Download WHAT IS GENE THERAPY? CHOOSING TARGETS FOR GENE

WHAT IS GENE THERAPY?
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 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?
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:
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?
What role does the protein encoded by the gene play within the cells of that tissue?
Exactly how do mutations in the gene affect the protein's function?
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 .
5.
Can you deliver the gene to cells of the affected tissue? The answer will come from several pieces of information,
including:


How accessible is the tissue? Is it fairly easy (skin, blood or lungs), or more difficult to reach (internal organs)?
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.
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?
The answer is that gene therapy poses one of the greatest technical challenges in modern medicine. It
is very hard to introduce new genes into cells of the body. Let's look at some of the main technical
issues in gene therapy.
Gene delivery and activation
Gene therapy will work only if we can deliver a normal gene to a large number of cells - say, several
million - in a tissue. And they have to be the correct cells, in the correct tissue. Once the gene reaches
its destination, it must be activated, or turned on to produce the protein encoded by the gene. Gene
delivery and activation are the biggest obstacles facing gene therapy researchers. Tools of the
Trade highlights some of the most common methods for addressing these challenges.
Introducing changes into the germline
Targeting a gene to the correct cells is crucial to the success of any gene therapy treatment. Just as
important, though, is making sure that the gene is not incorporated into the wrong cells. Delivering a
gene to the wrong tissue would be inefficient and could cause health problems for the patient.
For example, improper targeting could incorporate the therapeutic gene into a patient's germline, or
reproductive cells, which ultimately produce sperm and eggs. Should this happen, the patient would
pass the introduced gene on to his or her offspring. The consequences would vary, depending on the
type of gene introduced.
Immune response
Our immune systems are very good at fighting off intruders such
as bacteria, viruses and other biological substances. Gene
delivery vectors must be able to escape the body's natural
surveillance systems. Failure to do so can cause serious illness or
even death.
The story of Jesse Gelsinger illustrates this challenge well.
Gelsinger, who had a rare liver disorder, participated in a 1999
gene therapy trial at the University of Pennsylvania. He died of
complications from an inflammatory response shortly after
receiving a dose of experimental adenovirus vector. His death
halted all gene therapy trials in the United States for a time,
sparking a much-needed discussion on how best to regulate
experimental trials and report health problems in volunteer
patients.
Disrupting important genes in target cells
The best gene therapy is the one that lasts. Ideally, we would want a
gene that is introduced into a group of cells to remain there and
continue working.
For this to happen, the newly introduced gene must become a
permanent part of each cell's genome, usually by integrating, or
"stitching" itself, into the cell's existing DNA. But what happens if
the gene stitches itself into an inappropriate location, disrupting
another gene?
This happened recently in a gene therapy trial to treat several
children with X-linked Severe Combined Immune Deficiency
(SCID). People with this disorder have virtually no immune
protection against bacteria and viruses. To escape infections and
illnesses, they must live in a completely germ-free environment.
In the late 1990s, researchers tested a gene therapy treatment that
would restore the function of a crucial gene, gamma c, to cells of
the immune system. This treatment appeared very successful,
restoring immune function to most of the children who received it.
But later, two of these children developed leukemia. Researchers
found that the leukemia occurred because the newly transferred
gamma c gene had stitched itself into the wrong place, interrupting
the function of a gene that normally helps regulate the rate at which
cells divide. As a result, the cells began to divide out of control,
causing the blood cancer leukemia.
Although doctors have treated the children successfully with
chemotherapy, the fact that they developed leukemia during
treatment raises another important safety-related issue that gene therapy researchers must address.
ENHANCEMENT: GENE THERAPY AND SCIENCE FICTION
Scientists are looking to gene therapy as a way to treat medical conditions that have genetic origins.
But what if the same gene delivery techniques, once established, could be used to change human traits?
Some people say that gene therapy will open the door to genetic
enhancement and the creation of so-called "designer babies." In
theory, scientists could someday alter any physical or behavioral
trait that is controlled by genes.
In reality, genetic enhancement is not very likely, however. Here's
why:

Finding the right genes to alter: Scientists still know very little about the specific genes that
contribute to any given trait. And in fact, most human traits are controlled by multiple genes.

"Nature versus nurture": No human trait is determined solely by genes. Although genes can
contribute to varying degrees, environmental factors - the "who, what, when, where and hows" in life
- play a large role in determining how a person develops certain traits. So even if you knew which
genes to alter, you could not reliably predict how they would affect the individual.
Most traits are so complex that the concept of enhancement will likely remain in the science fiction
realm for the foreseeable future. But if you were dreaming up a plot for
your next sci-fi novel, your list might include:








Physical traits, including:
Appearance, such as hair or eye color
Height
Physical build, such as weight or muscle mass
Strength
Speed
Behavioral traits, including:
Intelligence
Temperament, such as "mellow" or "quick-tempered"
Personality, such as "shy" or "friendly"
But if you could change human traits, how would you do it?
The details of germline and embryonic gene delivery
Success in gene therapy depends on the efficient delivery of the correct gene to the correct cells in
the correct tissue. Once that's accomplished, you still need to make sure the gene gets to work and
continues working for the life of the cell. This is not an easy task.
The same can be said for genetic enhancement. If you could use gene delivery techniques to alter a
person's traits, you would have to know where and how to deliver the appropriate genes.
Delivering a gene to just one group of cells or one tissue in the body would not change most physical
and behavioral traits. To ensure an effect, you would need to deliver the gene to every single cell in the
body.
This would be impossible in adults, each of whom is comprised of about 100 trillion cells. (That's
100,000,000,000,000 cells!) The only possible way to alter a gene in every human cell would be to
make the change at the earliest stages of development, through germline or embryonic gene delivery:

Germline gene delivery refers to the permanent transfer of a gene into sperm or egg cells.

Embryonic gene delivery refers to the permanent transfer of a gene into the cells of an early embryo,
just after the sperm and egg unite.
In both cases, the delivered gene would become a permanent part of cells in the resulting adult.
Can you see how germline or embryonic gene delivery might be useful in gene therapy? We've
discussed these techniques as methods for genetic enhancement, but scientists are studying them as an
approach to treating genetic disorders at the earliest stages of a child's development.
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.
WHAT ARE SOME ISSUES IN GENE THERAPY?
We saw in Choosing Targets for Gene Therapy and Challenges in Gene Some Definitions
Therapy that gene therapy research is complex and has many variables. Though eth-i-cal: (adj.) 1. Relating to morals,
especially as concerning human conduct.
several clinical trials have shown promising results, much more research is
2. Morally correct.
needed to guarantee the safety and efficiency of gene therapy procedures. As
gene therapy comes closer to becoming a medical treatment for genetic diseases, le-gal: (adj.) 1. Of or based on law. 2.
other ethical, legal, and social issues must be kept in mind.
Appointed or required by law. 3.
Permitted by law.
What are the possible implications of gene therapy research to society? All of us
- researchers, policymakers and the public - have a responsibility to explore the
potential effects of gene therapy research on our lives so that we can make
informed decisions.





For each new application of gene therapy research, we must consider:
What are the benefits?
What are the risks?
Whom will the technology help? Whom will it potentially hurt?
What does gene therapy mean for me? For my family? For the people in my
community?
Why might others not share my view?
Ethical, legal and social issues
There are several types of issues to consider as we think about gene therapy:
so-cial: (adj.) 1. Of or relating to society
and its organization. 2. Concerned with
the mutual relations of human beings. 3.
Living in organized communities.
pol-i-cy: (n.) 1. Course or principle of
action adopted or proposed by a
government, party, business or
individual, etc.
Definitions adapted from the Oxford
Des k Dict i ona ry and Th esau rus .
Step Into Someone Else's
Shoes!
Ethical issues ask us to consider the potential moral outcomes of gene therapy
research.
Legal issues require researchers and the public to help policymakers decide
whether and how gene therapy research should be regulated by the government.
Social issues involve the impact of gene therapy research on society as a whole.
Some questions to ponder


When should gene therapy be used? Should it be used to treat critically ill patients?
Should it be used to treat babies and children?
What effect would gene therapy have on future generations if germline
(reproductive) cells were genetically altered? How might this alteration affect
human variation?

Who should decide what are "good" or "bad" uses of genetic modifications? How
do you define "normal" with regard to human beings?

What if we could alter human traits not associated with disease? Would it be okay
to use gene therapy to improve or enhance a person's genetic profile?

Who will have access to gene therapy, treatments and long-term follow-ups? Will
gene therapy and genetic enhancements create an advantage for those who can afford
it?
The questions raised here have no clear right or wrong answer. Your responses will
depend on your values, as well as on the opinions of those around you.
Would your views be the same if you
were a different person? How might they
change if you .
. had a medical condition that gene
therapy research might someday benefit?
. knew a family member or close friend
with such a medical condition?
. worked as a research scientist?
. were a prominent religious leader?
. were a policymaker involved in making
laws?
Can you think of other people who would
have a special interest in gene therapy
research? How might their views differ
from yours?