NAD+-mediated regulation of mammalian base excision repair

Abstract

The enzymes of the base excision repair (BER) pathway form DNA lesion-dependent, transient complexes that vary in composition based on the type of DNA damage. These protein sub-complexes facilitate substrate/product handoff to ensure reaction completion so as to avoid accumulation of potentially toxic DNA repair intermediates. However, in the mammalian cell, additional signaling molecules are required to fine-tune the activity of the BER pathway enzymes and to facilitate chromatin/histone reorganization for access to the DNA lesion for repair. These signaling enzymes include nicotinamide adenine dinucleotide (NAD⁺) dependent poly(ADP-ribose) polymerases (PARP1, PARP2) and class III deacetylases (SIRT1, SIRT6) that comprise a key PARP-NAD-SIRT axis to facilitate the regulation and coordination of BER in the mammalian cell. Here, we briefly describe the key nodes in the BER pathway that are regulated by this axis and highlight the cellular and organismal variation in NAD⁺ bioavailability that can impact BER signaling potential. We discuss how cellular NAD⁺ is required for BER to maintain genome stability and to mount a robust cellular response to DNA damage. Finally, we consider the dependence of BER on the PARP-NAD-SIRT axis for BER protein complex assembly.

Keywords: Base excision repair; Nicotinamide adenine dinucleotide; PARP1; Sirtuin.

Fig. 1 –. The PARP-NAD-SIRT signaling axis in base excision repair.

A graphical depiction of base excision repair in a mammalian cell. The critical protein-protein interactions and enzyme activities needed to conduct BER are additionally regulated by factors of the PARP-NAD-SIRT signaling axis to facilitate and regulate enzyme activities (glycosylases, APE1). Further, PARP-activation promotes chromatin and histone reorganization to allow critical BER protein complex assembly and disassembly. Cellular consequences of BER failure are also highlighted. Overall, NAD⁺ plays an essential role as a substrate for both PARP-family and SIRT-family enzymes.

Fig. 2 –. An essential role for NAD⁺ in base excision repair.

NAD⁺ metabolism is tightly regulated within the cell. Controlled activation of NAD⁺ consumers PARP1 and SIRT1-SIRT7 is essential to maintain high NAD⁺ levels associated with a healthy outcome. However, PARP1 hyperactivation and SIRT1-SIRT7 inhibition leads to a decrease in cellular NAD⁺ levels associated with a poor pathologic outcome for many disease states. Treatment with PARP1 inhibitors or NAD⁺ precursors can reverse the pathologic outcome by increasing cellular NAD⁺ levels.

Fig. 3 –. Laser micro-irradiation in the study of base excision repair.

Top panels show confocal fluorescent micrographs revealing the recruitment of XRCC1-dsRED (left) or EGFP-Polβ (right) following DNA damage via laser-induced micro-irradiation. The peak time of 45 seconds shows the fluorescent protein accumulating at the site of micro-irradiation (center image) and the lack of recruitment when PARP1 is inhibited by BMN673 (right image). Plots below show the kinetics of BER protein recruitment to the site of damage in the presence of DMSO (black lines) or the PARP1 inhibitor BMN673 (blue lines).