Depletion of Ptch2 Activates Canonical and Non-Canonical HH Signaling within the Niche Leading to Myeloproliferation, Stem Cell Exhaustion and Accelerates JAK2V617F Driven Disease

Abstract

MPNs often transform into secondary AML associated with poor prognosis and high mortality due to limited therapeutic options. Here we show that aberrant Hedgehog ligand secretion in MPN and CML not only induces constitutive Smoothened-dependent canonical HH signaling causing GLI1 transcription, but also causes PTCH1-dependent non-canonical HH signaling leading to constitutive ERK activation.Our investigations show, that depletion of the Ptch2 receptor in vitro and in vivo recapitulates overactivation of both pathways, while depletion of Ptch1 only reflects constitutive Gli1 activation pinpointing the important role of Ptch1 for Erk activation (Figure 1). Ptch2-/- mice develop a pronounced hematopoietic phenotype with leukocytosis driven by an increase in neutrophils, anemia, thrombocytopenia, loss of T cells combined with a strong increase of cKit+ progenitor and stem cells in the peripheral blood and increased extramedullary hematopoiesis causing splenomegaly reflecting a MPN phenotype. HSCs (Lin- cKit+ Sca1+) residing within the BM (bone marrow) showed enhanced cycling properties causing exhaustion and loss of LKS cells over time (Figure 2), but improved stress hematopoiesis after 5-FU treatment. Niche change experiments show that cytopenias and loss of LKS cells are caused by overactivated HH signaling within the niche cells, causing depletion of osteoblasts and alterations of essential niche factors like Cxcl12, Angiopoietin, Stem cell factor, Thrombopoietin or Jagged1 (Figure 2). In contrast, the hematopoietic Ptch2-/- is responsible for leukocytosis and even promotes HSC expansion and replating capacity in vitro.Interestingly, depletion of Ptch2 in the niche or within hematopoietic cells dramatically altered JAK2V617F-driven pathogenesis causing transformation of a non-lethal chronic myeloproliferative disease into an aggressive AML-like disease with up to 30% blasts in the peripheral blood (Figure 3). In contrast, the BCR-ABL-driven leukemia was exclusively accelerated by the cell intrinsic Ptch2-/-, but not by cell extrinsic HH activation.In conclusion, combined constitutive canonical and non-canonical HH activation induced by depletion of Ptch2 causes a MPN phenotype driven by cell intrinsic, but mainly cell extrinsic mechanisms and accelerates myeloproliferative diseases caused by JAK2V617F and BCR-ABL into acute leukemias. Therefore HH ligand-blocking antibodies or a combination of ERK and SMO inhibitors might be a potential treatment option to prevent transformation of MPNs. [Display omitted] [Display omitted] [Display omitted] DisclosuresNo relevant conflicts of interest to declare.