Human DNA repair enzyme O6-Methylguanine-DNA Methyltransferase in cellular regulation upon DNA damage

Abstract

SummaryAlkylation of DNA at the O6-position of guanine can lead to mutation and cell death. These DNA adducts are repaired by the repair protein MGMT (O6-methylguanine-DNA-methyltransferase). MGMT protein is conserved through evolution but the exact physiological function remains obscure. MGMT acts by transferring the O6-alkyl group to the cysteine residue at position 145 at its active site. The alkylated protein then falls from the DNA and is ubiquitinated and degraded subsequently. This repair process irreversibly inactivates the protein and hence the term a??suicidala?? repair.Active-site alkylated MGMT (R-MGMT) adopts a conformation change. This renders the protein susceptible to protease V8 cleavage. In addition, the altered conformation exposed the domain surrounding the residues 91-107 containing the monoclonal antibody 3C7 recognition epitope. We have thus far characterized the roles of R-MGMT in regulating the cell growth. We have showed that R-MGMT, with its exposed domain, containing the LXXLL motif, interacts directly with the estrogen receptor (ER) to repress ER mediated transcription and proliferation in cells expressing both MGMT and ER proteins. Furthermore, it was showed that the domain exposed in R-MGMT shares high homology to tumour suppressor protein p53 and R-MGMT can interact directly with MDM2 (an onco-protein). By performing p53-degradation assay, we found that R-MGMT can inhibit the function of MDM2 as an ubiquitin E3 ligase and hence stabilize the p53 protein. We have also found that the specific MGMT inhibitor O6 -benzylguanine (BG), which is used in early phase II clinical studies, can cause dephosphorylation of the serine-15 residue in p53 protein.