Abstract 3014: Location specificity in fusion-negative rhabdomyosarcoma driven by cell of origin

Abstract

Abstract Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma and despite aggressive treatment, clinical outcomes have not improved for three decades. There is a need to uncover the molecular underpinnings of RMS. Tumor location is a key prognostic indicator and although RMS occurs throughout the body, nearly 40% of all RMS occurs in the head and neck. It is unknown how the cell of origin affects location and therefore clinical outcome of RMS. Previously, we demonstrated that activation of Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under the control of the adipose protein 2 (aP2)-Cre recombinase transgene gives rise to aggressive skeletal muscle tumors in mice that resemble human fusion negative RMS (FN-RMS). Interestingly, tumors were anatomically restricted to the neck in aP2-Cre;SmoM2 mice. In this study we leverage the aP2-Cre;SmoM2 model of FN-RMS to interrogate how cell of origin affects tumor localization. By genetic fate mapping we determine that aP2-Cre labeled cells are non-myogenic and that aP2-Cre is not expressed in quiescent or activated muscle stem cells. Instead, we identify aP2-Cre expressing cells as endothelial cell progenitors within the muscle interstitium. Although aP2-Cre expressing endothelial cells were observed throughout the mouse, we observed that SmoM2 expression specifically drives embryonic expansion of aP2-Cre labeled cells only in the neck. SmoM2 expression reprograms endothelial progenitors resulting in a myogenic fate switch prior to terminal endothelial cell differentiation. We illustrate that endothelium and skeletal muscle within the head and neck arise from KDR expressing progenitors and that aP2-Cre is expressed after endothelial lineage commitment. Aberrant hedgehog activation in these aP2-Cre labeled endothelial progenitors results in TBX1 expression, a skeletal muscle specification factor in the head and neck, and subsequently MYOD1 expression, driving a partial myogenic program characteristic of FN-RMS. Further characterization of tumor cells isolated by flow cytometry revealed that FN-RMS in aP2-Cre;SmoM2 mice express TBX1, as well as PITX2, TCF21 and MSC, additional skeletal muscle specification factors in the head and neck. In contrast, PAX3, which specifies trunk and limb muscle, was not expressed in FN-RMS cells isolated from aP2-Cre;SmoM2 mice. Together, these results demonstrate that FN-RMS in the head and neck can arise from endothelial progenitor cells and suggest that aberrant activation of normal muscle development programs in non-myogenic cell types with developmental pliancy can drive location specific tumor formation. Citation Format: Catherine J. Drummond, Jason A. Hanna, Matthew R. Garcia, Daniel J. Devine, Alana J. Heyrana, David Finkelstein, Mark E. Hatley. Location specificity in fusion-negative rhabdomyosarcoma driven by cell of origin [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):Abstract nr 3014.