What is a transgenic animal?

After my last post, I realized the term “transgenic” gets thrown around a lot in our media. I’m willing to bet that many readers do not understand what transgenic means, why we’d want to make transgenic animals, or how something is made to be transgenic. Therefore, I’d like to take the time to explain this term here.

Whether we are discussing bacteria, flies, or mammals, the sequence of DNA in the cells controls the development of each organism. Many species have specific sequences of DNA that give that animal a unique trait or ability. In biology research, sometimes a special trait in one species would be advantageous in the animal model being used during experimentation. For example, some genes (genetic units in the DNA) might allow for a color change in a cell. If a scientist wants to utilize that trait, the gene from a host species can be taken from its DNA and incorporated into a recipient species. The newly created animal containing another organism’s DNA is called a transgenic animal.

In the case of the paper I reviewed in the last post (Zhang et al., 2009), the recipient mice were given new DNA sequences that enabled the researchers to cut the presenilin genes out of the DNA upon exposure to specific drugs. In addition, the transgenic mice received DNA that allowed the scientists to visualize the target neurons through a chemical reaction that caused the neurons to turn blue. Without the incorporation of the new DNA, these experiments would not have been possible.

How is a transgenic mouse made?

A transgenic mouse is made using common laboratory techniques. First, an embryo is taken from a pregnant animal. In the embryo, some of the cells that still have the ability to generate an entire animal (embryonic stem cells) are removed from the embryo. They are put into a dish and exposed to the new pieces of DNA (transgenes). Different methods can be employed to force the transgenes into the embryonic stem cell. Common methods include soaking the cells in the DNA solution, using electrical currents, and microinjection directly into the embryo.

Usually, the new DNA incorporated into the cell has a sequence that offers drug resistance. Therefore, the researchers can expose the cells to a drug and select cells that indeed receive and incorporate the transgene of interest. Next, the cells are re-injected into a fresh embryo so that the cells containing the transgene will contribute to the adult animal. These embryos are placed into a surrogate mother to grow.

The first set of animals from this technique will contain two types of cells: 1) cells from the recipient embryo (usually from a light coat mouse) and 2) cells containing the transgene (originally taken from a mouse with a dark coat). Therefore, the mice will look like they have a mixture of coat color, indicating that the mouse is composed of descendents of both cell types.

The hope is that reproductive cells (eggs or sperm) contain the transgene. After a series of crosses (mating mice together), the researchers will select mice with a uniform dark coat color, indicating that the mice are only composed of cells with the transgene. Voila! A transgenic animal!