Initiation of malignant transformation of normal cells by a carcinogenic agent involves a permanent,
heritable change in the gene expression of the transformed cell. This could come about by either direct genotoxic or mutational events, in which a carcinogenic agent reacts directly with DNA, or by indirect or ‘‘epigenetic’’ events that modulate gene expression without directly reacting with the base sequence of DNA. Most investigators favor the mutational theory of carcinogenesis—that is, that the initiating events involve a direct action on the genome. The mutational theory depends on three kinds of evidence:
1. Agents that damage DNA are frequently carcinogenic. As discussed previously, chemical carcinogens are usually activated to form electrophilic agents that form specific reaction products with DNA. The extent of formation of some of these reaction products, for example, alkyl-O6-guanine, has been shown to correlate with mutagenicity and carcinogenicity of certain chemical agents. Ultraviolet and ionizing
radiation also interact with DNA at doses that are carcinogenic.
2. Most carcinogenic agents are mutagens. A number of in vitro test systems using mutational events in microorganisms have been developed to rapidly screen themutagenic potential of various chemical agents. One of the best known of these, the Ames test, is based on certain characteristics of specially developed strains of the bacterium Salmonella typhimurium. The tester strain, amutant line that requires exogenous histidine for its growth (hisauxotroph), has a poor excision repair mechanism and an increased permeability to exogenously added chemicals. Using this system, together with a liver microsomal fraction that has the capacity to activate most chemical carcinogens metabolically, Ames and colleagues have shown that about 90% of all carcinogens tested are also mutagenic.36 Moreover,
few noncarcinogens show significant mutagenicity in this test system. Malignant transformation can be induced in a variety of cultured mammalian cells by agents that are mutagenic for the same cells. For example, carcinogenic polycyclic hydrocarbons cause mutations, as measured by induction of resistance to 8-azaguanine, ouabain, or elevated temperature, in Chinese hamster V79 cells if the cells are
cocultured with lethally irradiated rodent cells that can metabolize the hydrocarbons to their electrophilic, activemetabolite.37,38 In these studies,mutagenicity was obtained with the carcinogenic hydrocarbons 7,12- dimethylbenz(a)anthracene, benzo(a)pyrene, and 3-methylcholanthrene. There was no mutagenicity with a noncarcinogenic hydrocarbon, and the degree of mutagenicity was related to the degree of carcinogenicity of the chemicals in vivo.
3. Incidence of cancer in patients with DNArepair deficiencies is increased. In individuals with certain recessively inherited disorders, the prevalence of cancer is significantly higher than in the general population. 39 The connecting link between these disorders is the inability to repair certain kinds of physical or chemical damage to DNA. The high incidence of cancer in these diseases constitutes the best available evidence for a casual relationship between mutagenicity and carcinogenicity in humans.
One example of xeroderma pigmentosum (XP) is characterized by extreme sensitivity of the skin to sunlight and is the most widely studied of the repair-deficient human diseases. Virtually 100% of affected
individuals will eventually develop some form of skin cancer. In addition, heterozygotes who carry the XP gene but do not have the disease appear to have a higher incidence of nonmelanoma skin cancer.40
All individuals with XP are defective in repair of ultraviolet damage to DNA, and most of them have a defect in the excision repair pathway. The repair defect ranges from 50% to 90% repair efficiency in cells from different patients, and there is good correlation between the severity of the molecular defect and the
extent of the disease. The defect in most patients appears to be at the nicking or incision step of excision repair, although patients in one complementation group have normal excision repair and are defective
in postreplication repair.39 The XP cells are also less efficient at repairing chemically induced damage to their DNA.
