Conditional Deletion of Jak2 Reveals an Essential Role in Hematopoiesis throughout Mouse Ontogeny: Implications for Jak2 Inhibition in Humans

Abstract

Germline deletion of Jak2 in mice results in embryonic lethality at E12.5 due to impaired hematopoiesis. However, the role that Jak2 might play in late gestation and postnatal life is unknown. To understand this, we utilized a conditional knockout approach that allowed for the deletion of Jak2 at various stages of prenatal and postnatal life. Specifically, Jak2 was deleted beginning at either mid/late gestation (E12.5), at postnatal day 4 (PN4), or at ∼2 months of age. Deletion of Jak2 beginning at E12.5 resulted in embryonic death characterized by a lack of hematopoiesis. Deletion beginning at PN4 was also lethal due to a lack of erythropoiesis. Deletion of Jak2 in young adults was characterized by blood cytopenias, abnormal erythrocyte morphology, decreased marrow hematopoietic potential, and splenic atrophy. However, death was observed in only 20% of the mutants. Further analysis of these mice suggested that the increased survivability was due to an incomplete deletion of Jak2 and subsequent re-population of Jak2 expressing cells, as conditional deletion in mice having one floxed Jak2 allele and one null allele resulted in a more severe phenotype and subsequent death of all animals. We found that the deletion of Jak2 in the young adults had a differential effect on hematopoietic lineages; specifically, conditional Jak2 deletion in young adults severely impaired erythropoiesis and thrombopoiesis, modestly affected granulopoiesis and monocytopoiesis, and had no effect on lymphopoiesis. Interestingly, while the hematopoietic organs of these mutant animals were severely affected by the deletion of Jak2, we found that the hearts, kidneys, lungs, and brains of these same mice were histologically normal. From this, we conclude that Jak2 plays an essential and non-redundant role in hematopoiesis during both prenatal and postnatal life and this has direct implications regarding the inhibition of Jak2 in humans.

Genetic studies reveal an unexpected negative regulatory role for Jak2 in thrombopoiesis

“…This finding suggests that thrombocytopenia observed with certain JAK2 inhibitors is due to JAK2 inhibition in more proximal stem/progenitor cells consistent with conditional Jak2 knockout studies in hematopoiesis, 15,16 or potentially due to inhibition of other targets. Analogous to genetic Jak2 loss, Jak2 inhibition in PLTs/MKs may actually enhance PLT production, 30 but may be overruled by the impact of Jak2 inhibition on stem/ progenitors.…”

“…11 Germline deletion of Jak2 causes embryonic lethality by deficient definitive erythropoiesis, 12,13 which was confirmed by a conditional Jak2 allele and germline Cre-recombinase. 14 Conditional Jak2 loss in hematopoietic stem cells (HSCs)/progenitor cells induces anemia and thrombocytopenia, 15,16 whereas deletion of Mpl in platelets (PLTs) leads to thrombocytosis. [17][18][19] Although these studies highlight the significance of Jak-Stat signaling for steady state hematopoiesis, the specific requirement for Jak2 in different terminally differentiated myeloid lineages, including PLTs, has not been clarified.…”

JAK of all trades: JAK2-STAT5 as novel therapeutic targets in BCR-ABL1+ chronic myeloid leukemia

“…We are currently unable to predict the effects of JAK2 inhibition on stem and progenitor cells, although our limited information suggests a critical role for JAK2-dependent signaling in both cellular compartments. 106 Identifying the JAK2 vulnerabilities in stem/progenitor cells and defining the potential therapeutic window remains a major challenge. Further studies are required to understand the combined effects of BCR-ABL1 and JAK2 TKIs in mice and humans.…”

JAK2/STAT5 inhibition by nilotinib with ruxolitinib contributes to the elimination of CML CD34+ cells in vitro and in vivo

“…11 The central role of the JAK2/STAT5 axis is clearly demonstrated by the profound effects on hemopoiesis, resulting in embryonic lethality of JAK2 knockout (KO) mice. [12][13][14] Both JAK2 and STAT5 are constitutively active in BCR-ABL 1 cells 15,16 with evidence supporting a role for each in CML leukemogenesis. BCR-ABL 1 cell clones transfected with kinase inactive JAK2 mutant displayed reduced clonogenic potential and tumorogenic activity.…”

“…This suggests that the BM toxicity secondary to JAK2 inhibition in adults might be less than that observed during embryonic development in JAK2 KO mice. [12][13][14] This is to be expected given that chemical inhibition is less complete than gene deletion and evidence that JAK2 scaffolding functions, which would not be targeted by a JAK2 kinase inhibitor, might be important for its activity. 56 In conclusion, our work supports a role for JAK2 in primary CML SPC survival, and provides further preclinical evidence for studies combining TKI with RUX in CML patients.…”