PUMA promotes apoptosis of hematopoietic progenitors driving leukemic progression in a mouse model of myelodysplasia

Abstract

Myelodysplastic syndrome (MDS) is characterized by ineffective hematopoiesis with resultant cytopenias. Increased apoptosis and aberrantly functioning progenitors are thought to contribute to this phenotype. As is the case for other malignancies, overcoming apoptosis is believed to be important in progression toward acute myeloid leukemia (AML). Using the NUP98-HOXD13 (NHD13) transgenic mouse model of MDS, we previously reported that overexpression of the anti-apoptotic protein BCL2, blocked apoptosis and improved cytopenias, paradoxically, delaying leukemic progression. To further understand this surprising result, we examined the role of p53 and its pro-apoptotic effectors, PUMA and NOXA in NHD13 mice. The absence of p53 or PUMA but not NOXA reduced apoptosis and expanded the numbers of MDS-repopulating cells. Despite a similar effect on apoptosis and cell numbers, the absence of p53 and PUMA had diametrically opposed effects on progression to AML: absence of p53 accelerated leukemic progression, while absence of PUMA significantly delayed progression. This may be explained in part by differences in cellular responses to DNA damage. The absence of p53 led to higher levels of γ-H2AX (indicative of persistent DNA lesions) while PUMA-deficient NHD13 progenitors resolved DNA lesions in a manner comparable to wild-type cells. These results suggest that targeting PUMA may improve the cytopenias of MDS without a detrimental effect on leukemic progression thus warranting further investigation. Cell Death and Differentiation (2016) 23, 1049-1059 doi:10.1038/cdd.2015; published online 8 January 2016Myelodysplastic syndromes (MDS) are a genetically and clonally heterogeneous group of myeloid malignancies, likely arising from the hematopoietic stem cell. 1-3 Despite their genetic heterogeneity, they share common phenotypic features including low peripheral blood counts (cytopenias) in spite of a hypercellular bone marrow (ineffective hematopoiesis), dysplasia and a variable propensity for transformation to acute myeloid leukemia (AML). 4 Programmed cell death or apoptosis of white blood cells is a common feature, and is thought to contribute to the cytopenias particularly in early stages of the disease. The basis of ineffective hematopoiesis in MDS may also lie in the dysfunction of hematopoietic progenitors and their inability to support adequate bone marrow function. Apoptosis can be triggered by extrinsic factors, such as FAS ligand or TNF-α, or by cell-intrinsic factors such as ribosomal stress or increased reactive oxygen species. A cell extrinsic mechanism for apoptosis in MDS has been favored for many years; 5 however, recent evidence suggests a cell-intrinsic trigger, especially in del(5q) MDS for which there is evidence for activation of the tumor suppressor p53 by ribosomal dysfunction. 6,7 More aggressive stages of MDS have less apoptosis than earlier stages, 8 supporting the paradigm that acquired resistance to apoptosis of malignant progenitors is an important step in cancer progression. 9 Among hematological...

A fate worse than death: apoptosis as an oncogenic process

“…In a ramped-up process of Darwinian selection, rounds of these events, may lead to the development of aggressive and apoptosis resistant AML. Indeed, supporting this idea, inhibiting apoptosis, either through ectopic BCL-2 expression or loss of PUMA expression delays leukemia progression in mouse models of MDS 59,60 . As the authors suggest, if these findings hold These results, together with transcriptomic and in vitro co-culture experiments led the authors to propose a model whereby tumour cell apoptosis reprogram TAM towards a wound-healing type response that fuels tumourigenesis 68 .…”

Loss of Toll-like receptor 2 results in accelerated leukemogenesis in the NUP98-HOXD13 mouse model of MDS

“…These mice exhibit many features of human MDS, including bone marrow dysplasia, cytopenias, and increased apoptosis of hematopoietic stem and progenitor cells (HSPCs) and die of acute leukemia or severe cytopenias. [12][13][14][15] Notably, similar to the CD34 1 cells of patients with MDS, the HSPCs (lineage 2 c-Kit 1 Sca-1 1 [KSL] cells) of NHD13 mice display elevated TLR2 expression and signaling compared with wild-type (WT) controls (Figure 1A-B; supplemental Figure 1A-C, available on the Blood Web site). In this study, the NHD13 mice were crossed to mice lacking TLR2, and the effects of TLR2 loss on the development of cytopenias, leukemogenesis, and survival were assessed.…”

TLR2/6 signaling promotes the expansion of premalignant hematopoietic stem and progenitor cells in the NUP98–HOXD13 mouse model of MDS

“…The HSPCs of NHD13 mice have high rates of cell death [ 23 , 25 ], and this cell death (as measured by Annexin V staining, Figure 2I ) is modestly reduced in the TLR2/6 agonist-treated animals compared with the TLR1/2 agonist-treated ones (with a trending reduction in death compared with water-treated controls). The KSL population from NHD13 mice has previously been reported to be capable of serial replating in methylcellulose and to contain the transplantable, disease-initiating cells [ 26 – 28 ]. To confirm that the expanded KSL population in the PAM 2 CSK 4 -treated mice retains these properties, we sorted KSL cells from WT and NHD13 mice treated for 2 weeks with the TLR1/2 (PAM 3 CSK 4 ) or TLR2/6 (PAM 2 CSK 4 ) agonist, or water alone, and performed serial replatings in MethoCult.…”