br Ataxia telangiectasia and Rad related as a therapeutic
Ataxia–telangiectasia and Rad3 related as a therapeutic target Several concerns revolving around functional inhibition of ATR have hindered the exploitation of ATR as therapeutic target in cancer therapy and delayed the development of specific ATR kinase inhibitors. It was anticipated that pharmacological inhibition of ATR might not be tolerated in vivo since ATR has been shown to be an essential gene. ATR knockout in mice leads to early embryonic lethality (Brown and Baltimore, 2000, De Klein et al., 2000). In humans, mutations in ATR lead to a rare hereditary disorder, Seckel syndrome (O'Driscoll et al., 2003, Alderton et al., 2004). However, the ATR mutations causing Seckel syndrome are hypomorphic, with only a partial loss of gene function. While there are numerous human hereditary diseases which are caused by a loss of protein expression of DDR proteins like ATM (ataxia–telangiectasia) or NBS1 (Nijmegen breakage syndrome), it appears that mutations in ATR are only compatible with viability when heterozygous or hypomorphic. This further supports the concept that some ATR function is essential for the development and viability of multicellular organisms. Furthermore, ATR activity is required in all proliferating 8602 synthesis during normal S-phase to ensure proper DNA replication and maintenance of genomic stability. Ruzankina et al. demonstrated that depletion of ATR in adult mice leads to defects in tissue homeostasis through acute cellular loss in tissues in which continuous cell proliferation is required for maintenance (Ruzankina et al., 2007). Furthermore, in a mouse model of Seckel syndrome, the partial loss of ATR leads to the induction of substantial replication stress, leading to accelerated ageing thereby limiting the lifespan of the mice (Murga et al., 2010). These observations may be explained by the fact, that even in the absence of replication stress-inducing agents, some replication fork stalling can occur during normal replication, for example at common fragile sites or repetitive sequences (Mirkin & Mirkin, 2007). Common fragile sites are large chromosomal regions that are thought to be particularly difficult to replicate. It has been shown that ATR is critical for fragile-site stability and that ATR-deficient cells have high levels of fragile site breakage resulting in the induction of DNA double strand breaks (Casper et al., 2002). This finding is consistent with the observation that ATR knockout leads to chromosomal fragmentation and cell death which are thought to be the underlying reason for embryonic lethality (Brown & Baltimore, 2000). The observed impediments of normal DNA replication and induction of DNA DSBs following ATR depletion raise the possibility that pharmacological ATR inhibition could cause severe side effects due to toxicity on highly proliferative normal tissues, especially if ATR inhibition was combined with drugs that cause replication stress. However, several studies have indicated that ATR inhibition might be preferentially cytotoxic for cancer cells, thereby raising the possibility of a therapeutic window for ATR inhibitors in cancer therapy. A recent study in a mouse model of Seckel syndrome demonstrated that the detrimental effects of ATR-deficiency on cell viability may be ameliorated by p53 since loss of p53 function exacerbated the accumulation of replication stress when ATR signalling was compromised (Murga et al., 2010). Functional loss of p53 was also found to profoundly aggravate the severity of ATR loss in adult mice. Simultaneous depletion of p53 and ATR exacerbated tissue degeneration, accompanied by the induction of high levels of DNA damage, and accelerated lethality of the mice (Ruzankina et al., 2009). These findings point towards an important role of p53 in the cellular response to ATR inhibition and raise the possibility that p53-deficient tumours, which comprise a high proportion of cancer cases, may show increased sensitivity to ATR inhibition compared with non-tumour tissue.