Abstract 4340: Profiling the evolutionary history of giant cancer cells in sarcomas: Hopeful monsters or an evolutionary dead end

Abstract Background: As tumors evolve, they accumulate somatic mutations that serve as an archeological record of their evolutionary past. Undifferentiated pleomorphic sarcomas (USARCs) are malignant soft tissue tumors that often contain scattered giant cancer cells, or potential ‘hopeful monster cells’, histologically. As USARCs frequently undergo whole genome doubling (WGD) and chromothripsis (Steele et al, 2019), both mutational events are likely to contribute to the formation of giant cancer cells, which frequently display atypical nuclear features such as polyploidy, micronuclei and extrachromosomal DNA. Giant cancer cells are also a common histological finding in other sarcoma types, such as dedifferentiated liposarcomas. Aims: This study aims to leverage single cell sequencing approaches to determine the evolutionary history and genomic landscape of giant cancer cells in sarcomas. Methods: 112 giant cancer cells identified in 8 USARCs, 1 dedifferentiated liposarcoma (DDLPS) and 1 myxofibrosarcoma by histology (i. e. nuclear size and shape, as well as multi-nucleation) were isolated via laser capture microdissection thereby retaining important spatial and morphological information. Once isolated, single cell DNA was extracted and amplified using the Ampli1 whole genome amplification (WGA) kit and underwent WGS. DNA was also estimated at a single cell level in one USARC using FACS. Once sorted into different ploidy populations, single nuclei also underwent WGS. Allele specific copy number aberrations (CNAs) were inferred in all single cells. Bulk tumour and normal WGS was also performed. Results: LCM isolated giant cancer cells exhibited extreme sub-clonal heterogeneity with some demonstrating de novo chromothripsis events that was not present in bulk WGS. FACS analysis and copy number inference of single cancer cells in one USARC also revealed the presence of three ploidy populations, including a haploid, a near diploid and a WGD population. Single cell WGS and copy number profiling demonstrated that following near genome scale haploidisation, multiple haploid cells underwent successive rounds of WGD. Conclusions: Chromothripsis and WGD emerges as major contributors to intra-tumor heterogeneity in sarcoma giant cancer cells. Single cell LCM and WGS revealed that some giant cancer cells contained de novo chromothripsis events that were not observed in bulk WGS. Multiple sub-clonal WGD events also permitted one USARC to continue to explore its fitness landscape contributing to ongoing tumor heterogeneity and lethality. Citation Format: Amy Bowes, Tom Lesluyes, Sara Waise, Annelien Verfaillie, Adrienne Flanagan, Jonas Demeulemeester, Maxime Tarabichi, Nischalan Pillay, Peter Van Loo. Profiling the evolutionary history of giant cancer cells in sarcomas: Hopeful monsters or an evolutionary dead end 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 4340.