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 is the proteins 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 non-specific location within the genome to replace a non-functional 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.

Steps in Gene Therapy

gene therapy

In most gene therapy studies, a ‘normal’ gene is inserted into the genome to replace an ‘abnormal’, disease causing gene.

A carrier molecule called 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.

Besides virus-mediated gene-delivery systems, there are several non-viral 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 within tissues and requires large amounts of DNA.

Gene therapy techniques

There are several techniques for carrying out gene therapy. These include:

Gene augmentation therapy

  • This is used to treat diseases caused by a mutation that stops a gene from producing a functioning product, such as a protein.
  • This therapy adds DNA containing a functional version of the lost gene back into the cell.
  • The new gene produces a functioning product at sufficient levels to replace the protein that was originally missing.
  • This is only successful if the effects of the disease are reversible or have not resulted in lasting damage to the body.
  • For example, this can be used to treat loss of function disorders such as cystic fibrosis by introducing a functional copy of the gene to correct the disease (see illustration below).
gene augmentation therapy

Gene inhibition therapy

  • Suitable for the treatment of infectious diseases, cancer and inherited disease caused by inappropriate gene activity.
  • The aim is to introduce a gene whose product either:
    • inhibits the expression of another gene
    • interferes with the activity of the product of another gene.
  • The basis of this therapy is to eliminate the activity of a gene that encourages the growth of disease-related cells.
  • For example, cancer is sometimes the result of the over-activation of an oncogene? (gene which stimulates cell growth). So, by eliminating the activity of that oncogene through gene inhibition therapy, it is possible to prevent further cell growth and stop the cancer in its tracks.
gene inhibition therapy

Types :

There are two different types of gene therapy depending on which types of cells are treated: 

    • Somatic gene therapy: transfer of a section of DNA to any cell of the body that doesn’t produce sperm or eggs. Effects of gene therapy will not be passed onto the patient’s children.
    • Germline gene therapy: transfer of a section of DNA to cells that produce eggs or sperm. Effects of gene therapy will be passed onto the patient’s children and subsequent generations.
  • Challenges of gene therapy
    1. Delivering the gene to the right place and switching it on:
  • Delivering the gene to the right place and switching it on:
    • it is crucial that the new gene reaches the right cell
    • delivering a gene into the wrong cell would be inefficient and could also cause health problems for the patient
    • even once the right cell has been targeted the gene has to be turned on
    • cells sometimes obstruct this process by shutting down genes that are showing unusual activity.
  • Avoiding the immune response:
    • The role of the immune system is to fight off intruders.
    • Sometimes new genes introduced by gene therapy are considered potentially-harmful intruders.
    • This can spark an immune response in the patient, that could be harmful to them.
    • Scientists therefore have the challenge of finding a way to deliver genes without the immune system ‘noticing’.
    • This is usually by using vectors that are less likely to trigger an immune response.
  • Making sure the new gene doesn’t disrupt the function of other genes:
    • Ideally, a new gene introduced by gene therapy will integrate itself into the genome of the patient and continue working for the rest of their lives.
    • There is a risk that the new gene will insert itself into the path of another gene, disrupting its activity.
    • This could have damaging effects, for example, if it interferes with an important gene involved in regulating cell division, it could result in cancer.
  • The cost of gene therapy:
    • Many genetic disorders that can be targeted with gene therapy are extremely rare.
    • Gene therapy therefore often requires an individual, case-by-case approach. This may be effective, but may also be very expensive.

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