therapy is an experimental technique that uses genes to treat or prevent
disease. It involves inserting a gene into a patient’s cells instead of using
drugs or surgery.
Several approaches to gene therapy that are being tested are:
Replacing a mutated, disease
causing gene with a healthy gene.
Inactivating or ‘knocking out’ a
mutated gene that is functioning properly.
Introducing a new gene into the body
to help fight a disease.
How does it work?
Gene therapy is used to introduce a
healthy copy of a gene into the body to compensate for damaged or abnormal
If a mutated gene produces an abnormal protein, or if the protein is
missing, gene therapy can be used to introduce a normal copy of the gene to
restore the function of the protein.
A gene that is inserted directly into the cell does not function. A
carrier called a vector is genetically engineered
to carry the gene.
The vector can be injected or given intravenously (by IV) directly into
the specific tissue, where it is taken up by individual cells.
Another method is to expose the patient’s cells to the vector in a
laboratory. The cells are returned to the body. If this is successful, the new
gene carried by the vector will produce a functional protein.
There are two classes of vectors:
1) Recombinant Vectors:
Also called biological nanoparticles or viral vectors.
During viral replication, viruses introduce their genetic material into
the host cell.
They then utilize the cell machinery to replicate their DNA.
Retroviruses have their DNA copied into the host cell.
This characteristic is exploited by substituting the viral genetic
material with the therapeutic DNA.
A number of viruses have been used in human gene therapy, such as
retroviruses, adenoviruses, herpes samples, vaccinia and adeno-associated
Electroporation: An electric field is applied to cells to increase permeability
of the cell membrane, allowing DNA to enter the cell.
The gene gun: Used for delivery of
exogenous DNA to cells, a method known as biolistics.
Types of Gene Therapy
1)Somatic Gene Therapy and Germline
In somatic gene therapy, gene
defects in the somatic cells (such as blood cells) are corrected. These
corrections are not passed to the next generation.
In germline gene therapy, modifications
are made to the sperm and ova (germline cells). These changes will be passed to
In vivo and ex vivo gene
One method to introduce the exogenous gene is to isolate the cells so that
they can be modified in vitro. This way, the cells can be selected and cultured
to increase their numbers. The cell clones are then reintroduced into the body.
This treatment is known as ex vivo gene
In the second method, the gene is directly introduced into the cells by
the mutation. This method is called ex
vivo gene therapy.
Ex vivo gene therapy is suitable for treating disorders in blood cells.
Blood cells are derived from pluripotent stem cells in the bone marrow. They can
be easily isolated, cultured in vitro and returned to the body. This method has
been used in the treatment of adenosine deaminase (ADA) deficiency.
Clinical trials for treatment of Gaucher’s disease, Fanconi’s anaemia and
Hurler’s syndrome are underway.
Ex vivo treatment is not possible for treating diseases such as cystic
fibrosis and muscular dystrophy.
In the first case, epithelial lung cells are difficult to culture in vitro
as they divide slowly. Also, these cells are difficult to access. Another
reason is that it is difficult to repopulate the lungs of affected individuals
with cells cultured in vitro.
For these reasons, these disorders can be treated by
injecting the exogenous gene directly into the affected cells, i.e. by in vivo
treatment. Thus, muscular dystrophy can be treated by injecting the Dystrophin
gene into muscle cells, CF by introducing the CTFR gene into cells of the
Approaches to Gene Therapy
In straightforward cases, gene therapy involves inserting a functional
copy of a gene in a specific tissue.
Sometimes this approach is not enough to fix the problem, in which case
different methods have to be used.
Certain situations that
arise may be:
Some mutations in genes lead to production of a dominant negative protein.
This type of protein may block a normal protein from doing its job.
In this case, adding a functional copy of the gene will not help because
the protein will still be there.
This mutation makes a protein act
abnormally. A gain-of-function mutation activates a protein even when there is
no signal, leading to cancer.
Problems in gene regulations change
when, where and how much protein is produced.
1)Repairing mutations: A few techniques involve
replacing a defective copy of a gene with a healthy one.
The term SMaRT stands for
Spliceosome-Mediated RNA Trans-splicing.
Targets and repairs the Mrna transcripts copied from the mutated gene.
This technique just replaces the part of the mrna that contains the
mutation instead of replacing the whole gene.
This approach “turns off” a gene so
that no protein is made from it. Gene silencing can target a gene directly or
they can target mrna transcripts from a gene.
3)Triple Helix Forming
Oligonucleotide Gene Therapy targets the DNA sequence of a mutated gene to prevent its
transcription. Oligonucleotides bind specifically to the groove of double
stranded DNA, preventing its transcription.
4) RNA Interference takes advantage of the
cells virus killing machinery.
A short piece of RNA is introduced. This has a sequence complementary to
the mrna transcript of a gene.
The RNA piece binds to the complementary mrna, forming a double stranded
RNA molecule, which is destroyed by the cell since it mimics viral genetic
5) Ribozymes are RNA molecules that
act as enzymes. They are designed to target and destroy mrna transcripts
encoded by the mutated gene so no protein can be made from it.
6) Genetically modifying immune
cells to target specific molecules:
Immune cells can be modified to recognize a specific antigen using gene
therapy. When they are returned to the patient, the modifies cells find and
destroy any cell carrying the antigen.
Immune cells can also be modified to make certain molecules, such as a
drug. When returned to the body, the cells target and destroy specific antigen
as well as release the disease-fighting product.
Ethical and Moral
Issues of gene therapy
Gene therapy raises many ethical concerns because it involves changing the
body’s basic instructions.
Germline gene therapy is controversial as it may lead to long term side
effects in the foetus, or it may impact the foetus in unexpected ways. Also
since the therapy is used to treat unborn individuals, they cannot choose
whether they want to have the treatment or not.
It also raises the question of which disorders or traits require gene
therapy to be corrected. The function of many genes and the traits they
determine are still being studied. It will lead to the question of whether gene
therapy is required to treat what is considered an “undesirable” trait.