BY: Reddy Sailaja M (MSIWM030)
‘Transgenesis’ is a molecular method of introducing a foreign gene (of interest) into the genome of an organism to express the desired trait or characteristic and further pass the trait to the progeny successfully. The gene that is being introduced is called a ‘transgene’.
A transgenic animal or genetically modified animal is the one that is being introduced with a desired foreign gene into its genetic material through recombinant DNA technology, a molecular biology technique.
Transgenesis has been widely applied in most of the domestic animals, aquaculture and agriculture that aids in human welfare and development.
Ralph Brinster and Richard Palmiter were the pioneers in creating first transgenic animal – “Super mouse” in 1982 by introducing human growth hormone in the mouse genome. The offspring produced were larger in size than the parent.
Figure 1: Transgenic super mouse (right) produced by recombinant DNA technology
Pig, goat, sheep, fish, cattle and insects like Drosophila melanogaster (fruit fly) are the most common transgenic animals that are being used in basic and applied research for human welfare.
PRODUCTION OF TRANSGENIC ANIMALS
Two methods are principally followed to generate transgenic animals
- Embryonic stem cell method
- Pronucleus method by microinjection
- Embryonic stem cell method for Transgenesis:
- Inner cell mass of mammalian blastocysts contain embryonic stem cells (ESCs). ESCs have the ability to produce all kinds of organisms’ cells, including gametes.
- Desired gene is selected from the donor organism.
- Vector DNA is chosen that carries the desired DNA to the host cell.
- Vector contains promoter and other regulatory sequences that are crucial for transgene transfer, selection and expression in the host organism.
- ESCs were cultured along with the vector containing desired DNA
- Successfully transformed cells will be selected based on the selection methods like antibiotic resistance.
- The transformed cells are injected into inner cell mass of embryonic blastocysts of the mouse for further propagation.
- A pseudo pregnant mouse (stimulus of mating results in making mouse uterus receptive for the blastocysts due to hormonal changes) was prepared and the transformed blastocyst stage embryo was introduced into the uterus.
- Blastocyst would implant successfully and the mouse gives birth to pups. 10-20% pups will be having the transgene and is heterozygous in nature (only one copy of the gene was transformed and the other was wild).
- Heterozygous mice are allowed to mate to get homozygous offspring (1 in 4, Mendelian ratio) was selected and propagated further to generate transgenic trait.
Figure 2: Embryonic stem cell method for transgenic animal generation
- Microinjection method:
- Manipulation of the pronucleus is the most common method to create a transgenic animal and is first described by Gordon et al.
- Superovulating female is induced with specific hormones and the eggs are harvested.
- The male and female pronuclei are visible under microscope several hours after the sperm is allowed to enter into the oocyte. As male pronucleus is larger in size, the transgene is microinjected easily into it.
- Pronucleus stage is advantageous as it allows early incorporation of the transgene into the host DNA and the entire host cells could express it.
- Once the transgene is introduced, male and female pronuclei are allowed to fuse to form a fertilized egg.
- Once the blastocyst stage is reached, it was implanted into the pseudo pregnant mother and the progeny was checked for the transgene expression as in the ESCs method.
Figure 3: Generation of transgenic animal by microinjection method
Other techniques followed to generate transgenic animals are listed in table 1.
|Cre-lox technique||Ideal technique with more control over resulting phenotype; time-consuming|
|Viral vectors||difficult; largely restricted to avian species|
|Cytoplasmic injection||Less efficient than direct pronuclear microinjection|
|Primordial germ cells||Chimeric animals result|
|Nuclear transfer||Large potential for genetically modifying livestock|
|Spermatogonial manipulation||Transplantation into recipient testes|
Table 1: Other techniques used to generate transgenic animals
APPLICATION OF TRANSGENESIS
- Disease resistant transgenic animals:
Selection and cross breeding of animals is a natural way of producing superior quality livestock animals with respect to disease resistance, more milk production, larger size etc. However, maintaining these qualities to be passed to the generations is unpredictable.
- Neurodegenerative disease resistant animals: Spongiform encephalopathy (Scrapie disease) in sheep, bovine spongiform encephalopathy (BSE) (Mad cow disease) in cattle, Creutzfeldt Jacob disease in humans are some of the major neurodegenerative diseases. These diseases occur because of the expression and misfolding of “prion” protein. ‘Gene knock out’ of ‘prion protein through rDNA technology helps in generating prion protein free livestock and resistant to neurodegenerative disorders. RNA interference (RNAi) is the new rDNA technique that helps in knocking down of the desired gene by forming a double stranded DNA construct and suppressing its expression.
RNAi method has extensive applicability, one of which is to generate knock down transgenic animals that can survive RNA based viral infections like foot and mouth disease, classic swine fever and the most resent SARS-CoV-2 disease.
Figure 4: Prion free sheep (Denning et al 2001)
- Bacterial disease resistant cattle: Mastitis is a bacterial infection of the mammary gland in the cows that affects quality and quantity of the milk being produced. Scientists have developed transgenic cattle that express lysostaphin protein, which kills mastitis causing bacteria by cleaving their cell wall.
Similarly, lysozyme producing transgenic goats are also generated that prevents mastitis causing bacterial lysis and healthier mammary glands.
- Disease resistance in fishes: Catfish often prone to microbial infection and death. Cecropin B is a small protein expressed in Hyalophora cecropia moth that has anti-microbial protperties. Scientists have generated Cecropin gene expressing transgenic catfish that confers resistance against microbial infections.
- Disease resistant cattle against Brucellosis: Brucellosis is a deadly zoonotic disease, that can spread across animals without limit and even to humans. A large number of animals in American Bison area have been affected badly and the grazing cattle used to acquire the infection that lead to abortions, low fertility rates, reduced milk production etc. In humans, the disease is called undulant fever and its effects are severe. Recently, it was discovered that bovineNRAMP1 gene is efficient in providing resistance to brucellosis. Transgenic cattle with this gene offer protection against the disease.
Mastitis resistant transgenic cow (Agricultural research service, US)
Identification and integration of genes through transgenesis is the need of the hour that provides disease resistance and improved immune response in livestock and poultry. Scientists are focusing on genetic engineering based disease resistant animal models that would help livestock show resistance to diseases as shown in the table 2.
Table 2: Diverse applications of disease-resistant genetically engineered animals.
- Medical applications:
- Disease models: Understanding the disease and its effects are crucial for effective drug development and vaccine creation. Transgenic method is widely applied to generate disease models to understand causes and effects of human diseases. For example, mouse with various cancers or cystic fibrosis were produced through rDNA technology. These models give insights into the disease and further effective drug development and treatment.
- Understanding gene functions: Mouse/rat genetic composition is closely related to humans. Hence, mice or rat models are chosen to produce genetically modified organism with alteration of gene like, gene knock-out (removal of a particular gene) or gene knock-in (insertion of a gene) or damaging the gene.
This category of transgenic models helps to understand the crucial functions of the gene and its role in human development and disease. This method is widely used to produce transgenic animals with superior quality. For example, transgenic cow – with disease resistance and improved milk production.
- Production of therapeutic proteins and antibodies: Animals like horse, goat and cows were genetically modified to develop and secrete useful chemical substances like antibodies, and therapeutic proteins that help treat the human diseases efficiently. For example, transgenic cow that secretes egg proteins into its milk. These transgenic animals with therapeutic reagents production are also called ‘walking pharmacies’.
- Production of xenotransplants: Scientists have developed transgenic farm animals by ‘knocking out’ the gene that is responsible for eliciting immune response and rejection of an organ when introduced into the human body. For example, knock out transgenic pig organs can be now used for organ transplantation in humans. This method solves the issue of organ donor shortage and saves many critical lives.
- Transgenic fishes: Transgenic fishes are produced by introducing genes responsible for disease resistant/temperature tolerant/ better growth etc. For example, a company called Aqua Bounty Farms has requested United States Food and Drug Administration (USFDA) to approve its genetically modified salmon that has the ability to grow three times bigger size than normal within a year of its growth.
DISADVANTAGES OF TRANSGENIC ANIMALS
- Genetically modified animals (like disease laboratory study models) will show negative impact on ecosystem if they escape and released into the environment.
- May act as human disease reservoirs for critical pathogens like virus, prions etc.
- May cause severe allergies in humans as it is not a natural product.
- Genetically modified animals may show alterations in its behavior if the foreign gene undergoes any changes like mutations, leading to any unexpected harm to the mankind.
Even though, the chance of adverse effects is minimal, one can’t rule out completely.
ETHICAL CONCERNS OF TRANSGENIC ANIMALS
It is considered as unethical to produce transgenic animals because it is kind of violating animal rights and disrespect to animals.
Unless there is a balance maintained between need and the production of transgenic animals and effective application in human welfare like medical purpose, agriculture and scientific understanding and application.
From its origin, transgenic method has been creating revolutionary output for human well being by producing therapeutic proteins, superior quality breeds of animals and plants and xenografts. A transgenic animal has full potential to play a significant role in biomedical field. However, it is important to maintain ethical standards in effective usage of transgenic method for human welfare. As there is an equal chance of enormous harm that may cause to humans and the environment with the misuse of the technique.