The Parp1 Val762ala Polymorphism Reduce The Enzyme Activity

Poly(ADP-ribose) polymerases (PARPs, EC 2.4.2.30) catalyze the transfer of multiple ADP-ribose groups from nicotinamide-adenine dinucleotide (NAD~+) onto protein targets, thus building up a linear or branched homopolymer of repeating ADP-ribose units, i.e. poly(ADP-ribose) (PAR) (MeSH). More than 40 years after its discovery, poly(ADP-ribosyl)ation is recognized as a key post-translational modification. Human PARPs now constitute a large family of 18 proteins encoded by different genes and displaying a conserved catalytic domain, in which Poly(ADP-ribose) polymerase 1 (PARP1) is the prototypical member.PARP1 modifies a variety of nuclear proteins by poly(ADP-ribosyl)ation, and plays diverse roles in molecular and cellular processes. The targets of PARP1 include PARP1 itself, which is the primary target in vivo, core histones, the linker histone H1, and a variety of transcription-related factors that interact with PARP1. A common PARP1 single nucleotide polymorphism (SNP) at codon 762, resulting in the substitution of alanine (Ala) for valine (Val) in the catalytic domain has been implicated in susceptibility to cancer.To characterize the functional effect of this polymorphism on PARP1, we performed in vitro enzymatic analysis on PARP1-Ala762 and PARP1-Val762. We found that PARP1-Ala762 displayed 57.2% of the activity of PARP1-Val762 for auto-poly(ADP-ribosyl)ation and 61.9% of the activity of PARP1-Val762 for trans-poly(ADP-ribosyl)ation of histone H1, in the presence of activated DNA. For the modification of PARP1 itself and histone H1, the activity decrease of PARP1-Ala762 is consistent. With 3-AB inhibition, the enzymatic activity decrease of PARP1-Ala762 is consistent. The kinetic characterization revealed that the Km of PARP1-Ala762 was increased to a 1.2-fold of the Km of PARP1-Val762 for trans-poly(ADP-ribosyl)ation of histone H1. It could explain the reduced catalytic activity of PARP1-Ala762.Residue 762 might be implicated in the binding of substrate NAD~+. Crystal structure of the catalytic domain of human PARP1 reveals that Val762 is located in the fifth helix of the N-terminal PARP_Reg sub-domain, facing the pocket of active site. The loss of two methyl groups from valine to alanine increases the distance between residue 762 and glycine888, which is the closest neighbour of residue 762 in the active site. This steric change may loose the binding of NAD~+ in the pocket and impair the enzymatic activity.The PARP1 Val762Ala polymorphism, causing about 40% decrease of PARP1 activity, is common in Caucasian and Asian, which suggests that the level of poly(ADP-ribosyl)ation by PARP1 is different in population. The PARP1 Val762Ala polymorphism has been implicated in cancer susceptibility. The AA genotype is associated with a higher cancer risk (odds ratio around 2 referring to VV genotype), and the VA genotype shows a slightly but not significantly increased cancer risk. Therefore, the deficiency of poly(ADPribosyl)ation by PARP1 and the degree of the deficiency might be implicated in carcinogenesis of the PARP1 Val762Ala polymorphism carriers.Several PARP inhibitors have entered human clinical trials. The distribution of the PARP1 Val762Ala polymorphism in patients and the relevant low PARP1 activity should be taken into account for the clinical trials and the future therapy.