JAK2 Signaling Specifies Phenotypic Pleiotropy in Myeloproliferative Neoplasms.
Authors: Huang Lily , Yao Huiyu , Ma Yue 
Published: October 2019

Abstract
Abstract 2868
The Janus tyrosine kinase 2 (JAK2) plays an important role in hematopoiesis of multiple lineages. A gain-of-function JAK2 mutation, V617F, is the major determinant in myeloproliferative neoplasms (MPNs), a phenotypically diverse group of hematological diseases in which cells of the myelo-erythroid lineage are overproduced. JAK2 kinase inhibitors showed hematological toxicity in treating MPNs, calling for novel therapeutics that can target only the affected lineage while sparing others. This task is hindered by lack of understanding in how JAK2 signaling differentially regulates the generation of different blood cells.
We performed an unbiased screen for residues essential for JAK2 auto-inhibition, and identified a panel of novel gain-of-function JAK2 mutations in addition to V617F (1). Surprisingly, three activating JAK2 mutants with similar kinase activities in vitro elicited distinctive hematopoietic abnormalities in mice. Specifically, JAK2(K539I) results primarily in erythrocytosis, JAK2(N622I) predominantly granulocytosis, and JAK2(V617F) in both. These phenotypes are consistent with clinical data showing that patients with the V617F mutation exhibit erythrocytosis and granulocytosis, whereas those with mutations in exon 12 (where K539 resides) exhibit erythrocytosis only (2). Quantification of the hematopoietic stem and progenitor populations in mice expressing wild-type JAK2 or JAK2 mutants showed significant granulocytic skewing by JAK2(V617F) and JAK2(N622I) both in the bone marrow and spleen. In contrast, erythroid skewing by JAK2(K539I) was observed. Consistent with these results, qualitative and quantitative differences were observed in signaling events downstream of JAK2 in stem and progenitor cells from mice expressing different JAK2 mutants. JAK2 mutants also caused redistribution of hematopoietic stem and progenitors from the bone marrow to spleen. In later more differentiated compartments, JAK2(K539I) and JAK2(V617F) expanded erythroid precursor cells, including proerythroblasts and later precursors, to cause erythrocytosis, while JAK2(V617F) and JAK2(N622I) expanded myeloid precursors to cause granulocytosis. The expansion of these later compartments was at least in part due to a decrease in apoptosis. Together, our results showed that JAK2 mutants differentially skew early stem and progenitor compartments toward the erythroid or granulocytic lineage, and expand distinct precursor subsets to cause erythrocytosis or granulocytosis in mice. These results provide mechanistic basis for the phenotypic diversity observed in mice expressing different JAK2 mutants.
Our results show that differential JAK2 signaling regulates hierarchically early and late progenitor compartments to drive erythropoiesis vs. granulopoiesis. These results shed light on MPN biology and may facilitate the design of novel and more effective therapeutic agents that specifically target affected lineage without compromising other lineages.

Disclosures:
No relevant conflicts of interest to declare.