BRAF Inhibition and UVB Light Synergistically Promote Mus musculus Papillomavirus 1-Induced Skin Tumorigenesis.

Simple Summary: Outgrowth of skin tumors in cancer patients who receive therapies with BRAF inhibitors is frequently observed as an undesired side effect. Infection with certain human papillomaviruses may play a role in BRAF-inhibitor-associated skin tumor development. In this study, the impact of BRAF inhibitors and Ultraviolet B (UVB) light on tumor growth was investigated in laboratory mice after infection of their skin with a murine papillomavirus. We could show that the combination of BRAF inhibitor treatment and UVB exposure resulted in higher numbers of virus-induced tumors compared with virus-infected mice that had received only BRAF inhibitors, only UVB irradiation or no additional treatment. This shows that BRAF inhibitors, particularly when combined with UVB light, support skin tumor growth in mice and may represent a novel way by which BRAF inhibitors contribute to skin cancer development. The development of keratinocytic skin tumors, presumably attributable to paradoxical activation of the MAPK pathway, represents a relevant side effect of targeted therapies with BRAF inhibitors (BRAFis). The role of cutaneous papillomavirus infection in BRAFi-associated skin carcinogenesis, however, is still inconclusive. Employing the Mus musculus papillomavirus 1 (MmuPV1) skin infection model, the impact of BRAFis and UVB exposure on papillomavirus induced skin tumorigenesis was investigated in immunocompetent FVB/NCrl mice. Systemic BRAF inhibition in combination with UVB light induced skin tumors in 62% of the MmuPV1-infected animals. In contrast, significantly fewer tumors were observed in the absence of either BRAF inhibition, UVB irradiation or virus infection, as demonstrated by lesional outgrowth in 20%, 5% and 0% of the mice, respectively. Combinatory exposure to BRAFis and UVB favored productive viral infection, which was shown by high numbers of MmuPV1 genome copies and E1^E4 spliced transcripts and an abundance of E6/E7 oncogene mRNA and viral capsid proteins. BRAF inhibition, but not viral infection or UVB light, activated ERK1/2, whereas γH2AX expression, inducible by UVB light, remained unaltered by BRAFis. These results provide experimental evidence that BRAF inhibition and UVB irradiation synergistically promote MmuPV1-induced skin tumor development in vivo. This indicates an alternative pathway by which papillomavirus skin infection may contribute to BRAFi-associated skin tumorigenesis. [ABSTRACT FROM AUTHOR]