Abstract A20: A mouse model of postnatal Hedgehog-driven muscle tumors shows a common progenitor that relies on loss of Tp53 expression for malignant transformation

Introduction: Patients with nevoid basal cell carcinoma syndrome (NBCCS), a condition that presents with activating mutations in the Hedgehog pathway (HH), have increased risk of developing skeletal muscle tumors: fetal rhabdomyomas (FRMs) and translocation-negative rhabdomyosarcomas (RMS), that both appear with same age/gender and location distribution but have very different behavior. TP53 mutations have been related to RMS. The purpose of our study is to test whether loss of Tp53 in Nestin+ progenitors affects HH-driven muscle tumor phenotype. Experimental Design: We generated a mouse model (Nestin-FlpoER; R26MASTR/SMOM2-YFP;Tp53+/+/Tp53FL/+/Tp53FL/FL) using the MASTR allele engineered in our lab, that upon tamoxifen (Tm) administration allows forFlp-mediated induction of a GFP-cre fusion protein expression, so that mutant cells and their progeny can subsequently be identified by the continuous nuclear GFP labelling. Upon cre expression, R26SMOM2-YFP allele is recombined in mutant cells conferring constitutive HH pathway activation and neither (Tp53+/+), one (Tp53FL/+) or both (Tp53FL/FL) copies of Tp53 are deleted. Tm was injected subcutaneously at birth (P0) at 200 ug/g. Mice were sacrificed when unwell or at 400 days and tumors were processed for histologic analysis. Results:Tp53+/+, Tp53FL/+ and Tp53FL/FL groups were analyzed: incidence of muscle tumors (desmin+; myogenin+) was 33%/63%/6%, respectively. Median age at the time of death from muscle tumor was 150 days in the Tp53+/+, 124 days in Tp53FL/+, and 88 days in Tp53FL/FL group with a male-to-female ratio of 1.8/1 amongst all groups. In the Tp53+/+ group, all tumors were FRMs. In the Tp53FL/+ and Tp53FL/FL groups, tumors showed increasing degrees of undifferentiation and proliferation (Ki67+) in mutant cells (GFP+) acquiring a more aggressive phenotype compatible with RMS histology. Within these tumors, different degrees of differentiating cells (mostly GFP-) were also seen. Tumors arose in proximal limbs, chest, and abdominal wall regions and interestingly tumor genotype seemed to correlate with a craniocaudal distribution: Tp53+/+ tumors located mostly in hind-limbs and Tp53FL/+ and Tp53FL/FL located more cranially. The only other tumor type that arose was medulloblastoma: 27% and 63% of all tumors within the Tp53FL/+ and Tp53FL/FL groups, respectively. Complete loss of Tp53 (Tp53FL/FL) had inferior muscle tumor incidence compared to Tp53FL/+ mainly due to earlier onset of medulloblastoma. Interestingly, within this group, also 12% of mice (n=4) died from infectious-related complications within the genitourinary region, suggesting an underlying developmental defect. Conclusion: We present the first murine model of both FRM and RMS in which Tp53 loss is required for malignant transformation following HH pathway activation in postnatal progenitors. Citation Format: Estibaliz Lopez Rodrigo, Alexandra Joyner. A mouse model of postnatal Hedgehog-driven muscle tumors shows a common progenitor that relies on loss of Tp53 expression for malignant transformation [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr A20.