Genetic Engineering: A Look At Knockout Mice

A knockout mouse is a genetically engineered animal that has had a specific gene or genes silenced, that is turned off. The targeted genes have been disrupted by an artificial DNA sequence and this usually results in a change in the mouse's phenotype. The first knockout mouse was created in 1989.

By turning off a particular gene scientists hope to infer its function by observing subsequent changes in mouse behaviour or condition that deviate from normal. In this way scientists hope to be able to work out what the gene normally does.

Humans and mice share many genes and physiological processes and observing what happens to knockout mice has contributed to the study of heart disease, obesity, many cancers, and a range of other diseases.

Mouse models tend to be named after the gene that has been knocked out. For example, the p53 knockout mouse has had the p53 gene silenced. The gene codes for a protein that suppresses tumours. 5-HT1A and 5-HT1B knockout mice are created to lack or express reduced levels of serotonin receptors. Malfunctions and mutations of the serotonergic system have been linked with a range of psychiatric disorders.

To create knockout mice embryonic stem (ES) cells are harvested from early-stage mouse embryos. These have the potential to grow into any kind of adult cell, and so if a gene knockout is created in ES cells it will be present in most adult cells. To knockout the gene artificial DNA is inserted into the nuclei of ES cells which are then grown in the lab. After a few days these modified ES cells are injected into early-stage embryos which are then implanted into the uterus of a female mouse.

When the puppies are born naturally they will have some tissues containing the knockout genes. They will not be completely knockout mice as some cells will not have both relevant genes silenced. And so complete knockout mice are created by crossbreeding to generate lines where both copies of a gene in a nucleus have been altered.

The technology does have some limitations and principally these are: -

• 15% of knockouts are developmentally lethal. This means that the mice will not grow into adulthood and so studies are restricted to embryonic development, making it more difficult to relate the function of these genes to human health. Some genes also have a different function in the adult and the embryo.

• Knocking out a gene may not result in any observable change in behaviour, or the behaviour could be different from the behaviour in humans in which the same gene has been inactivated. For example, p53 is probably the most important gene in cancer research, as p53 mutations cause more than 50% of cancers. However, p53 knockout mice can develop tumours in different tissues to humans.

Despite the setbacks knockout mice are still the most powerful tool that scientists possess to study the function of a gene in a living animal.