A role for endogenous p21 in the protection of genomic instability from dysregulated translesion DNA synthesis

SABRINA MANSILLA; AGOSTINA BERTOLIN; MARIA BELEN DE LA VEGA; MARIE-JEANNE PILLAIRE; VALERIE BERGOGLIO; MARIA BELEN FEDERICO; MARIA BELEN VALLERGA; CHRISTOPHE CASAUX ; JEAN SEBASTIEN HOFFMANN; VANESA GOTTIFREDI

Brno

Conferencia; Structure-Specific Endonucleases in Genome Stability Meeting.; 2015

DNA damage is very frequent. At least 10.000 lesions accumulate per cell dai/y even in the absence of exogenous stress. So me of those DNA lesions are inevitably encountered by active replisomes during S-phase of the cell cycle. Therefore organisms evo/ved tolerance pathways that allow bypass ofdamaged or diffic ult -to-replicate temp/ates such as common fragi/e sites, thus ensuring cont inu ity of the replication process. Tranlesion synthesis (TLS) is the major tolerance pathway in mammalian ce lis. Specialized TLS DNA polymerases can be recru ited to replisomes when replicative polymerases sta /1 atgenomic DNA obstacles, as they can use a wide variety of DNA adducts as replication templates owing to its flexible active sites. A detrimental aspect ofTLS is its low process ivity and its intrinsic mutagenic nature, and thus the activity of these alternative polymerases must be spatia l/y and temporally restricted to synthesis on damaged or difficult-to-replicate DNA templates. We have found that persistent TLS blockage by the cyclin kinase inhibitor p21 after UV impairs the elongation of ongoing replication fork increasing both cell death and genomic instability. To impair TLS, p21 interacts with the reppication auxiliary platform PCNA on chromatin preventing t he foca l recruitment of specialized polymerases to replication factories. We have also found that the proteolysis of endogenous levels of p21 is required for the proper activation of TLS events after UV irradiation The implications of these findings are that the endogenous levels of p21 in cells transiting S phase are sufficient to keep TLS on hold during unperturbed replication. lndeed, 1 wiil present different lines of evidences showing that this may be the case: low basal levels of p21 in cycling cells are associated with replication factories and t hey are capable of modulating DNA replication choreography prevent ing replication stress andgenome instability during unperturbed S phase. All these p21 basal functions depend solely of its PCNA binding site. Together, these observations have identified an unanticipated bio logically relevant function for the endogenous p21 in cycling ce lis which are residual from the point of view of its cellcycle arresting abi lities; but can still regulate DNA replication by means of preventing the excessive recruitment of non-replicative DNA polymerases to elongating DNA.