BRCA1 is required for hMLH1 stabilization following doxorubicin-induced DNA damage.

Human DNA mismatch repair (MMR) is involved in the removal of DNA base mismatchesthat arise either during DNA replication or are caused by DNA damage. In thisstudy, we show that the activation of the MMR component hMLH1 in response todoxorubicin (DOX) treatment requires the presence of BRCA1 and that thisphenomenon is mediated by an ATM/ATR dependent phosphorylation of the hMLH1Ser-406 residue. BRCA1 is an oncosuppressor protein with a central role in theDNA damage response and it is a critical component of the ATM/ATR mediatedcheckpoint signaling. Starting from a previous finding in which we demonstratedthat hMLH1 is able to bind to BRCA1, in this study we asked whether BRCA1 mightbe the bridge for ATM/ATR dependent phosphorylation of the hMLH1 molecularpartner. We found that: (i) the negative modulation of BRCA1 expression is ableto produce a remarkable reversal of hMLH1 stabilization, (ii) BRCA1 is requiredfor post-translational modification produced by DOX treatment on hMLH1 which is, in turn, attributed to the ATM/ATR activity, (iii) the serine 406phosphorylatable residue is critical for hMLH1 activation by ATM/ATR via BRCA1.Taken together, our data lend support to the hypothesis suggesting an importantrole of this oncosuppressor as a scaffold or bridging protein in DNA-damageresponse signaling via downstream phosphorylation of the ATM/ATR substrate hMLH1.