Genetic alterations and DNA repair in human carcinogenesis.

A causal association between genetic alterations and cancer is supported by extensive experimental and epidemiological data. Mutational inactivation of tumor suppressor genes and activation of oncogenes are associated with the development of a wide range of cancers. The link between mutagenesis and carcinogenesis is particularly evident for cancers induced by chemical exposures, which, in some cases, lead to characteristic patterns of mutations. These "genotoxic," direct-acting carcinogens form covalent adducts with DNA, which cause mutations during DNA replication. The link between mutagenesis and carcinogenesis is also supported by the observation that DNA repair defects are associated with an increased cancer risk. Normally, DNA repair mechanisms serve to suppress mutagenesis by correcting DNA damage before it can lead to heritable mutations. It has been postulated that mutagenesis plays a role in both the initiation phase and the progression phase of carcinogenesis, and that an essential step in the carcinogenic process is the development of a mutator state in which the normal cellular processes that suppress mutagenesis become compromised. Given the link between mutations and cancer, attempts have been made to use the mutational profile of cancer cells as an indicator of the causative agent. While this may be a valid approach in some cases, it is complicated by the role of endogenous processes in promoting mutagenesis. In addition, many important carcinogenic agents may enhance mutagenesis indirectly through suppression of DNA repair functions or stimulation of inappropriate cell proliferation. Epigenetic phenomena may also suppress gene expression without causing overt changes in DNA sequence.