EXTH-27. POLYCOMB OVERACTIVITY DRIVES TUMORIGENESIS IN AGGRESSIVE CUSHING’S DISEASE

Abstract INTRODUCTION Aggressive Cushing’s disease (CD) tumors (Ki67 > 3%) exhibit rapid growth, poor postsurgical outcomes, and resistance to conventional therapies. Polycomb group (PcG) proteins are canonical epigenetic silencers that can drive tumor aggressiveness. We asked if PcG underlies rare but fatal pituitary tumor progression. METHODS We performed paired multiome analysis (single nucleus RNAseq and ATACseq) in 5 aggressive CD adenomas, and adjacent normal gland. We verified protein expression with multiplexed immunohistochemistry (mIHC) and PcG activity by H3K27me3 ChIPseq. Aggressive murine pituitary tumor cells ATT20 were treated using PcG inhibitors (DZNep, tazemetostat), HDAC inhibitor (vorinostat), or siRNA (Ezh2, Suz12). PcG binding to the POMC promoter was assessed by reverse chromatin immunoprecipitation (rChIP). We treated one patient off-label with oral vorinostat 400mg daily for 6 days prior to radiation. RESULTS PcG loci (EZH2 and EED) were more accessible in aggressive CD adenoma versus normal gland corticotrophs, and EZH2, SUZ12 and EED mRNA was robustly overexpressed. mIHC confirmed SUZ12, EZH2 and H3K27me3 overexpression, and ChIPseq confirmed polycomb overactivity in aggressive CD adenomas. Polycomb inhibition by DZNep, vorinostat, or siRNAs led to proliferative arrest and Pomc suppression in ATT20 cells (P < 0.05 versus controls). Vorinostat enriched Suz12 at the Pomc promoter (P < 0.01), confirming direct silencing. In a patient with aggressive CD and hypercortisolemia despite two prior surgeries, a single course of vorinostat decreased cortisol (24mcg/dL to 7.5mcg/dL) and ACTH (63.7pg/mL to 44.9pg/mL) prior to radiation therapy initiation. CONCLUSIONS PcG hyperactivity is associated with a more aggressive CD phenotype. We report here the therapeutic potential of HDAC inhibition against aggressive CD adenomas in a first-in-human application.