In this case, molecular methods can be used to supplement or replace a target allele present in one population with a preferred allele present in another. The key issue is, of course, the identification of the genes and alleles that should be targeted for GE in livestock species. Likely, many genes for which naturally occurring variation creates subtle phenotypic effects exist, whereas GE of these genes with novel alleles might provide profound improvements in animal health and performance. Undoubtedly, the discovery of these genes will come from research with model organisms where the effects of the allelic forms on fitness can be extensively studied, emphasizing the importance of support for basic agricultural research.Early research efforts in livestock GE were focused on increasing the efficiency and yield of production for a diversity of species. The use of more productive GE animals [i.e., animals that produce more units of output with the same or less inputs] should be given due consideration in the context of sustainability. Making a conscious choice to use less productive animals necessitates the use of more land per unit of product or the use of more animals to produce a constant amount of animal product or both. This choice could cause conflict when open land is scarce, or other uses compete with the use of land for agriculture. The use of the best available technologies and inputs to produce greater output per unit of input offers overt sustainability advantages . Rapid growth and increased production result in a reduced number of animals being required for a fixed amount of output. Additionally, the environmental footprint per unit of animal product is reduced for more productive animals, regardless of the agricultural production system used. Bradford estimated that a small incremental increase of 2% per year in average milk production per cow globally,raspberry cultivation pot with no change in cow numbers, would result in a 60% increase in the global milk supply by the year 2020. The observation by Palmiter et al. that supplementing the mouse genome with an extra GH gene increased muscle growth greatly stimulated research into this approach for enhancing the productivity of meat animals.
However, early transgenic animal GH supplementation experiments achieved mixed results. Whereas growth enhancement was observed in experiments with both fish and mammals , in some experiments increased GH concentrations compromised the health of animals . From this research, much was learned about the GH axis, methods for transgenesis, the need for controlled gene expression, and best practices for transgenic animal stewardship. One successful outcome of this early work was a transgenic salmon whose genome contains an extra copy of the salmon GH gene . After nearly 15 years of research and development, the GH-transgenic salmon are now in an advanced stage of regulatory review by the FDA and could constitute the first transgenic animal product approved for human consumption in the United States. These fish produce the same amount and kind of circulating GH as wild-type salmon, but they produce it throughout the entire year. This modification has resulted in fish that reach market weight faster and consume less food per kilogram of product than wild-type salmon because they process their food 10 to 30% more efficiently. The more efficient utilization of protein in the diet of an animal leads to a reduction in the excretion of nitrogenous waste . Concerns over the potential environmental impact of feral transgenic fish have been studied extensively and practically addressed by eliminating the possibility of gene flow from transgenic to wild salmon by implementing a redundant biological and physical containment production system, which exclusively utilizes sterile female fish. These fish are reared in land-based facilities with multiple redundant physical containment features. Because of the controlled production environment, the fish are neither exposed to disease challenges nor are they reservoirs for disease transmission as they might be in conventional salmon net pen aquaculture. The ability to grow these salmon in land-based facilities closer to population centers dramatically reduces transportation costs compared with conventional salmon aquaculture, affecting the economics and environmental footprint of salmon production.
Finally, because these fish can be raised in inland fisheries, they represent a unique food security opportunity, potentially rejuvenating a nearly extinct US Atlantic salmon industry. To address the sustainability of pork production, a line of GE pigs was developed with the ability to digest and metabolize natural P in their feed, which non-GE pigs cannot accomplish. These pigs referred to as Enviropigs, have a genome supplemented with a gene from Escherichia coli that produces phytase exclusively in their salivary glands . This genetic modification reduces the excretion of undigested P in pig feces by 30 to 60%, which will ameliorate surface water eutrophication from swine production, as well as eliminate the environmental footprint of phytase production as a feed supplement . Clinical analysis of the health of Enviropigs using hematology,As mentioned, a focus on animal welfare positively influences productivity, and, therefore, indirectly enhances the sustainability of animal production. Numerous targets of GE aim to deliver even more direct, simultaneous improvements in animal welfare and sustainability . For example, GE could provide a humane method for sex selection in dairy and egg industries, where cows and hens provide the animal product. The development of male animals could be avoided ab initio and eliminate inefficiencies in animal production and welfare concerns associated with sex selection and castration. Gene supplementation that feminizes male embryos or eliminates the production of male sperm in sires is technically feasible; the latter approach has the desirable outcome that the animals that are produced are not themselves genetically engineered. Based on the global importance of pork, researchers have developed GE pigs to improve the sustainability of production, enhance animal welfare, and add nutritional value . For example, the expression of bovine α-lactalbumin and IGF in the mammary gland of lactating sows results in increased milk production, which directly enhances animal welfare as demonstrated by improved growth, intestinal development, and overall survival of piglets at weaning .
In a striking example of the value of GE in enhancing animal welfare, Wall et al. at the USDA-ARS engineered Jersey cattle to express the antibacterial protein lysostaphin in their milk, an accomplishment that dramatically enhanced the resistance of these cows to infection by Staphylococcus aureus, the most common and most difficult to treat cause of mastitis. This genetic improvement, could not only improve the well being of around 2 million dairy cattle per year in the United States alone, but also could decrease the economic costs of mastitis, which are currently estimated to exceed $2 billion per year in the United States .One promising aspect of GE is the potential for the development of functional foods that enhance food safety, human nutrition, and health . For example, in China the nutritional value of bovine milk has been improved by GE to express human α-lactoglobulin and human lactoferrin,low round pots proteins normally found in human milk but missing from bovine milk . Given the increasing prevalence of obesity and cardiovascular disease in developed nations, changes in product composition in conjunction with improvements in dietary practices could contribute to improvements in consumer health. The amounts and type of fats in animal products are topics of frequent public discourse, and from the perspective of sustainability, improved feed conversion efficiency increases the ratio of lean-to-fat deposition in livestock. Net benefits include reduced production costs, improved product quality, reduced excretion of nitrogenous wastes into the environment, decreased grazing pressure on fragile landscapes, and reduced pressure on world feed supplies . A decrease in the prevalence of deleterious fats and cholesterol and an increase in the prevalence of MUFA and n-3 fatty acids are consistent with dietary recommendations for cardiovascular health and an objective difficult to achieve in the absence of GE. In fact, 3 proof-of-principle studies have been published: 1) GE goats that expressed a rat stearoyl-CoA desaturase in the mammary gland and yielded milk with a reduced saturated fatty acid content and increased content of CLA, a beneficial antioxidant fatty acid ; 2) a GE pig made transgenic for a Δ12 fatty acid desaturase gene from spinach that produced the PUFA, linoleic acid and α-linolenic acid , which are essential for human nutrition ; and 3) a GE pig that was developed to express an n-3 fatty acid desaturase capable of converting n-6 fatty acids to n-3 fatty acids . Although fish provide an excellent source of dietary n-3 fatty acids, which are important for fertility, cardiovascular health, immune system health, mental health, and cancer prevention , worldwide fisheries will be challenged to sufficiently supply n-3 fatty acids to the developing world. As the most widely consumed meat, pork logically should be considered as an alternative source of n-3 fatty acids. Consistent with this strategy, the pigs developed by Lai et al. produce increased content of n-3 fatty acids from n-6 analogs, and their tissues have a reduced ratio of n-6/n-3 fatty acids. Such animals may be useful as models for human health and for providing a dietary source that could enhance the health of consumers in developed and developing countries. The first FDA approval of a GE animal product, the anticoagulant ATryn , firmly established the importance and safety of engineered mammary gland-based protein expression systems. Additional GE projects in cattle and goats have targeted the mammary gland for the expression of proteins to enhance the welfare of animals, and the safety and stability of milk products .
Bacterial diarrhea, which is responsible for more than 2 million infant deaths per year in developing countries, results from campylobacter, salmonellae, shigellae, and some strains of E. coli infections. Transgenic goats that express the human lysozyme protein, a natural antimicrobial protein in breast milk, were developed to produce milk with an enhanced shelf life that would improve the gastrointestinal health of goat kids and children . Experiments in vitro and in vivo have established that milk from these goats has antimicrobial properties, whether pasteurized or not, and that this milk inhibits the enteric bacteria E. coli when fed to piglets . Approval of this product could make a significant contribution to the alleviation of hunger and disease .Recent estimates indicate that 31.7% of children aged 2 to 19 years are overweight and 16.9% are obese . The issue of obesity is complex, with numerous interrelated causes. It stands to reason that any problem with this degree of complexity cannot be solved through programs that address a single contributing factor. Multiple government agencies have echoed this logic in a call for integrative and innovative strategies that demonstrate promise in promoting healthy lifestyle choices among children. These strategies include implementation at multiple levels, from individual to family to community to society . The need for healthier school environments, improved dietary and physical activity behaviors at home and community engagement in efforts to improve the health and well-being of our nation’s children was emphasized in a recent joint initiative released by first lady Michelle Obama, the surgeon general and the Department of Health and Human Services . To help reach these goals, President Obama issued a memorandum on Feb. 9, 2010, calling for the establishment of a federal task force on childhood obesity . In an Institute of Medicine report, key stakeholders were urged to commit to childhood obesity prevention and to strive not only to develop innovative programs but to monitor the progress and evaluate the efficacy of new and existing obesity prevention policies and programs and work to disseminate promising practices for maximum impact . The committee cited nutrition education and gardening as examples of promising, innovative practices to increase fruit and vegetable consumption through both Farm to School programs and school gardens. Other organizations, including the Centers for Disease Control and Prevention, encouraged creating a school environment that supports regular physical activity and healthy eating habits . Conceptual frameworks are key to the development, implementation and evaluation of successful health programs, as they can provide a system for linking and evaluating the multiple components that influence health behavior . Effective, sustainable programs targeting obesity prevention for the individual are needed within the context of the socio-ecological model, which succinctly describes the relationship of choices made by an individual to the other spheres of influence within the environment and society.