Transgenic mice

ORIGINAL: Genome Trust

8/1/03. By Richard Twyman

Transgenic mice contain additional foreign DNA in every cell allowing them to be used to study gene function or regulation and to model human diseases.

A transgenic mouse contains additional, artificially-introduced genetic material in every cell. This often confers a gain of function, for example the mouse may produce a new protein, but a loss of function may occur if the integrated DNA interrupts another gene. A transgenic mouse a very useful system for studying mammalian gene function and regulation because analysis is carried out on the whole organism.

Transgenic mice are also used to model human diseases that involve the overexpression or misexpression of a particular protein.

Key principles

Transgenic mice contain extra genetic material integrated into the genome in every cell.

The extra genetic material is often described as foreign DNA, but it can come from any source, including another mouse.

To get the same foreign DNA sequence into every cell of the mouse, it is necessary to introduce the DNA into cells of the very early mouse embryo that will contribute to the germ line (the cells that produce eggs or sperm).

Photo: Gloria Kwon. Memorial Sloan-Kettering Institute

How does it work?

There are two major methods.

1- There are two major methods for producing transgenic mice. The first method, pronuclear microinjection, begins by isolating eggs just after they have been fertilised.

2- Using a fine needle, the foreign DNA is injected into the large male pronucleus, which is derived from the sperm. This is implanted in a foster mother and develops to term.

3- The DNA tends to integrate as many tandemly arranged copies at a random position in the genome, often after one or two cell divisions have occurred. Therefore, the resulting mouse may be only partially transgenic.

4- If the transgenic cells contribute to the germ line, then some transgenic eggs or sperm will be produced and the next generation of mice will be fully transgenic - where every cell contains a copy of the foreign DNA.

5- The second method of producing transgenic mice works by introducing DNA into embryonic stem cells. These are derived from the very early mouse embryo and can therefore differentiate into all types of cell when introduced into another embryo.

6- DNA introduced into embryonic stem cells may integrate randomly, as in the case of pronuclear microinjection. However, if the introduced DNA is similar in sequence to part of the mouse genome, it may undergo homologous recombination and integrate as a single copy at a specific site.

7- The embryonic stem cells need a host embryo in which to develop, and this is isolated from a mouse with different coloured fur.

8- The embryonic stem cells will colonise a host embryo, the resulting mouse being a chimera - a mixture of cells from brown and black mice. Embryonic stem cells often contribute to the germ line, resulting in the production of some sperm carrying the extra DNA.

9- When these transgenic sperm fertilise a normal egg, a transgenic mouse is produced with the same foreign DNA in every cell.

In 'pronuclear microinjection', the foreign DNA is introduced directly into the mouse egg just after fertilisation. Using a fine needle, the DNA is injected into the large male pronucleus, which is derived from the sperm. The DNA tends to integrate as many tandemly arranged copies at a random position in the genome, often after one or two cell divisions have occurred. Therefore, the resulting mouse is only partially transgenic. If the transgenic cells contribute to the germ line, then some transgenic eggs or sperm will be produced and the next generation of mice will be fully transgenic.

The second method is the introduction of DNA into embryonic stem cells (ES cells). These are derived from the very early mouse embryo and can therefore differentiate into all types of cell when introduced into another embryo.

DNA introduced into ES cells may integrate randomly, as in the case of pronuclear microinjection. However, if the introduced DNA is similar in sequence to part of the mouse genome, it may undergo 'homologous recombination' and integrate as a single copy at a specific site (see Knockout mice ).

ES cells will colonise a host embryo and often contribute to the germ line, resulting in the production of some sperm carrying the extra DNA. When these transgenic sperm fertilise a normal egg, a transgenic mouse is produced with the same foreign DNA in every cell.

There are two major methods for producing transgenic mice. The first method, 'pronuclear microinjection', begins by isolating eggs just after they have been fertilised.

How is it used?

In most cases the addition of foreign DNA to the genome results in a gain of function, such as the production of a new protein or the expression of an existing protein at a higher level or in a different range of cells. This is a generally useful approach for studying gene function or regulation, but can also be used to model human diseases caused by dominantly acting mutant proteins (such as Alzheimer's disease).

The addition of foreign DNA can also cause loss of function if it interrupts or disturbs the expression of an existing gene. This is one strategy used to generate knockout mice.