LAUSR

Deregulated gene expression is a major driver of melanoma metastasis, a process that results in the majority of melanoma-related deaths. Importantly, these alterations are not limited to protein-coding mRNAs, but also include noncoding RNAs. Despite many recent studies implicating circular RNAs (circRNAs) in cancer development, it is unknown if their deregulation contributes to melanoma metastasis. To identify circRNAs with putative roles in melanomagenesis, we performed RNA sequencing on a panel of melanoma and melanocyte cell lines. 21 differentially expressed circRNAs were validated by qPCR and Sanger sequencing of the backsplice junction. Further analyses of the ratio of the circular and cognate linear RNA transcripts and expression in a larger panel of cell lines identified circPMS1 as a circRNA that is upregulated in melanoma through enhanced backsplicing of the linear PMS1 mRNA. Using transposon-mediated delivery, we examined the effects of stable circPMS1 overexpression on melanoma cells. While proliferation and focus formation were not impacted, migration and invasion were enhanced by circPMS1. Moreover, tail vein injection of circPMS1 overexpressing melanoma cells increased lung metastasis. In addition to this transplant model, we generated the first genetically engineered mouse model of melanoma overexpressing a pro-tumorigenic circRNA. These mice are currently being analyzed. Mechanistically, we find that circularization is required for the pro-migratory effects of the circPMS1 overexpression construct. Additionally, circPMS1 harbors the canonical PMS1 start codon, a predicted ORF spanning over the backsplice junction, and a predicted IRES sequence. Indeed, inserting a flag-tag in the overexpression construct revealed circPMS1 to be translated into a truncated 25kD PMS1 protein as well as a 20kD protein likely arising from an in-frame downstream start codon. Importantly, these truncated circPMS1 proteins are endogenously expressed and increased in melanoma compared to melanocytes while the full-length PMS1 protein is evenly expressed. These findings suggest that circPMS1 promotes melanoma metastasis, at least in part, by coding for truncated circPMS1 proteins. Further work will establish the functions of the circPMS1-encoded proteins and their roles in driving melanoma progression and metastasis. Citation Format: Nicol Mecozzi, Olga Vera, Florian Karreth. circPMS1 is a pro-metastatic circular RNA in melanoma. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3743.