Regulation and Function of the HPV16 CircE7 RNA

High-risk human papillomaviruses (HPV), including HPV16, produce circular RNA that encompass the E7 oncogene (circE7). CircE7 can be detected in HPV16-positive cells and tumors, is preferentially localized to the cytoplasm, is N6-methyladenosine (m6A) modified and can be translated to produce the E7 oncoprotein. Here, we explored the regulation and function of circE7. Mutation of m6A motifs flanking the backsplice junction revealed a single essential m6A motif to be essential for circE7 formation. Mutation of this m6A motif promoted linear splicing of the E6*I splice site (226^409), suggesting that linear and circular E7 splicing are inversely regulated. Additionally, mutation of an IRES-like motif in circE7 significantly decreased E7 protein expression, without having significant effects on circE7 RNA levels. Knockdown of YTHDC1, but not other m6A-binding proteins, decreased both circE7 RNA and protein expression. BaseScope ISH was used to confirm the expression of circE7 in HPV positive head and neck squamous cell carcinoma cell lines. Using both qRT-PCR and BaseScope ISH, we found that serum and amino acid starvation significantly increased circE7 levels. Finally, we generated a HPV16 genome with two point mutations in the circE7 m6A motif (Mut2). Stable transduction of primary keratinocytes with Mut2 confirmed the loss of circE7 and increased expression of E6*I. The Mut2 HPV16 genome exhibited significantly decreased viral replication but an increased ability to transform primary keratinocytes. Our studies reveal that the precise regulation of circE7 and E6*I by m6A are critical for the ability of HPV16 to infect and transform keratinocytes.