Any mouse which has its genome altered using any genetic engineering technique is termed as genetically modified or a “transgenic” mouse. The power of introducing foreign DNA precisely and permanently into the genome of animals has enhanced the potential of biological research. Transgenic animals are used in many applications like understanding gene regulation, understanding the mechanism of diseases, development of vaccines, gene therapies, etc. and the progress in this field is substantial and has massively helped in acquiring more comparable animal models for biological research.
A laboratory mouse is biologically very similar to human beings and is always a favourite animal to be used in research studies. Currently, we have a huge variety of knocked-out and knocked-in mice strains which help in the study of genes connected to diseases and disorders. An example is the p53 knockout mice in which the “p53 gene” encoding the tumour suppressor protein is knocked out. This mouse model is used in studying carcinogens, teratogens, and cancer therapeutic agents. The other example is transmembrane protease serine type 2 (TMPRSS2) knockout mice. These mice are used in studying the role of TMPRSS2 cell surface protein during COVID-19 infection.
The first attempt in generating transgenic mice dates back to 1974. Rudolph Jaenisch and Beatrice Mintz are the pioneers of this technique and altered the mouse genome by introducing viral DNA into mouse blastocyst. The procedure they did to show this technique is noteworthy. To alter the genome, they used simian virus 40 (SV-40) viral DNA and microinjected it into mouse blastocyst. Later, they showed the presence of SV-40 specific DNA sequences in the genome of mice developed from these blastocysts and demonstrated the generation of first genome altered mice.
To get into details of this experiment, the blastocyst is a rapidly dividing ball of cells and is the earliest structure in mammalian development. They isolated these blastocysts from the female mouse uterus for the experiment. To isolate a greater number of blastocysts from mice they used a technique called superovulation. Superovulation produces a greater number of ova at once and in turn, more number of embryos from the same mice can be isolated after mating. Superovulation is induced by hormones like equine chorionic gonadotropin (eCG) which stimulates the follicular growth in ovaries and human chorionic gonadotropin (hCG) which stimulates ovulation.
After superovulation, the females were mated to males and zygotes were collected from the oviduct. These zygotes were cultured in lab conditions till they reached the blastocyst stage and were microinjected with SV-40 DNA into blastocoel cavity. After injecting the viral DNA into the blastocyst, the blastocysts were transferred into the uterine horns of different pseudopregnant females for normal growth and development of pups. After the birth of pups, the DNA was isolated from them and the presence of viral DNA in the mice genome was demonstrated.
Since then, innumerous improved methods have been developed to generate research-specific animal models including other species like cattle, sheep, goat, pig, dog, fish, etc. These experiments are not only interesting but revolutionary and are much important to be known. I will be back with another article narrating an unconventional research with the “first-time” theme soon. Stay tuned!
- Rudolf Jaenisch, Beatrice Mintz — Simian Virus 40 DNA Sequences in DNA of Healthy Adult Mice Derived from Preimplantation Blastocysts Injected with Viral DNA — 1974.