Repair of deaminated bases in DNA

Deamination of DNA bases can occur spontaneously, generating highly mutagenic lesions such as uracil, hypoxanthine, and xanthine. When cells are under oxidative stress that is induced either by oxidizing agents or by mitochondrial dysfunction, additional deamination products such as 5-hydroxymethyluracil (5-HMU) and 5-hydroxyuracil (5-OH-Ura) are formed. The cellular level of these highly mutagenic lesions is increased substantially when cells are exposed to DNA damaging agent, such as ionizing radiation, redox reagents, nitric oxide, and others. The cellular repair of deamination products is predominantly through the base excision repair (BER) pathway, a major cellular repair pathway that is initiated by lesion specific DNA glycosylases. In BER, the lesions are removed by the combined action of a DNA glycosylase and an AP endonuclease, leaving behind a one-base gap. The gapped product is then further repaired by the sequential action of DNA polymerase and DNA ligase. DNA glycosylases that recognize uracil, 5-OH-Ura, 5-HMU (derived from 5-methylcytosine) and a T/G mismatch (derived from a 5-methylcytosine/G pair) are present in most cells. Many of these glycosylases have been cloned and well characterized. In yeast and mammalian cells, hypoxanthine is efficiently removed by methylpurine N-glycosylase, and it is thought that BER might be an important pathway for the repair of hypoxanthine. In contrast, no glycosylase that can recognize xanthine has been identified in either yeast or mammalian cells. In Escherichia coli, the major enzyme activity that initiates the repair of hypoxanthine and xanthine is endonuclease V. Endonuclease V is an endonuclease that hydrolyzes the second phosphodiester bond 3' to the lesion. It is hypothesized that the cleaved DNA is further repaired through an alternative excision repair (AER) pathway that requires the participation of either a 5' endonuclease or a 3'-5' exonuclease to remove the damaged base. The repair process is then completed by the sequential actions of DNA polymerase and DNA ligase. Endonuclease V sequence homologs are present in all kingdoms, and it is conceivable that endonuclease V might also be a major enzyme that initiates the repair of hypoxanthine and xanthine in mammalian cells.