Analysis of RUNX-CBFβ as a relevant regulator of RSPO3 expression in breast cancer cells

Carla M Felcher¹, Camila D Arcuschin¹, Micaela N Stedile¹, Johanna M Tocci¹, Ignacio E Schor¹, John H Bushweller², Lucio H Castilla³, Edith C Kordon¹

We have recently determined that R-spondin3 (RSPO3), a secreted protein that potentiates Wnt signaling pathway, is a key modulator of tumor progression and stem cell behavior in basal breast cancer. Previous reports suggested the potential involvement of the RUNX-CBFβ axis on RSPO3 expression in mammary tumor cells. These preliminary observations were confirmed by our results showing that small molecules able to inhibit CBFβ-RUNX interaction caused reduction of RSPO3 mRNA and protein levels in MDA-MB231 breast cancer cells. These treatments also induced inhibition of cell migration, ability that was recovered upon addition of recombinant RSPO3. To further explore the mechanisms underlying the control exerted by RUNX-CBFβ on RSPO3, we performed an in silico analysis of publicly available data from two RUNX1 CHIP-seq reports and an ATAC-seq study from human breast cell lines. We aligned the emerging data with the occurrences of the RUNX1 DNA-recognition-motif in the Rspo3 locus. This approach revealed a few putative RUNX1 binding sites. Among them, an intronic Rspo3 region that seems to be particularly active in triple negative (TN) breast cancer cells deserves special attention. In summary, our results show that RUNX-CBFβ transcriptional activity might affect TN mammary tumors by controlling RSPO3 expression levels. More experiments are being carried out to determine the mechanisms involved and the impact of this pathway on TN breast cancer behavior.