Abstract LB493: A conserved enhancer locus in ecDNA and HSRs activates MYC transcription in group 3 medulloblastoma

Abstract MYC is amplified on extrachromosomal DNA (ecDNA) or homogenously staining regions (HSRs) in Group 3 medulloblastoma (G3-MB), conferring a poor prognosis, but the underlying mechanisms controlling MYC expression within ecDNA and HSRs are poorly understood. Using a structure-function approach, we identified and characterized a novel enhancer (ecMYC E1) that drives MYC activation specifically in G3-MB with MYC-amplified ecDNA or HSRs. The ecMYC E1 locus exhibits enhancer hallmarks exclusively in MYC-amplified G3-MB but not in other MYC-dependent cancer cell lines, including those with MYC amplification. Silencing of the ecMYC E1 enhancer significantly reduced MYC transcription, which was compensated by increases in ecDNA copy number, but not in HSR-driven G3-MB tumor. NeuroD1 and BRD4 interact with each other and bind to ecMYC E1, looping to the enhancer to the MYC promoter, and defining a novel mechanism that regulates amplified MYC gene expression within ecDNA or HSRs specifically in G3-MB. Citation Format: Jake D. Friske, Flore Cuisin, Paloma Guernalec, Hayden Malone, Stephanie Nance, Declan Bennett, Steven Burden, Ti-Cheng Chang, Hao Shi, Justin S. Williams, Virginia Valentine, Barbara Passaia, Bensheng Ju, Modupeore Adetunji, Paul Geeleher, Brian J. Abraham, Gang Wu, Chunliang Li, Martine F. Roussel. A conserved enhancer locus in ecDNA and HSRs activates MYC transcription in group 3 medulloblastoma abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (8Suppl): Abstract nr LB493.