C. Tumor Suppressor Genes and Anti-Oncogenes

An increasing number of genes (oncogenes and tumor suppressing genes) have been identified so far. These genes get dysregulated during carcinogenesis. Different molecular mechanisms and genetic alteration like gene deletion, mutation, or promoter silencing are the major reasons for gene dysregulation which along with some complex processes leads ultimately to cancer (an example of which has been depicted in supplementary figure 2). These processes of genetic changes and modification cause the deactivation or activation of multiple genes. As a result of these processes the cancer cell proliferates in an uncontrolled fashion and does not enter apoptosis and becomes extremely invasive in nature. Oncogenes get activated as a result of genetic alteration and are involved in promoting carcinogenesis. The tumor suppressing genes become inactivated during carcinogenesis. Current techniques in this field involve the inactivation of over expressed oncogenes by antisense molecules or dominant negative mutants or, alternatively, the reintroduction of tumor-suppressor genes that was lost or mutated. The principle here is not likely the complete reversal of tumor cells back to normal cells (this task is extremely difficult to accomplish as more than one mutation is usually involved in the generation of tumor cells during the transformation of a normal cell to a cancer cell) but the goal instead is to find out the weak point in the cell's regulatory balance which can be exploited and accordingly identify the genes having the highest impact on arresting cell-cycle or involved in apoptosis. Thus, overall understanding of the cellular mechanisms like signal transduction, regulation of cell cycle (as shown in supplementary figure 1), cellular apoptotic processes are extremely important. Targeting those genes which become dysfunctional in the tumor cells is usually the most efficient method to cure cancer. As already known, that mutation in more than one gene is involved in the development of cancer, hence by targeting all those genes specific and efficient treatment of cancer can be achieved. One of the main hurdles to this therapeutic method is the requirement of an efficient high gene-transfer technique. To some extent a weak or moderate bystander effect has been reported with respect to the p53 gene transfer. In those cases where the immune system is weak, an efficient high gene-transfer technique is extremely required to completely eradicate the tumors. With the current vectors available it is not possible to achieve such an efficient high gene-transfer, therefore this highly efficient tumor specific method depends on the quality of the vector that will be developed in the near future. Out of many tumor suppressor genes that have been identified p53 has shown promise in vivo and is being used in clinical investigations and gene therapy for cancer. Many other genes like caspases, PTEN, BRCA1 and proapoptotic genes like bax and bcl-x s are connected to the regulation of the G1-phase of the cell cycle. All these genes have been evaluated by conducting numerous animal experiments. The entry of cells into S-phase can be prevented by transferring the wild-type and truncated pRb (which in its active form binds to E2F-1, a gene regulatory protein) as well as by keeping pRb in its hypo-phosphorylated active state by cdk/cyclin inhibitors p16 and p21. The efficacy of arresting the cell-cycle for treating established tumors is sufficient or there is a need of a strong apoptotic induction obtained by the transfer of p16 and p27 is yet to be evaluated. Reversal of tumor cells to normal cells by using p53 or p21 can enhance the cells' susceptibility to radiation and chemo-therapy. Tumoricidal effect can be greatly increased by combined activation of a number of tumor suppressor genes as shown for the p53 and p16 genes together. Many clinical trials involving gene therapy directed against oncogenes or apoptosis suppressors like bcl-2 or bcl-xlh are being explored. These anti-oncogene approaches are mainly supported by the recent success of small molecular inhibitors (Gleevec or Imatinib mesylate) of oncogenes which inhibit the EGF receptor, the RAS pathway, and the ABL gene. Gene therapy is efficient in competing with small molecules as small molecules are able to efficiently target enzymes and receptors only, whereas the oncogene can be inhibited by relevant transdominant anti-oncogenes, antisense oligonucleotides, and the highly potent RNA-interfering nucleotides.