Cytogenetic analysis of malignant skin tumors induced in chemically treated TG-AC transgenic mice

TG.AC mice (which carry a v-Ha-ras transgene) rapidly develop papillomas in response to 12-O-tetradecanoylphorbol-13-acetate (TPA). Approximately 30% of the papillomas are associated with subsequent development of malignancies. Early-passage spindle-shaped tumor cells arising from explant cultures of TPA-induced tumors in TG.AC mice were tumorigenic when transplanted to syngeneic recipients. The v-Ha-ras transgene in the transplanted tumors was expressed at a high level. To identify possible genetic changes associated with the development of malignant tumors, explanted cells were cultured in vitro and assessed for karyotypic changes between the second and third passages by analyzing G-banded metaphase chromosomes. For comparison, skin malignancies were induced in nontransgenic FVB/N mice (parent strain) by 7,12-dimethylbenz[a]anthracene (DMBA) initiation and TPA promotion, and their G-banded metaphase chromosomes were analyzed. Trisomy (in at least 50% of about 30 metaphases) of chromosome 15 (in five of 15 tumors) and chromosome 6 (four of 15 tumors) was observed in TG.AC mice, independent of chemical treatment or tumor type. Of six tumors from DMBA/TPA-treated FVB/N mice, three had trisomy 10 or 15 (or both), and two appeared normal. The absence of trisomy 7 is notable because c-Ha-ras maps to that chromosome. The absence of trisomy 7 in the six FVB/N DMBA/TPA-induced skin malignancies contrasts with DMBA/TPA-induced karyotypic effects in SENCAR mice. Expression of the v-Ha-ras transgene may have precluded the requirement for endogenous mutant ras and allelic imbalance involving chromosome 7 in TG.AC mice, but it could not have in FVB/N mice. These results suggest the possibility that the observed trisomies are consequential, rather than causal, events in the development of TG.AC or FVB/N skin malignancies. Molecular genetic analysis will be required to understand the changes associated with tumorigenesis in this transgenic line as well as in the parent mouse line.