A JAK2 mutant to WT prothrombotic cross talk

In this issue of Blood, Liu et al assess the impact of JAK2^V617F clonal hematopoiesis on arterial thrombosis by combining an epidemiologic meta-analysis of 3 large cohorts with the study of JAK2^V617F murine models.¹ 

The JAK2^V617F mutation is the main driver of myeloproliferative neoplasms (MPNs), resulting in a constitutively activated JAK-STAT pathway and subsequent myeloid lineage proliferation. In 2014, the MPN research community was challenged by the unexpected finding of JAK2^V617F clonal hematopoiesis of indeterminate potential (CHIP).² Indeed, JAK2^V617F is one of the most common CHIP mutations, along with TET2, DNMT3A, and ASXL1 gene mutations. The JAK2^V617F CHIP concept has since become controversial with many in the field considering this state as an early or latent MPN, not reaching the World Health Organization definition criteria for overt MPN diagnosis.³ Recently, the term clonal megakaryocyte dysplasia with normal blood values has been suggested to represent a “gray-zone” disease with normal to minimally elevated blood counts and minor abnormal bone marrow histology findings in a subset of the so-called JAK2^V617F CHIP population.⁴ More recent molecular phylogeny studies suggest acquisition of the JAK2^V617F mutation decades prior to MPN diagnosis, highlighting the impact of the intrinsic and extrinsic components on JAK2^V617F clonal fitness.⁵ Thus, one could argue that many patients with MPN transition from a “CHIP” state to clinical manifestations and an overt MPN diagnosis. Clinically, JAK2^V617F CHIP is associated with an increased risk of thrombosis, including cardiovascular disease, ischemic stroke, and splanchnic vein thrombosis, a hallmark of overt MPN.^6-8 It remains unclear whether patients with JAK2^V617F CHIP (1) can be considered “healthy,” (2) require a specific diagnostic workup including bone marrow biopsy, and/or (3) need a dedicated management and follow-up plan. Moving from a statistical association with an increased thrombosis risk to a suggested mechanism and causal link in vivo is a key to more rational management of individuals with JAK2^V617F CHIP.

Arterial thrombosis has already been associated with JAK2^V617F CHIP, and the mechanisms underlying thrombosis in MPN have been studied in vivo using mainly pan-hematopoiesis JAK2^V617F transgenic murine models. Liu et al provide new insights on the effect of partial JAK2^V617F hematopoiesis on platelet activation and subsequent arterial thrombosis. They used bone marrow transplantation within hyperlipidemic Ldlr-deficient recipient mice to generate a relevant CHIP murine model with low mutational burden (1.5%) and no MPN-associated phenotypic changes. Their model shows increased platelet activation, resulting in accelerated arterial thrombosis. Both JAK2^V617F mutated and wild-type (WT) platelets show enhanced thrombin-induced activation in this model, highlighting the importance of studying somatic mutation diseases in a chimeric and not pan-mutated setting for increased clinical relevance. Interestingly, this finding is reminiscent of paracrine mechanisms of MPN transformation into acute myeloid leukemia (AML), where 30% of patients who were post-MPN AML were thought to have developed AML from neighboring non-JAK2^V617F hematopoietic clones. The authors shed light on an interesting paracrine mechanism of activation of WT platelets through a thromboxane-mediated cross talk with JAK2^V617F mutated platelets. In the in vivo CHIP model described, JAK2^V617F platelets play a “dominant negative” role, activating the WT platelets to synergistically result in JAK2^V617F CHIP-associated arterial thrombosis (see figure). Indeed, they also demonstrate a reactive oxygen species–mediated increase in cytosolic phospholipase A2 (cPLA2) activation as well as upregulation of COX-1 and COX-2 expression, concomitantly resulting in thromboxane A2 (TXA2) production by JAK2^V617F platelets. TXA2 production by mutant platelets leads to WT platelet activation. This mechanism poses the rationale for the potential use of aspirin to prevent arterial thrombosis in JAK2^V617F CHIP individuals. Interestingly, while showing increased thrombosis, JAK2^V617F CHIP models with 1.5% and 20% allelic frequencies did not show increased bleeding, unlike panhematopoietic overt MPN models, highlighting a potentially important clinical difference between JAK2^V617F CHIP and overt MPN.⁹ The findings suggest a therapeutic window for the use of aspirin in the JAK2^V617F CHIP setting, with a potentially lower bleeding risk. There was a 2.4-fold increased incidence of coronary artery disease events associated with JAK2^V617F CHIP, whereas non-JAK2 CHIP was associated with a lower incidence (1.09-fold), highlighting the distinctiveness of JAK2^V617F CHIP individuals who could potentially benefit from preventive low-dose aspirin therapy. Beyond CHIP, the mechanistic input brought on by this newly identified paracrine WT–mutant platelets cooperation also offers a potential explanation for the severe thrombosis in patients with overt MPN carrying low JAK2^V617F allelic frequency.

JAK2^V617F-associated thrombosis is multifactorial and features the interplay between diverse hematopoietic cellular components. Indeed, the role of JAK2^V617F mutated macrophages and erythrocytes has been explored mechanistically in murine models in recent work. Increased atherosclerosis was shown to occur through activation of the AIM2 inflammasome in macrophages and erythrophagocytosis and ferroptosis involving erythrocytes.^10,11 Liu et al demonstrate that JAK2^V617F clonal hematopoiesis was associated with young hyperreactive platelet production. The authors also leveraged a murine model where the JAK2^V617F mutation was mainly restricted to the megakaryocytic compartment through Gp1baCre-mediated expression, to elegantly pinpoint this lineage’s key role in arterial thrombosis acceleration. The model showed increased platelet counts, a mild hematocrit increase due to erythroid leaky expression, and no increase in leukocyte counts. Moreover, unlike venous thrombosis, for which JAK2^V617F neutrophils’ increased propensity for neutrophil extracellular trap (NET) formation played a critical role, the authors show no evidence of increased NETosis in arterial thrombosis in their JAK2^V617F clonal hematopoiesis murine models.¹² 

Overall, Liu et al address an important physiopathologic question with potentially major clinical consequences for patients harboring JAK2^V617F CHIP. They pose the rationale for further translational and clinical studies evaluating the JAK2^V617F-WT cross talk mechanistic underpinnings and the benefit/risk balance of aspirin use in individuals with JAK2^V617F CHIP.

Conflict-of-interest disclosure: L.B. has received research support from Gilead and Pfizer for research projects unrelated to the current study.