Two major classes of genes contribute to causing cancer i.e., Oncogenes and Tumour suppressor genes. Oncogenes must be activated to cause cancer. Tumour suppressor genes, which normally hold mitosis in check, must be inactivated or removed to eliminate control of the cell cycle and initiate cancer.
Oncogenes are genes that normally activate during cell division in specific situations. Oncogene activation at wrong place or time during cell division may lead to cancer. Oncogenes are not alien to the cell, they are normal, essential genes that have undergone a mutation. In its normal non mutated state, it is called proto-oncogene, a gene that can be transformed into an oncogene.
Activation of proto-oncogene to oncogene is achieved by different mechanisms like promoter and enhancer insertion, chromosomal translocation, gene amplification and point mutation.
Tumor suppressor genes as the term suggested it prevents or suppresses tumor formation by regulating cell division. Tumor suppressor genes are now recognized as key players in the genesis of cancer. Malfunctioning of tumor suppressor genes may lead to uncontrolled cell division. Researchers have identified about a half dozen tumor suppressor genes. Important tumor suppressor genes include RB I and p53, both of which are nuclear phosphoproteins and probably affect the transcription of genes involved in regulating events in the cell cycle.
Oncogene vs Tumor Suppressor Genes
1. Mutation in one of the two alleles is sufficient for activity as an oncogene and often act dominant to wild type.
2. Mutation often occurs in somatic tissues therefore not inherited
3. Conversion of protooncogene to oncogene is often a “gain of function” of protein that signals uncontrolled cell division.
4. Some tissue preference.
Tumor Suppressor Genes
1. Tumor suppressor gene malfunctioning is caused by mutations in both alleles or a mutation in one followed by a loss of or reduction to homozygosity in the second.
2. Mutation may occur in germ cell (can be inherited) or somatic cells.
3. ‘Loss of function’ mutation is the reason for tumor suppressor gene malfunctioning.
4. Strong tissue preference in the case of mant tumor suppressor genes (Example: effect of RB II gene in retina)