Abstract PO3-26-07: Epigenetic approaches to deliver a targeted radiotherapy for triple negative breast cancer
Authors: Brookes Katie , Read Martin , Wani Sajad , Jhiang Sissy , Stover Daniel , Ringel Mathew , McCabe Christopher , Campbell Moray 
Published: May 2024

Abstract
The sodium iodide symporter (NIS, encoded by SLC5A5) is the sole transporter of iodide, and successfully exploited in thyroid cancer to uptake radioiodide (RAI). NIS is essentially undetected in normal mammary epithelial cells but elevated during lactation, and has a heterogeneous expression profile in breast cancer (BRCA), as evidenced by TCGA cohort and also in a ~120 sample TMA we have profiled. Furthermore, pre-clinical studies support an epigenetic mechanisms that involves Retinoic Acid Receptor alpha (RARa) to regulate SLC5A5 expression. Therefore, we have applied an orthogonal approach to both elucidate and target mechanisms that control NIS expression and function to deliver a therapeutic strategy that targets this transporter as a novel radiotherapy in BRCA.
To understand the epigenetic control of expression we segregated samples in the TCGA-BRCA cohort into lower and upper quartile SLC5A5 expression and identified ~600 significant differentially expressed of genes (DEG). Interestingly, restricting analyses to either triple negative (TN) or tumors from African American (AA) patients doubled the number of DEGs. These DEGs were significantly associated with several transcription factors including LHX3, a pituitary development transcription factor regulated by thyroid signaling. The most significantly altered ~70 DEGs significantly clustered tumors by genomic ancestry and ERBB2/HER2 IHC status.
The DEGs also supported a footprint of RARa signaling include PRAME, a known RARa co-repressor associated with histone deacetylase, and reflects the finding that NIS expression is highly responsive to SAHA treatment. To investigate if PRAME expression suppresses RARa regulation of SLC5A5 we have established a panel of BRCA cell lines (AU565, SKB3 SUM52 and MDA-MB-231) stably transfected with either CRISPR-VP64 (activator) and CRISPR-KRAB (repressor) to modulate PRAME and measure the epigenomic impact, the expression of SLC5A5 and the ability to transport RAI.
In parallel we have applied a drug screen approach that targets non-canonical approaches to govern NIS expression, for example by targeting valosin-containing protein (VCP) that induces NIS proteolysis. In the first instance we have developed a therapeutic strategy to target NIS by inhibiting VCP. Specifically, we are screening a range of VCP inhibitors in combination with the HDAC inhibitor SAHA or the transcriptional potentiator, disulfiram metabolite Cu(DDC)2 in BRCA cell lines by testing the ability to augment RAI uptake. Remarkably, in MDA MB 231 cells, Cu(DDC)2 combined with the VCP inhibitor CB5339, resulted in a significant additive increase in RAI uptake and a prominent decrease in cell proliferation and survival.
These studies combined with future murine and PDX testing of CRISPR-edited tumors and drug exposures will develop a comprehensive analyses of the mechanisms that control SLC5A5 expression across breast cancer cell types, how this can be most effectively targeted to restore NIS expression, and exploited to promote RAI as a novel targeted BRCA radiotherapy.
Citation Format: Katie Brookes, Martin Read, Sajad Wani, Sissy Jhiang, Daniel Stover, Mathew Ringel, Christopher McCabe, Moray Campbell. Epigenetic approaches to deliver a targeted radiotherapy for triple negative breast cancer [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 PO3-26-07.