Cutaneous human papillomaviruses (HPVs) are considered as cofactors for non-melanoma skin cancer (NMSC) development, especially in association with UVB.,Extensively studied transgenic mouse models failed to mimic all aspects of virus-host interactions starting from primary infection to the appearance of a tumor.,Using the natural model Mastomys coucha, which reflects the human situation in many aspects, we provide the first evidence that only UVB and Mastomys natalensis papillomavirus (MnPV) infection strongly promote NMSC formation.,Using UVB exposures that correspond to UV indices of different geographical regions, irradiated animals developed either well-differentiated keratinizing squamous cell carcinomas (SCCs), still supporting productive infections with high viral loads and transcriptional activity, or poorly differentiated non-keratinizing SCCs almost lacking MnPV DNA and in turn, early and late viral transcription.,Intriguingly, animals with the latter phenotype, however, still showed strong seropositivity, clearly verifying a preceding MnPV infection.,Of note, the mere presence of MnPV could induce γH2AX foci, indicating that viral infection without prior UVB exposure can already perturb genome stability of the host cell.,Moreover, as shown both under in vitro and in vivo conditions, MnPV E6/E7 expression also attenuates the excision repair of cyclobutane pyrimidine dimers upon UVB irradiation, suggesting a viral impact on the DNA damage response.,While mutations of Ras family members (e.g.,Hras, Kras, and Nras) were absent, the majority of SCCs harbored-like in humans-Trp53 mutations especially at two hot-spots in the DNA-binding domain, resulting in a loss of function that favored tumor dedifferentiation, counter-selective for viral maintenance.,Such a constellation provides a reasonable explanation for making continuous viral presence dispensable during skin carcinogenesis as observed in patients with NMSC.
The failure to mount an effective DNA damage response to repair UV induced cyclobutane pyrimidine dimers (CPDs) results in an increased propensity to develop cutaneous squamous cell carcinoma (cSCC).,High-risk patient groups, such as organ transplant recipients (OTRs) frequently exhibit field cancerization at UV exposed body sites from which multiple human papillomavirus (HPV)-associated cSCCs develop rapidly, leading to profound morbidity and increased mortality.,In vitro molecular evidence indicates that HPV of genus beta-papillomavirus (β-PV) play an important role in accelerating the early stages of skin tumorigenesis.,We investigated the effects of UV induced DNA damage in murine models of β-PV E6 oncoprotein driven skin tumorigenesis by crossing K14-HPV8-E6wt mice (developing skin tumors after UV treatment) with K14-CPD-photolyase animals and by generating the K14-HPV8-E6-K136N mutant mouse strain.,Thymine dimers (marker for CPDs) and γH2AX (a marker for DNA double strand breaks) levels were determined in the murine skin and organotypic skin cultures of E6 expressing primary human keratinocytes after UV-irradiation by immunohistochemistry and in cell lines by In Cell Western blotting.,Phosphorylation of ATR/Chk1 and ATM were assessed in cell lines and organotypic skin cultures by Western blots and immunohistochemistry.,Skin tumor development after UV-irradiation in K14-HPV8-E6wt mice could completely be blocked through expression of CPD-photolyase.,Through quantification of thymine dimers after UV irradiation in cells expressing E6 proteins with point mutations at conserved residues we identified a critical lysine in the C-terminal part of the protein for prevention of DNA damage repair and p300 binding.,Whereas all K14-HPV8-E6wt animals develop skin tumors after UV expression of the HPV8-E6-K136N mutant significantly blocked skin tumor development after UV treatment.,The persistence of CPDs in hyperproliferative epidermis K14-HPV8-E6wt skin resulted in the accumulation of γH2AX foci.,DNA damage sensing was impaired in E6 positive cells grown as monolayer culture and in organotypic cultures, due to lack of phosphorylation of ATM, ATR and Chk1.,We showed that cells expressing E6 fail to sense and mount an effective response to repair UV-induced DNA lesions and demonstrated a physiological relevance of E6-mediated inhibition of DNA damage repair for tumor initiation.,These are the first mechanistical in vivo data on the tumorigenicity of HPV8 and demonstrate that the impairment of DNA damage repair pathways by the viral E6 protein is a critical factor in HPV-driven skin carcinogenesis.,The online version of this article (doi:10.1186/s12943-015-0453-7) contains supplementary material, which is available to authorized users.