Strategies of gene therapy

INTRODUCTION

• Gene therapy is when DNA is introduced into a patient to treat a genetic disease. The new DNA usually contains a functioning gene to correct the effects of a disease-causing mutation.

• Gene therapy is a strategy used to treat disease by correcting defective genes or modifying how genes they are expressed.

• Gene therapy will enable patients to be treated by inserting genes into their cells rather than administering drugs or subjecting them to surgery.

• A gene that is inserted directly into a cell usually does not function. Instead, a carrier called a vector is genetically engineered to deliver the gene.

• Gene therapy holds promise for treating a wide range of diseases, such as cancer, cystic fibrosis, heart disease, diabetes, hemophilia and AIDS.

TYPES OF GENE THERAPY

• 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. Somatic cells are nonreproductive, Often the effects of somatic cell therapy are short-lived. Somatic gene therapy can be broadly split into two categories:

• Ex vivo: where cells are modified outside the body and then transplanted back in again. cells from the patient’s blood or bone marrow are removed and grown in the laboratory.

• In vivo: where genes are changed in cells still in the body.

• 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.

• The therapy alters the genome of future generations to come, the use of germline therapy due to fears over unknown risks and long-term effects in future generations is inhibited in various countries.

• In addition, the therapy is very costly.

Gene therapy using ADENOVIRUS VECTOR

• A gene that is inserted directly into a cell usually does not function, instead a vector is used i.e. viruses such as retroviruses, integrate their genetic material (including the new gene) into a chromosome in the human cell.

• The vector can be injected or given intravenously (by IV) directly into a specific tissue in the body, where it is taken up by individual cells.

• If the treatment is successful, the new gene delivered by the vector will make a functioning protein.

Strategies of gene therapy

• Gene augmentation therapy

• Targeted killing of specific cells

• Targeted mutation correction

• Targeted inhibition of gene expression

I. GENE AUGMENTATION THERAPY

• It 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.

II.Targeted killing of specific cells

• Artificial cell killing and immune system assisted cell killing have been popular in the treatment of cancers.

• The aim is to insert DNA into a diseased cell that causes that cell to die.

• Genes are directed to the target cells and then expressed so as to cause cell killing.

• It can be done by two ways:

1. Direct killing: the inserted DNA contains a “suicide” gene that produces a highly toxic product which kills the diseased cell.

2. Indirect killing: uses immune-stimulatory genes to provoke or enhance an immune response against the target cell. The inserted DNA causes expression of a protein that marks the cells so that the diseased cells are attacked by the body’s natural immune system.

• It is essential with this method that the inserted DNA is targeted appropriately to avoid the death of cells that are functioning normally

III. Targeted mutation correction

• The repair of a genetic defect to restore a functional allele.

• Technical difficulties have meant that it is not sufficiently reliable to warrant clinical trails.

• In principle, it can be done at different levels: at the gene level or at the RNA transcript level.

IV. TARGETED INHIBITION OF GENE EXPRESSION

• suitable for treating infectious diseases and some cancers.

• The basis of this therapy is to eliminate the activity of a gene that encourages the growth of disease-related cells.

• If disease cells display an inappropriate expression of a gene a variety of different systems can be used specifically to block the expression of a single gene at the DNA, RNA or Protein levels.

• For example, cancer is sometimes the result of the over-activation of an oncogene. 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.

SUCCESSES

• Successes represent a variety of approaches—different vectors, different target cell populations, and both in vivo and ex vivo approaches for treating a variety of disorders.

• Immune deficiencies

• Hereditary blindness

• Hemophilia

• Cancer

• Blood disease

Immune deficiencies

• Several inherited immune deficiencies have been treated successfully with gene therapy.

• Most commonly, blood stem cells are removed from patients, and retroviruses are used to deliver working copies of the defective genes.

• Severe Combined Immune Deficiency (SCID) and Adenosine deaminase (ADA) deficiency.

Hereditary blindness

• Gene therapies are being developed to treat several different types of inherited blindness, especially degenerative forms, where patients gradually lose the light-sensing cells in their eyes.

• Most gene-therapy vectors used in the eye are based on AAV (adeno-associated virus).

• In one small trial of patients with a form of degenerative blindness called LCA (Leber congenital amaurosis), gene therapy greatly improved vision for at least a few years.

Hemophilia and Cancer• People with hemophilia are missing proteins that help their blood form

clots.

• In a small trial, researchers successfully used an adeno-associated viral vector to deliver a gene for Factor IX, the missing clotting protein, to liver cells. After treatment, most of the patients made at least some Factor IX, and they had fewer bleeding incidents.

• CANCER: herpes simplex 1 virus was used (which normally causes cold sores) has been shown to be effective against melanoma that has spread throughout the body.

Blood disease

• Patients with beta-Thalassemia have a defect in the beta-globin gene, which codes for an oxygen-carrying protein in red blood cells.

• In 2007, a patient received gene therapy for severe beta-Thalassemia. Blood stem cells were taken from his bone marrow and treated with a retrovirus to transfer a working copy of the beta-globin gene.

• The modified stem cells were returned to his body, where they gave rise to healthy red blood cells.

Challenges of gene therapy

• Delivering the gene to the right place and switching it on.

• Avoiding the immune response.

• Making sure the new gene doesn’t disrupt the function of other genes.

• cost of gene therapy.