The neuronal homeobox transcription factor HMX3 is a crucial vulnerability in MECOM-negative KMT2A::MLLT3 acute myelomonocytic leukemia [RNAseq_CD34_HSPC_ectopic_HMX3]

The KMT2A::MLLT3 fusion protein causes acute myeloid leukemia (AML) by activating the oncogenic transcription factor MECOM. However, MECOM expression occurs only in half of the cases. By integrating gene expression and enhancer activity data from patient cells, we identified neuronal homeobox transcription factor HMX3 as cell fate determining factor in MECOM-negative KMT2A::MLLT3 AML. HMX3 expression associated with younger age and KMT2A-rearranged leukemia in large AML cohorts (p<0.002). It was absent in other major genetic risk groups and healthy blood cells. Transcriptomic analyses revealed that HMX3 drives cancer-associated E2F, MYC, and cell cycle gene programs. Ectopic HMX3 expression completely inhibited monocytic but not granulocytic colony formation of healthy CD34+ cord blood cells. Its silencing in KMT2A::MLLT3 AML cell lines and patient cells resulted in cell cycle arrest, monocytic differentiation, and apoptosis. Thus, HMX3 is a leukemia-specific vulnerability that enhances proliferation and blocks differentiation of MECOM-negative KMT2A::MLLT3 leukemia. Overall design: RNA sequencing of primary HMX3-positive KMT2A::MLLT3 AML (P7, 4297) (WT), non-targeting control (1418), HMX3 KD (1) (shRNA, 2355), and HMX3 KD (2) (shRNA, 2416).