Abstract GS03-09: Characterization and proposed therapeutic exploitation of fusion RNAs in metastatic breast cancers

Abstract BACKGROUND: Large-scale genomic studies such as The Cancer Genome Atlas (TCGA) and Pan-Cancer Analysis of Whole Genomes (PCAWG) show that the breast cancer (BrCa) genome is dominated by structural variation (SV) rather than single base pair mutations, producing a fertile environment for gene fusions. In this study, we implement a rigorous, expression-based approach to create a comprehensive landscape of fusion RNAs in metastatic breast cancer (MBC). We find fusion RNAs—many of which are novel involving known oncogenes—are surprisingly common in the advanced setting and credential their use as base-editing therapeutic targets. METHODS: Two retrospective cohorts of MBC RNA-sequencing data were analyzed— Dana-Farber Cancer Institute CCPM (n = 252 cases, 276 specimens), MichiganCSER (n = 171 cases, 190 specimens)—with a Fusion MetaCaller that integrates 5 unsupervised fusion-finding algorithms. Fusion RNAs identified in at least 2 callers (High-Confidence) and absent in RNA-seq from normal tissue (Cancer-Specific) were classified as HCCS-Fusions. Further removal of common artifactual fusions was performed using public databases. Expression of each HCCS-Fusion was quantified using a supervised method (FusionInspector)—expressed HCCS-Fusions were defined as having a Fusion Fragment Per Million (FFPM) value > 0.1 and at least 10% of read counts mapping to the fusion breakpoint versus the flanking 5’ or 3’ partners’ exons. Normalized gene-level expression abundances were calculated to correlate transcriptomic features (gene expression, PAM50) with fusion RNAs. Recurrent, potentially pathogenic fusion RNAs were annotated using OncoKB and outlier expressed fusions (Q3 FFPM + [1.5 X IQR]) were interrogated. RESULTS: The frequency of HCCS-Fusions differed between subtypes with basal BrCa harboring the most per tumor followed by Her2, LumB and LumA—with a median HCCS-Fusion count of 13, 12, 7, 4 respectively. 64.5% of cases harbored a HCCS-Fusion in an OncoKB cancer-related gene. The most recurrent fusions involving a cancer-related gene were 5’ ESR1 fusions (14 cases)—all with in-frame breakpoints near exon6/7, disrupting ESR1’s ligand binding domain. 13 of 14 ESR1 fusions were called in Luminal B (LumB) metastases (Fisher’s exact enrichment p < 0.005 vs other subtypes)—defining an ESR1 fusion frequency of 6.5% in LumB disease. Beyond ESR1, we identify recurrent, low-frequency (2-4 cases) in-frame kinase fusions involving FGFR2, ADK, TLK2, PRKCA, BRAF, CHKA, CSNK1D, NEK11, TNIK—some potentially targetable with FDA-approved small molecule inhibitors—as well as recurrent, predicted loss-of-function fusion RNAs in NF1, MSI2, USP32, PTEN, and CDH1. Lastly, 33.6% of cases harbored at least one outlier expressed fusion RNA; including highly expressed in-frame fusions involving known BrCa mediators such as ERBB2, BRCA1, ARID1B, RPS6KB1/2, PIK3R3, AXIN1, TGFB1/2, FOXP1, PAK1, and CREBBP. CONCLUSIONS: Taken together, these results demonstrate that fusion RNAs in MBC—some recurrent, many highly expressed and unique to individual tumors—are common. We create the most comprehensive catalog of ESR1 fusions in MBC, better define their frequency, discover their enrichment in LumB-like tumors, and will discuss clinicopathologic and transcriptomic features associated with ESR1 fusion positive disease. We identify druggable fusions that would likely be missed by current testing standards, find recurrent loss-of-function fusion RNAs, and show that over one-third of metastatic cases harbor at least one outlier expressed fusion—many of which involve BrCa-related genes. In summary, we propose that fusion RNAs are a driving and perhaps overlooked mechanism of tumor evolution in therapy-resistant disease and postulate fusion transcripts present a compelling therapeutic opportunity in MBC. Preliminary data targeting fusion RNA breakpoints using a novel RNA base-editing approach will be discussed. Citation Format: Nolan Priedigkeit, Alinés Lebrón-Torres, Janny Liao, Jean-Baptiste Alberge, Stefania Morganti, Jakob Weiss, Jorge Gomez Tejeda Zanudo, Albert Grinshpun, Melissa Hughes, Karla Helvie, Kerry Sendrick, Kyleen Nguyen, Sarah Strauss, Janet Files, Maxwell Lloyd, Nikhil Wagle, Chip Stewart, Eric Winer, Bruce Johnson, Yvonne Li, Rinath Jeselsohn, Sara Tolaney, Daniel Abravanel, Nancy Lin, Heather Parsons, Gad Getz, Steffi Oesterreich, Adrian Lee, Todd Golub. Characterization and proposed therapeutic exploitation of fusion RNAs in metastatic breast cancers [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr GS03-09.