The Role of Papillomavirus Infection in Skin Carcinogenesis

Non-melanoma skin cancers, such as squamous cell cancer (SCC), represent one of the most common malignancies worldwide. The main risk factor is exposure to ultraviolet (UV) light. In addition, considerable evidence points to skin infection with human papillomaviruses of genus beta (beta-HPV) as causative co-factor in skin cancer development, particularly in transplant recipients under immunosuppressive treatment, but also in immunocompetent individuals. So far, definitive proof is lacking. Recently, we established a new mouse model for papillomavirus (PV) disease, in which Mus musculus papillomavirus 1 (MmuPV1) causes transient skin infection in immunocompetent and papilloma outgrowth in immunosuppressed laboratory mice after experimental transmission. These characteristics are reminiscent of beta-HPV in humans, where natural infection with beta-HPV generally causes asymptomatic infection in immunocompetent and development of lesions in immunosuppressed humans. Taking advantage of this new mouse model, we aim at studying the role of PV in skin cancer development. In particular, we will evaluate whether MmuPV1 infection can cause SCC development in immunocompetent and immunosuppressed mice. This will be evaluated in the presence or absence of UV light. The results will provide information whether PV infection alone can induce skin tumors in an immunocompetent and an immunosuppressed setting, or whether UV light is needed for cancer development. We will also investigate the still unknown biological functions and the ability of individual MmuPV1 genes to cause skin cancer. To investigate which mechanism(s) this virus employs to cause skin tumors, we will determine the interaction of the viral genes E6 and E7 with the tumor suppressor proteins p53 and pRb, the E6/E7 genes ability to prevent physiological cell death or to induce cell growth, and evaluate whether this virus can interfere with repair of DNA damage which occurs e.g. after UV exposure. All in vivo results in the mice will be confirmed in vitro by infection of cultured mouse keratinocytes and studied in more detail. Also, increased skin tumor formation is seen as side effect in humans after therapeutic administration of BRAF inhibitors. We will investigate whether MmuPV1 contributes to this phenomenon by studying the effects of MmuPV1 infection in BRAF inhibitor-treated mice and mouse keratinocytes. The results obtained herein will provide solid experimental proof for or against a causal role of skin PV in the development of skin cancer and settle a long-lasting, not yet fully understood issue in viral carcinogenesis. If skin PV were proven to play a carcinogenic role, this may change the existing approaches to skin cancer surveillance, prevention, and treatment in immunosuppressed and immunocompetent humans and provide better prevention and treatment options for skin cancer patients.

Infection with papillomaviruses can cause benign warts (papillomas) on skin and mucosae of humans and animals, but also epithelial malignancies, especially anogenital cancer, some oropharyngeal carcinomas and, in genetically predisposed individuals, cutaneous squamous cell cancers. Epidemiological and experimental evidence that certain human papillomaviruses (HPV) of genus-beta are involved as co-factor in the pathogenesis of cutaneous squamous cell cancers in immunosuppressed people and, possibly, in patients receiving targeted therapies are accumulating. However, definitive proof is lacking. Employing the Mus musculus papillomavirus-1 (MmuPV1) skin infection model we established and characterized a MmuPV1-induced mouse model for skin cancer. Herein we could show that MmuPV1 infection of the skin caused development of cutaneous squamous cell cancers in mice, whereas in the absence of viral infection malignancies did not develop. We could further emphasize the importance of the immune system, as only immunocompromised, but not immunocompetent mice developed cutaneous squamous cell cancers. Importantly, in our model we could show that the main risk factor for skin cancer development, UV light, was dispensable for cancer outgrowth, as cutaneous squamous cell cancers arose in the absence of UV irradiation. Our model furthermore corroborated the "hit-and-run mechanism" proposed for papillomavirus-induced skin carcinogenesis. This MmuPV1-induced skin cancer model opens future investigations into viral involvement, pathogenesis, and cancer surveillance, aiming at understanding and controlling the high incidence of skin cancer in immunosuppressed individuals. In addition, we investigated the capacity of the cathelicidin-related antimicrobial peptide (CRAMP), which belongs to the family of host defence antimicrobial peptides, the cathelicidins, to prevent MmuPV1 skin infection in mice. We could demonstrate that mice lacking CRAMP in their skin and immune cells consistently developed large papillomas on the skin after infection with MmuPV1 and concomitant immunosuppression. In contrast, the presence of CRAMP in control mice was protective against skin papilloma development. The CRAMP-mediated protection against MmuPV1 skin infection and associated disease was predominantly achieved by modulation of the host's immune system. Hence, our study indicates cathelicidins as important targets for future anti-papillomaviral compounds.