Transgene expression and repression in transgenic rats bearing the phosphoenolpyruvate carboxykinase-simian virus 40 T antigen or the phosphoenolpyruvate carboxykinase-transforming growth factor-alpha constructs

Transgenic Sprague-Dawley rats expressing either human transforming growth factor-alpha (TGF alpha) or simian virus 40 large and small T antigen (TAg) , each under the control of the phosphoenolpyruvate carboxykinase (PEPCK) p romoter, were developed as an approach to the study of the promotion of hep atocarcinogenesis in the presence of a transgene regulatable by diet and/or hormones. Five lines of PEPCK-TGF alpha transgenic rats were established, each genetic line containing from one to several copies of the transgene pe r haploid. genome. Two PEPCK-TAg transgenic founder rats were obtained, eac h with multiple copies of the transgene, Expression of the transgene was un detectable in the TGF alpha transgenic rats and could not be induced when t he animals were placed on a high-protein, low-carbohydrate diet. The transg ene was found to be highly methylated in all of these lines. No pathologica l alterations in the liver and intestine were observed at any time (up to 2 years) during the lives of these rats. One line of transgenic rats express ing the PEPCK-TAg transgene developed pancreatic islet cell hyperplasias an d carcinomas, with few normal islets evident in the pancreas. This transgen e is integrated as a hypomethylated tandem array of 10 to 12 copies on chro mosome 8q11. Expression of large T antigen is highest in pancreatic neoplas ms, but is also detectable in the normal brain, kidney, and liver. Mortalit y is most rapid in males, starting at 5 months of age and reaching 100% by 8 months. Morphologically, islet cell differentiation in the tumors ranges from poor to well differentiated, with regions of necrosis and fibrosis, Sp ontaneous metastasis of TAg-positive tumor cells to regional lymph nodes wa s observed. These studies indicate the importance of DNA. methylation in th e repression of specific transgenes in the rat, However, the expression of the PEPCK-TAg induces neoplastic transformation in islet cells, probably la te in neuroendocrine cell differentiation, T antigen expression during neop lastic development may result in a pervasive change in the islet cell growt h properties with selection of a transformed phenotype as a possible requir ement for cell viability.