PKR Inhibits Hematopoietic Stem Cell Differentiation

Recent work from our laboratory has demonstrated that increased expression and activation of the interferon-inducible, dsRNA-activated kinase (PKR) is associated with inferior survival for many hematologic malignancies including AML.  Mice expressing a PKR transgene (TgPKR) specifically in hematopoietic cells develop a preleukemic/myelodysplastic syndrome (MDS)-like disease consisting of peripheral blood (PB) cytopenias and bone marrow (BM) dysplasia with an increased frequency of BM blasts. In addition, TgPKR mice have a reduced number of BM Lin-, Sca+, cKit+ hematopoietic stem and progenitor cells (HSPCs) and an increased percentage of these cells are quiescent (G0) compared to WT mice.  New data now reveals that BM of TgPKR mice have a significantly reduced frequency of multipotent progenitor cells (MPPs) compared to wild type (WT) mice.  Furthermore, MPPs isolated from BM of TgPKR mice display a partial block of the cell cycle in G0/G1 compared to MPPs from WT mice.  In contrast, PKR knockout mice (PKRKO) have an increased number of HSPCs that are more frequently proliferating (S+G2/M) than WT mice. While the BM of PKRKO mice have a significantly increased frequency of MPPs there is a decreased frequency of both phenotypic LT- and ST-HSCs compared to either WT or TgPKR mice. Thus, in long term culture initiating cell (LTC-IC) assays BM of TgPKR mice has nearly double the colony forming capacity after >4 weeks in culture compared to BM of PKRKO mice, indicating that the TgPKR expressing mice have increased numbers of HSCs that are  functional in vitro but not in vivo.  Significantly, K562 cells with reduced PKR expression by siRNA knockdown display an increased rate and extent of PMA-induced megakaryocyte differentiation compared to control cells. Taken together these results suggest that increased PKR expression/activation as observed in high-risk MDS, AML and other hematologic malignancies may have a previously unrecognized function to block hematopoietic cell differentiation.  This may account, at least in part, for why TgPKR mice accumulate quiescent HSPCs that are unable to terminally differentiate in vivo.  Since we recently discovered that nuclear PKR inhibits DNA damage response signaling that results in an increased frequency of somatic mutations, we propose that increased PKR may contribute to leukemogenesis by promoting both genomic instability and a differentiation block in the HSPC populations.