Transgenic Mice Vs. Knockout Mice: The Difference Between Them

Mouse models have been used to mimic the human genome and study disease for decades. They are the best models to study human disease, as many diseases share a common pathogenic gene in humans and mice. For the same purpose, a wide variety of genetically modified mice models are being introduced to unlock the secret of the genome.  Over time, gene editing strategies have been refined and made more efficient. 

In this article, we will discover the topic: transgenic mice vs. knockout mice, what makes the difference?

Transgenic Mice Vs. Knockout Mice: What is a Knockout Mouse?

For studying inherited diseases or deformed protein functions, genes are inactivated by approaches like interrupting it with an artificial piece of DNA in a mouse – which is regarded as a knockout mouse. A knockout mouse model is an important animal model to study the role of genes that have been sequenced but whose functions have not been determined. Inactivating the target gene also influences the phenotype of mouse thereby providing information about the gene functions to researchers.

Transgenic Mice Vs. Knockout Mice: What is a Transgenic Mouse?

Transgenic mice are inserted with cDNA clones, and they express a specific protein. Still, protein function can be better understood by deleting a specific gene coding for that protein. Transgenic mouse models are injected with DNA into their fertilized eggs, and desired proteins are expressed in the adult life stages of the mouse. There is no targeted insertion of a gene in transgenic mice, and the gene of interest is integrated randomly into the genome.

Transgenic Mice Vs. Knockout Mice: The Difference Between Them

Transgenic mice are the best for studying overexpression of specific proteins, such as how an overexpressed gene and an excessive protein in the body would affect the phenotype. In contrast, knockout mice technology enables researchers to study the gene function by deleting or inactivating specific genes. Overall, targeted deletion of the gene is achieved in knockout strategy, which is done by homologous recombination of our mimicked null DNA with sister DNA. In this way, deletion is targeted to a specific genomic location. The difference between transgenic mice and knockout mice would be discussed below. 

Transgenic Mice Vs. Knockout Mice: The key differences between them is that:

  • In the knockout mouse, a gene is inactivated to study its function.
  • In the transgenic mouse, a gene is inserted and overexpressed to study protein function. 

Homologous recombination in embryonic stem (ES) cells gives rise to knockout mice, while in transgenic mice, different vectors (retrovirus) and approaches like microinjection are used. In knockout models, a gene is silenced by interrupting the gene. In the transgenic mouse, foreign DNA is added to study protein expression like overexpressing hormone from pituitary glands gave rise to gigantic mice. Knockout mice do not contain specific genes that are added. 

  1. Homologous Recombination for Creating Knockout Mouse

A knockout vector is constructed to be similar to the gene of interest but mutated to achieve loss of function in the resulting protein. After vector construction, knockouts are introduced into the mouse embryonic stem (ES) cells and resulting gene knockout cell lines having DNA insert are further screened. Then the screened cells are inserted into the embryo, creating chimeras, which are then implanted in a pseudo-pregnant mouse. This mouse gives birth to chimeric offspring having that knockout gene.

  1. Mechanism of Creating Transgenic Mice

There are three methods by which a transgenic mouse is made:

  1. Retroviral Infection: The introduction of DNA is in embryonic developmental stages through retroviral vectors. Due to the limitations in using a retroviral vector, this method is not commonly used. 
  2. Direct microinjection: The fertilized embryos of the mouse are taken, and pronuclear microinjection of DNA is performed. As there is non-homologous recombination, the insertion would be random. Then after microinjection, the embryos are implanted into foster mothers to produce transgenic variants. 
  3.  Targeted manipulation: It involves targeted manipulation of embryonic stem cells by introducing loss- or gain-of-function mutations. Then these stem cells are added to other blastocysts of a pseudo-pregnant mouse to produce chimeras, which are then crossed with wild-type mice to produced heterozygous transgenic species.

Transgenic Mice Vs. Knockout Mice: Conclusion

Knockout models are best for studying the function(s) of a gene, as they provide information about protein expression and how protein affects phenotype. We can study human diseases with knockout mouse models, as mice and humans share many common disease-causing genes & pathological mutations. Conditional knockout (cKO) models are being used today for cancer research, which is a refined knockout strategy.

Transgenic mice are the best for studying overexpression of a gene because they have random insertion of DNA, and protein can be overproduced by overexpressing the gene. This allows for research into the function of the protein, so its influence on pathology could be better understood. 
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