**4. Role of RUNX2 in melanoma progression and acquired resistance to BRAFi**

RUNX2 was initially described as one of the transcription factors whose expression was significantly correlated with elevated levels of the non-canonical signaling member of the WNT family, WNT5A, following chronic treatment (over 10 weeks) with the BRAF inhibitors PLX4720 and PLX4032 [29]. We previously showed that RUNX2-deficient melanoma cells, displayed a significant down-regulation of leading receptor tyrosine kinases, EGFR, IGF-1R, PDGFRβ and AXL. Our finding strongly suggested a critical role for RUNX2 in mediating intrinsic RTK-associated pro-oncogenic properties in melanoma. In addition, we demonstrated a significant up-regulation of RUNX2 expression and concomitant up-regulation of EGFR, IGF-1R and AXL in melanoma cells rendered resistant to PLX4720 [30]. We then reported that PLX4720-resistant cells developed in an *in vivo* context exhibit an increase in RUNX2 levels when re-exposed to PLX4720 *in vitro.* These findings strongly suggest that RUNX2 could play a critical role in acquired resistance to PLX4720. In order to address the relevance of these findings in human melanoma, clinical data from a cohort containing samples from untreated tumors and tumors treated with vemurafenib and dabrafenib respectively [31] were analyzed. Probes for all three main RUNX2 transcripts were represented on the array. We found that the expression of RUNX2 isoform 3 is significantly higher in vemurafenib-treated patients compared to the untreated group (p = 0.0024). These results showing the up-regulation of RUNX2 in melanoma lesions from patients treated with vemurafenib, strongly suggest that chronic exposure to BRAFi (PLX4720/vemurafenib) could favor RUNX2 up-regulation, leading to RTK up-regulation and the induction of acquired drug resistance to BRAFi [30].

The mechanism(s) leading to RUNX2 up-regulation in BRAFi-resistant melanoma cells have yet to be discovered. One possible mechanism would involve WNT5A and the WNT5A-mediated activation of the PI3K/AKT pathway [29]. As RUNX2 expression is increased by the PI3K/AKT pathway signaling [28, 30], elevated WNT5A expression and subsequent AKT pathway activation could result in RUNX2 overexpression. Therefore, any kinase rewiring that leads to hyperactivated PI3K/AKT signaling in melanomas resistant to BRAFi [32] would provide a favorable context for high RUNX2 expression.
