Human Papillomavirus E6 Triggers Upregulation of the Antiviral and Cancer Genomic DNA Deaminase APOBEC3B

Several recent studies have converged upon the innate immune DNA cytosine deaminase APOBEC3B (A3B) as a sig- nificant source of genomic uracil lesions and mutagenesis in multiple human cancers, including those of the breast, head/neck, cervix, bladder, lung, ovary, and other tissues. A3Bis upregulated in these tumor types relative to normal tissues, but the mecha- nism is unclear. Because A3B also has antiviral activity in multiple systems and is a member of the broader innate immune re- sponse, we tested the hypothesis that human papillomavirus (HPV) infection causes A3B upregulation. We found that A3B mRNA expression and enzymatic activity were upregulated following transfection of a high-risk HPV genome and that this effect was abrogated by inactivation of E6. Transduction experiments showed that the E6 oncoprotein alone was sufficient to cause A3Bupregulation, and a panel of high-risk E6 proteins triggered higher A3Blevels than did a panel of low-risk or noncancer E6 proteins. Knockdown experiments in HPV-positive cell lines showed that endogenous E6is required forA3Bupregulation. Anal- yses of publicly available head/neck cancer data further support this relationship, as A3Blevels are higher in HPV-positive can- cers than in HPV-negative cancers. Taken together with the established role for high-risk E6 in functional inactivation of TP53 and published positive correlations in breast cancer between A3Bupregulation and genetic inactivation of TP53, our studies suggest a model in which high-risk HPV E6, possibly through functional inactivation of TP53, causes derepression of A3Bgene transcription. This would lead to a mutator phenotype that explains the observed cytosine mutation biases in HPV-positive head/neck and cervical cancers. IMPORTANCE The innate immune DNA cytosine deaminase APOBEC3B (A3B) accounts for a large proportion of somatic muta- tions in cervical and head/neck cancers, but nothing is known about the mechanism responsible for its upregulation in these tumor types. Almost all cervical carcinomas and large proportions of head/neck tumors are caused by human papillomavirus (HPV) infection. Here, we establish a mechanistic link between HPV infection and A3Bupregulation. The E6 oncoprotein of high-risk, but not low-risk, HPV types triggers A3Bupregulation, supporting a model in which TP53 inactivation causes a dere- pression ofA3Bgene transcription and elevated A3B enzyme levels. This virus-induced mutator phenotype provides a mechanis- tic explanation for A3B signature mutations observed in HPV-positive head/neck and cervical carcinomas and may also help to account for the preferential cancer predisposition caused by high-risk HPV isolates.