Transgenic Animals | Zoology Optional Notes for UPSC PDF Download

Transgenic Animals

Animals, containing inserted foreign genes (transgenes) transferred by using techniques of genetic engineering are called transgenic animals.

Construction of transgene

A transgene must contain the gene of interest along with extra DNA sequences (promoter sequence and sequence for termination), which control the function of the foreign gene in the transgenic animal. Three parts are required to construct a transgene:

• A tissue specific promoter sequence that determines the tissue specific function.
• The sequence of the gene coding for amino acids of the desired protein.
• The sequence responsible for the termination of the expressed protein.

Purification of transgenic DNA constructs

A sucrose gradient or a gel-purification method can be adopted to obtain purified and clean DNA that is dissolved in a microinjection buffer.

Common Methods for Creating Transgenic Animals

• Microinjection of the cloned gene(s) into the pronucleus of a fertilized egg
• Injection of recombinant embryonic stem cells into embryos
• Use of retroviruses

Microinjection Method

The most common method of producing transgenic animals is microinjection. These steps are required:

• Collection of eggs from a super ovulated female animals.
• Proceed to fertilize eggs in vitro and before the two pro-nuclei fuse inside the new zygote, the male pronucleus is microinjected with the recombinant DNA.
• The recombinant DNA is injected into the male pronucleus because it is larger than the female pronucleus.
• Following microinjection, the embryo is cultured to the blastocyst stage in vitro, and then placed back into a pseudo-pregnant female.

Embryonic stem cell from blastocyst

Embryonic Stem cells are isolated from a blastocyst. The blastocyst consists of an inner cell mass of embryonic stem (ES) cells and an outer trophoblast. The transgenic DNA is then inserted into them via viruses or microinjection. The ES cells are injected back into another blastocyst for further development. That blastocyst is then implanted into a surrogate mother as before to create transgenic pups.

Retrovirus mediated gene transfer

In this method, some viruses can be used for transfer the transgenes. Retroviruses are more efficient at integrating foreign DNA, although the integration site for retroviruses is often random. A retrovirus is a virus that contains its genetic information in RNA rather than DNA, and this virus can be engineered to contain a transgene. The biggest advantage of viral delivery is the fact that the cellular infection rate is very high. Virus is then used to infect a cell, such as an ES cell, with its RNA. An enzyme from the virus (reverse transcriptase) copies the RNA to double stranded DNA. An 8-cell embryo is infected with a virus containing a transgene. Then the embryo is inserted into a foster mother as usual.

Uses of Transgenic Animals

Many transgenic animals have been engineered for different applications:

Disease Models:

Various transgenic animals have been developed to mimic the human symptoms of disease. Examples of such models include the OncoMouse (mouse model for cancer study), the AIDS mouse, the Alzheimer’s mouse, and Parkinson’s fly etc.

• Oncomouse: Mouse model to study cancer made by inserting activated oncogenes.
• Alzheimer’s Mouse: Made by using amyloid precursor gene into fertilized eggs of mice. (In the Alzheimer’s patient brain, dead nerve cells are entangled in a protein called amyloid).
• Drosophila fly for Parkinson’s disease: This fly has a mutation of the α-synuclein gene linked to inheritable Parkinson’s disease. Parkinson’s fly shows disease-specific characteristics that are seen in humans during the progression of the disease, such as loss of motion control and loss of dopamine.

Transgenic animals are also helpful in studying animal diseases such as “mad cow” disease (bovine spongiform encephalopathy) and infection of the udder (mastitis). Several other transgenic animals such as mice, rats, fishes, drosophila etc. are designed as study models for various human diseases such as Parkinson's, Alzheimer’s, other neurodegenerative and metabolic diseases like diabetes, osteoporosis, and infectious diseases HIV/AIDS/Influenza etc.

Transgenic animals for pharmaceutical products:

Transgenic Goats:

• The first human therapeutic protein, Antithrombin III was derived from the milk of genetically engineered goats.
• Human antithrombin III (hAT) is a serum glycoprotein that controls blood clots by inactivating the clotting factor thrombin, as well as inhibiting other clotting factors.
• Spider silk proteins isolated from the milk of transgenic goats.

Transgenic Sheep:

• Transgenic sheep is designed to produce Alpha-1-antitrypsin (AAT) (Alpha-1-antitrypsin Deficiency in humans generally results in problems with the lungs and respiratory functions).

Transgenic Cows:

• Designed to produce lactoferrin and interferon in their milk that helps to protect the body from infections and strengthens the immune system.

Xenotransplanters:

There are animals that have been engineered to produce histocompatible organs that can be implanted in humans without fear of rejection by the human body. This technique has been used for producing pigs as xenotransplanters, but the use of those organs has not been approved yet.

Transgenic Animals as Food Sources:

To meet the daily increasing demand for food, animals have been designed to grow larger than their wild-type counterparts without requiring extra food.

• Super fish: Developed for increased growth and size using inserted growth hormone gene into fertilized egg.
• Super Salmon: Transgenic salmon have been shown to grow faster than normal salmon and can reach marketable size earlier.

Scientific research models:

Well-known examples of scientific research models are:

• ANDi, the transgenic monkey: ANDi is one of the most important biological models. A harmless gene for green fluorescence protein (GFP) was inserted into ANDi’s rhesus genome using an engineered virus.
• Glow fish: Genetically modified freshwater Zebra fish developed by introducing a fluorescent protein gene from a Jellyfish into the embryo of the fish.
• Influenza-Resistant Mouse: Influenza-resistant mice were created to study the use of genetic alterations for effecting disease resistance against influenza. These mice overproduce Mx protein, which is known to act as an antiviral agent.
• Smart mouse: Biological model designed to overexpress NR2B receptor in the synaptic pathway. This makes them learn faster like juveniles throughout their life.

Several other disease-resistant, high productive animals are produced for agriculture and industrial purposes.