Abstract 214: Patient-specific mapping of oncogenic structural changes in pediatric osteosarcoma
Authors: Clugston Andrew S. , Ostrowski Megan , Wu Coco , Sayles Leanne , Leung Stanley G. , Ramani Vijay , Sweet-Cordero E. Alejandro , Breese Marcus R. 
Published: April 2023

Abstract
We have leveraged long-read and spatial sequencing modalities to identify patient-specific oncogenic alterations for treatment of pediatric osteosarcomas. Osteosarcoma (OS) is the most common primary bone malignancy in children and young adults, with over 500 cases identified each year in the United States. OS is archetypal of cancers driven by aneuploidy and structural rearrangement rather than point mutations. These structural variants (SVs) can alter gene expression stoichiometrically, but the functional consequences of many SVs stem from their effect on multidimensional genome organization. SVs can enable enhancer hijacking, alter boundaries of topologically associated domains (TADs), and move gene loci to different regulatory compartments affecting transcription. SVs can also form extrachromosomal DNA amplicons (ecDNA), which can be megabases in size and can incorporate both genes and regulatory elements to result in substantial increases in gene transcription and intratumor heterogeneity. Complex SVs may have profound implications for prognosis and treatment in OS and other cancers. By incorporating long-read optical genome mapping (OGM) and chromatin conformation capture sequencing (HiC), our goal is to detect complex tumor-specific SVs that will inform and refine pediatric OS subtypes, revealing new tumor-specific chemotherapy treatment options.
We have generated a unique panel of 11 OS patient-derived xenograft (PDX) cell lines which have been previously characterized regarding response to targeted therapies using whole genome (WGS), RNA (RNA-seq), and chromatin accessibility (ATAC-seq) sequencing. To understand the complex genomic reorganization that occurs in OS, we have integrated OGM and WGS data for six cell lines, and OGM, WGS, and HiC data for two. SVs were detected using OGM, HiC, and WGS modalities, and we observed 59-61% concordance between SVs called using OGM and HiC, versus 22-45% when comparing with WGS calls. PDX lines assayed using OGM exhibit between 5,135 and 7,514 SVs each, and translocation numbers ranged from 212 to 1,422. These lines exhibit between dozens of potential ecDNA amplicons, including one amplicon which may explain that line’s specific and exceptionally high expression of YAP1 and BIRC2 transcripts. HiC interaction maps for two PDX lines reveal genomic compartments and TAD boundaries in detail, with 3,713 unique TAD boundaries identified between them using 50kb genomic bins. We observe that 60% of these boundaries are consistent between these lines, and 27% exhibit differences that suggest boundary loss or gain. These changing boundaries are associated with numerous SVs and/or copy number discrepancies, and with genomic regions containing genes with dysregulated RNA transcript levels. Incorporating OGM and HiC modalities adds substantially to our ability to detect oncogenic SVs and the genes they affect.
Citation Format: Andrew S. Clugston, Megan Ostrowski, Coco Wu, Leanne Sayles, Stanley G. Leung, Vijay Ramani, E. Alejandro Sweet-Cordero, Marcus R. Breese. Patient-specific mapping of oncogenic structural changes in pediatric osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 214.