Oncogenes in transgenic mice

Interaction of c-myc with transforming growth factor alpha and hepatocyte growth factor in hepatocarcinogenesis

Double transgenic mice bearing fusion genes consisting of mouse albumin enhancer/promoter-mouse c-myc cDNA and mouse metallothionein 1 promoter-human TGF-alpha cDNA were generated to investigate the interaction of these genes in hepatic oncogenesis and to provide a general paradigm for characterizing the interaction of nuclear oncogenes and growth factors in tumorigenesis. Coexpression of c-myc and TGF-alpha as transgenes in the mouse liver resulted in a tremendous acceleration of neoplastic development in this organ as compared to expression of either of these transgenes alone. The two distinct cellular reactions that occurred in the liver of the double transgenic mice prior to the appearance of liver tumors were dysplastic and apoptotic changes in the existing hepatocytes followed by emergence of multiple focal lesions composed of both hyperplastic and dysplastic cell populations. These observations suggest that the interaction of c-myc and TGF-alpha, during development of hepatic neoplasia contributes to the selection and expansion of the preneoplastic cell populations which consequently increases the probability of malignant conversion. These studies have now been extended to examine the interaction of hepatocyte growth factor (HGF) with c-myc during hepatocarcinogenesis in the transgenic mouse model. While sustained overexpression of c-myc in the liver leads to cancer, coexpression of HGF and c-myc in the liver delayed the appearance of preneoplastic lesions and prevented malignant conversion. Similarly, tumor promotion by phenobarbital was completely inhibited in the c-myc/HGF double transgenic mice whereas phenobarbital was an effective tumor promoter in the c-myc single transgenic mice. The results indicate that HGF may function as a tumor suppressor during early stages of liver carcinogenesis, and suggest the possibility of therapeutic application for this cytokine. Furthermore, we show for the first time that interaction of c-myc with HGF or TGF-alpha results in profoundly different outcomes of the neoplastic process in the liver.

Targeting gene expression to specific cardiovascular cell types in transgenic mice

Transgenic techniques, which allow the introduction of exogenous genes into the genome of experimental animals, promise to bridge the gap between the in vitro observations made by molecular and cellular biologists on cardiac and vascular cells in tissue culture and the physiology and pathology of the whole organ system. One such application of these techniques is tissue targeting: by genetic manipulation to direct expression of a protein--such as a signaling peptide, a growth factor receptor, or an oncogene involved in cell growth--to a tissue where it normally would not be expressed (or where expression is tightly controlled) by fusing it to the transcriptional control sequences of another gene normally expressed in that tissue. In the cardiovascular system, regulatory sequences for cardiomyocyte-specific proteins, vascular endothelium-specific proteins, and smooth muscle-specific proteins can be used to target heterologous genes to their respective tissues in transgenic animals. The effects that such perturbations have on organ physiology and intracellular and intercellular communication can be observed by applying established physiological and molecular approaches. In this review, we highlight some tissue-specific genes from cardiac and vascular cell types whose regulatory sequences may be used to target heterologous proteins; we discuss neutral "reporter" proteins and signal transduction components as paradigms for the application of this technique; and we briefly touch on the potentials and pitfalls of transgenic approaches to molecular physiology.

Mice bearing the E mu-myc and E mu-pim-1 transgenes develop pre-B-cell leukemia prenatally

Previously, it has been shown that E mu-pim-1 transgenic mice are predisposed to T-cell lymphomas, whereas E mu-myc transgenic mice are predisposed to pre-B-cell lymphomas. Here we show that double-transgenic E mu-myc E mu-pim-1 mice exhibit pre-B-cell leukemia in utero. Upon transplantation into recipient mice, embryo-derived double-transgenic leukemic cells frequently progressed to highly malignant monoclonal tumors, indicating that additional (epi)genetic events had occurred during the progression of the disease.

Mice bearing the E mu-myc and E mu-pim-1 transgenes develop pre-B-cell leukemia prenatally

Previously, it has been shown that E mu-pim-1 transgenic mice are predisposed to T-cell lymphomas, whereas E mu-myc transgenic mice are predisposed to pre-B-cell lymphomas. Here we show that double-transgenic E mu-myc E mu-pim-1 mice exhibit pre-B-cell leukemia in utero. Upon transplantation into recipient mice, embryo-derived double-transgenic leukemic cells frequently progressed to highly malignant monoclonal tumors, indicating that additional (epi)genetic events had occurred during the progression of the disease.

The ras and myc oncogenes cooperate in tumor induction in many tissues when introduced into midgestation mouse embryos by retroviral vectors

Midgestation embryos were infected with replication-defective retroviral vectors that either transduced the myc oncogene, the ras oncogene, or both oncogenes simultaneously. The myc virus induced tumors in diverse organs at a very low frequency and with a long latency period, while approximately 20% of the mice derived from embryos infected with the ras virus developed tumors in the skin with a latency of 4-8 weeks. In contrast, infection of embryos with the ras/myc double oncogene virus resulted in 27% of the animals developing rapidly growing and malignant tumors in a great variety of tissues after a median latency period of 2-3 weeks. All tumors were of monoclonal origin, as shown by Southern analysis using the provirus as a molecular marker. Our results are consistent with the hypothesis that the ras and myc oncogenes cooperate in transforming cells, but that additional alterations are necessary for realization of the fully malignant phenotype. Our observations also suggest that a much wider range of cell types become targets for malignant transformation when the embryos are exposed to the myc and the ras oncogenes simultaneously than when exposed to the same oncogenes separately. Infection of mouse embryos with vectors carrying different oncogenes or oncogene combinations may be an efficient and rapid method for evaluating the spectrum of cell types at risk for malignant conversion following mutation of a protooncogene to a transforming gene.