Selective utilization of non‐homologous end‐joining and homologous recombination for DNA repair during meiotic maturation in mouse oocytes

Abstract

DNA double-strand breaks (DSBs) are highly toxic lesions that can cause genomic instability and can be repaired by non-homologous end-joining (NHEJ) and homologous recombination (HR) pathways. Despite extensive studies about DSB repair pathways, the roles of each pathway during meiotic maturation in oocytes are not well understood. Here we show that oocytes selectively utilize NHEJ and HR to repair DSBs during meiotic maturation. Inhibition of NHEJ impaired the meiotic maturation of oocytes with DNA damage by activating the spindle assembly checkpoint (SAC) with a concomitant increase in metaphase I (MI) arrest and DNA damage levels. In contrast, oocytes with DNA damage bypassed SAC-mediated MI arrest despite the presence of fragmented DNA when HR was inhibited. Notably, this bypass of SAC arrest by HR inhibition was associated with a loss of centromere integrity and subsequent impairment of chromosome architecture. Our results demonstrate that, while NHEJ is critical for the meiotic maturation of oocytes with DNA damage, HR is essential to maintain centromere integrity against DNA damage during meiotic maturation, revealing distinct roles of NHEJ and HR during meiotic maturation in mouse oocytes. | INTRODUCTIONDNA double-strand breaks (DSBs) are one of the most deleterious types of DNA damage and often occur naturally through various endogenous and exogenous processes. However, cells are able to cope with DSBs using well-equipped repair pathways to maintain genome integrity. DSBs are primarily repaired by two mechanisms: non-homologous end-joining (NHEJ) and homologous recombination (HR). 1 NHEJ is initiated by the recognition and binding of the Ku70/80 heterodimer to DNA ends, followed by recruitment of the main NHEJ repair factors, including DNA-dependent protein kinase catalytic subunit (DNA-PKcs), Artemis and DNA ligase IV. 2 Because DSB ends are directly ligated without the use of a homologous template, NHEJ is generally considered error-prone. By contrast, HR tends to be error-free because broken ends are repaired using homologous sequences as templates. Also, HR is a more complex process involving 5 0 -end resections to generate 3 0 -single-stranded (ss) DNA overhangs. 3 This processing is initiated by the binding of the heterotrimeric Mre11-Rad50-Nbs1 (MRN) complex to DSBs, which coordinates tethering and short-range nucleolytic degradation of DSB ends by activating the endo-and exonuclease activity of Mre11. After 5 0end-resection, the 3'-ssDNA overhangs are rapidly coated by RPA, which is subsequently displaced by Rad51, forming presynaptic filaments that facilitate the search for homologous DNA templates for Crystal Lee and Jiyeon Leem contributed equally to this work.

Genome Instability and DNA Repair in Somatic and Reproductive Aging

“…The DSBs in oocytes are mainly repaired by two pathways, HR and NHEJ. Due to the presence of the sister chromatid in meiotic prophase oocytes, HR is considered to be the predominant repair pathway in arrested primordial follicles (131). Blocking apoptosis in primordial follicles can activate HR to remove DSBs efficiently (132), and key components of HR, such as RAD51 and BRCA2, are highly expressed in primordial follicle oocytes (133).…”

Distinct characteristics of the DNA damage response in mammalian oocytes

“…However, it was also reported that “clean” DSBs with complementary overhangs can be accurately repaired by the classic NHEJ pathway, showing nearly equal efficiencies in the presence of a homologous template 84 , 89 . In oocytes, NHEJ was found to be the preferential repair pathway during MII, as the inhibition of NHEJ-related elements was associated with increased DNA damage signals and decreased developmental competence 78 , 79 . Moreover, oocytes exposed to etoposide at the prophase stage undergo an increase in SAC-mediated MI arrest upon NHEJ inhibition, along with partially impaired K-MT attachment 78 (Fig.…”

“…In oocytes, NHEJ was found to be the preferential repair pathway during MII, as the inhibition of NHEJ-related elements was associated with increased DNA damage signals and decreased developmental competence 78 , 79 . Moreover, oocytes exposed to etoposide at the prophase stage undergo an increase in SAC-mediated MI arrest upon NHEJ inhibition, along with partially impaired K-MT attachment 78 (Fig. 3 ).…”

“…Considering the difference in the mechanisms involved between the HR and NHEJ pathways, it can be speculated that each pathway plays a differential role in the DDR throughout cell cycle progression. Indeed, there is evidence showing that each repair pathway plays distinct roles in repairing DSBs during meiotic maturation 78 . In contrast to NHEJ, when HR is inhibited, the oocyte cell cycle is not affected, despite significant chromosome fragmentation possibly resulting from compromised centromere integrity, suggesting crosstalk between the highly repetitive sequences in the centromere and the HR machinery 78 (Fig.…”

BRCA Mutations and Fertility Preservation

“…Finally, BRCA2 is thought to be involved in mitosis following phosphorylation by CHK1/2 and PLK1. During the metaphase to anaphase transition, the attachment of duplicated chromosomes to the spindle relies on the spindle assembly checkpoint (SAC) [ 48 ]. This complex is composed, among other proteins, of budding uninhibited by benzimidazole-related 1 (BUBR1) protein which needs to be acetylated to interact with the anaphase promoting complex (APC/C).…”