What is Gene Therapy?


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Imagine that you accidentally broke one of your neighbor's windows. What would you do? You could:

  1. Stay silent: no one will ever find out that you are guilty, but the window doesn't get fixed.
  2. Try to repair the cracked window with some tape: not the best long-term solution.
  3. Put in a new window: not only do you solve the problem, but also you do the honorable thing.

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 proteinencoded 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?

  1. Stay silent: ignore the genetic disorder and nothing gets fixed.
  2. Try to treat the disorder with drugs or other approaches: depending on the disorder, treatment may or may not be a good long-term solution.
  3. Put in a normal, functioning copy of the gene: if you can do this, it may solve the problem!

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.

Choosing Targets for Gene Therapy

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:

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  1. Does the condition result from mutations in one or more genes? For you to even consider gene therapy, the answer must be "yes."
  2. Which genes are involved? If you plan to treat a genetic flaw, you need to know which gene(s) to pursue. You must also have a DNA copy of that gene available in your laboratory. The best candidates for gene therapy are the so-called "single-gene" disorders - which are caused by mutations in only one gene.
  3. What do you know about the biology of the disorder? To design the best possible approach, you need to learn all you can about how the gene factors into the disorder. For example:
  4. Which tissues are affected?
  5. What role does the proteinencoded by the gene play within the cells of that tissue?
  6. Exactly how do mutations in the gene affect the protein's function?
  7. Will adding a normal copy of the gene fix the problem in the affected tissue? This may seem like an obvious question, but it's not. What if the mutated gene encodes a protein that prevents the normal protein from doing its job? Mutated genes that function this way are called dominant negative and adding back the normal protein won't fix the problem. Learn more about how researchers are trying to address dominant negative mutated genes in New Approaches to Gene Therapy.
  8. Can you deliver the gene to cells of the affected tissue? The answer will come from several pieces of information, including:
  9. How accessible is the tissue? Is it fairly easy (skin, blood or lungs), or more difficult to reach (internal organs)?
  10. What is your best mode of delivery? You can examine the pros and cons of potential delivery methods in Tools of the Trade.
If you can answer "yes" to Questions 4 and 5, then the disorder may be a good candidate for a gene therapy approach.

Gene Delivery: The Key to Gene Therapy

In Choosing Targets for Gene Therapy, we saw that cystic fibrosis is a good candidate for gene therapy. This is because:

  • We know which gene is mutated in the disorder.
  • We have a normal copy of that gene available.
  • We understand the biology of the disease, including which tissue types are affected and how they are affected.
  • We can predict that adding the normal gene back to the cells that make up the affected tissues will restore a needed function.

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.

Tools of the Trade

Genes are made of DNA. Successful gene delivery requires an efficient way to get the DNA into cells and to make it work. Scientists refer to these DNA delivery "vehicles" as vectors.

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Choosing the best vector
There is no "perfect vector" that can treat every disorder. Like any type of medical treatment, a gene therapy vector must be customized to address the unique features of the disorder.
Part of the challenge in gene therapy is choosing the most suitable vector for treating the disorder.
Below, find out more about the most commonly used types of gene therapy vectors.

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Viral vectors
Mother Nature is a brilliant scientist! Over the last three billion years or so, she's developed an incredibly efficient means of delivering foreign genes into cells: the virus.
Usually when we think of viruses, we think of them causing diseases such as the common cold, the flu, HIV/AIDS and SARS. When faced with the problem of gene delivery, scientists looked to viruses. Why reinvent the wheel if there's a perfectly good one out there? If we can modify viruses to deliver genes without making people sick, we may have a good set of gene therapy tools.
General advantages of viral vectors:
  • They're very good at targeting and entering cells.
  • Some viral vectors might be engineered to target specific types of cells.
  • They can be modified so that they can't replicate and destroy the cell.
General drawbacks of viral vectors:
  • A virus can't "expand" to fit a piece of genetic material larger than it is naturally built to carry. Therefore, some genes may be too big to fit into a certain type of virus.
  • Viruses can cause immune responses in patients, resulting in two potential outcomes:
  • Patients may get sick.
  • A patient's immunity to a virus may prevent him from responding to repeated treatments.
However, modern viral vectors have been engineered without most of the proteins that would cause an immune response.
Non-Viral Vectors
Although viruses can effectively deliver genetic material into a patient's cells, they do have some limitations. It is sometimes more efficient to deliver a gene using a non-viral vector, which has fewer size constraints and which won't generate an immune response.
Non-viral vectors are typically circular DNA molecules, also known as plasmids. In nature, bacteria use plasmids to transfer genes from cell to cell.
Scientists use bacteria and plasmids to easily and efficiently store and replicate genes of interest from any organism.

From Research to Trials

Taking gene therapy from the laboratory to the clinic involves many steps. Before trying a therapy on human patients, researchers must:

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  • Understand the biology behind the disorder
  • Develop the treatment approach
  • Test its effectiveness in biological models of the disease
  • Establish its safety in humans
Each of these steps requires the efforts of expert researchers and physicians, as well as funding to support the research, approval by regulatory agencies, and time to obtain and analyze results. Applying gene therapy can take a long time.
How long? Days, months, years?
Let's look at a real-life example: development of a gene therapy treatment for a disorder called Adenosine Deaminase (ADA) deficiency. In the box below, you can examine the steps needed to bring a therapy to the clinic. Milestones in developing the ADA gene therapy are boxed in blue.
Click on the "plus" icons to expand a topic, or click "expand all" to see the entire procedure.
EXPAND ALLCOLLAPSE ALL
STEP 1: Learn about the disease
Is the disorder a good candidate for gene therapy? To find out, study the disease.

1) Get money for the project
  1. Government research granting agencies
  2. Private investors
  3. Pharmaceutical companies

2) Get approval for the project
  1. From your organization's Institutional Review Board

3) Perform clinical research
  1. Diagnose and classify the disorder
  2. What are the signs and symptoms?
  3. Who gets the disorder? (Males or females? Specific ethnic groups?)
  4. At what age does the disorder appear?
  5. Clinical research on ADA deficiency began in the 1960s.
  6. Examine current treatments for the disorder
  7. What treatments are available?
  8. How effective are they?
  9. What are their drawbacks?
  10. What types of new treatments are needed?
  1. 1976: Blood transfusions show limited effectiveness and potential immune system complications (graft-versus-host disease)
  2. 1987: First report of successful PEG-ADA treatment
  1. Publish your results in a peer-reviewed journal

4) Perform biological research
  1. Genetics
  2. Which gene(s) are involved in the disorder?
  3. What are the specific disease-causing mutations in these genes?
  1. 1974: Scientists map the ADA gene to a region on human Chromosome 20.
  2. 1986: The sequence of the ADA gene was first published.
  1. Cell biology
  2. What is the normal job of the protein encoded by the gene?
  3. How do mutations in the gene affect the protein's function?
  4. How does the altered protein influence cell and tissue function?
  5. 1960s-1980s: Scientists study the ADA protein, which works as an enzyme. For ADA deficiency, the protein was identified before the gene!
  6. Animal models
  7. What can you learn from animal models of the disorder?
  8. Publish your results in a peer-reviewed journal

5) DECISION: Is the disorder a good candidate for gene therapy?
  1. Use the criteria defined in Choosing Targets for Gene Therapy to decide
  2. If you answer yes, proceed to Step 2
STEP 2: Design a gene therapy

1) Use your knowledge of the disorder to design a gene therapy
  1. Which tissue do you need to target?
  2. How will you deliver the gene to the tissue? Which approach is more appropriate, ex vivo or in vivo?
  3. 1985 - ADA deficiency affects cells of the immune system, which are present in the blood and produced in the bone marrow. Therefore, an ex vivo approach was deemed the safest and easiest method for gene therapy. This approach involves extracting cells from the patient's bone marrow, treating them, and then putting them back into the patient.
  4. How will you ensure that the gene gets into cells and starts working?
  5. What can go wrong?
  6. How will you know if sonething has gone wrong?
  7. What will you do if something goes wrong?

2) Test the therapy in appropriate models of the disease
  1. Test it in cells grown in the laboratory
  2. Test it in appropriate animal models
  3. Publish your results in a peer-reviewed journal
  1. 1985 - Researchers show that a retrovirus-based approach can deliver the ADA gene into cells taken from ADA patients.
  2. 1988 - Researchers show that the gene can be transferred safely and efficiently into the white blood cells of animals.

3) DECISION: Does your therapy look promising?
  1. If you think your therapy will work safely in human patients, proceed to Step 3
STEP 3: Get money and approval for clinical trials

1) Get money for the trials
  1. Government research granting agencies
  2. Private investors
  3. Pharmaceutical companies

2) Get approval for the trials
  1. From your organization's Institutional Review Board
STEP 4: Phase One clinical trial

1) Establish safety and dosage limits in a small group of people (20-80)
  1. Participants are usually terminally ill individuals who have not responded to other treatment.
  2. Is the treatment safe?
  3. At which doses is the treatment safe?
  4. What are the adverse side effects, if any?
  5. How will you address these side effects?
  6. Will the benefits of the treatment outweigh the risks?
  7. Publish your results in a peer-reviewed journal
  8. 1989 - Researchers establish the safety of an ex vivo ADA gene therapy in human cancer patients.

2) DECISION: Does your therapy still look promising?
  1. If you have established the safety and determined the dosage limits of the treatment, proceed to Step 5.
STEP 5: Phase Two clinical trial

Test the efficacy and safety in a larger group of people (100-300)
  1. Can you deliver the gene effectively to target cells?
  2. Do target cells express the delivered gene?
  3. Do you observe health improvements in study participants?
  4. Short-term?
  5. Long-term?
  6. Continue to measure side effects
  7. Publish your results in a peer-reviewed journal
  1. 1990 - The first clinical trial of ADA gene therapy in the United States began, using two children diagnosed with ADA deficiency. The children's immune status improved after they received the gene therapy; however, the therapy worked for only a few months and had to be repeated several times over the next two to three years. In the years following the treatment, periodic tests confirmed that the childrens reengineered cells are surviving and producing the ADA enzyme. Over the long term, however, the procedures have shown mixed success. In 2003, the researchers reported that some of the first patient's cells still expressed the delivered gene. The second patient developed an immune response to the retrovirus gene delivery system, and few of her cells still express the transferred ADA gene.
  2. 1993 - In a different study, researchers performed gene therapy using the umbilical cord blood stem cells from two infants born with ADA deficiency. Both patients continue to express the transferred ADA gene in their immune cells. (For more about umbilical cord blood stem cells, see the Stem Cells in the Spotlight module.)

2) DECISION: Is your therapy effective in a larger group of people?
  1. If so, proceed to Step 6
STEP 6: Phase Three clinical trial

1) Test the therapy in a large group of people (1,000-3,000)
  1. Give treatment in a "double-blind" scenario. Neither the treating physicians nor the patient knows whether the treatment is authentic or a "placebo" (control). This ensures the validity of results.
  2. Publish your results in a peer-reviewed journal

2) DECISION: Is your treatment successful?
  1. If so, proceed to Step 7
  2. 2003 - No phase three human clinical trials for ADA deficiency have yet been conducted.
STEP 7: Get FDA approval for general clinical use

1) Write proposals, fill out paperwork, answer questions and wait for approval
  1. If approved, use your therapy to treat patients and proceed to Step 8
STEP 8: Phase Four clinical trial

1) Further test the efficacy and optimal use of the treatment in general use
  1. Publish your results in a peer-reviewed journal
Why does gene therapy approval take so long?
It's been more than 30 years since physicians began studying ADA deficiency and more than 15 years since efforts to develop a gene therapy began in earnest. To date, no ADA gene therapy has been successful enough to become common medical practice.
Why has this process taken so long? Gene therapy techniques were just emerging when researchers began designing gene therapies for ADA deficiency. Although researchers employed the latest technologies at the time, the first therapies were far from perfect. Several major obstacles must be overcome before successful gene therapies can be developed. These obstacles are discussed in Challenges in Gene Therapy.
As we learn more about how the human body works at the molecular level, gene therapies will become more and more effective.

Challenges in Gene Therapy

Gene therapy is not a new field; it has been evolving for decades. Despite the best efforts of researchers around the world, however, gene therapy has seen only limited success. Why?