Genome Instability, DNA Repair, and Cancer at a Glance
- DNA can be damaged by physical agents (ultraviolet or ionizing radiation) or chemical agents in the environment.
- DNA damage may lead to mutations (changes in DNA sequence).
- The ability of cells to repair DNA damage and to maintain genome stability is of utmost importance to prevent malignant transformation.
- Different agents induce different types of DNA damage, which in turn require different responses and repair pathways.
- A number of hereditary disorders are characterized by genome instability due to defects in genes involved in DNA repair or DNA damage signaling.
- Many different laboratory tests can be used to diagnose genome instability and/or DNA repair defects.
- Inherited or acquired genome instability is associated with an increased cancer risk.
The integrity of the genome of all living organisms is constantly threatened by exogenous and endogenous DNA-damaging agents. Exogenous DNA-damaging agents include physical agents, such as ultraviolet (UV) or ionizing radiation (IR), and a wide variety of chemical agents, such as components of cigarette smoke. Endogenous DNA damage arises from regular metabolic processes within the cell, mediated, for example, by reactive oxygen species. Maintaining the stability of the genome is of utmost importance to all living organisms. Therefore, since early evolution, all organisms ranging from prokaryotes to eukaryotes have been equipped with mechanisms that react to and repair DNA damage and thereby maintain genomic stability. The types of damage produced include alterations in the structure of nucleotides, DNA strand breaks, DNA cross-links, and DNA adducts. Different types of DNA-damaging agents induce different types of DNA damage (Table 110-1), which in turn require different responses and repair pathways for processing (Table 110-2).1
Table 110-1 Cellular Damage Induced by Physical and Chemical Agents |Favorite Table|Download (.pdf)
Table 110-1 Cellular Damage Induced by Physical and Chemical Agents
Ultraviolet (UV) radiation
Dipyrimidine cyclobutane dimers (TT, TC, CT, or CC), pyrimidine-pyrimidone (6–4) photoproducts (mostly TC), DNA-protein cross-links
DNA single- and double-strand breaks, oxidative base damage
Psoralens plus UVA
DNA-psoralen monoadducts, DNA interstrand cross-links (binds to T at TA sequences)
DNA interstrand cross-links
Reactive oxygen species
Oxidative base damage (8-oxo-deoxyguanine, thymine glycol), cyclopurines (A or G) making bulky lesions
Table 110-2 Types of DNA Damage and Associated DNA Repair Pathways |Favorite Table|Download (.pdf)
Table 110-2 Types of DNA Damage and Associated DNA Repair Pathways
Type of DNA Damage
DNA Repair Pathway
DNA photoproducts (CPDs, 6,4-PP)
Nucleotide excision repair (NER)
Oxidative base modifications (e.g., 8-oxoG)
Base excision repair
Incorrect DNA base pairing
DNA double-strand breaks
Nonhomologous end joining, homologous recombination (i.e., recombination repair)
DNA cross-links (interstrand and intrastrand)
Persistent DNA lesions
Translesion (bypass) DNA synthesis