APOBEC3A Functions as a Restriction Factor of Human Papillomavirus

Abstract

Human papillomaviruses (HPVs) are small DNA viruses causally associated with benign warts and multiple cancers, including cervical and head-and-neck cancers. While the vast majority of people are exposed to HPV, most instances of infection are cleared naturally. However, the intrinsic host defense mechanisms that block the early establishment of HPV infections remain mysterious. Several antiviral cytidine deaminases of the human APOBEC3 (hA3) family have been identified as potent viral DNA mutators. While editing of HPV genomes in benign and premalignant cervical lesions has been demonstrated, it remains unclear whether hA3 proteins can directly inhibit HPV infection. Interestingly, recent studies revealed that HPV-positive cervical and head-and-neck cancers exhibited higher rates of hA3 mutation signatures than most HPV-negative cancers. Here, we report that hA3A and hA3B expression levels are highly upregulated in HPV-positive keratinocytes and cervical tissues in early stages of cancer progression, potentially through a mechanism involving the HPV E7 oncoprotein. HPV16 virions assembled in the presence of hA3A, but not in the presence of hA3B or hA3C, have significantly decreased infectivity compared to HPV virions assembled without hA3A or with a catalytically inactive mutant, hA3A/E72Q. Importantly, hA3A knockdown in human keratinocytes results in a significant increase in HPV infectivity. Collectively, our findings suggest that hA3A acts as a restriction factor against HPV infection, but the induction of this restriction mechanism by HPV may come at a cost to the host by promoting cancer mutagenesis. IMPORTANCEHuman papillomaviruses (HPVs) are highly prevalent and potent human pathogens that cause >5% of all human cancers, including cervical and head-and-neck cancers. While the majority of people become infected with HPV, only 10 to 20% of infections are established as persistent infections. This suggests the existence of intrinsic host defense mechanisms that inhibit viral persistence. Using a robust method to produce infectious HPV virions, we demonstrate that hA3A, but not hA3B or hA3C, can significantly inhibit HPV infectivity. Moreover, hA3A and hA3B were coordinately induced in HPV-positive clinical specimens during cancer progression, likely through an HPV E7 oncoprotein-dependent mechanism. Interestingly, HPV-positive cervical and head-and-neck cancer specimens were recently shown to harbor significant amounts of hA3 mutation signatures. Our findings raise the intriguing possibility that the induction of this host restriction mechanism by HPV may also trigger hA3A-and hA3B-induced cancer mutagenesis. Human papillomaviruses (HPVs) are small, nonenveloped DNA viruses known as one of the most prevalent sexually transmitted pathogens. Among almost 200 different genotypes, ϳ24 high-risk HPV genotypes are causally associated with multiple human cancers, including nearly all cervical cancers and a portion of head-and-neck squamous cell carcinomas (1). From 1988 to 2004, the incidence of HPV-associ...

APOBEC3 genes: retroviral restriction factors to cancer drivers

“…The preponderance of A3 mutations in HPV driven cervical cancer, together with the observation that A3 mutations are enriched in the HPVassociated subset of head and neck squamous cell carcinoma (HNSC), suggest a possible off-target response to the virus [37,38,43,44]. Consistent with this, HPV16 infection has recently been shown to upregulate A3A and A3B mRNA expression in a keratinocyte cell line, and both are upregulated in pre-invasive cervical lesions [19,45]. However, within cervical cancer and HNSC the enrichment of A3 signature mutations is not related to the expression of A3 genes, at least as seen in the tumor biopsy [43,44].…”

“…While A3G resides in the cytoplasm, where it becomes incorporated into retroviral virions, A3 enzymes such as A3A, A3B and A3C enter the nucleus, where they are able to target other pathogens including human papillomaviruses (HPV) and also retrotransposons; mobile elements within the genome which appear to be targeted by multiple APOBEC enzymes via a variety of mechanisms [12][13][14][15][16][17][18][19]. With nuclear localization however, comes the risk of off-target activity against host genomic DNA and the potential for mutagenesis.…”

APOBEC-mediated genomic alterations link immunity and viral infection during human papillomavirus-driven cervical carcinogenesis

“…comparison to levels in normal tissues, and this upregulation is possibly due to HPV16 oncoproteins E6/ E7 (31). A previous study noted the expression of three genes of the APOBEC3A family, hA3A, hA3B, and hA3H, in skin keratinocytes, which are the main host cells for HPV infection (32).…”

APOBEC3A damages the cellular genome during DNA replication

“…A3A is among the most potent of the human APOBEC3 deaminases, and can restrict nuclear DNA viruses and retrotransposition events. 8,[34][35][36] We recently showed that the mechanism for A3A inhibition of L1 retrotransposition occurs by deamination of ssDNA transiently exposed during L1 integration. 9 We have also previously shown that A3A can act on the cellular genome to induce DNA breaks that activate the DNA damage response (DDR).…”