A comparison of the properties of different retroviral vectors containing the murine tyrosinase promoter to achieve transcriptionally targeted expression of the HSVtk or IL-2 genes

: To target therapeutic genes specifically to melanoma cells, we have constructed recombinant retroviruses where transcriptional control of the murine interleukin-2 (mIL-2) or herpes simplex virus thymidine kinase (HSVtk) genes is provided by the 5' promoter region of the murine tyrosinase gene. Tissue-specific expression of these genes is observed both at the mRNA and protein levels in the B16 melanoma line compared with NIH3T3 fibroblasts. Thus, B16 cells infected with one such retrovirus containing the HSVtk gene exhibited a > 90% reduction in colony-forming efficiency after exposure to 1 microgram/ml ganciclovir, relative to controls, whereas similarly infected NIH3T3 cells showed < 10% reduction in colony-forming efficiency under comparable conditions. The degree of preservation of tissue-specific expression from the internal tyrosinase promoter depended upon the exact molecular design of the vector, possibly as a consequence of the interference between closely juxtaposed promoters within the provirus. Our results show that retroviral vectors can be prepared with the capacity to regulate expression of inserted genes specifically in a particular cell type and may be useful for developing efficient, targeted vectors for the in vivo delivery of genetic therapies for malignant melanoma.