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

The KMT2A::MLLT3 fusion protein causes acute myeloid leukemia (AML) by activating the oncogenic transcription factor MECOM. However, MECOM expression occurs in only half of the KMT2A::MLLT3 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). HMX3 was not expressed 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+ adult cells. Silencing of HMX3 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.