Residual HSPC in the Leukemia Microenvironment Are Reprogrammed Via Extracellular Vesicle Trafficking
Authors: Abdelhamed Sherif , Hornick Noah I , Kurre Peter 
Published: October 2019

Several groups have shown that leukemic cells create a self-reinforcing bone marrow (BM) niche that functionally impairs normal hematopoietic stem and progenitor cells (HSPC) indirectly through stroma-secreted factors. We recently demonstrated an alternative mechanism whereby extracellular vesicles (EVs) from acute myeloid leukemia (AML) patients and cell lines, but not BM CD34 controls, suppress their clonogenicity through EV trafficking of microRNA that directly downregulate critical transcription factors (c-Myb and HoxA9). Here, we aimed to clarify the fate of residual HSPC in in vivo AML xenografts, as well as ex vivo intrafemural (IF) injection and in vitro exposure of EVs experiments. Among KSL cells we observed a significant increase in the frequency of the long-term hematopoietic stem cell (SLAM, CD150+CD48−) subpopulation, but not the multipotent progenitors even at low levels of AML infiltration or direct IF injection of EVs. The HSPC pool redistribution was accompanied by cell cycle alterations in residual HSPC that showed AML EVs consistently induced quiescence (G0) in KSL (cKit+Sca1+Lin−) HSPC populations. When we assessed their DNA damage, residual HSPC showed a distinct increase in the gH2AX foci relative to control non-engrafted mice as well as the transcriptional upregulation of Rad51 and P21 genes along with gains in phosphorylation of the tumor suppressor p53. Yet, the reprogrammed KSL showed no evidence of apoptosis indicated by the lack of upregulation of the p53 target, Puma, and Annexin V staining, nor evidence of senescence (P16 and Sparc transcripts). To gain additional insight, we performed a tandem mass tag (TMT) proteomic profiling of AML-EV exposed HSPC with or without exposure to EVs derived from AML cells. The results showed significant enrichment of DNA methylation regulatory pathway such as DNMT1, HELLS and UHRF1 as well as inflammatory pathways including IL1b, NOS, CEBPB and NFkB pathway-targets, confirmed by transcriptional profiling of KSL from xeno-transplanted mice. Based on our recent report that miR-1246 is one of the most highly enriched miRNA in AML derived EVs and proceeded to determine its target transcripts using an attenuated RISC complex (RISC-Trap), followed by high-throughput sequencing. Bioinformatics analysis identified a set of 27 miR-1246-specific targets relative to control microRNAs. Strikingly, the target set was selectively enriched for a panel of negative cell-cycle regulator genes (CDK1, CDK7, CDK11, CCNF, HDAC2 and GATA3) as well as the DNA methylation regulators (DNMT1 and HELLS).Collectively, our results demonstrated that residual HSPC in the AML BM are phenotypically reprogrammed and suppressed in their proliferation along with DNA damage accumulation via paracrine EV microRNA trafficking. Our study provides insight into HSPC fates in the AML niche and echoes observations of cell competition, as a mode of non-cell autonomous regulation where p53 activation in the reprogrammed cells leads to a progressive decline in proliferation and fitness. We propose that AML EV trafficking of miR-1246 specifically may contribute to the altered fate of residual HSPC via transcriptional regulation of proliferation-related genes.

Disclosures
No relevant conflicts of interest to declare.