TherapyHyeonJoo Cheon

GENE THERAPY

HYEONJOO CHEON

Question 1

What is gene therapy?

Question 2

What are commonly used viral vectors for gene therapy? What are the pros and cons for each viral vector system?

Question 3

How are viruses modified for gene therapy?

Question 4

What is suicide gene therapy?

Question 1 What is gene therapy?

Answer 1

Gene therapy is the delivery of genetic materials (RNA or DNA) into cells so that genetic properties of cells change. One or multiple aberrant genes can be replaced or their expression levels adjusted. Gene therapy encompasses treatments using monoclonal antibodies or small-molecule chemicals to correct pathological phenotypes caused by gene mutations.

Hall EJ, Giaccia AJ. Gene therapy. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:432–437.

Question 2 What are commonly used viral vectors for gene therapy? What are the pros and cons for each viral vector system?

Answer 2

1. Retroviral vector

Pros: Retroviral vector can be incorporated into the host genome, and thus the incorporated genes are replicated during cell division.

Cons: The infection rate is low, because only dividing cells can be infected with retroviruses. Integration is random and can activate proto-oncogenes.

2. Lentiviral vector

Pros: (a) Therapeutic genes are incorporated into the host genome and replicated during cell division. Lentiviruses are a subclass of retroviruses. (b) The infected cell ratio is high since lentiviruses infect dividing and nondividing cells.

3. Adenoviruses

Pros: Adenoviruses infect both dividing and quiescent cells. Infected cell ratio is high.

Cons: (a) Adenoviral DNA is not integrated into host genome. Thus, it is not replicated during cell division, and the therapeutic effects are short lived. (b) Adenoviruses induce immune responses, which limit repeat treatments.

Hall EJ, Giaccia AJ. Gene therapy. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:432–437.

Thomas CE, Ehrhardt A, Kay MA. Progress and problems with the use of viral vectors for gene therapy. Nat Rev Genet. 2003;4(5):346–358.

Question 3 How are viruses modified for gene therapy?

Answer 3

Viral vectors are engineered so that they cannot produce pathogenic viruses. A part of the viral genome critical for viral replication is deleted from viral vectors, so that they do not replicate.

Hall EJ, Giaccia AJ. Gene therapy. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:432–437.

Question 4 What is suicide gene therapy?

Answer 4

Suicide gene therapy needs two components. The first component is a gene that converts an inert prodrug into a toxic agent. The gene, contained in a viral vector, is injected into the tumor. The encoded protein is not toxic by itself if there is no prodrug in the tumor. The second component is a prodrug, which is converted into a toxic agent by the injected gene product. The prodrug, which is nontoxic in itself, is administered systemically. The prodrug does not harm the nontumor region, but is converted into a toxic agent in the tumor that expresses the converter gene. The tumor cells will be killed by the toxic agent upon conversion from the prodrug.

Hall EJ, Giaccia AJ. Gene therapy. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:432–437.

Question 5

How would you describe two commonly used suicide gene therapy systems?

Question 6

What is the advantage of suicide gene therapy as an adjunct over chemotherapeutic agents?

Question 7

A “bystander effect” in which neighboring cells are killed can be seen with suicide gene therapy. What are the causes of the bystander effect?

Question 8

Adenovirus varieties are used not only as a viral vector for gene therapy but also can function as an oncolytic virus. How does adenoviral protein E1b contribute to virus propagation in infected human cells expressing wild-type p53?

Question 5 How would you describe two commonly used suicide gene therapy systems?

Answer 5

1. The most commonly used suicide gene system consists of herpes simplex virus thymidine kinase gene (HSK-tk, a gene injected into the tumor) plus ganciclovir (GCV, a prodrug administered systematically). In cells containing the thymidine kinase gene (HSK-tk), the prodrug (GCV) is activated to become a toxic agent. HSV-tk phosphorylates GCV, converting it to a nucleoside analogue that inhibits DNA synthesis. This system inhibits repair of DNA double-strand breaks.

2. An alternative suicide gene system involves cytosine deaminase (CD, a gene injected into the tumor), which converts 5-fluorocytosine (5-FC, a prodrug administered systemically) to 5-fluorouracil (5-FU, a toxic agent that kills tumor cells). The 5-FU induces DNA strand breaks.

Hall EJ, Giaccia AJ. Gene therapy. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:432–437.

Question 6 What is the advantage of suicide gene therapy as an adjunct over chemotherapeutic agents?

Answer 6

In suicide gene therapy, the cytotoxic molecules are produced locally within the tumor by the gene directly injected into the tumor. Thus, it can avoid systemic toxicity that is often observed with traditional chemoradiotherapy.

Hall EJ, Giaccia AJ. Gene therapy. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:432–437.

Question 7 A “bystander effect” in which neighboring cells are killed can be seen with suicide gene therapy. What are the causes of the bystander effect?

Answer 7

1. The toxic agent produced in the initially transduced cells is transported to neighboring cells through gap junctions.

2. When the initially transduced cells die and are lysed, the toxic agent is released and spread to surrounding cells.

3. When tumor cells die in response to toxic agents, additional antigens are released, inducing the activation of T cells and B cells that can attack tumor cells, including those in remote locations.

Hall EJ, Giaccia AJ. Gene therapy. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:432–437.

Namm JP, Li Q, Lao X, et al. B lymphocytes as effector cells in the immunotherapy of cancer. J Surg Oncol. 2012;105(4):431–435.

Ribas A, Butterfield LH, Glaspy JA, Economou JS. Current developments in cancer vaccines and cellular immunotherapy. J Clin Oncol. 2003;21(12):2415–2432.

Question 8 Adenovirus varieties are used not only as a viral vector for gene therapy but also can function as an oncolytic virus. How does adenoviral protein E1b contribute to virus propagation in infected human cells expressing wild-type p53?

Answer 8

Viral protein E1b interacts with and inactivates a normal p53 protein, blocking p53-induced apoptosis. The delayed apoptosis allows the virus to replicate, package its genome, lyse the cells, and spread to neighboring cells.

Ridgway PJ, Hall AR, Myers CJ, Braithwaite AW. P53/E1b58kDa complex regulates adenovirus replication. Virology. 1997;237:404–413.

Yew PR, Berk AJ. Inhibition of p53 transactivation required for transformation by adenovirus early 1B protein. Nature. 1992;357(6373):82–85.

Question 9

Adenovirus with E1b gene deleted or inactivated is used to target only cancer cells. How would you explain the mechanism by which it kills only cancer cells and spares normal cells?

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