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The protein E2F is a transcription factor that binds to DNA to stimulate the synthesis of proteins necessary for cell division. When E2F is bound to the RB protein, however, it cannot bind to DNA. Thus, when functioning normally, the RB protein prevents a cell from dividing by binding to E2F. When RB is absent or inactivated, that restraint is lost, and E2F is constantly available to trigger cell division.
The p53 gene
The p53 protein was discovered in 1979. It resides in the nucleus, where it regulates cell proliferation and cell death. In particular, it prevents cells with damaged DNA from dividing or, when damage is too great, promotes apoptosis. Cells exposed to mutagens (chemicals or radiation capable of mutating the DNA) need time to repair any genetic damage they sustain so that they do not copy errors into the DNA of their daughter cells. When mutations occur, normal levels of the p53 protein rise, which slows the transition of the cell cycle from the G1 phase to the S phase. That extra time allows DNA repair mechanisms to effectively restore the DNA sequences to normal. The brakes on the cell cycle—high p53 levels—are then removed, and the cell proceeds to divide.
If there is a large amount of genetic damage, p53 triggers a series of biochemical reactions that cause the cell to self-destruct. Total functional inactivation of the p53 gene will cause genetic damage to accumulate in the cell and will also fail to set off apoptosis in severely injured cells.
Both radiation therapy and chemotherapy can kill tumour cells by stimulating apoptosis. Some tumours that have lost p53 function are more resistant to therapy because of the cells’ diminished capacity to trigger cell death. (See Diagnosis and treatment of cancer: Therapeutic strategies.)
Inactivation of the p53 gene occurs through mutation of one allele, and loss of the other accounts for 70 percent of cases of colon carcinoma, 30 to 50 percent of cases of breast cancer, and 50 percent of cases of lung cancer. In two other types of cancer, inactivation of the p53 gene occurs not through mutation and loss of the alleles but through binding of the p53 protein with another protein (called an antagonist) that disables p53 function. One such antagonist, called MDM2, is involved in sarcomas. Other antagonists are the “early proteins” produced by cancer-causing strains of the human papillomavirus (see Cancer-causing agents: Human papillomaviruses).
Other tumour suppressor genes
Other tumour suppressor genes that have been discovered through the study of familial cancers include the BRCA1 and BRCA2 genes, which are associated with about 5 percent of hereditary breast cancers; the APC gene, linked to familial adenomatous polyposis coli (a hereditary form of colorectal cancer that causes thousands of polyps to form in the colon, some of which can become cancerous); the WT1 gene, involved in Wilms tumour of the kidney; the VHL gene, associated with kidney cancer and von Hippel-Lindau disease; and the NF1 and NF2 genes, responsible for certain forms of neurofibromatosis.
Tumour suppressor genes discovered through the study of hereditary cancers also play a role in sporadic cancers. For example, hereditary melanoma is associated with a loss of function of the tumour suppressor gene called MTS1 (from multiple tumour suppressor), which also goes awry in a variety of sporadic tumours. MTS1 codes for a protein called p16. When functioning properly, the p16 protein prevents the cell cycle from progressing from the G1 stage to the S stage through an interaction with the RB protein. In cells in which p16 function is lost, the transition from G1 to S is not slowed. That transition point in the cell cycle seems to be extremely important to cellular health, since about 80 percent of human tumours exhibit a problem there.
DNA repair defects
DNA repair mechanisms are involved in maintaining the integrity of DNA, which often acquires errors during replication. The gene products that oversee the maintenance of DNA integrity help to detect the damage and activate and direct the repair machinery, thereby disabling mutagenic molecules before they permanently damage the DNA. In general, those genes, referred to as the “caretakers of the genome,” behave similarly to tumour suppressor genes. When the cellular mechanisms that repair errors in the DNA are damaged—through acquired or inherited alterations—the rate of genetic mutation increases by several orders of magnitude.
Defects in two mismatch repair genes, called MSH2 and MLH1, underlie one of the most-common syndromes of inherited cancer susceptibility, hereditary nonpolyposis colon cancer. That form of colorectal cancer accounts for 15 to 20 percent of all colon cancer cases. Inherited or acquired alterations in the mismatch repair genes allow mutations—specifically point mutations and changes in the lengths of simple sequence repetitions—to accumulate rapidly (behaviour referred to as a mutator phenotype). Since that defect is inherited by all the cells in the body, it is not known why some organs are more susceptible to cancer development than others.
Another type of repair system that can malfunction is one that corrects defects inflicted on DNA by ultraviolet radiation, a major constituent of sunlight (see Cancer-causing agents: Radiation). That kind of radiation damage involves the fusion of two nucleotide bases called pyrimidines to form a “pyrimidine dimer.” Normally, the repair system removes the dimer from the DNA and replaces it with two undamaged nucleotides. Malfunction of the repair pathway, on the other hand, is responsible for two inherited disorders, xeroderma pigmentosum and Cockayne syndrome.