DNA repair and protection in the female germline Event

Event details

Jacinta is a final year PhD student supervised by Professor Brett Nixon, Laureate Professor John Aitken and Dr Elizabeth Bromfield at Newcastle, Australia. Jacinta’s PhD is focused on determining the mechanisms that the oocyte and embryo use to protect themselves against chemotherapeutic drugs, such as etoposide, and whether such mechanisms may be further exploited to protect these cells from this and other damaging agents. 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. To improve our understanding of these systems, the aim of this research project was to determine whether mature (MII) oocytes are able to utilize these resources to detect, respond and subsequently mount effective protective and/or reparative strategies to mitigate the impact of genotoxic insult. For this purpose, DNA double strand breaks (DSB) were elicited using the topoisomerase II inhibitor, etoposide (ETP); which led to a rapid and significant increase in DSB (P = 0.0002). Utilizing this model of ETP challenge, we documented 2 distinct responses, namely: (i) the MII oocyte engaged a reparative cascade known as the non-homologous end joining DNA pathway to protect the integrity of the maternal genome and (ii) oocytes experience a developmental change in their vulnerability to ETP, with fertilisation leading to a rapid increase in the expression of permeability glycoprotein (PGP) and its subsequent translocation to the oolemma whereupon it acts as a transporter to minimize the intracellular accumulation of ETP. In support of these responses, we were able to document a significant reduction in DSB lesions 4h post-ETP treatment. Notably, this repair was completely abrogated by pharmacological inhibition of key elements of the canonical non-homologous end joining DNA pathway (i.e. DNA-PKcs and DNA ligase IV), thus providing the first evidence implicating this reparative cascade in the protection of the maternal genome. Similarly, we were able to utilize dye exclusion assays conducted in the presence of a selective PGP pharmacological inhibitor (PSC833) to confirm this multidrug efflux transporter does provide a first line of defence to protect the zygote against genotoxic agents that would otherwise be capable of eliciting DSB formation. Our collective data therefore encourages a reappraisal of the paradigm the oocyte/embryo is largely refractory to DSB DNA repair and protection.