Identifying ADP-ribosylation targets by chemical genetics

A posttranslational protein modification with increasing relevance for cancer biology is poly(ADP-ribosyl)ation (PARylation) (1). Inhibitors of PARylation show promising effects either as monotherapeutic agents or as chemo- or radiosensitizers to support classical DNA damaging cancer therapies. The targets of these inhibitors are enzymes of the family of poly(ADP-ribose) polymerases (PARPs, also known as ARTDs) (2). PARP inhibitors are analogs of NAD+, which PARP enzymes use as substrate to synthesize poly(ADP-ribose) (PAR). PAR can be covalently attached to or interact non-covalently with target proteins (3,4). In humans, 17 genes encode for PARP/ARTD enzymes, of which 5 can generate PAR (PARP1/2/4 and TNKS1/2), while all others either catalyze mono-ADP-ribosylation (MARylation) or appear to be inactive. PARPs localize to various cellular compartments and participate in a multitude of cancer-relevant cellular functions, such as DNA damage responses, transcription, chromatin organization and regulation of cell death (1).