Specific killing of DNA damage-response deficient cells with inhibitors of poly(ADP-ribose) glycohydrolase

Polly Gravells,Emma Grant,Kate M Smith,Dominic I James,Helen E Bryant

doi:10.1016/j.dnarep.2017.02.010

Abstract

Poly(ADP-ribosylation) of proteins following DNA damage is well studied and the use of poly(ADP-ribose) polymerase (PARP) inhibitors as therapeutic agents is an exciting prospect for the treatment of many cancers. Poly(ADP-ribose) glycohydrolase (PARG) has endo- and exoglycosidase activities which can cleave glycosidic bonds, rapidly reversing the action of PARP enzymes. Like addition of poly(ADP-ribose) (PAR) by PARP, removal of PAR by PARG is also thought to be required for repair of DNA strand breaks and for continued replication at perturbed forks. Here we use siRNA to show a synthetic lethal relationship between PARG and BRCA1, BRCA2, PALB2, FAM175A (ABRAXAS) and BARD1. In addition, we demonstrate that MCF7 cells depleted of these proteins are sensitive to Gallotannin and a novel and specific PARG inhibitor PDD00017273. We confirm that PARG inhibition increases endogenous DNA damage, stalls replication forks and increases homologous recombination, and propose that it is the lack of homologous recombination (HR) proteins at PARG inhibitor-induced stalled replication forks that induces cell death. Interestingly not all genes that are synthetically lethal with PARP result in sensitivity to PARG inhibitors, suggesting that although there is overlap, the functions of PARP and PARG may not be completely identical. These data together add further evidence to the possibility that single treatment therapy with PARG inhibitors could be used for treatment of certain HR deficient tumours and provide insight into the relationship between PARP, PARG and the processes of DNA repair.

Highlights

An early response to DNA damage is the addition of poly(ADPribose) (PAR) to proteins [1,2]
We and others demonstrated that poly(ADP-ribose) polymerase (PARP) inhibitors can kill homologous recombination repair (HRR) deficient tumours [19,20]
We demonstrate that each of the genes above is required for efficient HRR, suggesting that, as with BRCA2, it is the lack of HRR proteins at Poly(ADP-ribose) glycohydrolase (PARG) inhibitor-induced replication stalling that induces cell death

Summary

Introduction

An early response to DNA damage is the addition of poly(ADPribose) (PAR) to proteins [1,2] This post-translational modification has been implicated in repair of single [3,4,5,6,7] and double strand DNA breaks [8,9,10,11] and in the restart of stalled or collapsed DNA replication forks [12,13,14]. We and others demonstrated that PARP inhibitors can kill homologous recombination repair (HRR) deficient tumours [19,20] This is because, in the absence of PARP activity, increased numbers of replication forks collapse and HRR becomes essential for cell survival [14,19,21]. Inhibition of PARP has become a successful and increasingly promising therapeutic approach for certain tumour types [22]

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Conclusion