Knockout mice are genetically modified laboratory animals engineered to have one or more of their genes inactivated, or “knocked out.” This allows scientists to investigate the role of specific genes in biological processes and diseases by observing changes when a gene is absent or non-functional.
Defining Knockout Mice
A gene is a fundamental unit of heredity, carrying instructions that guide the development and function of an organism. In the context of knockout mice, “knocking out” a gene means intentionally disabling or deleting it from the mouse’s genome. This targeted inactivation prevents the gene from producing its corresponding protein, effectively removing its influence.
By comparing a knockout mouse with a normal mouse, researchers can deduce the role of the inactivated gene. If the knockout mouse exhibits a specific change in its traits or health, it suggests the missing gene normally contributes to that characteristic or process.
The Process of Creation
Creating a knockout mouse is a multi-step process that relies on advanced genetic engineering techniques. One primary method involves the use of embryonic stem (ES) cells, which are cells capable of developing into any cell type in the body. Researchers introduce a specially designed DNA construct into these ES cells, aiming to replace or disrupt the target gene through a process called homologous recombination.
Homologous recombination is a natural cellular repair mechanism allowing genetic material exchange between similar DNA sequences. Scientists design a “targeting vector” with sequences matching the gene to be knocked out, along with artificial DNA to disrupt it.
Once the modified ES cells are identified, they are injected into early-stage mouse embryos, specifically into a structure called a blastocyst. These injected blastocysts are then implanted into a surrogate mother mouse. The resulting offspring, known as chimeric mice, contain a mixture of cells from both the original embryo and the modified ES cells. To establish a pure line of knockout mice, these chimeric mice are bred with normal mice, and their offspring are carefully screened to identify those that have inherited the inactivated gene in all their cells, including their reproductive cells.
This technology, specifically the principles of introducing gene modifications into mice, earned Mario Capecchi, Martin Evans, and Oliver Smithies the Nobel Prize in Physiology or Medicine in 2007.
Diverse Research Applications
Knockout mice are widely used across scientific disciplines to study human biology and disease. In cancer research, for example, these models help scientists understand the function of genes involved in tumor growth and suppression. By inactivating genes like p53, a tumor suppressor, researchers can study how its absence contributes to different types of cancers, such as lymphomas and sarcomas.
Beyond cancer, knockout mice have proven valuable in studying neurological disorders. They allow researchers to investigate the role of specific genes in conditions like Alzheimer’s and Parkinson’s disease, offering insights into their molecular mechanisms and potential therapeutic targets. These models also aid in understanding cardiovascular diseases, metabolic disorders (like diabetes and obesity), and immune system functions. Modeling human diseases in mice allows scientists to test potential treatments and develop new therapies.
Ethical Considerations and Other Models
The use of animals in research, including genetically modified mice, involves ethical considerations. Research institutions adhere to strict guidelines and regulations to ensure the humane care and treatment of these animals. Oversight bodies, such as Institutional Animal Care and Use Committees (IACUCs), review and approve all animal research protocols to minimize discomfort and ensure scientific necessity.
While traditional knockout mice remain valuable, scientific advancements have led to other genetic mouse models. Knock-in mice, for instance, involve replacing a gene with a modified version or inserting a new gene, rather than just inactivating it. Transgenic mice carry foreign DNA that is typically integrated randomly into their genome, often leading to the overexpression of a gene. More recently, CRISPR/Cas9 technology offers a more precise and efficient way to create genetic modifications, including knockouts, by directly editing the mouse genome.