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RUNX1-EVI1 blocks RUNX1 and EVI1 driven cell fate
activators and co-repressors to affect gene expression, depending on the genomic context.9,33-35 In accordance with this notion, we did not find RUNX1-EVI1 behaving solely as a repressor or activator of gene expression, with further variation based on the differentiation stage. In early HE, when Mecom expression reaches its peak, we observed a bias towards repression of RUNX1-EVI1 target genes and an activation of the hematopoietic program, indicating that the fusion protein interfered with both the repressive and activating function of EVI1. Conversely, we found a bias towards gene activation in HP for multiple programs where Runx1 expression was upregulated. This result suggests that RUNX1-EVI1 interfered with the repressive activity of RUNX1 which is known to co-oper- ate with other factors to shut down the endothelial gene expression program.36,37 This idea is further supported by our finding that downregulated gene expression in HP can largely be accounted for by lost RUNX1 binding. In con- trast, upregulated gene expression is independent of new RUNX1 binding and is therefore likely caused by other transcription factors. These may include PU.1, which is a known mediator of EVI1 function in myeloid malignan- cy38,39 and which was precociously upregulated following induction of RUNX1-EVI1. PU.1 is a master myeloid reg- ulator which co-operates with RUNX1 in normal hematopoiesis,40,41 and also has roles in cell cycle regula- tion in stem cells.42 Alongside gene expression being upregulated, open chromatin sites gained in HP were enriched for PU.1 motifs and PU.1 target genes such as Csf1r43 were upregulated.
In conclusion, we found that RUNX1-EVI1 disrupts the
function of the endogenous RUNX1 and EVI1 in a develop- mental program specific fashion leading to loss of cell cycle control and an inability of hematopoietic precursor cells to execute and maintain regulated cell fate commitment deci- sions. Our results explain why RUNX1-EVI1 is associated with particularly poor prognosis. It adds to a growing num- ber of oncogenes that as sole drivers are incompatible with hematopoietic stem cell function.44 Our results also high- light the fact that whilst targeting transcription factors such as RUNX1 is a therapy currently being developed,45 cross- talk between multiple transcription networks in the pres- ence of several mutated or mis-expressed transcription fac- tors must also be considered.
Disclosures
No conflicts of interest to disclose.
Contributions
SK performed experiments and data analysis and wrote the paper; PK performed data analysis; EK performed experiments and CB conceived the study and wrote the paper.
Acknowledgments
The authors would like to thank the Genomics Birmingham Sequencing Facility for their expert sequencing service, and the University of Birmingham Flow Cytometry service and Dr Mary Clarke for expert cell sorting.
Funding
SGK received funding from Kay Kendall Leukemia Fund and PK received funding from Bloodwise, awarded to CB.
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