Insights in the PARP1 poisoning effect

Cellular and organism viability requires rapid cellular answers external changes and high adaptational capacities to external changes. Among the short term rapid variations that occurs in a cell, one can cite post-translational modifications. Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes that uses nicotinamide adenine dinucleotide (NAD+ proteins post-translationally. PARP1 was the first protein discovered to have a poly(ADP-ribosyl)-ation (PARylation) activity.Often called as the “guardian angel of DNA” , this protein has been actively studied for more than 40 years and implied in many medicinal chemistry programs for several therapeutic areas encompassing cancer, cardiac ischemia, stroke, inflammation and diabetes. Yet, many questions on PARP1 remains without answer. PARP1 is activated by the recognition of DNA strand breaks. How does PARP1 recognizes DNA lesions? How is the signal conducted to the catalytic domain? PARP1 has a structure composed of a DNA-binding domain, a catalytic domain and an auto-modification domain. The auto-modification domain contains acceptor residues for auto-PARylation by PARP1 to increase its activity by a thousand fold. The DNA-binding domain is made up of 2 zinc-finger domains, named ZN1 (N-terminal domain) and ZN3. Another zinc-finger domain named ZN2 exists but is dispensable for protein's activity. Either does the so-called AD auto-modification domain. The catalytic domain (CAT), responsible of the PARylation activity, is C-terminal. Finally, a last domain, called WGR, is an essential domain but of unknown function. Olaparib (AZD2281) and Veliparib (ABT-888) are PARP inhibitors in advanced clinical trials. Olaparib has passed phase II for advanced ovarian and breast cancers with BRCA1/2 mutation. Veliparib is investigated in phase II as a chemopotentiator of irinotecan for advanced or refractory solid tumor. They are highly effective PARP catalytic inhibitors with IC50 values reaching the low nanomolar range. Their structure includes a nicotinamide moiety that competes with NAD+ Olaparib is known to trap PARP1 on DNA, which is called a “poisoning effect”. On the contrary, Veliparib (ABT-888) is known not to have this poisoning effect. The trapping of PARP1 on DNA is very effective, but the molecular principles underlying this poisoning effect are unknown. Understanding them could lead to more potent clinical inhibitors of PARP1. ) as a substrate to transfer negatively charged ADP-ribose moieties on targets. This study aims at understanding the different effects induced by Olaparib and Veliparib on PARP1 at a molecular scale. We will try to find the pathways and cross-talks between domains from the CAT to the DNA-binding domain that, in one case, lead to a stabilization of the interactions between PARP1's ZN1 and ZN3 domains with DNA (Olaparib), and in one another case, to a destabilization of the interactions between PARP1's ZN1 and ZN3 domains with DNA (Veliparib).