PS996 TCF4 IS IMPORTANT FOR MUTANT RUNX1‐MEDIATED CELL IMMORTALIZATION AND PREDICTS OUTCOME IN ACUTE MYELOID LEUKEMIA

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Background:Aberrations of the RUNX1 transcription factor, either mutations (RUNX1 mut) or translocations t(8;21) creating a RUNX1‐RUNXT1 fusion gene, are frequently observed in AML. However, the pathomechanisms underlying RUNX1 mut AML and explaining its poor prognosis in contrast to t(8;21) cases are still poorly understood.Aims:As recent data suggested deregulation of the transcription factor 4 (TCF4) gene to play a potential role, we studied the importance of TCF4 in the context of RUNX1 mut and t(8;21).Methods:Chip‐sequencing data was analyzed to identify binding of RUNX1 on the TCF4 promoter and Luciferarse reporter assays were used to assay impact of RUNX1 on TCF4 promoter activity.CD34+ UCB cells were transduced with a pRRL construct overexpressing the RUNX1 R201Q mutant (RUNX1mut) or an empty vector (EV). Simultaneously, TCF4‐RNAi (TCF4kd) or a non‐targeting control (NT) was co‐transduced. These double transduced CD34+ UCB cells were cultured on MS5 bone marrow stromal layer and proliferation assays, colony forming unit assays and long‐term culture‐initiating cells assays were performed. TCF4 expression values were derived from a previously reported cohort of 436 AML patients of which 330 had a known RUNX1 status. Survival curves were calculated by the Kaplan‐Meier method and compared using the logrank test. Multivariate survival analysis was carried out using the Cox proportional hazards model, and covariates included were RUNX1 mutation and white blood cell count (WBC > 100 ∗109/L) with and without TCF4 expression (lowest 75% vs highest 25%).Results:While TCF4 is found upregulated in RUNX1 mut in contrast to t(8;21) AML, Chip‐seq and transactivation assays revealed that RUNX1 binds and regulates the TCF4 promoter activity. RUNX1 wild type (RUNX1wt) and the RUNX1‐RUNX1T1 fusion protein repressed TCF4 promoter activity, which was lost in RUNX1mut AML. In CD34+ cells RUNX1 mut enhanced colony‐formation and increased long‐term culture initiating cells, whereas RNA interference based TCF4 downregulation in RUNX1 mut cells abolished this effect. Being required for stem cell maintenance activity in RUNX1 mut AML, TCF4 upregulation accounted for the poor prognostic outcome of RUNX1 mut in AML patients, as RUNX1 mut lost its significance in multivariate analysis if TCF4 expression was included.Summary/Conclusion:In summary, TCF4 is an important downstream target of RUNX1mut AML contributing to leukemogenesis and poor response to standard AML therapy.