Abstract 3951: Timing the development of chemoresistance in relapsed pediatric cancer

Abstract Survivors of pediatric cancer face lifelong battles with severe morbidities, which includes a significant risk of recurrence. Pre-existing genetic variation within primary tumors may offer certain cell populations an evolutionary advantage, increasing the likelihood that some will be resistant to treatment. But the inherent complexity of the tumor means finding these markers of resistance has yet to be fully explored across childhood cancer. To address this, we developed a pipeline that combines clonal evolution reconstruction with mutational signature extraction at the subclonal level to elucidate changes in mutational processes. Using this pipeline to analyze 1, 743 pediatric tumor genomes we found a common pattern of parallel evolution in most pediatric cancers, irrespective of their tissue of origin. Further, we detected therapy-associated mutational signatures linked to specific treatments such as temozolomide, platinum, fluorouracil, and thiopurine in post-treatment tumor subpopulations. Of 235 samples with known treatment exposure, 37. 9% displayed one or more treatment-associated signatures. In the context of the phylogenetic tree, we identified specific subpopulations with therapy signatures. In multi-sample cases, resistant subpopulations had ancestors present in primary tumors at the time of diagnosis, suggesting certain subpopulations possessed the ability to withstand chemotherapy-induced pressures from an early stage and then expanded following treatment. Finally, we observed differences in the timing of mutational signatures between antimetabolite and alkylating agent treatments, reflecting the mechanisms of these drugs. Antimetabolite agents showed subclonal mutational signatures occurring in later branches, likely due to dependence on cell cycling. In contrast, alkylating agents displayed signatures in both early and late subpopulations, as they are unaffected by cell cycle state. Thus, investigating mutational signatures at the subclonal level unveils new insights into the clonal dynamics of pediatric cancers and the development of chemoresistance. This research offers valuable insights for identifying pediatric cancer patients at greater risk of recurrence, enabling the development of more effective treatment plans to enhance their quality of life and overall survival rates. Citation Format: Sasha Blay, Mehdi Layeghifard, Scott Davidson, David Chen, Astra Schwertschkow, Vijay Ramaswamy, Michael Taylor, Elli Papaemmanuil, Anita Villani, David Malkin, Ludmil Alexandrov, Mark Cowley, Adam Shlien. Timing the development of chemoresistance in relapsed pediatric cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 3951.