Targeted disruption of the cell-cycle checkpoint gene ATR leads to early embryonic lethality in mice

Checkpoints of DNA integrity are conserved throughout evolution, as are the kinases ATM (Ataxia Telangiectasia mutated) and ATR (Ataxia- and Rad-relat ed), which are related to phosphatidylinositol (PI) 3-kinase [1-3]. The ATM gene is not essential, but mutations lead to ataxia telangiectasia (AT), a pleiotropic disorder characterised by radiation sensitivity and cellular c heckpoint defects in response to ionising radiation [4-6]. The ATR gene has not been associated with human syndromes and, structurally, is more closel y related to the canonical yeast checkpoint genes rad3(Sp) and MEC1(Sc) [7, 8]. ATR has been implicated in the response to ultraviolet (UV) radiation a nd blocks to DNA synthesis [8-11], and may phosphorylate p53 [12,13], sugge sting that ATM and ATR may have similar and, perhaps, complementary roles i n cell-cycle control after DNA damage. Here, we report that targeted inacti vation of ATR in mice by disruption of the kinase domain leads to early emb ryonic lethality before embryonic day 8.5 (E8.5). Heterozygous mice were fe rtile and had no aberrant phenotype, despite a lower ATR mRNA level. No inc rease was observed in the sensitivity of ATR(+/-) embryonic stem (ES) cells to a variety of DNA-damaging agents. Attempts to target the remaining wild -type A TR allele in heterozygous ATR(+/-) ES cells failed, supporting the idea that loss of both alleles of the ATR gene, even at the ES-cell level, is lethal. Thus, in contrast to the closely related checkpoint gene ATM,ATR has an essential function in early mammalian development.