JAK2V617F-mutant vascular niche contributes to JAK2V617F clonal expansion in myeloproliferative neoplasms

Abstract

The myeloproliferative neoplasms (MPNs) are characterized by hematopoietic stem/progenitor cell (HSPC) expansion and overproduction of blood cells. The acquired mutation JAK2V617F plays a central role in these disorders. Mechanisms responsible for MPN HSPC expansion is not fully understood, limiting the effectiveness of current treatments. Endothelial cells (ECs) carrying the JAK2V617F mutation can be detected in patients with MPNs, suggesting that ECs are involved in the pathogenesis of MPNs. Here we report that JAK2V617F-bearing primary murine ECs have increased cell proliferation and angiogenesis in vitro compared to JAK2WT ECs. While there was no difference between JAK2V617F and JAK2WT HSPC proliferation when co-cultured with JAK2WT EC, the JAK2V617F HSPC displayed a relative growth advantage over the JAK2WT HSPC when co-cultured on JAK2V617F EC. In addition, the thrombopoietin (TPO) receptor MPL is up regulated in JAK2V617F ECs and contributes to the maintenance/expansion of the JAK2V617F clone over JAK2WT clone in vitro. Considering that ECs are an essential component of the hematopoietic niche and most HSPCs reside in the perivascular niche, our studies suggest that the JAK2V617F-bearing ECs form an important component of the MPN vascular niche and contribute to mutant stem/progenitor cell expansion, likely through a critical role of the TPO/MPL signaling axis.

JAK2V617F Megakaryocytes Promote Hematopoietic Stem/Progenitor Cell Expansion in Mice Through Thrombopoietin/MPL Signaling

“…Results from our previous studies and current work provide the first evidence that TPO and MPL have important role(s) in the vascular niche function and altered TPO/MPL signaling can contribute to JAK2V617F‐bearing MK‐induced HSPC expansion via its effect on the vascular niche function. First, we demonstrated that the EC MPL receptor is important for the maintenance/expansion of the JAK2V617F clone over the JAK2WT clone in vitro . Second, we showed that TPO and MPL are important for the expression of key vascular niche factor CXCL12 in marrow ECs in vivo.…”

“…Previously, using an in vitro competitive growth assay in which both JAK2WT and JAK2V617F HSPCs were cultured together in the presence of conditioned medium collected from WT or MPL −/− murine ECs, we found that there were significantly more JAK2WT cells than JAK2V617F cells in the presence of MPL −/− EC conditioned medium compared to in WT EC conditioned medium, suggesting that the EC MPL receptor is important for the maintenance/expansion of the JAK2V617F cells over JAK2WT cells [42]. These observations led us to hypothesize that some secreted factors from vascular ECs contribute to the JAK2V617F clonal expansion in MPNs, and that TPO/MPL signaling is critical for this vascular niche function.…”

The marrow stem cell niche in normal and malignant hematopoiesis

“…[14][15][16] The JAK2 V617F mutation expands the perivascular niche Next, we attempted to determine the cellular basis of how the JAK2 V617F mutation alters ECs to promote neoplastic hematopoiesis in MPNs. First, we found that the JAK2 V617F ECs (isolated from the lungs of Tie2 + /FF1 + mice) displayed significantly increased cell proliferation, wound closure (as a measure of cell migration), tube formation (as a measure of angiogenesis), and decreased apoptosis (after irradiation) compared with normal ECs in vitro, 10 indicating that the kinase mutant might act to expand the vascular niche in JAK2 V617F -positive Tie2 + /FF1 + mice, and alters its function, findings that also characterize the marrow of patients with MPNs. [17][18][19] Based on the known chemokine-and cytokinemediated mechanisms by which C-X-C motif chemokine ligand 12 (CXCL12)-abundant reticular (CAR) niche cells support hematopoiesis, we then measured the expression levels of CXCL12 and stem cell factor (SCF), the two essential niche factors important for HSC maintenance and survival, [20][21][22][23] in freshly isolated marrow ECs from normal and Tie2 + /FF1 + mice.…”

“…In contrast, JAK2 V617F Lin − /cKit + cells displayed a relative growth advantage over the corresponding normal Lin − /cKit + cells when cocultured on JAK2 V617F -positive ECs. 10 These results suggested that the malignant niche differentially supports the proliferation of malignant stem cells over normal cells.…”

“…At the end of the 6-day culture, there were significantly more normal cells than JAK2 V617F cells in the presence of c-Mpl −/− ECCM compared with that seen when cultured with wild-type ECCM, suggesting that EC c-Mpl is important for the maintenance/expansion of the JAK2 V617F cells over normal cells in vitro. 10 This observation led us to hypothesize that certain secreted factors from the vascular ECs contribute to the JAK2 V617F clonal expansion in MPNs and that c-Mpl signaling is critical for this perivascular niche function. Indeed, we found that CXCL12 expression was significantly downregulated in c-Mpl −/− or Thpo −/− marrow ECs compared with marrow ECs from normal mice.…”