IRX-related homeobox gene MKX is a novel oncogene in acute myeloid leukemia

Homeobox genes encode transcription factors which organize differentiation processes in all tissue types including the hematopoietic compartment. Recently, we have reported physiological expression of TALE-class homeobox gene IRX1 in early myelopoiesis restricted to the megakaryocyte-erythroid-progenitor stage and in early B-cell development to the pro-B-cell stage. In contrast, sister homeobox genes IRX2, IRX3 and IRX5 are aberrantly activated in the corresponding malignancies acute myeloid leukemia (AML) and B-cell progenitor acute lymphoid leukemia. Here, we examined the role of IRX-related homeobox gene MKX (also termed IRXL1 or mohawk) in normal and malignant hematopoiesis. Screening of public datasets revealed silent MKX in normal myelopoiesis and B-cell differentiation, and aberrant expression in subsets of AML and multiple myeloma (MM) cell lines and patients. To investigate its dysregulation and oncogenic function we used AML cell line OCI-AML3 as model which strongly expressed MKX at both RNA and protein levels. We found that IRX5, JUNB and NFkB activated MKX in this cell line, while downregulated GATA2 and STAT5 inhibited its expression. MKX downstream analysis was conducted by siRNA-mediated knockdown and RNA-sequencing in OCI-AML3, and by comparative expression profiling analysis of a public dataset from MM patients. Analysis of these data revealed activation of CCL2 which in turn promoted proliferation. Furthermore, MKX upregulated SESN3 and downregulated BCL2L11, which may together underlie decreased etoposide-induced apoptosis. Finally, myeloid differentiation genes CEBPD and GATA2 were respectively up- and downregulated by MKX. Taken together, our study identified MKX as novel aberrantly expressed homeobox gene in AML and MM, highlighting the function of IRX1 in normal myelopoiesis and B-cell development, and of IRX-related genes in corresponding malignancies. Our data merit further investigation of MKX and its deregulated target genes to serve as novel markers and/or potential therapeutic targets in AML patient subsets.