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. 2014 Dec 23;5(6):e02234-14.
doi: 10.1128/mBio.02234-14.

Human papillomavirus E6 triggers upregulation of the antiviral and cancer genomic DNA deaminase APOBEC3B

Valdimara C Vieira, Brandon Leonard  1 , Elizabeth A White  2 , Gabriel J Starrett  1 , Nuri A Temiz  1 , Laurel D Lorenz  3 , Denis Lee  3 , Marcelo A Soares, Paul F Lambert  3 , Peter M Howley  2 , Reuben S Harris  4
Affiliations

Human papillomavirus E6 triggers upregulation of the antiviral and cancer genomic DNA deaminase APOBEC3B

Valdimara C Vieira et al. mBio. .

Abstract

Several recent studies have converged upon the innate immune DNA cytosine deaminase APOBEC3B (A3B) as a significant 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. A3B is upregulated in these tumor types relative to normal tissues, but the mechanism is unclear. Because A3B also has antiviral activity in multiple systems and is a member of the broader innate immune response, 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 A3B upregulation, and a panel of high-risk E6 proteins triggered higher A3B levels than did a panel of low-risk or noncancer E6 proteins. Knockdown experiments in HPV-positive cell lines showed that endogenous E6 is required for A3B upregulation. Analyses of publicly available head/neck cancer data further support this relationship, as A3B levels are higher in HPV-positive cancers 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 A3B upregulation 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 A3B gene 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 mutations 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 A3B upregulation. The E6 oncoprotein of high-risk, but not low-risk, HPV types triggers A3B upregulation, supporting a model in which TP53 inactivation causes a derepression of A3B gene transcription and elevated A3B enzyme levels. This virus-induced mutator phenotype provides a mechanistic 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.

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Figures

FIG 1
FIG 1
APOBEC3B upregulation by transfection of full-length HPV genomes. (A) Histograms reporting APOBEC family member mRNA levels in NIKS transfected with a full-length HPV16 or HPV18 or a control plasmid (Cont.). Each histogram bar shows the mean expression level of each APOBEC family member normalized to TBP (error bars report standard deviations from triplicate assays). (B) Image of the results of a representative DNA cytosine deaminase assay performed with cell extracts from the same cells as in panel A. The single-stranded DNA substrate was treated with reaction buffer as a negative control (−) and recombinant APOBEC3A as a positive control (+).
FIG 2
FIG 2
HPV18 E6 is necessary for APOBEC3B upregulation. (A) A3B and (B) E6 mRNA levels in NIKS transfected with full-length HPV18 (WT), with HPV18 with a stop codon truncating the E6 open reading frame (E6-STOP), or with a control plasmid (Cont.). Each histogram bar shows the mean mRNA expression level normalized to TBP (error bars report standard deviations from triplicate assays). (C) Image of the results of a representative DNA cytosine deaminase assay performed with cell extracts from the same cells as in panels A and B. The single-stranded DNA substrate was treated with reaction buffer as a negative control (−) and recombinant APOBEC3A as a positive control (+).
FIG 3
FIG 3
Upregulation of APOBEC3B by expression of HPV E6. A3B mRNA levels in N/TERT-1 cells transduced with HPV E6 from different high-risk types (HPV16, -18, -33, -45, and -52), low-risk types (HPV6b and -11), or noncancer types (HPV2a and -57) or with an empty vector (Cont.). Each histogram bar shows the mean A3B expression level normalized to TBP (error bars report standard deviations from triplicate assays). Low-risk/noncancer E6 proteins did not cause significant A3B upregulation compared to high-risk E6 proteins (P < 0.01; two-tailed Student’s t test).
FIG 4
FIG 4
HPV E6 knockdown reduces endogenous A3B expression. E6 mRNA levels (A) and A3B mRNA levels (B) in CaSki cells transfected with siRNA targeting the HPV16 early transcript (#1 or #2) or with a nontargeting control siRNA (Cont.). Each histogram bar shows the mean E6 or A3B expression level relative to TBP (error bars report standard deviations from triplicate assays).
FIG 5
FIG 5
APOBEC3B overexpression in HPV-positive head/neck tumors. (A) A3B mRNA levels in HPV-positive and HPV-negative head/neck cancers (HPV positive, n = 69; HPV negative, n = 23; P = 0.0006). (B) A3B mRNA levels in the subset of patients in panel A reported as never-smokers (HPV positive, n = 6; HPV negative, n = 10; P = 0.0013). Each histogram bar shows the average A3B expression level normalized to TBP, with values derived from TCGA RNA-seq data sets (error bars report the standard deviations).
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