Induction of iPS Cells Using Transient Episomal Expression Vectors: Validation of Toxicity Testing in iPS-Derived Hepatocytes.
One of the biggest scientific breakthroughs of the past decade has been the isolation of human embryonic stem (hES) cells, which hold vast potential to treat conditions including diabetes, Parkinson’s, spinal cord injury, hearing loss, and cardiac disease. However, the ethical and political issues surrounding the use of human embryos have hindered development of these therapies. Recently, innovative publications have demonstrated that human skin cells can be reprogrammed into cells that are very similar to hES cells. These “induced pluripotent stem cells,” or iPSC, hold similar potential to hES cells but avoid the use of human embryos. Furthermore, since skin cells are obtained more easily than embryos, iPSC facilitate the generation of patient- or disease-specific cell lines.
Currently, viruses are used to transmit the genes necessary for reprogramming the skin cells, and these viruses permanently integrate into the DNA of the cells. Although the genes may be shut off after iPSC are induced, at least one of these genes, c-Myc, is capable of causing tumors when reactivated in cells derived from the iPSC. Here we propose a novel method of generating iPSC that does not use viruses and, therefore, avoids insertion of the reprogramming genes into the cells. These genes will only exist temporarily, long enough to allow the skin cells to change into iPSC. Once iPSC have formed, the reprogramming genes will be "kicked out" of the cells, eliminating the potential of tumor formation from reactivation of c-Myc. Furthermore, this method allows finer control over the amount of each reprogramming gene delivered to the cells than does the viral method, which means this method should be more efficient at producing iPS cells.
Genetic differences between individuals, particularly differences in certain liver genes, lead to differences in how patients respond or react to drugs. Currently, few good cellular models of liver are available, especially those that model the genetic diversity of human populations. Normal liver cells isolated from the body last in culture only a very short time, during which they gradually lose appropriate drug processing activity. Transformed liver cell lines can be cultured indefinitely due to activation of a gene normally involved in cancer, but because of this gene, it is unclear whether these cells respond to drugs in the same way that normal liver cells do. We plan to use our new iPSC method to derive three patient-specific lines that represent variation in some of the important drug metabolizing liver enzymes. The advantage of using iPSC is that they are a renewable source of normal, non-embryo-derived cells that can be turned into liver cells, thus allowing earlier identification of drug-induced liver toxicity, decreasing the number of animal studies needed for drug development, and permitting study of the effects of genetic variation on drug metabolism. This should lead to the development of safer and more effective drugs.
Drug-induced liver toxicity is the leading cause of liver failure in California, as is the case in the rest of the U.S. as well (Goldkind and Laine 2006). This includes drugs that were approved by the U.S. Food and Drug Administration (FDA) but subsequently removed from the market. In fact, the FDA estimates that increasing efficiency of early detection of toxicity by only 10% would save the pharmaceutical industry $100 million per drug, on average, of R&D development costs. In addition to the generalized liver toxicity, a significant reason for these adverse events results from the genetic differences (“polymorphisms”) between individuals with respect to certain drug metabolizing genes found in the liver. What the industry needs is a more robust clinically predictive drug screening system for the early detection of generalized and polymorphism-specific liver toxicity. Our iPSC-derived liver cells will be the basis of both types of screens by allowing the generation of polymorphism-specific cell lines. Eventually a panel of these donor-specific cell lines could be generated that represent most of the variation seen in humans. This should result in earlier identification of drug-induced liver toxicity, keeping many of the more dangerous clinical drugs from ever reaching the market and leading to the development of safer and more effective drugs for all Californians.
This proposal has been submitted by a California company whose mission is to develop and commercialize ES cell-based assays for the pharmaceutical industry. The Company has many years of experience working with human and mouse embryonic stem cells as tools for drug discovery and development, and has a successful pharmaceutical partnership using ES cells for diabetes drug screening. Partnering and licensing of the technology developed in this proposal, as well commercialization of new and safer drugs, would bring revenue and additional jobs to California. Intellectual property gains from patents arising from these technologies are also potentially significant for California, due to the licensing revenue fees that would go back to the state. The cell lines developed would also be the basis of additional academic and corporate collaborations, increasing California’s leadership role in stem cell applications.
Goldkind, L. and Laine, L. (2006). A systematic review of NSAIDs withdrawn from the market due to hepatotoxicity: lessons learned from the bromfenac experience. Pharmacoepidemiology and Drug Safety. 15:213-220.