NKL Homeobox Gene MSX1 Reactivates An Oncogenic Network In Lymphoid and Myeloid Malignancies

Homeobox genes encode conserved transcription factors (TFs) which regulate fundamental cellular processes during development. Many members of the NKL homeobox gene subfamily are aberrantly expressed in T-cell leukemia and compromise cell differentiation. NKL homeobox gene MSX1 is expressed during embryonic hematopoiesis and its deregulation in Hodgkin lymphoma suggests an oncogenic role of this gene in hematopoietic malignancies. After screening 114 leukemia/lymphoma cell lines by microarray profiling, we detected MSX1 overexpression in three examples each from T-cell acute lymphoblastic leukemia (T-ALL) and mantle cell lymphoma (MCL), and one from acute myeloid leukemia (AML). In silico analysis by R-based statistical tools identified conspicuous expression of MSX1 in 11% of pediatric T-ALL patients, and in 3% each of MCL and AML patients. Thus, we found aberrant MSX1 expression in subsets of both lymphoid and myeloid malignancies. Focusing on MCL and AML we excluded chromosomal rearrangements by classical and molecular cytogenetics at the MSX1 locus underlying overexpression in affected cell lines. However, comparative expression profiling data indicted aberrant histone acetylation involving PHF16 and RTN1, together with TFs FOXC1, HLXB9 and TAL1, as activators of MSX1 transcription. Their involvement was confirmed by siRNA-mediated knockdown and overexpression studies. Reciprocal regulation of MSX1 involved CCND1 and NOTCH signalling. Reporter gene analyses demonstrated that CCND1 and CDKN2D are direct transcriptional targets of MSX1 and its repressive cofactor histone H1C. Fluorescence in situ hybridization showed that t(11;14)(q13;q32) in MCL results in detachment of CCND1 from its corresponding repressive MSX1 binding site. In conclusion, we uncovered a regulatory network around MSX1 in leukemia/lymphoma cells, involving factors and pathways implicated in embryonic hematopoiesis. The reciprocal regulation of MSX1 and the NOTCH pathway in B-cells parallels that of MSX2 in T-cells. These data support the view of a recurrent genetic network involved in hematopoietic development which is reactivated in malignant transformation.