Explain production of transgenic mice using stem cells.
Answers
Transgenic animals are animals (most commonly mice) that have had a foreign gene deliberately inserted into their genome. Such animals are most commonly created by the microinjection of DNA into the pronuclei of a fertilised egg which is subsequently implanted into the oviduct of a pseudopregnant surrogate mother. This results in the recipient animal giving birth to genetically modified offspring. The progeny are then bred with other transgenic offspring to establish a transgenic line. Transgenic animals can also be created by inserting DNA into embryonic stem cells which are then micro-injected into an embryo which has developed for five or six days after fertilisation, or infecting an embryo with viruses that carry a DNA of interest. This final method is commonly used to manipulate a single gene, in most cases this involves removing or 'knocking out' a target gene. The end result is what is known as a ‘knockout’ animal.
Since the mid-1980s transgenic mice have become a key model for investigating disease. Mice are the model of choice not only because there is extensive analysis of its completed genome sequence, but its genome is similar to the human. Moreover, physiologic and behavioural tests performed on mice can be extrapolated directly to human disease. Robust and sophisticated techniques are also easily available for the generic manipulation of mouse cells and embryos. Another advantage of mice is the fact that they have a short reproduction cycle. Other transgenic species, such as pig, sheep and rats are also used, but their use in pharmaceutical research has so far been limited due to technical constraints. Recent technological advances, however, are laying the foundation for wider adoption of the transgenic rat.
Transgenic rodents play a number of critical roles in drug discovery and development. Importantly, they enable scientists to study the function of specific genes at the level of the whole organism which has enhanced the study of physiology and disease biology and facilitated the identification of new drug targets. Due to their similarity in physiology and gene function between humans and rodents, transgenic rodents can be developed to mimic human disease. Indeed, an array of transgenic mice models have been produced for this purpose. Mice are being used as models, for example, to study obesity, heart disease, diabetes, arthritis, substance abuse, anxiety, ageing, Alzheimer's disease and Parkinson's disease. They are also used to study different forms of cancer. In addition, transgenic pigs are being investigated as a source of organs for transplants, which if proven clinically safe could overcome some of the severe donor organ shortages. The development of transgenic animals has recently been transformed by the emergence of the new gene editing tool CRISPR which greatly reduced the number of steps involved in the creation of transgenic animals, making the whole process much faster and less costly.
This section on transgenic mice was jointly written by Lara Marks and Dmitriy Myelnikov. For more information see D. Myelnikov, 'Transforming mice: technique and communication in the making of transgenic animals, 1974-1988', unpublished PhD, Cambridge University, 2015.