Abstract 4126: DNA methyltransferase inhibitors target a YES1-YAP1 signaling axis in childhood rhabdomyosarcoma

We have previously shown that the SRC family kinase (SFK) member, YES1, is highly expressed and rapidly activated upon IGF-1R blockade in rhabdomyosarcoma (RMS), the most common soft tissue sarcoma in children and adolescents. YAP1 (YES-associated protein) is also highly expressed in human RMS tumors where it functions as an oncoprotein when the upstream Hippo tumor suppressor pathway is silenced, driving cell growth and survival. Thus far, efforts to pharmacologically inhibit YAP1 have been unsuccessful. One activator of the Hippo pathway, RASSF1a, is epigenetically silenced in many cancer types including sarcomas. Here we show a loss of RASSF1a expression in RMS cell lines via promoter methylation. Treatment with DNA methyltransferase inhibitors (DNMTis) can re-activate RASSF1a expression, resulting in YAP phosphorylation and inactivation. DNMTi treatment also significantly inhibits RMS cell growth, which can be rescued by overexpression of a constitutively active YAPS127A. In the alveolar variant (aRMS) in particular, DNMTis increase apoptosis and decrease both YES1 expression and activation. This suggests DNMTi treatment can inhibit both YAP1 and YES1 signaling. To further investigate the connection between YES1 and YAP1 we performed proximity ligation assays and discovered that YES1 and YAP1 interact in the nucleus of RMS cell lines. Furthermore, dasatinib treatment, a known SFK inhibitor, prevents YAP1 nuclear accumulation suggesting a role for YES1 in regulating YAP1 cellular localization. YAP1 is also tyrosine phosphorylated, which may be one way YES1 regulates YAP1. Lastly, cells expressing a constitutively active YAPS127A have increased activation of SFK suggesting a positive feedback loop whereby YAP1 activity can further activate YES1 through an unknown mechanism. These data uncover a signaling axis between the YES1 and YAP1 oncoproteins in RMS and suggest an alternative way to silence YES1 and YAP1 through use of DNMTis. We will further analyze this signaling through shRNA knockdown of YES1 and evaluate combination treatment of a DNMTi and dasatinib in vitro and in vivo. Citation Format: Katherine Slemmons, Choh Yeung, Josh Baumgart, Amy McCalla-Martin, Lee Helman. DNA methyltransferase inhibitors target a YES1-YAP1 signaling axis in childhood rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstra