PRAME-mediated retinoid resistance in keratinocyte carcinomas is overcome by EZH2 inhibition

Preferentially Expressed Antigen of Melanoma (PRAME) is a cancer-testis antigen that is expressed in subsets of basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC), collectively termed keratinocyte carcinomas (KC). PRAME functions as a transcriptional co-repressor of retinoid signaling by guiding enhancer of zeste homologue 2 (EZH2), a repressive histone methyltransferase, to retinoic acid response elements (RARE) found in gene promoters. The functions and therapeutic relevance of PRAME expression in KC have not been investigated. In the present study, we evaluated the implications of PRAME expression upon KC development and response to retinoid-based therapeutics. High PRAME expression in tumors was negatively correlated with epidermal differentiation gene signatures. Similarly, PRAME overexpression (OE) downregulated epidermal differentiation gene signatures, and impaired epidermal differentiation. PRAME OE also attenuated retinoid-induced RARE activation, growth suppression and differentiation responses. Conversely, low-PRAME tumors and PRAME-depleted KC cells possessed enrichment of epidermal differentiation gene signatures. PRAME depletion also restored retinoid-induced RARE activation, suppression of KC growth, keratinization suppression in SCC cells and cell death signaling in BCC cells. Furthermore, EZH2 inhibitors potentiated the retinoid-induced RARE activation and suppress the growth of PRAME-expressing BCC and cSCC cells. We demonstrate that PRAME confers retinoid resistance in KC, and we contend that PRAME-mediated retinoid resistance in KC may be overcome by targeting EZH2. Overall design: To investigate the impacts of PRAME expression upon keratinocyte and keratinocyte carcinoma phenotypes, PRAME was overexpressed in Ker-CT keratinocyte and CAL-27 squamous cell carcinomas. PRAME was also knocked out in the basal cell carcinoma cell line UW-BCC1 using CRISPR-Cas9 gene editing. We further evaluated transcriptional response to all-trans retinoid acid (ATRA) compared to DMSO (vehicle) treatment.