Extensive damage to maternal DNA during meiosis causes infertility, birth defects and abortions. However, it is unknown if fully grown oocytes have a mechanism to prevent the creation of DNA-damaged embryos. Here we show that DNA damage activates a pathway involving the spindle assembly checkpoint (SAC) in response to chemically induced double strand breaks, UVB and ionizing radiation. DNA damage can occur either before or after nuclear envelope breakdown, and provides an effective block to anaphase-promoting complex activity, and consequently the formation of mature eggs. This contrasts with somatic cells, where DNA damage fails to affect mitotic progression. However, it uncovers a second function for the meiotic SAC, which in the context of detecting microtubule–kinetochore errors has hitherto been labelled as weak or ineffectual in mammalian oocytes. We propose that its essential role in the detection of DNA damage sheds new light on its biological purpose in mammalian female meiosis.

DNA damage acquired during meiosis can lead to infertility and miscarriage. Hence, it should be important for an oocyte to be able to detect and respond to such events in order to make a healthy egg. Here, the strategies taken by oocytes during their stages of growth to respond to DNA damaging events are reviewed. In particular, recent evidence of a novel pathway in fully grown oocytes helps prevent the formation of mature eggs with DNA damage. It has been found that fully grown germinal vesicle stage oocytes that have been DNA damaged do not arrest at this point in meiosis, but instead undergo meiotic resumption and stall during the first meiotic division. The Spindle Assembly Checkpoint, which is a well-known mitotic pathway employed by somatic cells to monitor chromosome attachment to spindle microtubules, appears to be utilised by oocytes also to respond to DNA damage. As such maturing oocytes are arrested at metaphase I due to an active Spindle Assembly Checkpoint. This is surprising given this checkpoint has been previously studied in oocytes and considered to be weak and ineffectual because of its poor ability to be activated in response to microtubule attachment errors. Therefore, the involvement of the Spindle Assembly Checkpoint in DNA damage responses of mature oocytes during meiosis I uncovers a novel second function for this ubiquitous cellular checkpoint. Reproduction (2016) 152 R15-R22cryopreservation of embryos and oocytes (Maltaris et al. 2007, ASRM 2013, Roness et al. 2014, Skaznik-Wikiel et al. 2015. Unfortunately, these methods cannot be applied to all. One limitation of this technique is that a partner, or willingness to use a donor, is required to provide sperm. Hormonal suppression of ovaries during cancer treatment is another option for women; however, the use of such drugs has potential associated risks such as interference with the cancer treatment or survival of eggs with DNA damage (Roness et al. 2014).However, the major limitation of cryopreservation methods is that they can only be used in post-pubertal women. Therefore, there are currently no established options for young pre-pubertal girls (Skaznik-Wikiel et al. 2015). Experimental options include ovarian tissue cryopreservation, but this has a variety of risks associated with it (Maltaris et al. 2007, Skaznik-Wikiel et al. 2015.In this review, we will focus on the various strategies that oocytes elicit, in the adult, upon damage to their DNA. This includes the apoptosis of primordial follicles, evasion of the G2/M checkpoint and a metaphase arrest induced by DNA damage. Programmed doublestrand breaks (DSBs) occur in foetal life during meiotic recombination and pose a potential threat to oocytes if left unrepaired. However, only responses to exogenous sources of DNA damage will be discussed here. R16 J K Collins and K T JonesReproduction (2016) 152 R15-R22www.reproduction-online.org Primordial follicle apoptosis after DNA damageAt birth, the reserve of oocytes has been established and is held within primordial follicles ar...

Mouse oocytes carrying DNA damage arrest in meiosis I, thereby preventing creation of embryos with deleterious mutations. The arrest is dependent on activation of the spindle assembly checkpoint, which results in anaphase-promoting complex (APC) inhibition. However, little is understood about how this checkpoint is engaged following DNA damage. Here, we find that within minutes of DNA damage checkpoint proteins are assembled at the kinetochore, not at damage sites along chromosome arms, such that the APC is fully inhibited within 30 min. Despite this robust response, there is no measurable loss in k-fibres, or tension across the bivalent. Through pharmacological inhibition we observed that the response is dependent on Mps1 kinase, aurora kinase and Haspin. Using oocyte-specific knockouts we find the response does not require the DNA damage response kinases ATM or ATR. Furthermore, checkpoint activation does not occur in response to DNA damage in fully mature eggs during meiosis II, despite the divisions being separated by just a few hours. Therefore, mouse oocytes have a unique ability to sense DNA damage rapidly by activating the checkpoint at their kinetochores.

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