FMS-like Tyrosine Kinase 3 (FLT3) mutated Acute Myeloid Leukemia (AML) is a common form of AML which poses challenges to treating physicians. In particular, the FLT3 Internal Tandem Duplication (FLT3-ITD) mutation which is associated with poor prognosis in this group of patients.1 Ironically, it has been reported that in FLT3-ITD AML patients, there is a high expression of the tumor suppressor Runt Related Transcription Factor 1 (RUNX1).2 The impact of the co-expression of RUNX1 and FLT3-ITD is not currently understood.
Kira Behrens from the Heinrich Pette Institute, Leibniz Institute for Experimental Virology, Hamburg, Germany, and colleagues published an article ahead of print in The Journal of Experimental Medicine which investigated the effects of high expression levels of RUNX1 on the induction of AML and also explored the interaction between FLT3-ITD mutations and RUNX1.3
The key results of the study were:
- Mice transplanted with FLT3-ITD and RUNX1 transfected Hematopoietic Stem Cell and Progenitor Cells (HSPCs) developed AML compared to Myeloproliferative Neoplasm (MPN) development in mice transplanted with only FLT3-ITD transfected HSPCs
- In RUNX1 deficient HSPCs, there was a shorter latency and increased incidence of AML
- Treatment of MV4:11 cells (FLT3-ITD positive cell) with sunitinib (FLT3 inhibitor), reduced RUNX1 protein levels
- Mutant RUNX1 with altered Tyrosine (Tyr) phosphorylation sites in the Inhibitory Domain (ID) were not sensitive to sunitinib inhibition
- Inactivation of RUNX1 lead to an upregulation of genes involved in differentiation and a downregulation of genes involved in ribosome biogenesis
- Hhex gene expression increases upon activation of RUNX1
- Inhibition of FLT3-ITD with sunitinib blocks RUNX1-mediated Hhex expression
The findings of this study reveal that there is an uncharacterized synergy between RUNX1 and FLT3-ITD mutation in the induction of AML. In summation, RUNX1 can act as a tumor suppressor by suppressing the induction of AML, but after activation by FLT3-ITD (via the promotion of RUNX1 phosphorylation in the Tyr ID), and it can block differentiation via Hhex expression which promotes development of AML.
The authors highlighted that their study is the first to demonstrate the dual tumor suppressor and oncogenic function of RUNX1 in AML. Behrens et al. concluded by stating that “therapies that can reverse this differentiation block will offer significant therapeutic efficacy in AML patients with FLT3-ITD mutations”. Furthermore, they noted that ablation of RUNX1 has a selective toxicity for leukemic stem cells but not normal HSPCs thus suggesting that inhibition of RUNX1 in combination with FLT3 inhibitors might be an effective therapeutic strategy for AML patients.
Acute myeloid leukemia (AML) is induced by the cooperative action of deregulated genes that perturb self-renewal, proliferation, and differentiation. Internal tandem duplications (ITDs) in the FLT3 receptor tyrosine kinase are common mutations in AML, confer poor prognosis, and stimulate myeloproliferation. AML patient samples with FLT3-ITD express high levels of RUNX1, a transcription factor with known tumor-suppressor function. In this study, to understand this paradox, we investigated the impact of RUNX1 and FLT3-ITD coexpression. FLT3-ITD directly impacts on RUNX1 activity, whereby up-regulated and phosphorylated RUNX1 cooperates with FLT3-ITD to induce AML. Inactivating RUNX1 in tumors releases the differentiation block and down-regulates genes controlling ribosome biogenesis. We identified Hhex as a direct target of RUNX1 and FLT3-ITD stimulation and confirmed high HHEX expression in FLT3-ITD AMLs. HHEX could replace RUNX1 in cooperating with FLT3-ITD to induce AML. These results establish and elucidate the unanticipated oncogenic function of RUNX1 in AML. We predict that blocking RUNX1 activity will greatly enhance current therapeutic approaches using FLT3 inhibitors.