WHAT IS GENE THERAPY?
Imagine that you accidentally broke one of your neighbor's windows. What would you do? You could:
What does this have to do with gene therapy?
You can think of a medical condition or illness as a "broken window." Many medical conditions result from flaws, or mutations, in one or more of a person's genes. Mutations cause the protein encoded by that gene to malfunction. When a protein malfunctions, cells that rely on that protein's function can't behave normally, causing problems for whole tissues or organs. Medical conditions related to gene mutations are called genetic disorders.
So, if a flawed gene caused our "broken window," can you "fix" it? What are your options?
If it is successful, gene therapy provides a way to fix a problem at its source. Adding a corrected copy of the gene may help the affected cells, tissues and organs work properly. Gene therapy differs from traditional drug-based approaches, which may treat the problem, but which do not repair the underlying genetic flaw.
But gene therapy is not a simple solution - it's not a molecular bandage that will automatically fix a disorder. Although scientists and physicians have made progress in gene therapy research, they have much more work to do before they can realize its full potential. In this module, you'll explore several approaches to gene therapy, try them out yourself, and figure out why creating successful gene-based therapies is so challenging.
Gene therapy could potentially treat certain disorders at the source by repairing the underlying genetic flaws. Many disorders or medical conditions might be treated using gene therapy, but others may not be suitable for this approach.
How do you know whether a disorder is a good candidate for gene therapy?
For any candidate disorder, you need to answer the following questions:
If you can answer "yes" to Questions 4 and 5, then the disorder may be a good candidate for a gene therapy approach.
In Choosing Targets for Gene Therapy, we saw that cystic fibrosis is a good candidate for gene therapy. This is because:
Now all we have to do is deliver the gene into the proper cells and put it to work. This is not an easy job. Gene delivery is one of the biggest challenges in the field of gene therapy.
What are some of the hallmarks of successful gene delivery?
1. TARGETING the right cells. If you want to deliver a gene into cells of the liver, it shouldn't wind up in the big toe. How can you ensure that the gene gets into the correct cells?
2. ACTIVATING the gene. A gene's journey is not over when it enters the cell. It must go to the cell's nucleus and be "turned on," meaning that its transcription and translation are activated to produce the protein product encoded by the gene. For gene delivery to be successful, the protein that is produced must function properly.
3. INTEGRATING the gene in the cells. You might want the gene to stay put and continue working in the target cells. If so, you need to ensure that the gene integrates into, or becomes part of the host cell's genetic material, or that the gene finds another way to survive in the nucleus without being trashed.
4. AVOIDING harmful side effects. Anytime you introduce an unfamiliar biological substance into the body, there is a risk that it will be toxic or that the body will mount an immune response against it. If the body develops immunity against a specific gene delivery vehicle, future rounds of the therapy will be ineffective.
Explore some of the gene delivery methods that researchers have developed in Tools of the Trade.Supported by a Science Education Partnership Award (SEPA) [No. 1 R25 RR16291-01] from the National Center for Research Resources, a component of the National Institutes of Health, Department of Health and Human Services. The contents provided here are solely the responsibility of the authors and do not necessarily represent the official views of NCRR or NIH.
Mario R. Capecchi, Ph.D., of the University of Utah, has won the 2007 Nobel Prize in Physiology or Medicine. Capecchi shares the prize with Oliver Smithies of University of North Carolina, Chapel Hill and Sir Martin Evans of Cardiff University in the UK.
The prize recognizes Capecchi's pioneering work on "knockout mouse" technology, a gene-targeting technique that has revolutionized genetic and biomedical research, allowing the creation of animal models for hundreds of human diseases.
During the 1980s, Capecchi devised a way to change or remove any single gene in the mouse genome, creating strains of mice that pass the altered gene from parent to offspring. In the years since, these "transgenic" and "knockout" mice have become commonplace in the laboratory.
Capecchi's pioneering work in gene targeting has taught us much about how the body builds - and rebuilds - itself. He has given scientists worldwide the tools to make important discoveries about human diseases from cancer to obesity.
And he has raised a key question for the future of human medicine: if we can replace a perfectly good gene with a mutated one, can we also go the other way, replacing problem genes with those that work?
As a child, he wandered homeless in Italy. As a researcher, his first attempts at gene targeting were deemed "not worthy of pursuit" by the National Institutes of Health. Capecchi is an individual whose personal life proves that, while some events are not probable, anything is possible. Read Mario's story.
What makes an arm an arm? Capecchi's research team is working on answering that question using gene targeting. They have systematically "knocked out" a set of genes in mice, called homeotic genes, which govern body patterning during development. For example, one of the lab's most recent genetic discoveries may explain why we lack spare ribs. Find out more about how homeotic genes work in Genes Determine Body Patterns.
YOUR GOAL: You are studying how a particular gene, named OhNo, might play a role in panic attacks. You want to study mice with this gene turned off. To "knockout" the OhNo gene you will replace it with a mutated copy that doesn't work. Here's how:
Isolate embryonic stem cells that originated from male brown mice with a normal OhNo gene (blue).
To these cells, add a copy containing a mutated, inactive OhNo gene (red), and a drug resistance marker gene (pink).
By mechanisms that are not completely understood yet, similar genes will swap places. The mutant OhNo gene plus drug resistance marker gene is now incorporated into the genome and the normal version is kicked out. This is called homologous recombination.
Cells that haven't incorporated the inactive OhNo gene don't have the drug resistance marker gene (pink).
Adding the drug kills cells without the marker, leaving you with a clean batch of cells that all have an inactive version of the OhNo gene.
By transplanting stem cells that carry the inactive ohNo gene into a white mouse embyro, you'll create what is called a chimera. Chimeras have patches of cells throughout their bodies that grew from white mouse cells and patches that grew from brown stem cells. Some of the cells that have the inactive OhNo gene may develop into reproductive cells.
Chimeras are easy to identify because they have both brown and white patches of fur.
If a male chimera has some reproductive cells (sperm) that originated from the brown stem cells, he will produce some brown offspring when mated with a white female.
Half of the brown offspring will have a copy of the inactive OhNo gene in all of their cells - including their reproductive cells. These mice have one normal copy of the OhNo gene (not shown) and one inactive copy. So half of their reproductive cells will contain a normal copy and half will contain an inactive copy.
These mice can be identified by PCR and then bred with each other.
One fourth of your resulting offsping will have two copies of the "knocked-out" or inactive OhNo genes. You can now study these mice to determine how lacking the OhNo gene may affect panic attacks.