The Runx genes are important in development and cancer, where they can act either as oncogenes or tumour suppressors. We compared the effects of ectopic Runx expression in established fibroblasts, where all three genes produce an indistinguishable phenotype entailing epithelioid morphology and increased cell survival under stress conditions. Gene array analysis revealed a strongly overlapping transcriptional signature, with no examples of opposing regulation of the same target gene. A common set of 50 highly regulated genes was identified after further filtering on regulation by inducible RUNX1-ER. This set revealed a strong bias toward genes with annotated roles in cancer and development, and a preponderance of targets encoding extracellular or surface proteins, reflecting the marked effects of Runx on cell adhesion. Furthermore, in silico prediction of resistance to glucocorticoid growth inhibition was confirmed in fibroblasts and lymphoid cells expressing ectopic Runx. The effects of fibroblast expression of common RUNX1 fusion oncoproteins (RUNX1-ETO, TEL-RUNX1 and CBFB-MYH11) were also tested. Although two direct Runx activation target genes were repressed (Ncam1 and Rgc32), the fusion proteins appeared to disrupt the regulation of downregulated targets (Cebpd, Id2 and Rgs2) rather than impose constitutive repression. These results elucidate the oncogenic potential of the Runx family and reveal novel targets for therapeutic inhibition.

The Runx genes play paradoxical roles in cancer where they can function either as dominant oncogenes or tumor suppressors according to context. We now show that the ability to induce premature senescence in primary murine embryonic fibroblasts (MEF) is a common feature of all three Runx genes. However, ectopic Runx-induced senescence contrasts with Ras oncogene-induced senescence, as it occurs directly and lacks the hallmarks of proliferative stress. Moreover, a fundamental role for Runx function in the senescence program is indicated by the effects of Runx2 disruption, which renders MEFs prone to spontaneous immortalization and confers an early growth advantage that is resistant to stress-induced growth arrest.

The mammalian Runx gene family (Runx1-3) are transcription factors that play essential, lineage-specific roles in development. A growing body of evidence implicates these genes as mutational targets in cancer where, in different contexts, individual family members have been reported to act as tumour suppressors, dominant oncogenes or mediators of metastasis. We are exploring these paradoxical observations by ectopic expression of RUNX genes in primary murine embryonic fibroblasts where, in common with a number of other dominant oncogenes, RUNX1 induces senescence-like growth arrest in the presence of an intact p19 ARF -p53 pathway. We now report that, in MEFs lacking functional p53, RUNX1 has apparently prooncogenic effects on cell growth that include cytoskeletal reorganization, reduced contact inhibition at confluence and accelerated tumour expansion in vivo. On the other hand, RUNX1 conferred no obvious growth advantage at low cell density and actually delayed entry of primary MEFs into S phase. We also found that ectopic RUNX1 interferes with the morphological and growth responses of p53-null MEFs to TGFb indicating that these effects are mediated by overlapping pathways. These observations help to elucidate the context-dependent consequences of loss and gain of Runx activity.

A role for the RUNX genes in cancer failsafe processes has been suggested by their induction of senescence-like growth arrest in primary murine fibroblasts and the failure of RAS-induced senescence in Runx2 deficient cells. We now show that RUNX1 induces senescence in human primary fibroblasts. High affinity DNA binding is necessary but not sufficient, as shown by the functional attenuation of the truncated RUNX1/AML1a isoform and the TEL-RUNX1 fusion oncoprotein. However, a similar phenotype was potently induced by the RUNX1-ETO (AML1-ETO) oncoprotein, despite its dominant negative potential. Detailed comparison of H-RASV12, RUNX1 and RUNX1-ETO senescent phenotypes showed that the RUNX effectors induce earlier growth stasis with only low levels of DNA damage signalling and a lack of chromatin condensation, a marker of irreversible growth arrest. In human fibroblasts, all effectors induced p53 in the absence of detectable p14ARF, while only RUNX1-ETO induced senescence in p16INK4a null cells. Correlation was noted between induction of p53, reactive oxygen species and phospho-p38, while p38MAPK inhibition rescued cell growth markedly. These findings reveal a role for replication-independent pathways in RUNX and RUNX1-ETO senescence, and show that the context-specific oncogenic activity of RUNX1 fusion proteins are mirrored in their distinctive interactions with failsafe responses.