Abstract
The unique biology of the oocyte means that accepted paradigms for DNA repair and protection are not of direct relevance to the female gamete. Instead, preservation of the integrity of the maternal genome depends on endogenous protein stores and/or mRNA transcripts accumulated during oogenesis. The aim of this study was to determine whether mature (MII) oocytes have the capacity to detect DNA damage and subsequently mount effective repair. For this purpose, DNA double strand breaks (DSB) were elicited using the topoisomerase II inhibitor, etoposide (ETP). ETP challenge led to a rapid and significant increase in DSB (P = 0.0002) and the consequential incidence of metaphase plate abnormalities (P = 0.0031). Despite this, ETP-treated MII oocytes retained their ability to participate in in vitro fertilisation, though displayed reduced developmental competence beyond the 2-cell stage (P = 0.02). To account for these findings, we analysed the efficacy of DSB resolution, revealing a significant reduction in DSB lesions 4 h post-ETP treatment. Notably, this response was completely abrogated by pharmacological inhibition of key elements (DNA-PKcs and DNA ligase IV) of the canonical non-homologous end joining DNA repair pathway, thus providing the first evidence implicating this reparative cascade in the protection of the maternal genome.
Highlights
All cells within the human body encounter potentially thousands of DNA lesions on a daily basis owing to their exposure to a variety of internal and environmental factors[1]
While it is not yet clear why immature oocytes fail to activate major DNA damage response or repair factors, it has yet to be determined whether mature metaphase II (MII) phase oocytes are recalcitrant to DNA repair, or whether this later developmental stage is equipped to resolve DNA lesions and mitigate the possibility of DNA damaged oocytes participating in fertilisation[17]
Comparative proteomic analyses of mouse MII oocytes performed in an independent study have identified a total of 53 proteins putatively involved in the DNA damage response and DNA repair related processes, including: nucleotide excision repair (NER), single strand break repair (SSB), double strand break repair (DSB) and base excision repair (BER)[20]
Summary
All cells within the human body encounter potentially thousands of DNA lesions on a daily basis owing to their exposure to a variety of internal and environmental factors[1]. Unlike somatic cells that can dynamically upregulate the synthesis of DNA damage response machinery following genotoxic insult[9,10], the mature ovulatory stage oocyte is transcriptionally silent and incapable of mounting an equivalent response Instead, these cells must depend on endogenous stores of pre-synthesised proteins and/or mRNA transcripts accumulated during oogenesis to enact DNA repair[11]. The SAC may act to protect against both aneuploidy and the inheritance of DNA damage by preventing the production of abnormal mature oocytes and subsequent embryos[12] While it is not yet clear why immature oocytes fail to activate major DNA damage response or repair factors, it has yet to be determined whether mature MII phase oocytes are recalcitrant to DNA repair, or whether this later developmental stage is equipped to resolve DNA lesions and mitigate the possibility of DNA damaged oocytes participating in fertilisation[17]. Such mechanistic understanding is of critical importance in protecting the genetic integrity of the oocyte to allow for its unimpeded transition through embryogenesis
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