Oocytes Progress beyond Prophase in the Presence of DNA Damage

Abstract

In the female germline, DNA damage has the potential to induce infertility and even to lead to genetic abnormalities that may be propagated to the resulting embryo [1, 2]. The protracted arrest in meiotic prophase makes oocytes particularly susceptible to the accumulation of environmental insults, including DNA damage. Despite this significant potential to harm reproductive capacity, surprisingly little is known about the DNA damage response in oocytes. We show that double-strand breaks in meiotically competent G2/prophase-arrested mouse oocytes do not prevent entry into M phase, unless levels of damage are severe. This lack of an efficient DNA damage checkpoint is because oocytes fail to effectively activate the master regulator of the DNA damage response pathway, ATM (ataxia telangiectasia mutated) kinase. In addition, instead of inhibiting cyclin B-CDK1 through destruction of Cdc25A phosphatase, oocytes utilize an inhibitory phosphorylation of Cdc25B. We conclude that oocytes are the only nontransformed cells that fail to launch a robust G2 phase DNA damage checkpoint and that this renders them sensitive to genomic instability.

The DNA Damage Response in Mammalian Oocytes.

“…This has been determined by the presence of γH2AX at the DSB sites. A DDC is only established following very severe DNA damage inflicted by high concentrations of Etoposide or the DNA intercalating agent Doxorubicin, causing a significant delay in M-phase entry (Marangos and Carroll, 2012). Similar observations were also seen with the use of another DSB-inducing agent Neocarzinostatin (Yuen et al, 2012).…”

“…The lack of Cdc25A destruction appears to be independent of ATM activity on account of the fact that Cdc25A is still present following high levels of DNA damage when ATM and Chk1 are active. However, the inability of DSBs to block Cdc25B activity seems to be ATM/Chk1-dependent since high levels of DNA damage cause a dramatic inhibitory phosphorylation of the phosphatase (Marangos and Carroll, 2012). Cdc25B inactivation could explain the sustained prophase arrest observed following significant levels of damage.…”

“…In fully grown oocytes, DNA damage in the form of DSBs, that would normally cause G2 arrest in somatic cells, does not affect the timing and rate of entry into M-phase (Marangos and Carroll, 2012). Although, a DDC is not being established efficiently, DNA damage detection is effective.…”

“…The molecular basis for the absence of a reliable DDC in response to DSBs appears to be due to a limited ability to activate ATM kinase (Marangos and Carroll, 2012). The lack of ATM activity also affects the activation levels of downstream effectors such as Chk1.…”

“…Low levels of expression of ATM in fully grown oocytes could be the reason for limited ATM activity. Another possibility could be the distinct chromatin configuration in fully grown oocytes (Marangos and Carroll, 2012). The fully grown oocyte is subjected to chromatin histone modifications such as deacetylation and methylation which are crucial for chromatin condensation and transcriptionally inactive heterochromatin formation (Mattson and Albertini, 1990;De La Fuente, 2006;Ma et al, 2012).…”