RUNX1-ETO Stimulates Wnt Signaling by Inhibiting the Function of ETO Family Member Proteins.

The t(8;21) is one of the most frequent chromosomal translocations associated with acute myelogenous leukemia (AML). This translocation generates a fusion protein, RUNX1-ETO, consisting of the N-terminus of RUNX1 fused to a nearly full-length ETO protein. The RUNX1-ETO fusion protein stimulates the expression of genes that are regulated by Wnt signaling. The Wnt signaling pathway plays a key role in embryonic development and aberrations to this pathway are frequently involved in tumor formation. Therefore, we sought to define the molecular mechanism by which RUNX1-ETO may stimulate Wnt signaling. We have demonstrated that the ETO family member, Mtgr1, functions as a corepressor for TCF4 and that the levels of the TCF-regulated gene, c-Myc, are upregulated in Mtgr1-null mice. Here we show that the Xenopus homolog of Mtgr1, XETOR, can impair Wnt signaling and induce ventralization in a Xenopus axis duplication assay, a classical assay used to define the hierarchy of components in the Wnt pathway. Specifically, microinjection of in vitro transcribed XETOR mRNA was performed in the marginal zone of both dorsal blastomeres at the 2 to 4 cell stage with increasing amounts of XETOR. Embryos were monitored through stage 26. Compared to control embryos, the embryos injected with XETOR mRNA were ventralized and failed to develop head structures. Conversely, although each of the ETO family member proteins associated with TCF4, RUNX1-ETO failed to bind to TCF4 in co-immunoprecipitation experiments. Mtgr1 was originally identified as a RUNX1-ETO-associated protein. Therefore, we tested whether the fusion protein impairs the action of Mtgr1 as a co-repressor for TCF4. RUNX1-ETO associated with Mtgr1, and Mtgr1 failed to associate with TCF4 when RUNX1-ETO was co-expressed. Thus, RUNX1-ETO appears to stimulate TCF-dependent transcription by interfering with the action of the ETO family of transcriptional corepressors.