A transgenic mouse is a laboratory mouse that has been genetically engineered to carry a foreign piece of DNA within its genome. This intentional modification allows scientists to introduce a new gene, or alter an existing one, to study its effects on the whole organism. These animals serve as valuable tools in biomedical research, providing a living system to investigate how specific genes function and contribute to health and disease. By observing the consequences of these genetic changes, researchers can gain insights into complex biological processes that would be impossible to study in humans directly.
The creation of these specialized mice has become a common practice in laboratories worldwide. Their use enables the detailed exploration of genetic pathways and the development of models for human conditions. This approach has accelerated our understanding of the genetic underpinnings of numerous diseases, paving the way for new diagnostic and therapeutic strategies. The ability to manipulate the mouse genome with precision has significantly advanced many areas of biological and medical science.
Preparing the Genetic Blueprint
Before any genetic modification can occur, scientists must design and prepare the “transgene.” This is the specific segment of DNA, often a human gene, that is intended for insertion into the mouse’s genome. The design of this transgene includes not just the gene itself but also regulatory sequences that control when and where in the body the gene will be activated.
This piece of DNA is then packaged into a delivery vehicle known as a “vector.” The vector, a piece of circular DNA called a plasmid, is engineered to carry the transgene and facilitate its entry into the mouse’s cells. The choice of vector is important for ensuring the stability and successful delivery of the genetic material.
Mice are the most common model organism for this type of research. Their genome shares a high degree of similarity with the human genome, making them relevant for studying human diseases. Mice have a short gestation period and reproduce quickly, allowing scientists to study multiple generations in a relatively short time. Their small size and well-understood biology also make them manageable and cost-effective for laboratory settings.
The Pronuclear Injection Method
One of the primary techniques for creating transgenic mice is pronuclear injection. This method begins with collecting fertilized eggs from a female mouse. At this early stage, the genetic material from the sperm and egg exist in two separate spheres called pronuclei, before they fuse. This brief window provides an opportunity for direct genetic intervention.
Using a microscopic glass needle, a scientist injects a solution containing many copies of the transgene into one of the pronuclei. The fine needle pierces the egg without causing significant damage. During the egg’s natural DNA repair process, the injected transgene will hopefully be integrated into the chromosomes.
After injection, the modified eggs are surgically transferred into a surrogate mother mouse that has been hormonally prepared to receive them. She carries the embryos to term and gives birth to a litter of pups. A feature of this method is that the transgene inserts into the mouse’s genome at a random location. This means not all pups will carry the new gene, and the location of insertion can affect how the gene is expressed, making the technique unpredictable.
The Embryonic Stem Cell Method
A more targeted approach involves the use of embryonic stem (ES) cells. This method starts with harvesting ES cells from an early-stage mouse embryo, known as a blastocyst, and growing them in a lab dish. These cells are pluripotent, meaning they have the potential to develop into any cell type, which allows for precise genetic modifications.
The transgene is introduced into the ES cells while they are cultured in the dish. Scientists then select the cells that have successfully incorporated the transgene into the desired location in their genome. This is often achieved through homologous recombination, which allows the new gene to replace an existing one. This targeting ability is an advantage over the random insertion of pronuclear injection.
Once the desired ES cells are identified and multiplied, they are injected into a normal mouse blastocyst. This creates a “chimeric” mouse, composed of cells from both the original blastocyst and the modified ES cells. These chimeric mice are then bred, and if the modified ES cells contributed to the germline, they can pass the transgene to their offspring. This method is useful for creating “knockout” mice, where a gene is inactivated, or “knock-in” mice, where one gene is replaced with another.
Modern Gene Editing with CRISPR-Cas9
A recent development in creating transgenic animals is the CRISPR-Cas9 system. It is often described as “molecular scissors” that can be programmed to cut DNA at a specific sequence within the genome. This system consists of a DNA-cutting enzyme called Cas9 and a guide RNA that directs the enzyme to the target location.
The CRISPR-Cas9 components can be injected directly into a fertilized mouse egg, similar to the pronuclear injection method. Instead of random insertion, the system travels to the predetermined site in the genome and makes a precise cut. The cell’s natural repair mechanisms then take over, and scientists can guide these processes to either delete an existing gene or insert a new transgene at the cut site.
This technique simplifies the process and improves targeting precision over older methods. It bypasses the labor-intensive steps of the embryonic stem cell method while avoiding the random integration of pronuclear injection. The efficiency of CRISPR-Cas9 allows for complex genetic alterations, such as multiple gene edits at once, accelerating genetic research.
Applications in Scientific Research
The primary use of transgenic mice is to serve as models for human diseases. By introducing a gene associated with a condition like Alzheimer’s, cancer, or diabetes, scientists can recreate aspects of the disease in mice. These models allow researchers to observe disease progression and test the effectiveness and safety of potential new drugs before human trials.
Transgenic mice are also valuable for fundamental biological research to understand a single gene’s function. Using “knockout” mice, scientists can observe what happens in a gene’s absence, which helps reveal its role in processes like development or behavior. Conversely, creating mice that overexpress a gene can show the effects of having too much of a specific protein.
These genetically engineered mice have a role in pharmacology and toxicology. Before a new drug is approved, it must undergo safety testing. Transgenic mice can be designed with specific human metabolic enzymes or receptors, making them better predictors of how a drug might affect a person. This allows for screening new compounds for potential toxicity, contributing to safer medications.