Abstract 3501: Concurrent amplification of TBXT and highly activated enhancers in extrachromosomal DNA (ecDNA) drives chordoma tumorigenesis

Abstract

Abstract Chordoma, a rare bone cancer characterized by limited actionable clinical targets, exhibits a pivotal hallmark in the activation of the developmental transcription factor gene TBXT (Brachyury) through its association with highly active enhancers. TBXT amplification emerges as a major causative factor in familial chordomas, yet the amplification mechanism and its consequential impact on genomic instability remain inadequately elucidated. In this study, we conducted a comprehensive analysis of multiple genomic and epigenomic sequencing datasets, including WGS-seq, H3K27ac ChIP-seq, and ATAC-seq in six chordoma cell lines, and HiC data in two of them, to unravel the mechanisms of structural variations and their biological implications. Notably, our focus extends to JHC7 cells, which exhibit extrachromosomal amplification of a 2.4M region encompassing TBXT and the associated enhancers. The existence of such extrachromosomal DNA (ecDNA) in JHC7 was further validated through DNA FISH and Sanger sequencing. Comparing JHC7 with CH22, a chordoma cell line lacking TBXT amplification, revealed heightened chromatin relaxation and increased DNA rearrangements in JHC7. Interestingly, FISH results showed significant genomic ploidy heterogeneity within JHC7 cells, a finding demonstrated by nucleus FACS sorting. Single-cell multiomic analysis, combining snRNA-seq and snATAC-seq, demonstrated heterogeneous gene expression and distinct copy number variations among JHC7 cells. However, the amplification of the TBXT locus was observed in nearly all cells examined, indicating the rapid genomic evolution in the cultured JHC7 cells. These results collectively show that TBXT is extensively amplified through the formation of ecDNA, providing a fitness advantage to chordoma cells and concurrently exacerbating genomic instability. The findings in this study provide an insight for understanding the pathogenesis of chordoma and may inform the development of targeted therapeutic strategies. Citation Format: Bo Zhao, Martin Barrio Ines, Lingqun Ye, Xizi Wu, Alice Soragni, Andrew Futreal, Kadir Akdemir. Concurrent amplification of TBXT and highly activated enhancers in extrachromosomal DNA (ecDNA) drives chordoma tumorigenesis [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(6_Suppl):Abstract nr 3501.