Abstract 625: Early tumor priming in Li-Fraumeni Syndrome muscle using a rhabdomyosarcoma model.

Abstract Li-Fraumeni Syndrome (LFS), caused by germline TP53 mutations, markedly increases the risk of developing rhabdomyosarcoma (RMS) and other childhood cancers, often emerging during early neonatal life. This early onset, in conjunction with findings that genomic alterations occur well before cancer onset in LFS, strongly suggests malignant transformation in LFS begins during embryogenesis, within developmental windows where differentiation is normally tightly regulated. Despite the developmental connection to LFS tumorigenesis, mechanisms through which inherited TP53 dysfunction perturbs embryonic muscle development to create tumor-permissive states are not well understood. Using LFS-associated RMS (LFS-RMS) as a model, we examined how germline Trp53 mutations alter myogenesis and predispose LFS muscle to tumorigenesis. We performed single-nuclei RNA sequencing (snRNA-seq) across LFS-RMS tumors, matched distal muscle, and healthy embryonic, postnatal muscle tissue from Trp53R172H/+ (R172H) and wild-type (WT) littermates. To map accompanying genomic changes, we also generated deep (800x) whole-exome sequencing (WES) from these samples. The two datasets were integrated to identify genomic alterations that aligned with developmental transcriptional shifts observed in LFS. We identified a reproducible myogenic LFS-RMS signature enriched for WNT signaling, RNA metabolic pathways, and glycolytic reprogramming. This signature was present in LFS-RMS tumors and elevated in R172H embryos during the peak of myogenesis (E12-E14). Further analyses revealed delayed myogenic maturation in LFS embryos and the presence of progenitor-like cell states that become enriched in LFS-RMS, indicating that tumor-relevant states arise during LFS embryogenesis. By integrating WES with snRNA-seq, we found that transcriptionally unstable developmental periods in LFS embryos overlap with early genomic alterations, suggesting that transcriptional and genomic disruptions emerge in parallel. By defining these molecular patterns, our work identifies early transcriptomic and genomic predictors of LFS-RMS and how germline Trp53 mutations reshape developmental programs. Ongoing efforts include incorporating extra-embryonic tissues such as placenta and amniotic fluid to evaluate their potential as accessible prenatal cancer risk indicators. Ultimately, characterizing developmental progression in a germline Trp53 mutant model will reveal the sequence of events that drive LFS-RMS. From these findings we can better and define windows of vulnerability and work towards a prenatal screening tool for LFS. Citation Format: Ashby Kissoondoyal, Paula Rosanna Quaglietta, Brianne Laverty, David Malkin. Early tumor priming in Li-Fraumeni Syndrome muscle using a rhabdomyosarcoma model abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 625.