Abstract 3022: Synthetic essentiality between PTEN and core dependency factor PAX7 dictates rhabdomyosarcoma indentity
Authors: Langdon Casey G. , Gadek Katherine E. , Garcia Matthew R. , Reed Kristin B. , Bush Madeline , Hanna Jason A. , Drummond Catherine J. , Maguire Matthew C. , Leavey Patrick J. , Finkelstein David , Jin Hongjian , Rehg Jerold E. , Hatley Mark E. 
Published: July 2021

Rhabdomyosarcoma (RMS) is an embryonal tumor resembling developing skeletal muscle and the most common pediatric soft-tissue sarcoma. RMS is molecularly defined by the presence or absence of a fusion oncoprotein corresponding with the histological subtypes alveolar and embryonal RMS, respectively. Embryonal, or fusion-negative, RMS (FN-RMS) is heterogeneous in its molecular alteration profile; the major exception is the near universal PTEN promoter hypermethylation found in FN-RMS corresponding with decreased PTEN expression. PTEN's functional role in FN-RMS remains unclear as does PTEN's role in defining tumor fate decisions. Organismal cancer models can help elucidate these decisions by defining the potential tumor fate landscape that can occur in transformed multipotent progenitor cells. Our laboratory leverages a highly penetrant, early onset model of FN-RMS driven by the transdifferentiation of endothelial progenitors into skeletal muscle-like RMS cells by Hedgehog pathway activation. Therefore, our model is uniquely primed to empirically determine the core regulatory factors critical in FN-RMS initiation. Here, we conditionally deleted Pten in these cells (ASPcKO). ASPcKO tumors presented earlier than wild-type tumors and more closely resemble human FN-RMS with a less differentiated skeletal muscle state. These were unique characteristics of ASPcKO tumors as mice with homozygous loss of other tumor suppressors - Cdkn2a, Trp53, and Rb1 - did not recapitulate these phenotypes. We further probed the downstream transcriptional outputs of ASPcKO tumors revealing a profound increase in expression of the neural developmental transcription factors Dbx1 and Pax7. These outputs are functionally important as human FN-RMS patient-derived xenografts are dependent on both DBX1 and PAX7. Subsequently, we also show that DBX1 is a downstream transcriptional target of PAX7 highlighting how Pten loss engages a unique transcriptional program for tumor maintenance. PAX7 is also a core FN-RMS regulatory circuit component. To further interrogate the role of PAX7 on tumor initiation, we concomitantly deleted Pten and Pax7 in our FN-RMS model and found not only that Pax7 loss rescues the survival kinetics observed when Pten is lost, but also alters the developmental trajectory of the tumors that do develop. Instead of Smoothened trans-differentiating our aP2-Cre expressing primordial endothelial cell into a skeletal-muscle like FN-RMS, Pten and Pax7 loss dictates these endothelial cells to give rise to tumors with smooth muscle-like differentiation, including human-like leiomyosarcoma. Together, this synthetic essential interaction between Pten and Pax7 in FN-RMS stresses the importance of the bifunctional role of PAX7 in tumor initiation and maintenance and how specific tumor suppressor loss can engage developmental transcriptional programs to alter tumor fate.

Citation Format: Casey G. Langdon, Katherine E. Gadek, Matthew R. Garcia, Kristin B. Reed, Madeline Bush, Jason A. Hanna, Catherine J. Drummond, Matthew C. Maguire, Patrick J. Leavey, David Finkelstein, Hongjian Jin, Jerold E. Rehg, Mark E. Hatley. Synthetic essentiality between PTEN and core dependency factor PAX7 dictates rhabdomyosarcoma indentity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3022.