The IRX1/HOXA connection: insights into a novel t(4;11)-specific cancer mechanism

Introduction: MLL fusion proteins cause the ectopic transcriptional activation of specific HOXA genes. In case of t(4;11) leukemia, this picture becomes challenged, because they separate for yet unknown reasons into “HOXA ^hi” and “HOXA ^lo” patients. HOXA ^lo patients overexpress the homeobox gene IRX1. This was associated with an even higher relapse rate and worse outcome.

Methods: We used transient and stable transfection experiments, gene expression profiling experiments, chromatin-immunoprecipitation experiments, Western blots and diverse PCR techniques.

Results: We demonstrate that IRX1 proteins interact with the MLL-AF4 fusion protein at target gene promoters to counteract its activator function for gene transcription. This causes the downregulation of HOXA gene transcription. In addition, IRX1 causes the overexpression of HOXB4 and EGR1 – 3. HOXB4 is a known downstream target of c-KIT, WNT and TPO signaling in hematopoietic stem cells (HSC's). HOXB4 is necessary for inducing and maintaining HSCs by regulating a complex network of transcription factors. EGR proteins are also important as they are able to induce a p21-dependent circuit which is necessary for stem cell quiescence.

Conclusion: We conclude that IRX1 acts in a dominant-negative manner over MLL-AF4, but allow t(4;11) cells to generate a leukemic stem cell compartment.

This work is funded by grant MA 1876/11 – 1 from the DFG and R14/02 from the Deutsche José Carerras Leukämie-Stiftung.