Innate Immune Signal Pathway Activation Induced By Combined Loss of Del(5q) MDS Genes, Tifab and Mir-146a, Accelerates Hematopoietic Stem Cell Exhaustion Driven By Chronic Exposure to a TLR Ligands

Deletion of the long arm of chromosome 5 (del(5q)) is the most frequently observed chromosomal abnormality in patients with myelodysplastic syndrome (MDS). We previously reported that deletion of TRAF-interacting protein with forkhead-associated domain B (TIFAB) and miR-146a, two haploinsufficient genes in del(5q) MDS, activates the Toll-like receptor (TLR) signaling cascade in hematopoietic stem/progenitor cells (HSPC) by increasing TRAF6 protein stability and mRNA translation, respectively (Varney et al., JEM 2015 and Varney et al., Leukemia 2017). To investigate the epistasis of TIFAB and miR-146a, we generated a mouse model in which Tifab and miR-146a were simultaneously deleted (Tifab^-/-; miR-146a^-/-, herein dKO). Consistent with increased TRAF6 expression, dKO mice exhibit phosphorylation of IRAK1 and IRAK4 kinases, indicating activation of the TLR/innate immune pathway in bone marrow (BM) HSPC of dKO mice. To determine whether combined deletion of Tifab and miR-146a exaggerates the hematopoietic-specific defects observed in either Tifab- or miR-146a-deficient mice in vivo, BM cells from WT, Tifab^-/-, miR-146a^-/-, and dKO mice were transplanted into lethally-irradiated syngeneic recipient mice. As previously reported, a minor subset of Tifab^-/- and ~ 50% of miR-146a^-/- mice succumbed to a bone marrow failure (BMF)-like disease. For dKO mice, 80% of mice developed a more rapid and penetrant BMF-like disease as compared to loss of Tifab or miR-146a alone. Moribund dKO mice presented with leukopenia, severe anemia, and thrombocytopenia. Since TLR activation is a common feature of MDS due to overexpression and secretion of damage-associated molecular patterns (DAMPs) within the BM microenvironment (Monlish et al., Front Immunol 2016), we explored the biological effect of systemic TLR activation on hematopoietic stem cell (HSC) function of dKO mice following exposure to chronic low dose lipopolysaccharide (LPS), a TLR4 agonist. After 4 weeks, the proportion of myeloid cells in peripheral blood (PB) of LPS-treated dKO mice was significantly increased as compared to WT mice exposed to LPS. BM cellularity of dKO mice was dramatically decreased by LPS exposure while that of WT mice was only slightly affected by LPS. The proportion of CD48^-CD150⁺Lineage^-Sca1⁺c-Kit⁺ (LSK) of dKO mice was significantly reduced by exposure to LPS as compared to WT mice, which correlated with loss of HSC quiescence. We next performed competitive repopulation assays using WT and dKO BM cells exposed to LPS. As expected, the chimerism of donor-derived PB cells from LPS-treated WT mice was significantly lower than that from WT control mice. Intriguingly, LPS-treated dKO mice showed significantly reduced chimerism of donor-derived PB and CD48^-CD150⁺ LSK BM cells as compared to vehicle-treated dKO mice or WT mice exposed to LPS. Importantly, the impaired hematopoietic reconstitution was more pronounced following serial BM transplantation. These results indicate that deletion of Tifab and miR-146a diminishes the long-term repopulating potential of HSC driven by in vivo TLR stimulation. To uncover the molecular consequences that explain the functional HSC defect of dKO mice exposed to TLR activation, RNA-sequencing was performed on LSK from dKO and WT mice treated with LPS. Following TLR activation, we observed differential expression of distinct genes in dKO LSK as compared to WT LSK. Pathway network analyses revealed changes in chromatin organization, epigenetic regulations, and cellular senescence in WT LSK exposed to LPS as compared to control WT cells. In contrast, dKO LSK exposed to LPS exhibited enrichment in pathways associated with axon guidance, carbohydrate metabolism, and megakaryocyte development and platelet production as compared to control dKO cells. These results suggest the differential impact of TLR stimulation on hematopoiesis in Tifab;miR-146a-deficient mice. Collectively, our findings indicate that innate immune signal pathway activation induced by combined loss of Tifab and miR-146a accelerates HSC exhaustion driven by chronic exposure to TLR ligands via the activation of distinct signal pathways. These findings have important implications in the role of TLR stimulation in the cellular consequence of MDS HSC.