Gene+Therapy

=**Objectives**=


 * 1. Differentiate between somatic and germ cell gene therapy.** (p.326)

Germ line gene therapy is the modification of germ cells such that offspring arising from these cells have the inserted gene in every cell. Somatic cell therapy is the genetic modification of any other cell in the body.


 * 2. Differentiate between ex vivo and in vivo gene therapy.** (p.362)

Ex vivo gene therapy refers to when target cells are removed from the body and genetic materials transfered to the target cells in the laboratory. These modified cells are amplified or placed under selection, then returned to the patient.

In vivo gene therapy refers to when genetic material is transfered directly into the patient.


 * 3. Describe the advantages and disadvantages of viral gene therapy vectors.** (p.366)

__Advantages of Viral Vector Systems__
 * High efficiency of gene delivery
 * Some can permenently intergrate their genomes

__Disadvantages of Viral Vector Systems__
 * Elicit host immune response
 * Labor-intensive to produce large quantities
 * Infectious agent


 * 4. Describe the advantages and disadvantages of non-viral gene therapy vectors.** (p.364)

__Advantages of Non-Viral Systems__
 * Minimal host immune response
 * Cost effective for producing large amounts

__Disadvantages of Non-Viral Systems__
 * Inefficient gene delivery
 * No vectors thus far delivered have resulted in permanent changes


 * 5. Describe the molecular defect that causes severe combined immune deficiency syndrome and why this disease was chosen as the first disease to be experimentally treated by gene therapy.** (p.371)

SCIDS is caused by an adenosine deaminase deficiency that catalyzes the irreversible deamination of adenosine to inosine. This results in a buildup of dATP and deoxyadenosine in immature lymphocytes that causes cells to die, leaving host without B and T cells. ADA deficiency results from mutations or deletions in the ADA gene, making it ideal for the first experimental use of gene therapy because the therapy targets one specific gene.


 * 6. Describe the rationale for the treatment of cancer with gene therapy.** (p.376)

Cancer is a complex disease resulting from multiple gene mutations. The idenfitication of specific mutations that contribute to malignancy identifies potential gene therapeutic targets and it may be possible to deliver functional genes to tumors to reverse cancer phenotype. However, such therapy may require the reversal of the defect in a large number of tumor cells to affect a cure though temporary expression of the gene may be sufficinet to kill the tumor. In order to be widely used, the gene therapy should reduce or delay mortality and/or be more effective than existing cytotoxic chemotherapy agents.


 * 7. Briefly describe the construction of a viral gene therapy vector.** (p.368-369)

Use 2 viruses. Virus A has no viral functional replication and structural genes; however, it does have a piece of foreign DNA encoding the therapeutic gene with necessary viral origins of replication and packaging signals. Virus B contains the viral functional replication and structural genes; however, it does not contain the viral origin of replication and packaging signals.

As a result, neither virus can produce virus on its own. Virus A can repliate itself but cannot express viral replication or structural genes. Virus B cannot replicate itself, but can express viral replication and structural genes. If a eukaryotic "packaging cells" in culture can be infected with both viruses and integrate into the host chromosome, then the two viruses will complement each other defects and allow the "packaging cell" to carry the vector-producing DNAs on their cellular chromosomes and continuously produce gene therapy vectors.


 * 8. Explain how acycloguanosine and the herpes simplex virus thymidine kinase gene are utilized in cancer gene therapy.** (p.378)

Herpes simplex virus thymidine kinase (HSV-tk) gene is a target for cancer gene therapy. HSV-tk is an analog of cellular thymidine kinase but it phosphorylates acycloguanosine, something cellular thymidine kinase doesn't do. Phosphorylated acycloguanosine gets incorporated into DNA and functions as a chain terminator because it is missing a 3' hdyroxyl position. Delivery of acycloguanosine and viral delivery of the HSV-tk gene to cancer cells can result in the expression of HSV-tk, resulting in the formation of phosphorylated acycloguanosine and termination of DNA synthesis. Bystander cell killing can result due to the diffusion of phosphorylated acycloguanosine to adjacent tumor cells via gap junctions.