Abstract 46: Alternative lengthening of telomeres in patient-derived xenograft models of soft tissue sarcomas with complex genomic profile

Abstract Background: Cancer cells require telomere maintenance mechanisms (TMM) to achieve replicative immortality. About 85% of cancers utilize telomerase-mediated elongation of telomeres, and 15% of cancers use alternative lengthening of telomeres (ALT). In sarcoma ALT positivity (ALT+) is observed in 10-80% of samples, mainly in tumors with high genomic instability. They are characterized by the presence of extra-chromosomal telomeric repeats called C-circles and frequent loss of function of chromatin remodeling proteins alpha-thalassemia/mental retardation syndrome X-linked (ATRX) and/or death domain-associated protein (DAXX). To expand the utility of patient-derived xenografts (PDX) to therapeutic approaches targeting ALT pathway we aimed to characterize the type of TMM and related characteristics in PDX of sarcoma with high genomic instability. Methods: Out of 92 established and well-characterized PDX from 27 subtypes of soft tissue sarcoma (STS) from our XenoSarc panel 22 models with complex genomic profile were selected for the analysis: 7 models of leiomyosarcoma (LMS), 6 of dedifferentiated liposarcoma (DDLPS), 4 of undifferentiated pleomorphic sarcoma (UPS) and myxofibrosarcoma (MFS), and 1 model of high grade sarcoma not otherwise specified (HG). The level of C-circles as an ALT surogate marker was quantified based on detection of products of rolling circle amplification of C-circles by Phi-polymerase. Samples with C-circle levels ≥ 0.1 were classified as ALT+. Expression levels of ATRX and DAXX were evaluated using immunohistochemistry (IHC) and low coverage whole genome sequencing (lcWGS) was performed to compare potential differences in genomic landscapes of ALT+ and ALT- PDX models. Results: C-circle analysis revealed 14 out of 22 PDX models analyzed to be ALT+ (63%), including 5/7 LMS, 2/6 DDLPS, 4/4 UPS, 2/4 MFS, and 1/1 HG models. Out of 14 ALT+ models, loss of ATRX and DAXX expression was detected in 9 and 2 PDXs, respectively. High quality lcWGS data were successfully generated, which are currently being analyzed to detect differences between ALT+ and ALT- PDX, and will be integrated with IHC results. Conclusion: We documented a high prevalence of ALT+ in STS PDX models with complex genomic profile. An on-going molecular characterization of these models will further support genotype-specific evaluation of TMM-associated characteristics of PDXs, such as telomeric features, will add to the validity of these ALT+ models. We consider our PDX models an appropriate platform to study candidate drugs that can target ALT pathway in STS. Citation Format: Agnieszka Wozniak, Nicola Biondi, Kimberly Verbeeck, Oliver Bechter, Priya Chudasama, Patrick Schöffski. Alternative lengthening of telomeres in patient-derived xenograft models of soft tissue sarcomas with complex genomic profile [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 46.