8.2: Genetic Engineering in Animals

Biotech Focus

In the late 1990s, researchers at the National University of Singapore added the gene encoding the green fluorescent protein from jellyfish to the genome of wild zebrafish (Danio rerio). Under natural white and UV light, these fish glowed a bright green. Recognizing a commercial application, Alan Blake and Richard Crockett founded the GloFish brand in 2001 to market and sell these genetically modified fish. After two and a half years of research and consultation with experts, GloFish were introduced to the US market in late 2003 to be sold as tropical aquarium fish. Because aquarium fish are not used as a food source, the US Food and Drug Administration (FDA) determined that GloFish did not pose a greater environmental risk than unmodified zebrafish and opted not to regulate the sale of these fish. However, some environmentalists became concerned about the possibility of GloFish escaping into the wild and thriving. In 2014, a single GloFish was spotted in Florida, and in 2015, groups of GloFish were spotted in Brazil. These fish likely escaped from ornamental fish farms in both locations. How their presence is affecting natural ecosystems remains unknown.

Introduction

Genetic engineering in animals involves modifying an animal’s DNA in order to introduce desirable traits that improve the animals health, or enhances its productivity. By using tools like CRISPR-Cas9, recombinant DNA technology, and cloning, scientists can insert, delete, or modify specific genes in animals for various applications. Genetic engineering of animals has revolutionized research, agriculture (i.e., livestock management), the treatment of diseases, medicine and the pharmaceutical industry. Animals have been genetically modified for disease resistance, faster growth, and improved nutritional value. Transgenic animals are now used routinely to produce pharmaceutical proteins, and genetically-engineered pigs are being developed for the transplantation of organs into humans (i.e., xenotransplantation). Research on engineering animals also helps scientists study genetic diseases and drug development. Despite its benefits, genetic engineering in animals raises ethical concerns, including animal welfare, ecological risks, and regulatory challenges. As this technology advances, careful evaluation will be needed to balance innovation with ethical responsibility.

Learning Objectives

Genetically-modified animals have found use in a wide variety of industries, including agriculture and medicine. Transgenic animals form the core of "biopharming", disease management, and xenotransplantation. At the end of this page, you will be able to:

• Define appropriate terms like GMO and transgenic
• Outline the basic steps in making a GMO
• Describe how genetically-modified animals can be used a bioreactors to produce pharmaceuticals
• Describe how genetically-modified animals are used to manage diseases
• Describe how genetically-modified animals are used in xenotransplantation

Genetically modified organisms (GMOs)

Genetically-modified organisms (GMOs) are organisms whose genetic material has been altered through genetic engineering techniques. This alteration has been performed to express desirable traits or improve resilience of an organism. When the gene being introduced comes from a different organism, the GMO is often referred to as a transgenic organism. This means that all transgenic organisms are GMOs, but not all GMOs are transgenic.

The history of GMOs dates back to the early 1980s, when the human insulin gene was introduced into the genome of the bacteria E.coli. These genetically-engineered bacteria were quickly approved for medical use, revolutionizing diabetes treatment. In agriculture, the "Flavr Savr" tomato became the first genetically modified crop to be approved for commercial sale in 1994. Engineered to delay ripening, it sparked the rapid development of other GMO crops, such as pest-resistant corn and herbicide-tolerant soybeans. Since then, GMO applications have expanded into numerous fields, including medicine, biofuels, and environmental management, becoming an integral part of biotechnology.

• 1982: FDA approves the first consumer GMO product developed through genetic engineering: human insulin to treat diabetes.
• 1986: The federal government establishes the Coordinated Framework for the Regulation of Biotechnology. This policy describes how the U.S. Food and Drug Administration (FDA), U.S. Environmental Protection Agency (EPA), and U.S. Department of Agriculture (USDA) work together to regulate the safety of GMOs.
• 1990s: The first wave of GMO produce created through genetic engineering becomes available to consumers: summer squash, soybeans, cotton, corn, papayas, tomatoes, potatoes, and canola.
• 2003: The World Health Organization (WHO) and the Food and Agriculture Organization (FAO) of the United Nations develop international guidelines and standards to determine the safety of GMO foods.
• 2005: GMO alfalfa and sugar beets are available for sale in the United States.
• 2015: FDA approves an application for the first genetic modification in an animal for use as food, a genetically engineered salmon.
• 2016: Congress passes a law requiring labeling for some foods produced through genetic engineering and uses the term “bioengineered,” which will start to appear on some foods.

To create a GMO, researchers study and identify a gene responsible for giving an organism a desired trait. This in itself can be difficult because the degree to which a single gene affects a trait depends on the desired trait. Once a gene has been identified, there are several steps that are followed in the laboratory:

• the gene is copied from an organism through polymerase chain reaction (PCR)
• the copied gene is ligated into a cloning plasmid to create a recombinant plasmid
• the recombinant plasmid is introduced into another organism (i.e., the "host" organism)
• the host organism is allowed to develop under laboratory conditions
• the resulting GMO is studied extensively in the laboratory to ensure that it exhibits the desired trait and does not pose a threat to the unmodified organism found in nature.

The "host" organism whose genome will be altered can range from something as simple as unicellular bacteria and yeast, to multicellular, complex organisms such as animals and plants. The way in which the gene is introduced into the host organism will depend on the host organism. For animals, recombinant plasmids are typically introduced into cultured embryonic stem (ES) cells through a process called transfection. In transfection, the DNA is incorporated into small particles, called liposomes, in order to facilitate the entrance of the DNA into the cell. Once expression of the introduced gene is confirmed, the modified ES cells are then microinjected into an early stage embryo (i.e. the blastocyst) for incorporation into its inner cell mass. Alternatively, recombinant plasmids can be directly microinjected into into a fertilized egg (i.e. the zygote), or the gene can be inserted into the zygote's genome using CRISPR. Following implantation of the genetically altered embryo into a surrogate female, the offspring are assessed for their expression of the new gene (Figure \(\PageIndex{1}\)). For more information about transfection, go to Chapter 13.10: Liposomal transfection into mammalian cells.

Figure \(\PageIndex{1}\): GMO animal production. One method of producing a genetically-modified organism (GMO) is shown in this figure. The gene is introduced into embryonic stem cells (ES cells) that have been isolated from the host organism, such as a mouse. The selection of genetically modified mouse ES cells are is performed to ensure a purified population. Microinjection of these modified ES cells into a mouse embryo results in the incorporation of the cells into the inner cell mass of the embryo. Incorporation is followed by implantation of the modified embryo into a surrogate female for continued development. Testing of the offspring for expression of the gene (red mice) identifies the GMO. (GMO animal production; Patricia Zuk CC BY 4.0; figure created in BioRender. Zuk, P. (2024))

Genetically-Engineered Animals in Medicine

Animals as Bioreactors

Drug production, regenerative therapies, xenotransplantation, and disease modeling all use genetically-engineered animals. For example, spider silk genes have been inserted into goats, allowing them to produce silk proteins in their milk, which can be used for medical sutures and bulletproof materials. This technology, known as biopharming or "pharming", has also been used for the insertion of human proteins into animal genomes, essentially turning these modified animals into "living bioreactors". For example, insertion of the gene encoding anti-thrombin, a natural blood protein that prevents excessive blood-clotting, into the genome of goats has enabled them to produce this therapeutic protein in their milk (Figure \(\PageIndex{2}\)). The resulting product, known as ATryn® is currently used to treat conditions like deep vein thrombosis, hereditary anti-thrombin deficiency or for preventing clots during surgery or childbirth.

Figure \(\PageIndex{2}\): Biopharming of human anti-thrombin. The human anti-thrombin gene is isolated from a human donor and a goat is used for the donation of an ovum. The gene is microinjected into a goat ovum (pink cell) to created a genetically-modified ovum (blue cell). The ovum is then fertilized and implanted into a recipient goat (shown in white). The resulting transgenic goat (shown in gray) is bred for milk production. The milk, containing the anti-thrombin, can then be processed and the clotting agent extracted and purified. (Biopharming by Patricia Zuk, CC BY 4.0; figure created in BioRender. Zuk, P. (2025))

Additional examples of biopharming and the use of animals as bioreactors include:

• Transgenic cows for the production of human insulin and human growth hormone in their milk
• Transgenic cows modified with human immunoglobulin genes and challenged with specific pathogens to produce human antibodies
• Transgenic goats engineered to produce spider silk proteins in their milk for medical sutures and biomaterials
• Transgenic rabbits and the production of the hormone erythropoietin (EPO)
• Transgenic sheep production of alpha-1 anti-trypsin for the treatment of cystic fibrosis and emphysema
• Transgenic sheep and pigs in the production of clotting factor IX

Such applications, while exciting, require the breeding of large amounts of animals and the isolation of specific tissues like blood. As a result, many have called into question the ethics of using transgenic animals.

Disease Management

The use of animals as bioreactors for pharmaceutical production is an established method for the treatment and/or management of a disease. However, there are additional approaches like the production of disease-resistant animals through genetic engineering. For example, the genetic manipulation of cattle is being used to protect herds from deadly infections like foot-and-mouth disease. Genetic engineering of mosquitoes is revolutionizing how the spread of mosquito-borne diseases like malaria, dengue, Zika, and chikungunya is limited in affected populations. Genetically-modified mosquitoes have been created that prevent their reproduction (e.g. Oxitec's Friendly™ Aedes Mosquitoes). The males contain a "self-limiting gene" that, when expressed in their offspring, causes them to die before they reach adulthood. The release of these males into the wild population eventually leads to population collapse. Since their approval and introduction, decreases of over 90% have been measured Aedes aegypti mosquito populations in Brazil, Florida, and the Cayman Islands. Disease management through vaccine production has also seen significant advancements using genetic engineering. Chickens can be genetically modified to express a specific pathogenic protein in their eggs. The protein can then be isolated and purified from the egg and used for immunization, offering a faster, more efficient, and cost-effective way to manufacture vaccines for human and animal diseases. Since a single chicken can lay over 300 eggs per year, large amounts of vaccines can be made for a lower cost versus traditional vaccine culture techniques. These vaccines can also be quickly produced for viruses that show a rapid rate of mutation (e.g. influenza, COVID-19).

Xenotransplantation

Xenotransplantation is the process of transplanting cells, tissues, or organs from one species to another, typically from animals to humans (Figure \(\PageIndex{3}\)). This technology is being explored as a solution to the shortage of human organ donors for patients in need of transplants. To increase success rates, animal organs can be genetically-modified through somatic cell nuclear transfer (SCNT) to knock out immunoreactive porcine genes and to express human immune proteins, in the hope of decreasing the risk of immune rejection. Pigs have become the primary choice for xenotransplantation because their organs are similar in size and function to human organs. Prior to use, those pig genes that would illicit an immune reaction are silenced. In addition, the presence of the PERV (Porcine Endogenous Retrovirus) genome within the genome of the pig is removed to prevent infection of human recipients. For this, scientists are exploring the use of CRISPR to silence or edit out these genes. Human genes are then inserted to help prevent immune rejection and blood clotting.

Figure \(\PageIndex{3}\): Xenotransplantation. The modification of pig organs for human transplantation is done in two phases: Somatic Cell Nuclear Transfer (top box in blue) and the creation and transplantation of transgenic pig organs (bottom box in green). In SCNT, a somatic cell (pink) is isolated from a pig. Genetic manipulation of the somatic cell's DNA is performed to knock-out genes that could cause immune rejection in humans. Introduction of the modified DNA into an enucleated oocyte is accomplished by microinjection of the nucleus taken from the modified somatic cell. The oocyte is activated to initiate embryonic development (not shown) and then implanted into a surrogate sow. The resulting transgenic offspring is then used in the creation and transplantation of transgenic pig organs. In this process, the transgenic offspring undergo repeated cycles of additional genetic manipulation (e.g. knock-out of porcine genes, insertion of transgenes), SCNT, and breeding to produce transgenic pigs that express human immune antigens. The adult transgenic pigs are used for the harvesting of genetically-modified organs, such as the heart and kidneys. These organs are used for transplantation into humans. (Xenotransplantation by Patricia Zuk, CC BY 4.0; figure created in BioRender. Zuk, P. (2025))

In 2021, the first genetically-modified pig heart was successfully transplanted. In 2024, a modified pig kidney was transplanted and functioned in a brain-dead patient for over a month. In 2024, physicians at Massachusetts General Hospital transplanted a genetically-edited pig kidney into a 62 year old living patient. Genetically-modified pig hearts were first transplanted into living patients in 2022 and 2023. Most recently, in 2025, a genetically-modified pig liver was transplanted into a brain-dead human recipient in order to monitor its function and potential rejection. As the technology improves, pig organs are likely to see increased use in transplant facilities.

While xenotransplantation provides hope for alleviating organ shortages, there are several problems associated with this technology. Even with careful gene editing, rejection of the modified organ is still a risk. The presence of PERV in the pig genome presents concerns about unanticipated cross-species infections. In response, researchers have begun to explore the use of modified primate organs in place of pig organs. However, primate use is likely to increase the opposition people feel about using animals for human organ transplants.

Molecular Lab Protocols

•  Lab Technique: Liposomal transfection into mammalian cells

Key Concepts

Genetically-modified animals form the core of areas like "biopharming", disease management, and xenotransplantation, offering advancements in medicine, agriculture, and research.

Some important concepts to remember are:

• Genetically-modified organisms (GMOs) are organisms whose genetic material has been altered through genetic engineering techniques
• The introduction of foreign genes into an organism produces a GMO known as transgenic
• The method used to introduce genetic information into an organism depends on the organism itself
• In medicine, transgenic animals such as goats, cows, and chickens are modified to produce therapeutic proteins in a process known as biopharming
• Transgenic animals can also be used in the production of vaccines, and in the management of diseases
• Transgenic animals can also be engineered for the production of disease-resistant animals
• Genetically-modified pig organs with reduced potential for immune rejection are being developed for xenotransplantation
• Innovations using transgenic animals raise important ethical considerations

Glossary

 

Biopharming - the use of genetically engineered plants or animals to produce valuable medical or therapeutic compounds such as drugs, vaccines, and antibodies; also called pharmaceutical farming or "pharming"

Blastocyst - an embryonic stage comprised of a hollow, fluid-filled ball of cells; develops about 5–6 days after fertilization in humans and consists of a thin outer layer of cells and an inner cell mass made of embryonic stem cells (ES cells)

ES cells (Embryonic Stem Cells) - totipotent stem cells derived from the inner cell mass of a blastocyst, an early-stage embryo

GMO (Genetically-Modified Organism) - an organism whose genetic material has been altered through genetic engineering techniques

Inner cell mass - a group of cells found within the blastocyst stage of embryonic development; a source of ES cells

Transfection - the process of introducing foreign DNA (or RNA) into eukaryotic cells using chemicals, like liposomes or calcium phosphate

Transgenic organism - an alternate term used for a GMO; often used to refer to genetically-modified animals

Xenotransplantation - the process of transplanting cells, tissues, or organs from one species to another, typically from animals to humans