Transgenic livestock for agriculture and biomedical applications

Abstract

The classical definition of a genetically engineered or "transgenic" animal would be "those animals modified by recombinant DNA (rDNA) techniques" according to the U.S. Food and Drug Administration (United States Food and Drug Administration, Guidance #187, 2009). Thus, the introduction or deletion of any gene by using recombinant DNA would fall into this category. Since the first reports of stable germline transmission of integrated DNA in 1982, hundreds of thousands of transgenic animals have been produced, the vast majority of which are mice used in biomedical research. However, there are also thousands of agrarian and aquaculture species that have been produced for a wide variety of purposes from production of human biopharmaceuticals, xenotransplantation and to enhance economically important production traits. We can separate the transgenic animals produced in our laboratory into 3 major classes; production agriculture, biopharmaceutical production in milk and biomedical models of human disease. This brief review will highlight some of those projects in each category. Transgenic livestock in production agriculture: Production agriculture projects in our laboratory have utilized RNA interference to enhance meat production through improved muscle growth. Myostatin is a negative regulator of muscle cell proliferation during fetal development. It is know that inactivating mutations in a number of species, including cattle, develop a muscle overgrowth phenotype [1]. Although the increased muscle can be considered a positive trait, the increase in dystocia related to the size of the calves at birth limit the utilization of these naturally occurring myostatin mutations in production agriculture. RNA interference has been utilized to overexpress a short hairpin RNA (shRNA) with homology to the myostatin mRNA in vivo. Direct injection into the perivitelline space of bovine zygotes with recombinant lentiviral vectors promoted the efficient integration of the shRNA expression constructs into the bovine genome. The expression and cellular processing of the myostatin shRNA led to degradation of the myostatin mRNA and an overt muscling phenotype in transgenic cattle [2]. Subsequent studies in pigs also show a significant increase in fetal pig weights compared to transgenic controls. The advantage of this approach is that shRNA mediated degradation of the myostatin mRNA over a range of values, typically between 20-80% of controls. This approach can be used to decrease the production of myostatin in a way that may lead to increase muscling without associated dystocia problems. Enhanced resistance to endemic diseases, especially where vaccination strategies are ineffective or difficult to achieve, could dramatically increase the production capacity and thus the profitability of animal production systems. We have used RNAi to investigate host/pathogen interactions by direct suppression of viral proteins or immune system modulators, which decrease viral replication and improve disease resistance. Direct targeting of viral mRNA can reduce but not eliminate vesicular stomatitis virus or equine infectious anemia virus in vitro [3]. Likewise, suppression of the immunomodulatory components of the innate immune system have shown positive results, but not complete protection of the cells from viral infection. Current and future work will continue to refine these efforts with the long term goal of producing animals with enhanced innate immunity to a wide variety of viral and bacterial pathogens. Short-term investigations in collaboration with the USDA Plum Island Animal Disease Center have focused on therapeutic approaches to prevent spread of diseases such as Foot and Mouth Disease Virus during outbreaks. Transgenic livestock for biopharmaceutical production: Mammals have the remarkable ability to produce large quantities of proteins in their mammary glands and secrete these proteins during lactation. This natural protein production "factory" can be utilized to produce proteins that benefit both animals and humans through genetic engin