Chapter
19
Gene
Therapy
Gene therapy:
Is the treatment
of genetic disease by delivering replacement genes to correct the
genetic
deficiency.
Gene therapy: Three Examples:
1. adenosine deaminase deficiency
•Severe
combined
immune deficiency can be caused by adenosine deaminase (
•Lack
of
•Presence
of the
toxins destroy T cells and causes susceptibility to infections and
cancer.
•Replacement
of
Treatment:
•Injections
of PEG-ADA
(enzyme from cow stabilized with polyethylene glycol) given to first
child in
1986:
–Increased
–Increased
T cell survival
–Improved
immune function
•The
–Increased
T cells present
with functioning
•Umbilical
cord blood is
used to isolate stem cells. Stem cells
are treated to replace mutated
–T
cells with normal allele
accumulate in patient.
Gene therapy:
2. ornithine transcarbamylase
•Deficiency
of
ornithine transcarbamylase (OTC) is
inherited as an X-linked recessive mutation.
•OTC
normally
breaks down amino acids present in dietary protein.
•Lack
of OTC
allows build up of ammonia which damages brain function.
•Low
protein
diets and ammonia-binding drugs are used to treat OTC deficiency.
•Clinical
trials
to treat OTC deficiency were established using adenovirus as a vector
for the
normal OTC gene.
•Adenovirus
has
been used in over 330 gene therapy trials in 4,000 patients.
Jesse Gelsinger had a mild OTC deficiency. When he turned 18, Jesse volunteered for the
OTC gene therapy trial and was accepted as the 17th volunteer.
Four days after gene therapy Jesse died of
massive immune
reaction and associated complications.
OTC and Jesse Gelsinger
Jesse’s story led to immediate halt of this
trial and
review of all gene therapy trials to determine what happened and how to
prevent
future tragedies.
•Autopsy
results
suggest that Jesse had had an infection with parvovirus which may have
sensitized his immune system so it attacked the adenovirus used to
treat the
OTC.
•The
adenovirus
did not target liver cells as intended but macrophages, immune cells
that set
off an immune response.
•Reform
of gene
therapy trials continues.
•Weekly
reports
of adverse effects have been instituted.
3.
Canavan disease is caused by aspartoacylase
enzyme deficiency
•Neurons
release
N-acetylaspartate (NAA).
•NAA
is normally
broken down by the enzyme aspartoacylase to harmless components.
•Enzyme
deficiency creates NAA buildup which destroys oligodendrocytes.
•Lack
of
oligodendrocytes prevents myelin formation.
•Without
myelin,
neurons cease functioning.
Causes brain degeneration in children.
First observed as developmental delay,
inability to stand
or sit, poor muscle control and vision and lack of reaction to
surroundings.
It is a good gene therapy candidate:
•Gene
and
protein are well known.
•Window
of time
exists for treatment.
•Only
the brain
is affected.
•Brain
scans can
be used to monitor treatment.
•There
is no
existing treatment.
Gene therapy trials
There are three types of experiments:
Ex vivo gene therapy
cells are altered outside of patient’s body and
returned.
In situ gene therapy
healthy gene and vector DNA is injected at
isolated site
on body that is easily accessible (e.g. skin tumor).
In vivo gene therapy
vector and gene is
introduced into the body systemically.
Requirements for approval of a clinical trial
•Knowledge
of
defect and how it causes symptoms
•An
animal model
•Success
in growing human cells in vitro
•No
alternative
therapies or group of patients for whom therapies are not possible
•Safe
experiments
Bioethical concerns about gene therapy
•Does
the
participant of the trial truly understand the risks?
•If
the gene
therapy is effective, how will recipients be chosen assuming it is
expensive?
•Should
rare or
common disorders be the focus of gene therapy trials?
•What
effect
should deaths among volunteers have on research efforts?
Scientific concerns about gene therapy
•Which
cells
should be treated?
•What
proportion
of target cells must be corrected to alleviate or halt disease
progression?
•Is
overexpression of the therapeutic gene dangerous?
•Is
it harmful
if the altered gene enters other types of cells?
•How
long will
the treated cells function? (How long lasting is the treatment?)
•Will
the immune
system attack the altered cells?
Germline versus somatic gene therapy
Somatic gene therapy
•Involves
alteration of the DNA of somatic cells implicated in the disease.
•Changes
are not
heritable.
Germline gene therapy
•Involves
alteration of the DNA of a gamete or fertilized egg.
•Changes
are
heritable; passed from treated individual to offspring.
•Currently
there
is no germline gene therapy done in humans.
Sites of somatic gene therapy
Gene therapy for cancer treatment
Two
strategies
have been developed using recombinant methods for the treatment of
cancer.
•Suicide
gene therapy
involves expressing a protein in cancer cell that will kill them.
•Cancer
vaccines allow tumor
cells to produce proteins activating the immune response.
Gene therapy for cancer treatment
Gene therapy for treating skin cancer
Delivery systems for gene therapy
Adeno-associated virus (AAV) integrates into
specific
chromosomal sites infecting both dividing and nondividing cells. Expression is long-term, nontoxic. Can carry small genes. e.g. cystic fibrosis,
thalassemias
Adenovirus (AV) is a large virus that can carry
large
genes. It infects dividing and
nondividing cells. Expression is
transient. Invokes an immune system
reaction. e.g. cystic fibrosis, hereditary emphysema
Herpes virus can infect nerve cells and has
long term
expression. e.g brain tumors
Retroviruses provide a stable but imprecise
integration method. Infects predominantly
dividing cells. Expression is long term
and nontoxic. This method has the longest
history. e.g.
Gaucher disease, HIV infection,
Drug treatment can modify genetic disease