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

MANSILLA SF, BERTOLIN AP, DE LA VEGA MB, PILLAIRE MJ, BERGOGLIO V, FEDERICO MB, VALLERGA MB, CAZAUX C, HOFFMANN JS AND GOTTIFREDI V

Brno

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

ICRC-ERA-HumanBridge-Framework Programme of the European Union

A role for endogenous p21 in the protection of genomic instability from dysregulated translesion DNA synthesis Mansilla SF, Bertolin AP, de la Vega MB, Pillaire MJ, Bergoglio V, Federico MB, Vallerga MB, Cazaux C, Hoffmann JS and Gottifredi V.1Fundación Instituto Leloir. IIBBA. CONICET. Buenos Aires. Argentina. 2Cancer research Center Toulouse, FranceDNA damage is very frequent. At least 10.000 lesions accumulate per cell daily even in the absence of exogenous stress. Some of those DNA lesions are inevitably encountered by active replisomes during S-phase of the cell cycle. Therefore organisms evolved tolerance pathways that allow bypass of damaged or difficult-to-replicate templates such as common fragile sites, thus ensuring continuity of the replication process. Tranlesion synthesis (TLS) is the major tolerance pathway in mammalian cells. Specialized TLS DNA polymerases can be recruited to replisomes when replicative polymerases stall at genomic DNA obstacles, as they can use a wide variety of DNA adducts as replication templates owing to its flexible active sites. A detrimental aspect of TLS is its low processivity and its intrinsic mutagenic nature, and thus the activity of these alternative polymerases must be spatially 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 replication auxiliary platform PCNA on chromatin preventing the focal 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 un perturbed replication. Indeed, I will 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 they are capable of modulating DNA replication choreography preventing replication stress and genome instability during unperturbed S phase. All these p21 basal functions depend solely of its PCNA binding site. Together, these observations have identified an unanticipated biologically relevant function for the endogenous p21 in cycling cells which are residual from the point of view of its cell cycle arresting abilities; but can still regulate DNA replication by means of preventing the excessive recruitment of non-replicative DNA polymerases to elongating DNA.