Dermal Delivery of Constructs Encoding Cre Recombinase to Induce Skin Tumors in Pten^LoxP/LoxP;Braf^CA/+ Mice.

Current genetically-engineered mouse melanoma models are often based on Tyr::CreER^T2-controlled MAPK pathway activation by the BRAF^V600E mutation and PI3K pathway activation by loss of PTEN. The major drawback of these models is the occurrence of spontaneous tumors caused by leakiness of the Tyr::CreER^T2 system, hampering long-term experiments. To address this problem, we investigated several approaches to optimally provide local delivery of Cre recombinase, including injection of lentiviral particles, DNA tattoo administration and particle-mediated gene transfer, to induce melanomas in Pten^LoxP/LoxP;Braf^CA/+ mice lacking the Tyr::CreER^T2 allele. We found that dermal delivery of the Cre recombinase gene under the control of a non-specific CAG promoter induced the formation of melanomas, but also keratoacanthoma and squamous cell carcinomas. Delivery of Cre recombinase DNA under the control of melanocyte-specific promoters in Pten^LoxP/LoxP;Braf^CA/+ mice resulted in sole melanoma induction. The growth rate and histological features of the induced tumors were similar to 4-hydroxytamoxifen-induced tumors in Tyr::CreER^T2;Pten^LoxP/LoxP;Braf^CA/+ mice, while the onset of spontaneous tumors was prevented completely. These novel induction methods will allow long-term experiments in mouse models of skin malignancies.