The DNA Damage Checkpoint Eliminates Mouse Oocytes with Chromosome Synapsis Failure

Abstract

SUMMARY Pairing and synapsis of homologous chromosomes during meiosis is crucial for producing genetically normal gametes, and is dependent upon repair of SPO11-induced double stranded breaks (DSBs) by homologous recombination. To prevent transmission of genetic defects, diverse organisms have evolved mechanisms to eliminate meiocytes containing unrepaired DSBs or unsynapsed chromosomes. Here, we show that the CHK2 (CHEK2)-dependent DNA damage checkpoint culls not only recombination-defective mouse oocytes, but also SPO11-deficient oocytes that are severely defective in homolog synapsis. The checkpoint is triggered in those oocytes that accumulate a threshold level spontaneous DSBs (~10) in late Prophase I, the repair of which is inhibited by presence of HORMAD1/2 on unsynapsed chromosome axes. Furthermore, Hormad2 deletion rescued fertility of oocytes containing a synapsis-proficient, DSB repair-defective mutation in a gene (Trip13) required for removal of HORMADs from synapsed chromosomes, suggesting that many meiotic DSBs are normally repaired by intersister recombination in mice.

Signaling to TRP53 and TAp63 from CHK1/CHK2 is responsible for elimination of most oocytes defective for either chromosome synapsis or recombination

“…These experiments indicate that unrepaired meiotic DSBs, when present at levels above the threshold to trigger their elimination (RINALDI et al 2017), ultimately cause DNA damage signaling to TRP53 and TAp63. Additionally, we conclude that one or both of these proteins can be activated not only by CHK2, but also another kinase.…”

“…To address the question of whether a CHK2-independent pathway exists that can eliminate oocytes bearing unrepaired DSBs, we utilized two mutant models, Trip13 Gt and a Spo11 null (Spo11 -). Virtually all Trip13 Gt/Gt oocytes are eliminated due to failure to repair SPO11-dependent DSBs (LI and SCHIMENTI 2007) by the end of pachynema (RINALDI et al 2017). Chk2 deficiency rescued ~ 1/3 of these oocytes, and these rescued oocytes gave rise to viable offspring (BOLCUN-FILAS et al 2014).…”

“…indicative of the active form) and TRP53 (which is stabilized by phosphorylation) in various genotypes of neonatal (3-5 days post-partum) ovaries treated with zero or 3Gy of ionizing radiation (IR). This level of IR induces ~40 DSBs, as measured by RAD51 foci (a proxy for DSBs) on oocyte meiotic chromosomes (RINALDI et al 2017). Three hours later, proteins were extracted for Western blot analysis.…”

“…Approximately 61% of Spo11 -/oocytes, which lack any appreciable degree of homolog synapsis and are incapable of forming programmed meiotic DSBs, reach this threshold, and almost the entire ovarian reserve (primordial oocytes) is depleted by 3 weeks after birth. Chk2 deletion rescued oocyte numbers to ~25% of WT, indicating that most are either eliminated by an alternative pathway, or that they succumb nonspecifically from a catastrophically high number of DSBs (up to ~100, with an average of ~50/cell) (RINALDI et al 2017). Similarly, Chk2 deficiency rescued Trip13 Gt/Gt oocytes to ~1/3 of WT levels (BOLCUN-FILAS et al 2014), raising the possibility that the same CHK2-independent pathway may be active in both cases.…”

“…Surprisingly, this mechanism is also highly dependent on the DNA damage checkpoint. The mechanistic basis for this is the formation of a threshold level (~10) of spontaneously-arising DSBs that are SPO11-independent (CAROFIGLIO et al 2013;RINALDI et al 2017). Approximately 61% of Spo11 -/oocytes, which lack any appreciable degree of homolog synapsis and are incapable of forming programmed meiotic DSBs, reach this threshold, and almost the entire ovarian reserve (primordial oocytes) is depleted by 3 weeks after birth.…”