Abstract 4435: PRAME-mediated retinoid resistance is overcome by EZH2 inhibition in keratinocyte carcinoma

Abstract Retinoids have exhibited efficacy as treatments and prophylactics for both types of keratinocyte carcinoma (KC): basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC). However, the practical efficacy of these compounds is limited by retinoid resistance mechanisms. Preferentially Expressed Antigen in Melanoma (PRAME) is a cancer-testis antigen ectopically expressed in a subset of BCC and cSCC tumors. In gametes, PRAME binds Enhancer of Zeste Homologue 2 (EZH2), an epigenetic silencer, selectively targeting transcriptional repression to retinoic acid response elements (RARE) in gene promoters. The functions and therapeutic relevance of PRAME expression in KC tumors have not been investigated. A combination of transcriptomic analyses, 2D and 3D cell culture approaches were undertaken to evaluate the contributions of PRAME to KC pathogenesis and treatment response. Pharmacological retinoid sensitization strategies were also investigated. Epidermal differentiation and cytokine signaling pathways were downregulated in high PRAME-expressing tumors and in PRAME overexpressing (OE) cell lines. Conversely, low-PRAME tumors and PRAME depleted cell lines exhibited an enrichment of epidermal differentiation and cytokine signaling gene signatures. PRAME OE impaired epidermal differentiation in 3D organotypic skin culture models. Additionally, PRAME OE impaired RARE activation and conferred resistance to retinoid-induced growth suppression. PRAME depletion restored RARE activation and restored retinoid-induced growth suppression. Mechanistically, PRAME was determined to abrogate the anti-keratinization effect of retinoid treatment in SCC cells and organoid models. PRAME also abrogated the cell death promoting effect of retinoid treatment in BCC cells. Targeting of EZH2, using pharmacological or genetic strategies, in combination with retinoid treatment, resulted in a selective repression of cell growth in KC cell lines. Similar effects were observed when performing co-treatments using cell lines representative of cutaneous melanoma and lung squamous cell carcinoma. This study illustrates potential roles for PRAME in modulating differentiation programs and demonstrates a role for PRAME in conferring resistance to retinoid-based therapeutics. Furthermore, this study offers a preclinical demonstration of a drug repurposing strategy including retinoid compounds and EZH2 inhibitors to suppress the growth of PRAME-expressing cells. Citation Format: Brandon Liam Ramchatesingh, Ivan V. Litvinov. PRAME-mediated retinoid resistance is overcome by EZH2 inhibition in keratinocyte carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4435.