Tumorigenesis by transected cells in nude mice: a new method for detecting cellular transforming genes

We have demonstrated that NIH 3T3 cells freshly transfected with either a cloned retroviral provirus or cell DNA derived from virally-transformed cells are able to induce tumors when injected subcutaneously into nude mice. Furthermore, cells transfected with DNA derived from at least three transformed human cell lines are able to induce tumors. These latter tumors contain human DNA sequences and DNA isolated from at least some of them is able to induce both foci and tumors in subsequent DNA transfection. Our data suggests that tumor induction by transfected 3T3 cells could serve as a powerful system for the selection of cells transformed by dominant cellular oncogenes. This method oviates the requirement that oncogenes induce clearly defined morphologically-transformed foci in order to be detected, and eliminates the need to maintain morphologically normal cells in tissue culture for many weeks, as well as the necessity of microscopically screning large numbers of tissue culture dishes. The tumors which arise in nude mice grow progressively, have been readily transplantable to other nude mice, and have been readily explantable into tissue culture. In addition, both DNA and RNA can be isolated directly from the mouse tumors to screen for the presence and expression of transfected sequences. We are currently examining other DNA samples from both human tumor-derived cell lines and primary human tumors to determine if this assay will detect and identify additional oncogenes. We are also studying the suitability of other normal cell lines as recipients in this assay, since cell lines which do not show readily discernible morphological transformation in monolayer culture may be suitable as tumor inducers following transfection. This assay should provide a convenient alternative method for detecting transforming genes and may help to increase the number of such sequence which can be identified and analyzed.