Direct Reversal The simplest of the human DNA repair pathways involves the direct reversal of the highly mutagenic alkylation lesion O6-methylguanine (O6-mG) by the product of the MGMT gene (O6-methylguanine DNA methyltransferase). The O6-mG adduct is generated in low levels by the reaction of cellular catabolites with the guanine residues in the DNA.
Correction of the lesion occurs by direct transfer of the alkyl group on guanine to a cysteine residue in the active site of MGMT in a "suicide" reaction. The inactivated alkyl-MGMT protein is then degraded in an
ATP-dependent ubiquitin proteolytic pathway.
This energetically expensive repair mechanism for the correction of a relatively simple alkyl-adduct implies O6-mG is extremely detrimental to the cell. Accordingly, a number of chemotherapeutic agents that attack the O6 position of guanine have been developed and are in clinical use.
BER Base excision repair (BER) is a multi-step process that corrects non-bulky damage to bases resulting from oxidation, methylation, deamination, or spontaneous loss of the DNA base itself. These alterations, although simple in nature, are highly mutagenic and therefore represent a significant threat to genome fidelity and stability.
BER has two subpathwaysboth of which are initiated by the action of a DNA glycosylase that cleaves the N-glycosidic bond between the damaged base and the sugar phosphate backbone of the DNA. This cleavage generates an apyrimidinic/apurinic (AP) or abasic site in the DNA. Eight DNA glycosylases with partially overlapping base adduct specificity have been identified in humans. Alternatively, AP sites can also arise by the spontaneous hydrolysis of the N-glycosidic bond. In either case, the AP site is subsequently processed by AP Endonuclease 1 (APE1) which cleaves the phosphodiester backbone immediately 5' to the AP site, resulting in a 3' hydroxyl group and a transient 5' abasic deoxyribose phosphate (dRP). Removal of the dRP can be accomplished by the action of DNA polymerase beta (Pol b), which adds one nucleotide to the 3' end of the nick and removes the dRP moiety via its associated AP lyase activity. The strand nick is finally sealed by a DNA ligase, thus restoring the integrity of the DNA. Replacement of the damaged base with a single new nucleotide as described above is referred to as "short-patch" repair and represents approximately 80-90% of all BER.
Correction of the lesion occurs by direct transfer of the alkyl group on guanine to a cysteine residue in the active site of MGMT in a "suicide" reaction. The inactivated alkyl-MGMT protein is then degraded in an
ATP-dependent ubiquitin proteolytic pathway.
This energetically expensive repair mechanism for the correction of a relatively simple alkyl-adduct implies O6-mG is extremely detrimental to the cell. Accordingly, a number of chemotherapeutic agents that attack the O6 position of guanine have been developed and are in clinical use.
BER Base excision repair (BER) is a multi-step process that corrects non-bulky damage to bases resulting from oxidation, methylation, deamination, or spontaneous loss of the DNA base itself. These alterations, although simple in nature, are highly mutagenic and therefore represent a significant threat to genome fidelity and stability.
BER has two subpathwaysboth of which are initiated by the action of a DNA glycosylase that cleaves the N-glycosidic bond between the damaged base and the sugar phosphate backbone of the DNA. This cleavage generates an apyrimidinic/apurinic (AP) or abasic site in the DNA. Eight DNA glycosylases with partially overlapping base adduct specificity have been identified in humans. Alternatively, AP sites can also arise by the spontaneous hydrolysis of the N-glycosidic bond. In either case, the AP site is subsequently processed by AP Endonuclease 1 (APE1) which cleaves the phosphodiester backbone immediately 5' to the AP site, resulting in a 3' hydroxyl group and a transient 5' abasic deoxyribose phosphate (dRP). Removal of the dRP can be accomplished by the action of DNA polymerase beta (Pol b), which adds one nucleotide to the 3' end of the nick and removes the dRP moiety via its associated AP lyase activity. The strand nick is finally sealed by a DNA ligase, thus restoring the integrity of the DNA. Replacement of the damaged base with a single new nucleotide as described above is referred to as "short-patch" repair and represents approximately 80-90% of all BER.
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