CANCER GENE THERAPY II
This article describes current research into the applications of cancer gene therapy. Scientists have amassed a vast amount of knowledge on the subject, and are vigorously pursuing many options, but positive results have been slow in coming. There are many clinical trials in progress using gene therapy, and the consensus is that the method will eventually prove valuable.
Current thinking indicates that there are three broad categories in which gene therapy can be applied: (1) restore proper gene function, (2) stimulate the body's immune system to destroy the cancer, and (3) introduce a gene that causes the conversion of a non-toxic substance into a toxic substance.
Restoring proper gene function
I indicated in the previous article that cancer is caused by mutations in genes that control the cell division cycle. Oncogenes are mutations that cause an excessive stimulation of the cell cycle, while mutations in tumor suppressor genes can result in a loss in function in controlling this excessive activity. A major target of gene therapy is the tumor suppressor gene, p53. It has been found that there is a mutation of the p53 gene in about 50% of all cancers. When the DNA of the cell is damaged, p53 protein begins to accumulate. If the DNA damage is not too severe, p53 directs the formation of another protein, p21, which blocks further progression of the cell cycle until the DNA can be repaired. If the DNA cannot be repaired, p53 directs the cell to commit suicide through a process called apostasies. A clinical study now in progress involves the replacement of defective p53 in patients with lung cancer. A delivery vehicle was prepared by inserting the p53 gene into an adenovirus vector. The adenovirus was injected repeatedly into the patient's tumors. The majority of the patients showed a stabilization of the disease but not a cure. One of the most promising approaches to control oncogene activity is using anti sense drugs. I will discuss antisense in detail in another article, but for now I will simply say that the antisense oligonucleotide (section of DNA) is complimentary in structure to the oncogene. Consequently, the antisense oligonucleotide binds to the oncogene, preventing the formation of its protein product. The oncogene then becomes inactive, and cannot cause over stimulation of the cell cycle. Preliminary results indicate that, similar to p53 therapy, the cancer can be controlled, but it is difficult to effect a cure. As a rule, it appears that every cancer cell must receive the replacement gene in order to cure the cancer.
Stimulation of the Immune System
There are two basic assumptions underlying the use of gene transduction (transfer) for immunotherapy of cancer. The first assumption is that unique antigens specific to the cancer exist. In many instances they can be hard to find. The second assumption is that the immune system is intact, capable of recognizing the antigen, and mounting an attack against the cancer cell. Cancers thought to be most sensitive to immunotherapy include melanoma (skin cancer), renal cell carcinoma (kidney), colorectal cancer and non-small cell lung cancer. Currently, the most active area of research involves introducing one of the cytosine genes into the cancer cell. As you will recall, cytosine's are hormones secreted by the immune cells that serve to communicate with and stimulate other parts of the immune system. After the cancer cell receives the cytosine gene, it secretes large amounts of the cytosine, which in turn stimulate an immune response to an antigen present on the cancer cell. There are several options to deliver the cytosine gene. The original method was ex vivo, or out of the body. Tumor cells are removed at surgery, grown in culture, transfected with the gene, and finally re injected into the patient. A second approach is to introduce the cytosine gene into one of the viral or non-viral vectors previously mentioned. The vector is then injected directly into the tumor. Recently, there has been interest in transferring genes encoding tumor-associated antigens or cytosines directly into dendrite's cells. Dendrite cells are necessary to trigger the immune response to foreign antigens. Since dendritic cells often do not recognize the antigens on cancer, the insertion of these genes may prime the cells into action. It is well known that cytotoxic T cells must be stimulated by two signals before it can react to a cell as foreign. The first signal is the binding of antigen to a receptor on the T cell. The second signal is a molecule such as B7 that is produced by the infected antigen-presenting cell that encounters the T cell. This lack of a second signal is thought to be the reason for a lack of response to many cancers. A recent development involves the preparation of a plasmid vector containing genes for the two cytosines, interleukin-12 and interferon alpha. The interferon improves the expression of antigen on the tumor cell surface, while the interleukin provides a co-stimulatory signal. The strategy behind this technique is to introduce a gene into tumor cells that encodes a protein that converts an otherwise non-toxic pro-drug into a toxic substance. In effect, you are causing the cancer to commit suicide. Two genes currently under investigation code for enzymes that are not found in humans, so they are foreign
Thymidine kinase gene is found in herpes simplex virus. It was found that this gene phosphorylates the compound gancicivir, causing it to inhibit the synthesis of DNA, resulting in cell death. The gene is inserted into brain tumors and certain other cancers via a retrovirus carrier. Gancicivir is then given systemically. Cytosine deaminase is found in the bacterium E. coli. This enzyme can convert 5-flurocytosine into the toxic chemotherapeutic agent, 5-flurouracil. In this manner, large amounts of 5-fluorocytosine can be administered to the patient without causing harm to the normal body cells, while delivering a toxic dose specifically to cancer cells. The advantage of both of these chemosentization genes is that they can apparently kill by a bystander effect,that is, not every cell in the tumor needs to be transduced by the gene in order to eradicate the tumor completely.