KLF4 Controls Leukemic Stem Cell Self-Renewal in MLL-AF9-Induced Acute Myeloid Leukemia

Acute myeloid leukemia (AML) develops from sequential mutations which transform hematopoietic stem and progenitor cells (HSPCs) in the bone marrow into leukemic stem cells (LSCs) which drive the progression of frank leukemia. Especially poor outcomes in elderly patients coupled with frequent relapse have led to a dismal 28.3% 5-year survival, warranting the need for innovative therapeutic approaches. Successful targeted therapy will selectively eliminate LSCs, which possess distinct characteristics enabling self-renewal and chemotherapeutic resistance, while sparing normal HSPCs. We theorized that KLF4, a zinc finger transcription factor, maintains key self-renewal pathways in LSCs due to its known importance in preserving stemness in embryonic and cancer stem cells. KLF4 alters gene transcription through its activating and repressing domains as well as remodeling chromatin through various epigenetic mechanisms, and work from our lab has demonstrated that loss of KLF4 in leukemia driven by the BCR-ABL fusion oncogene results in depletion of LSCs (Park et. al in revision) while enhancing self-renewal of hematopoietic stem cells.

To address this hypothesis, mice featuring floxed Klf4 gene (Klf4^fl/fl) were crossed with transgenic Vav-iCre mice to produce mice with hematopoietic-specific deletion of Klf4 (Klf4^Δ/Δ). The murine t(9;11)(p21;q23) translocation (MLL-AF9 or MA9) transduction model has previously been shown to reflect clinical disease attributes, and represents the MLL-rearranged human patient subset with particularly poor prognosis and relatively higher levels of KLF4. Lin⁻Sca-1⁺c-Kit⁺ (LSK) cells from Klf4^fl/fl and Klf4^Δ/Δ mice were transduced with retrovirus containing MA9 and GFP reporter and transplanted into lethally-irradiated wild-type (WT) mice to generate trackable Klf4^fl/fl and Klf4^Δ/ΔAMLs. Recipients of both MA9Klf4^fl/fl and Klf4^Δ/Δ cells developed a rapid expansion of leukemic cells with myeloid immunophenotype by flow cytometric analysis (CD11b⁺Gr-1⁺; 68-91%), characterized as AML with latency of approximately 44.5 days. To quantify the defect induced by loss of KLF4 in the leukemic stem cell population, we performed secondary transplant of multiple limiting-dilution cell doses of primary transformed leukemic bone marrow from moribund mice. Klf4^Δ/Δ AML mice exhibited significantly improved survival in all dose-cohorts, in some cases presenting no detectable leukemic cells at completion of monitoring (225 days). Limiting dilution analysis using the ELDA online software tool demonstrated a 7-fold reduction from 1 in 513 in Klf4^fl/fl to 1 in 3836 in Klf4^Δ/Δ AML bone marrow cells capable of leukemic initiation function (p<0.001), a hallmark of LSCs. Using the ERCre-tamoxifen inducible deletion system, Klf4 deletion 15 days post-transplant of AML significantly improved survival of Klf4^Δ/Δ mice compared to controls, demonstrating KLF4 promotes maintenance of disease. Plating of leukemic bone marrow from Klf4^Δ/Δ mice in methylcellulose medium revealed a reduction in serial colony-forming ability, further supporting a defect in self-renewal. To further determine the mechanisms connected to this reduction in functional LSCs, we isolated leukemic granulocyte-macrophage progenitors (L-GMPs), a population previously reported to be highly enriched for functional LSCs and representing a comparable cellular subset in human clinical samples, from Klf4^fl/fl and Klf4^Δ/Δ AMLs and conducted RNA-Seq to identify potential transcriptional targets of KLF4 with therapeutic promise.

Taken together, these data suggest a novel function of the stemness transcription factor KLF4 in the preservation of leukemic stem cells in AML. Whereas prior models based on KLF4 expression in human cell lines and bulk AML samples have proposed a tumor suppressive role, our work suggests KLF4 supports expansion of leukemic cells with a stem cell phenotype and serial assays suggest an effect on LSC self-renewal. Further studies are being conducted to define the transcriptional and epigenetic mechanisms governing these findings. Understanding the molecular changes induced by loss of KLF4 presents promise for development of new therapies selectively targeting LSCs.