Regulators of the DDR have therefore become attractive targe

Regulators of the DDR have therefore become attractive targets for cancer therapy primarily through two potential approaches. First, to be used as chemo- or radiosensitisers to increase the effectiveness of standard genotoxic treatments and to help prevent or overcome the development of resistance. Second, to exploit defects in DDR mechanisms as potentially targetable weaknesses through synthetic lethal approaches. While defects in DDR components may, on the one hand, give cancer cells a growth advantage, allowing them to survive and proliferate despite oncogene-induced replication stress and genomic instability, they may also drive a reliance of cancer cells on any remaining DDR pathways in order to survive DNA damage. Targeting of such remaining pathways may therefore be selectively toxic to cancer cells with mutations in certain DDR genes. The potential of this approach was first demonstrated in cells harbouring mutations in the breast and ovarian cancer susceptibility genes BRCA1 and BRCA2, which were shown to be highly sensitive to small molecule inhibitors of poly(ADP-ribose)-polymerase (PARP), a DDR protein that is involved in the detection and repair of DNA single strand breaks by omecamtiv excision repair (Bryant et al., 2005, Farmer et al., 2005). The PARP inhibitor olaparib has since shown promise for the treatment of BRCA1/2 mutated breast or ovarian cancer in clinical trials (Fong et al., 2009, Audeh et al., 2010, Tutt et al., 2010). Further studies have demonstrated that inhibition of other components of the DDR machinery can sensitise cancer cells to DNA damaging treatments, including DNA-PKcs (Zhao et al., 2006), ataxia–telangiectasia mutated (ATM) (Rainey et al., 2008, Golding et al., 2012), ataxia–telangiectasia and Rad3 related (ATR) (Fokas et al., 2012, Pires et al., 2012, Prevo et al., 2012, Huntoon et al., 2013), or their downstream targets CHK1 and CHK2 (Matthews et al., 2007, Blasina et al., 2008, Mitchell et al., 2010, Riesterer et al., 2011).
The phosphatidylinositol-3 kinase-related kinase (PIKK) family ATM and ATR are members of the phosphatidylinositol 3-kinase-related kinase (PIKK) family of serine/threonine protein kinases, which also comprises DNA-dependent protein kinase catalytic subunit (DNA-PKcs/PRKDC), mammalian target of rapamycin (MTOR/FRAP) and suppressor of morphogenesis in genitalia (SMG1). The cellular functions of these protein kinases range from regulation of the DNA damage response (DDR) to cell survival, proliferation, metabolism, differentiation, motility and nonsense-mediated mRNA decay (Lempiäinen and Halazonetis, 2009, Jackson and Bartek, 2010, Ciccia and Elledge, 2011). A sixth member of the PIKK family, transformation/transcription associated protein (TRRAP), serves as component of various histone acetyltransferase complexes and plays a role in the epigenetic regulation of transcription, but possesses no kinase activity (Lempiäinen & Halazonetis, 2009). The members of the PIKK kinase family show considerable similarities in their domain architecture and extensive sequence homology, particularly in their C-terminal kinase domain and the flanking FAT (FRAP–ATM–TRRAP) and FATC (FAT C-terminal) domains (Fig. 1) (Keith and Schreiber, 1995, Bosotti et al., 2000). Even though the functions of the FAT and FATC domains are not yet fully understood, both domains have been implicated in the regulation of kinase activity. The N-terminal region is poorly conserved between PIKK family members and is believed to be important for the interaction with various substrates and adapter proteins (Fernandes et al., 2005). In ATR, the N-terminus also contains the binding site for ATRIP (ATR-interacting protein), which regulates the localisation of ATR to sites of replication stress and DNA damage and is essential for ATR signalling (Cortez et al., 2001, Zou and Elledge, 2003).
Ataxia–telangiectasia mutated activation and downstream signalling