The current focus of our laboratory is using transgenic mouse models to understand the molecular mechanisms of ras mediated tumorigenesis. The transgenic mouse model we are currently employing for this work is Tg.AC. Tg.AC is an important basic research model because 30% of human tumors have been estimated to involve Ras signaling. At the programmatic level, the Tg.AC mouse model is being evaluated by the National Toxicology Program (NTP) as an adjunct to the conventional two-year bioassay. The short-term 26-week bioassay relies on the empirical observation that Tg.AC transgenic mice produce skin papillomas when topically treated with chemical carcinogens. Created on the FVB/N mouse, only one of five founder lines displayed the unique characteristics now associated with the Tg.AC mouse. Tg.AC transgenic mice carry a transgene consisting of an oncogenic v-Ha-ras coding region flanked 5' by a mouse zeta-globin promoter and 3' by an SV-40 polyadenylation sequence. Located proximal to the centromere on chromosome 11, the transgene is transcriptionally silent until activated by full thickness wounding, UV light, or chemical carcinogens. This discriminating tumorigenic response does not occur when Tg.AC mice are treated with structurally similar non-carcinogenic chemicals. Expression of the transgene is a required early event that drives cellular proliferation resulting in clonal expansion and tumor formation. Genetic and epigenetic mechanism may contribute to this induced response In our molecular studies of the transgene integration site we discovered a new ABC oxysterol transport gene. This gene appears to transport cholesterol and may be responsible for the transcriptional activation of the ras gene in Tg.AC mice. We are continuing to investigate the role ABCa13 plays in the transcriptional activation of the transgene. To date we have determined that the transgene has compromised the expression of the transporter. This interference of expression results in three discrete phenotype changes in Tg.AC and in Tg.AC mice incapable of ras expression (NR). The first and most obvious phenotype is a significant change in body weight as a result of reduced fat deposition. The second phenotype is a dysregulation in ratios of saturated to mono and di-unsaturated C16-C18 fatty acids. The third is a tremendous difference in cellular cholesterol. Peritoneal mononuclear lymphocytes of NR Tg.AC mice have 15-20 fold lower cholesterol levels. Also arachidonic acid levels are also significantly decreased in the NR mice. Having presently completed our molecular characterization the transgene integration site and its relationship to the ABCa13 gene, we next will investigate its role in tumorigensis in Tg.AC responder and NR mice. These experiments will provide insight into the role of ABCA13 in lipid maintenance and their role in skin tumorigenesis.
