Gene therapy

Defective genes are responsible for the development of some disease. Gene therapy is the introduction of new genetic material into specific cells for correcting their defective genes.

Different approaches to gene therapy are investigated and include:

 1. Genetic modification of the immune system:

• Active immunization.

   - Immunization with modified tumor cells with genes that enhance the immunogenicity with cytokine genes like IL2 or non-cytokine genes like CD80.

  - Immunization with genes encoding for tumor antigens recognized by T cells like MART-I in melanoma.

• Genetic modification of the effector immune cells.

 - Increasing the recognition of tumor cells by the immune system. To allow host immune lymphocytes to recognize new tumor target genes.

 - Enhancing the survival of immune cells like the introduction of tumor specific lymphocytes with IL2 that maintain their survival. 

 - Increase the antitunmor effect of host immune cells like the introduction of lymphocytes with TNF gene in an attempt to deliver more cytokine on tumor site.

 - Decrease the undesirable toxic effect of effector cells like in the case of donor lymphocytes transduced with suicide genes in BMT patients to avoid GVHD.

 2. Modification of tumors with genes that have direct antitumor activity:

• Tumor suppressor genes. Replacing missing or mutated genes. Some cancer cells grow because certain genes are missing or are permanently shut off. An example is the tumor suppressor gene p53 that keeps the genomic integrity and prevent tumor growth. Many types of cancer showed a missing or inactive p53 gene. Researcher are investigating the replacement this missing with a functioning one to prevent cancer cell growth.

• Stopping mutated genes (oncogenes) that allow cancer growth and spread. Examples include BCR/ABL in CML and BCL2 in lymphoma.

• Suicide genes that result in tumor cell killing by introducing toxic products. An example is HSV-TH gene in brain tumors.

      3. Selective replication of viruses into tumor cells. An example is the introduction of E1B defective adenovirus capable of replication in p53 deficient tumor cells only (head and neck cancers).

     4. Chemoprotection of Hematopoietic stem cells. This is the transduction of multiple drug resistance (MDR) gene into the bone marrow cells to allow the maximum use of different chemotherapy regimens.

      5. Antiangiogenic therapy. It is the introduction of genes that prevent cancer cells from generating new blood vessels (angiogenesis). Examples of that like the inhibition of the proangiogenic cytokine VEGF.

 Methods for gene transfer:

1. Viral vector.

• Retroviral vector: The genetic material in retroviruses is in the form of RNA molecules while it is a DNA in host cells. Viral RNA molecule fare transformed into DNA by an enzymes carried in the virus, called reverse transcriptase.  The DNA is then incorporated into the host genome of the host cell by another viral enzyme known as  integrase.

• Adenoviruses: Their genetic material is in the form of double-stranded DNA.

• Adeno-associated viruses: They are from the parvovirus family,and their genetic material is in the form of single stranded DNA. They replicate only in the presence of a helper virus like adenovirus.

• Envelope protein pseudotyping of viral vectors: This type requires a favorable interaction between a protein on the surface of the virus and a protein on the surface of the cell.For example, the lentivirus Simian immunodeficiency virus coated with envelope proteins, G-protein, from another virus known as Vesicular Stomatitus virus. This chimeric vector is known as VSV G-pseudotyped lentivirus.

• Pox (Vaccinia virus) vector: This refres to the incorporation of large amount of foreign DNA into a large viral genome without adversely influencing its infectivity.

   2.    Non viral methods for gene transfer.

• Naked DNA: DNA is directly injected into host cells in vivo like in muscles or thyroid cells.

• Gene Gun: It is a gold beads that are coated with DNA.

• Oligonucleotides: Certain synthetic sequence specific single antistrand DNA is injected into the host cells to stop the genes involved in the disease development. Some uses small RNA molecules for silencing of the responsible gene siRNA.

• Lipoplexes and polyplexes: These are new molecules that have the ability to protect and enhance the delivery of the DNA during the transfection process. Simply, plasmid DNA are covered with lipids to form molecules like micelle or liposomes. Examples for this type include cationic lipids that are positively charged molecules forming complexes with the neagtively charged DNA.