What is gene therapy?

Genes, which are carried on chromosomes, are the basic physical and functional units of heredity. Genes are specific sequences of bases that encode instructions on how to make proteins. Although genes get a lot of attention, it’s the proteins that perform most life functions and even make up the majority of cellular structures.
When genes are altered so that the encoded proteins are unable to carry out
their normal functions, genetic disorders can result.

Gene therapy is a technique for correcting defective genes responsible for
disease development. Researchers may use one of several approaches for correcting
faulty genes:

• A normal gene may be inserted into a nonspecific location
  within the genome to replace a nonfunctional gene. This approach is most common.


• An abnormal gene could be swapped for a normal gene
  through homologous recombination.


• The abnormal gene could be repaired through selective
  reverse mutation, which returns the gene to its normal function.


• The regulation (the degree to which a gene is turned
  on or off) of a particular gene could be altered.

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How does gene therapy work?

In most gene therapy studies, a "normal" gene is inserted into the
genome to replace an "abnormal," disease-causing gene. A carrier molecule
called a vector must be used to deliver the therapeutic gene to the patient's
target cells. Currently, the most common vector is a virus that has been genetically
altered to carry normal human DNA. Viruses have evolved a way of encapsulating
and delivering their genes to human cells in a pathogenic manner. Scientists
have tried to take advantage of this capability and manipulate the virus genome
to remove disease-causing genes and insert therapeutic genes.

Target cells such as the patient's liver or lung cells are infected with the
viral vector. The vector then unloads its genetic material containing the therapeutic
human gene into the target cell. The generation of a functional protein product
from the therapeutic gene restores the target cell to a normal state.

Some of the different types of viruses used as gene therapy vectors:

• Retroviruses- A class of viruses that can create
  double-stranded DNA copies of their RNA genomes. These copies of its genome
  can be integrated into the chromosomes of host cells. Human immunodeficiency
  virus (HIV) is a retrovirus.


• Adenoviruses- A class of viruses with double-stranded
  DNA genomes that cause respiratory, intestinal, and eye infections in humans.
  The virus that causes the common cold is an adenovirus.


• Adeno-associated viruses- A class of small,
  single-stranded DNA viruses that can insert their genetic material at a specific
  site on chromosome 19.


• Herpes simplex viruses- A class of double-stranded
  DNA viruses that infect a particular cell type, neurons. Herpes simplex virus
  type 1 is a common human pathogen that causes cold sores.

Besides virus-mediated gene-delivery systems, there are several nonviral options
for gene delivery. The simplest method is the direct introduction of therapeutic
DNA into target cells. This approach is limited in its application because it
can be used only with certain tissues and requires large amounts of DNA.

Another nonviral approach involves the creation of an artificial lipid sphere
with an aqueous core. This liposome, which carries the therapeutic DNA, is capable
of passing the DNA through the target cell's membrane.

Therapeutic DNA also can get inside target cells by chemically linking the
DNA to a molecule that will bind to special cell receptors. Once bound to these
receptors, the therapeutic DNA constructs are engulfed by the cell membrane
and passed into the interior of the target cell. This delivery system tends
to be less effective than other options.

Researchers also are experimenting with introducing a 47th (artificial human)
chromosome into target cells. This chromosome would exist autonomously alongside
the standard 46 --not affecting their workings or causing any mutations. It
would be a large vector capable of carrying substantial amounts of genetic code,
and scientists anticipate that, because of its construction and autonomy, the
body's immune systems would not attack it. A problem with this potential method
is the difficulty in delivering such a large molecule to the nucleus of a target
cell.

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What is the current status of gene therapy research?

The Food and Drug Administration (FDA) has not yet approved any human gene
therapy product for sale. Current gene therapy is experimental and has not proven
very successful in clinical trials. Little progress has been made since the
first gene therapy clinical trial began in 1990. In 1999, gene therapy suffered
a major setback with the death of 18-year-old Jesse Gelsinger. Jesse was participating
in a gene therapy trial for ornithine transcarboxylase deficiency (OTCD). He
died from multiple organ failures 4 days after starting the treatment. His death
is believed to have been triggered by a severe immune response to the adenovirus
carrier.

Another major blow came in January 2003, when the FDA placed a temporary halt
on all gene therapy trials using retroviral vectors in blood stem cells. FDA
took this action after it learned that a second child treated in a French gene
therapy trial had developed a leukemia-like condition. Both this child and another
who had developed a similar condition in August 2002 had been successfully treated
by gene therapy for X-linked severe combined immunodeficiency disease (X-SCID),
also known as "bubble baby syndrome."

FDA's Biological Response Modifiers Advisory Committee (BRMAC) met at the end
of February 2003 to discuss possible measures that could allow a number of retroviral
gene therapy trials for treatment of life-threatening diseases to proceed with
appropriate safeguards. FDA has yet to make a decision based on the discussions
and advice of the BRMAC meeting.

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What factors have kept gene therapy from becoming an effective treatment
for genetic disease?

• Short-lived nature of gene therapy- Before gene
  therapy can become a permanent cure for any condition, the therapeutic DNA
  introduced into target cells must remain functional and the cells containing
  the therapeutic DNA must be long-lived and stable. Problems with integrating
  therapeutic DNA into the genome and the rapidly dividing nature of many cells
  prevent gene therapy from achieving any long-term benefits. Patients will
  have to undergo multiple rounds of gene therapy.


• Immune response- Anytime a foreign object is
  introduced into human tissues, the immune system is designed to attack the
  invader. The risk of stimulating the immune system in a way that reduces gene
  therapy effectiveness is always a potential risk. Furthermore, the immune
  system's enhanced response to invaders it has seen before makes it difficult
  for gene therapy to be repeated in patients.


• Problems with viral vectors- Viruses, while
  the carrier of choice in most gene therapy studies, present a variety of potential
  problems to the patient --toxicity, immune and inflammatory responses, and
  gene control and targeting issues. In addition, there is always the fear that
  the viral vector, once inside the patient, may recover its ability to cause
  disease.


• Multigene disorders- Conditions or disorders
  that arise from mutations in a single gene are the best candidates for gene
  therapy. Unfortunately, some the most commonly occurring disorders, such as
  heart disease, high blood pressure, Alzheimer's disease, arthritis, and diabetes,
  are caused by the combined effects of variations in many genes. Multigene
  or multifactorial disorders such as these would be especially difficult to
  treat effectively using gene therapy..

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What are some recent developments in gene therapy research?

• Universityof California, Los Angeles, research team
  gets genes into the brain using liposomes coated in a polymer call polyethylene
  glycol (PEG). The transfer of genes into the brain is a significant achievement
  because viral vectors are too big to get across the "blood-brain barrier."
  This method has potential for treating Parkinson's disease.


• RNA interference or gene silencing may be a new way
  to treat <st1:place>Huntington</st1:place>'s. Short pieces of double-stranded
  RNA (short, interfering RNAs or siRNAs) are used by cells to degrade RNA of
  a particular sequence. If a siRNA is designed to match the RNA copied from
  a faulty gene, then the abnormal protein product of that gene will not be
  produced.


• New gene therapy approach repairs errors in messenger
  RNA derived from defective genes. Technique has potential to treat the blood
  disorder thalassaemia, cystic fibrosis, and some cancers.


• Gene therapy for treating children with X-SCID (sever
  combined immunodeficiency) or the "bubble boy" disease is stopped
  in <st1:country-region><st1:place>France</st1:place></st1:country-region>when
  the treatment causes leukemia in one of the patients.


• Researchers at Case<st1:PlaceName>Western Reserve</st1:PlaceName><st1:PlaceType>University</st1:PlaceType>and
  Copernicus Therapeutics are able to create tiny liposomes 25 nanometers across
  that can carry therapeutic DNA through pores in the nuclear membrane.


• Sickle cell is successfully treated in mice. See Murine
  Gene Therapy Corrects Symptoms of Sickle Cell Disease from <st1:date
  Year="2002" Day="18" Month="3">March 18, 2002</st1:date>, issue of The
  Scientist.

What are some of the ethical considerations for using gene
therapy?

 --Some Questions to Consider...

• What is normal and what is a disability or disorder,
  and who decides?


• Are disabilities diseases? Do they need to be cured
  or prevented?


• Does searching for a cure demean the lives of individuals
  presently affected by disabilities?


• Is somatic gene therapy (which is done in the adult
  cells of persons known to have the disease) more or less ethical than germline
  gene therapy (which is done in egg and sperm cells and prevents the trait
  from being passed on to further generations)? In cases of somatic gene therapy,
  the procedure may have to be repeated in future generations.


• Preliminary attempts at gene therapy are exorbitantly
  expensive. Who will have access to these therapies? Who will pay for their
  use?

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Source: http://www.ornl.gov/

http://www.the-scientist.com/