What is transgenesis? Describe the methods and applications of transgenesis in animals.

Methods of Transgenesis in Animals

Creating transgenic animals involves several sophisticated techniques, broadly categorized into microinjection and vector-mediated gene transfer.

1. Microinjection

This was the earliest method used for generating transgenic animals. It involves directly injecting the desired gene (transgene) into the pronucleus of a fertilized egg. The injected egg is then implanted into a surrogate mother.

• Procedure: DNA containing the transgene is introduced directly into the male pronucleus of a zygote using a fine glass micropipette.
• Advantages: Relatively simple and can be used for a wide range of species.
• Disadvantages: Low efficiency, requires skilled technicians, and can cause damage to the embryo.

2. Vector-Mediated Gene Transfer

This method utilizes viral vectors or other carriers to deliver the transgene into the animal cells.

• Viral Vectors: Viruses like retroviruses, adenoviruses, and adeno-associated viruses (AAVs) are engineered to carry the transgene. These viruses infect cells and integrate their genetic material, including the transgene, into the host genome.
• Sperm-Mediated Gene Transfer: The transgene is introduced into sperm cells, which then fertilize the egg, delivering the gene to the offspring.
• Embryonic Stem (ES) Cell-Mediated Gene Transfer: ES cells are genetically modified in vitro with the transgene. These modified ES cells are then injected into blastocysts (early-stage embryos). The ES cells integrate into the developing embryo, creating a chimeric animal. Breeding these chimeric animals can produce fully transgenic offspring.

3. CRISPR-Cas9 Gene Editing

A more recent and precise method, CRISPR-Cas9 allows for targeted gene insertion or modification. It utilizes a guide RNA to direct the Cas9 enzyme to a specific location in the genome, where it cuts the DNA. The transgene can then be inserted at the cut site.

• Advantages: High precision, efficiency, and versatility.
• Disadvantages: Potential for off-target effects (unintended modifications at other genomic locations).

Applications of Transgenesis in Animals

Transgenesis has a broad spectrum of applications, impacting various fields.

1. Biomedical Research

Transgenic animals serve as valuable models for studying human diseases.

• Disease Models: Animals can be engineered to express genes associated with human diseases like cancer, Alzheimer's disease, and cystic fibrosis, allowing researchers to study disease mechanisms and test potential therapies.
• Drug Discovery: Transgenic animals are used to assess the efficacy and safety of new drugs.
• Xenotransplantation: Genetically modified pigs are being developed to provide organs for transplantation into humans, addressing the shortage of donor organs.

2. Agriculture and Livestock Improvement

Transgenesis can enhance agricultural productivity and improve livestock traits.

• Increased Growth Rate: Transgenic salmon expressing a growth hormone gene exhibit faster growth rates. (Example: AquAdvantage Salmon)
• Enhanced Milk Production: Transgenic cows have been engineered to produce milk with increased protein content.
• Disease Resistance: Transgenic livestock can be made resistant to specific diseases, reducing the need for antibiotics.
• Improved Meat Quality: Transgenesis can alter the composition of meat, improving its nutritional value and tenderness.

3. Industrial Applications

Transgenic animals can be used to produce valuable proteins and pharmaceuticals.

• Biopharming: Transgenic animals, particularly livestock, can be engineered to produce therapeutic proteins like antibodies, enzymes, and hormones in their milk, blood, or eggs.
• Production of Rare Proteins: Transgenic animals can be used to produce proteins that are difficult or expensive to obtain through traditional methods.

Application	Example	Benefit
Biomedical Research	Transgenic mice with human amyloid precursor protein gene	Model for studying Alzheimer’s disease
Agriculture	Bt cotton (though plant transgenesis, principle applies)	Insect resistance, reduced pesticide use
Biopharming	Transgenic goats producing human antithrombin	Production of a protein used to treat hereditary antithrombin deficiency