Embryonic stem cells (ESCs) need to maintain their genomic integrity in response to DNA damage to safeguard the integrity of the organism.
DNA double strand breaks (DSBs) are one of the most lethal forms of DNA damage and, if not repaired correctly, they can lead to cell death,
genomic instability and
cancer. How human ESCs (hESCs) maintain genomic integrity in response to agents that cause DSBs is relatively unclear. In the present study we aim to determine the hESC response to the
DSB inducing agent
camptothecin (
CPT). We find that hESCs are hypersensitive to
CPT, as evidenced by high levels of apoptosis.
CPT treatment leads to DNA-damage sensor
kinase (ATM and
DNA-
PKcs) phosphorylation on
serine 1981 and
serine 2056, respectively. Activation of ATM and
DNA-
PKcs was followed by
histone H2AX phosphorylation on Ser 139, a sensitive reporter of DNA damage. Nuclear accumulation and ATM-dependent phosphorylation of p53 on
serine 15 were also observed. Remarkably, hESC viability was further decreased when ATM or
DNA-
PKcs kinase activity was impaired by the use of specific inhibitors. The
hypersensitivity to
CPT treatment was markedly reduced by blocking p53 translocation to mitochondria with
pifithrin-μ. Importantly, programmed cell death was achieved in the absence of the
cyclin dependent kinase inhibitor, p21(Waf1), a bona fide p53 target gene. Conversely, differentiated hESCs were no longer highly sensitive to
CPT. This attenuated apoptotic response was accompanied by changes in cell cycle profile and by the presence of p21(Waf1). The results presented here suggest that p53 has a key involvement in preventing the propagation of damaged hESCs when genome is threatened. As a whole, our findings support the concept that the phenomenon of apoptosis is a prominent player in normal embryonic development.