DNA crosslink repair safeguards genomic stability during pre-meiotic germ cell development
Germ cells are single-handedly responsible for the transmission of genetic and epigenetic information (the instructions to make new life) from one generation to the next. It is therefore, of paramount importance that germ cells maintain the stability of their genome. Failure to do so can result in the transmission of de novo mutations to offspring. Germline de novo mutations are the basis of genetic disease and the foundation of genetic diversity within a species, and the substrate on which evolution acts. However, the timings and molecular origins of germline mutagenesis are not fully understood. Here we describe a fundamental role for the structure-specific endonuclease XPF-ERCC1 in Fanconi-mediated DNA crosslink repair during germ cell development in utero. Inactivation of Fanconi-mediated DNA repair results in an almost complete loss of gamete production in mice. This repair pathway is critical for the normal development of primordial germ cells (PGCs), which are the earliest germ cells to arise during embryogenesis. We find that loss of crosslink repair leads the accumulation of DNA double-strand breaks in PGCs during a narrow temporal window of development. PGCs with DNA damage phosphorylate p53 and are lost through apoptosis. The loss of PGCs in crosslink repair-deficient embryos coincides with the unique process of epigenetic reprogramming, in which PGCs erase epigenetic restraints and parental imprints during the acquisition of germline totipotency. However, we find that the remaining crosslink repair-deficient PGCs undergo the major hallmarks of epigenetic reprogramming. Furthermore, we demonstrate the both XPF-ERCC1 and Fanconi-mediated DNA crosslink repair are dispensable for meiosis in mice. Furthermore, the data presented within this Thesis sheds light on the fertility defects observed in human patients with Fanconi anaemia, and provides a plausible explanation for germ cell attrition and infertility in a complex human disease. Finally, we describe an essential role for components of the translesion synthesis (TLS) machinery during PGC development in mice. TLS is a mechanism of DNA damage tolerance whereby the high-fidelity replicative polymerases that become obstructed by DNA lesions are replaced by specialised low-fidelity polymerases capable of bypassing the lesion. Most strikingly, we find that PGCs unable to modify the replication processivity factor, PCNA, a prerequisite for efficient TLS, fail to undergo DNA demethylation, a key feature of epigenetic reprogramming and thus germ cell development.
Germ cell, DNA repair, Stem cells, DNA Crosslink repair, Fanconi anaemia
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