but we believe JAK2ex13InDel may confer a high risk of thrombosis,” said leading study author Ami Patel, MD, assistant professor at the University of Utah and researcher at the Huntsman Cancer Institute. “This would not be unexpected, as both hypereosinophilia and polycythemia are independently associated with elevated thrombotic risk in myeloproliferative neoplasms.”
The mutation was first reported in a patient presenting with an unusual MPN that combined features of PV and CEL. The investigators then identified 3 additional patients with JAK2 insertion/deletion mutations, all of whom had eosinophilia and 1 of whom exhibited a similar PV/CEL phenotype, raising the question of a specific PV/CEL overlap syndrome associated with the JAK2 insertion/deletion mutation.
The investigators performed functional studies and computational modeling on all 4 enrolled patients and found an association between JAK2ex13InDel and deregulation of JAK pathway signaling: Like JAK2V617F, JAK2ex13InDel also activated signal transducer and activator of transcription 5 (STAT5), as well as extracellular signal-regulated kinase (ERK), while transforming parental Ba/F3 cells to growth factor independence.
JAK2ex13InDel did not appear to require an exogenous homodimeric type 1 cytokine receptor to facilitate the transformation of Ba/F3 cells. The JAK2ex13InDel was able to activate ß common chain family cytokine receptor signaling in the absence of ligand, including from the following receptors:
- interleukin (IL)-3 receptor
- IL-5 receptor
- granulocyte-macrophage colony stimulating factor receptor
The maximum effect was found for IL-5 receptor, which was considered consistent with the clinical phenotype for eosinophilia.
JAK signaling inhibitors ruxolitinib and momelotinib were chosen for cell proliferation experiments, which were conducted both in the presence and absence of IL-3. Compared with controls, the 50% inhibitory concentration of ruxolitinib in JAK2ex13InDel Ba/F3 cells was increased tenfold, whereas the 50% inhibitory concentration of momelotinib was increased twofold. Absence of IL-3 augmented the inhibitory effect of ruxolitinib on growth of Ba/F3 JAK2ex13InDel cells. According to the researchers, this experiment suggests that JAK2ex13InDel may transform parental Ba/F3 cells to the stage of cytokine independence to a greater degree than JAK2V617F.
According to Dr. Patel, because patients with a dual phenotype of PV/CEL may be particularly vulnerable to thrombotic events, cytoreductive strategies should be strongly considered.
“Although hydroxyurea can be used for cytoreduction, our findings suggest that targeted JAK2 inhibition with ruxolitinib could disrupt the inflammatory signaling associated with this mutation, ultimately serving as a more effective therapy by specifically inhibiting eosinophil production,” noted Dr. Patel.
“As more information emerges, we hope to define the clinical course and specific risks of [patients with JAK2ex13InDel] in greater detail.”
—Michael Deininger, MD
Screening for end-organ damage with an echocardiogram and a chest CT scan also is advised, given the propensity of eosinophils to infiltrate various tissues. Low-dose aspirin should be considered if a PV phenotype is present, said Dr. Patel.
One surprising finding, according to study co-author Anca Franzini, PhD, also of Huntsman Cancer Institute, was the heterozygous nature of endogenous erythroid colonies (EECs) for JAK2ex13InDel. “This is in contrast to PV, in which EECs are often homozygous for JAK2 mutations,” she said. “I think this illustrates the complexity of MPNs in general and raises questions about how zygosity affects phenotype.”
Another novel finding, added Dr. Franzini, was the very distinct clinical phenotypes that resulted from JAK2ex13InDel and JAK2V617F, despite the 2 genes undergoing similar conformational changes and using similar mechanisms of cellular signaling activation. “Patients with JAK2ex13InDel invariably have hypereosinophilia,” stated Dr. Franzini, compared with only a small minority of patients with JAK2ex13InDel.”
Michael Deininger, MD, principal investigator and senior director of Transdisciplinary Research at Huntsman Cancer Institute, added that the study didn’t address “whether JAK2ex13InDel alone is the primary driver of eosinophilic differentiation, or whether there is some antecedent mutational event that not only establishes the clonal hematopoiesis observed in the index case, but also somehow influences cell fate.”
“Alternatively, although JAK2ex13InDel uses the same regulatory circuit as JAK2V617F, the two mutants are distinct in their downstream signaling, and this must have a structural basis within the JAK2 kinase,” he said. “Clarifying this mechanism may provide new insights into JAK2 regulation.”
The implications of this study are limited by the small number of enrolled patients, Dr. Deininger noted. “As more information emerges, we hope to define the clinical course and specific risks of these patients in greater detail,” he said. “Trials investigating the use of ruxolitinib in hypereosinophilic syndromes are under way and will define the utility of and JAK2 inhibitors in this setting.”
The authors report no relevant conflicts of interest.
Patel AB, Franzini A, Leroy E, et al. JAK2ex13InDel drives oncogenic transformation and is associated with chronic eosinophilic leukemia and polycythemia vera. Blood. 2019;134:2388-2398.