Enhancing survival of mouse oocytes following chemotherapy or aging by targeting Bax and Rad51

The DNA double-strand break repair proteins γH2AX, RAD51, BRCA1, RPA70, KU80, and XRCC4 exhibit follicle-specific expression differences in the postnatal mouse ovaries from early to older ages

PURPOSE

Ovarian aging is closely related to a decrease in follicular reserve and oocyte quality. The precise molecular mechanisms underlying these reductions have yet to be fully elucidated. Herein, we examine spatiotemporal distribution of key proteins responsible for DNA double-strand break (DSB) repair in ovaries from early to older ages. Functional studies have shown that the γH2AX, RAD51, BRCA1, and RPA70 proteins play indispensable roles in HR-based repair pathway, while the KU80 and XRCC4 proteins are essential for successfully operating cNHEJ pathway.

METHODS

Female Balb/C mice were divided into five groups as follows: Prepuberty (3 weeks old; n = 6), puberty (7 weeks old; n = 7), postpuberty (18 weeks old; n = 7), early aged (52 weeks old; n = 7), and late aged (60 weeks old; n = 7). The expression of DSB repair proteins, cellular senescence (β-GAL) and apoptosis (cCASP3) markers was evaluated in the ovaries using immunohistochemistry.

RESULT

β-GAL and cCASP3 levels progressively increased from prepuberty to aged groups (P < 0.05). Notably, γH2AX levels varied in preantral and antral follicles among the groups (P < 0.05). In aged groups, RAD51, BRCA1, KU80, and XRCC4 levels increased (P < 0.05), while RPA70 levels decreased (P < 0.05) compared to the other groups.

CONCLUSIONS

The observed alterations were primarily attributed to altered expression in oocytes and granulosa cells of the follicles and other ovarian cells. As a result, the findings indicate that these DSB repair proteins may play a role in the repair processes and even other related cellular events in ovarian cells from early to older ages.

Effects of Endocrine Interventions Targeting ERα or PR on Breast Cancer Risk in the General Population and Carriers of BRCA1/2 Pathogenic Variants

There is evidence suggesting that endocrine interventions such as hormone replacement therapy and hormonal contraception can increase breast cancer (BC) risk. Sexual steroid hormones like estrogens have long been known for their adverse effects on BC development and progression via binding to estrogen receptor (ER) α. Thus, in recent years, endocrine interventions that include estrogens have been discussed more and more critically, and their impact on different BC subgroups has increasingly gained interest. Carriers of pathogenic variants in BRCA1/2 genes are known to have a high risk of developing BC and ovarian cancer. However, there remain open questions to what extent endocrine interventions targeting ERα or the progesterone receptor further increase cancer risk in this subgroup. This review article aims to provide an overview and update on the effects of endocrine interventions on breast cancer risk in the general population in comparison to BRCA1/2 mutation carriers. Finally, future directions of research are addressed, to further improve the understanding of the effects of endocrine interventions on high-risk pathogenic variant carriers.

Research progress on the prevention and treatment of chemotherapy-induced ovarian damage

Chemotherapy is a main treatment option for malignant tumors, but it may cause various adverse effects, including dysfunction of female endocrine and fertility. Chemotherapy-induced ovarian damage has been concerned, apart from ovarian preservation, the prevention and treatment of ovarian dysfunction are widely studied. In this article, the mechanisms of ovarian injury caused by chemotherapy, including the apoptosis of follicle and supporting cells, follicle "burn out", ovarian stromal and microvascular damage; and influencing factors, including age at diagnosis and initial low pre-treatment anti-Müllerian hormone levels, toxicity, dose and regimen of chemotherapy drugs are reviewed based on the latest research results and clinical practice. The article also discusses measures and frontier therapies for prevention and treatment of ovarian injury, including the application of gonadotropin releasing hormone agonists or antagonists, tyrosine kinase inhibitors, antioxidants, sphingosine-1-phosphate, ceramide-1-phosphate, mammalian target of rapamycin inhibitors, granulocyte-colony stimulating factor, stem cell therapy and artificial ovary, etc.

Genome Instability and DNA Repair in Somatic and Reproductive Aging

Genetic material is constantly subjected to genotoxic insults and is critically dependent on DNA repair. Genome maintenance mechanisms differ in somatic and germ cells as the soma only requires maintenance during an individual's lifespan, while the germline indefinitely perpetuates its genetic information. DNA lesions are recognized and repaired by mechanistically highly diverse repair machineries. The DNA damage response impinges on a vast array of homeostatic processes and can ultimately result in cell fate changes such as apoptosis or cellular senescence. DNA damage causally contributes to the aging process and aging-associated diseases, most prominently cancer. By causing mutations, DNA damage in germ cells can lead to genetic diseases and impact the evolutionary trajectory of a species. The mechanisms ensuring tight control of germline DNA repair could be highly instructive in defining strategies for improved somatic DNA repair. They may provide future interventions to maintain health and prevent disease during aging.

The Dominant Mechanism of Cyclophosphamide-Induced Damage to Ovarian Reserve: Premature Activation or Apoptosis of Primordial Follicles?

Cyclophosphamide (CPM), a part of most cancer treatment regimens, has demonstrated high gonadal toxicity in females. Initially, CPM is believed to damage the ovarian reserve by premature activation of primordial follicles, for the fact that facing CPM damage, primordial oocytes show the activation of PTEN/PI3K/AKT pathways, accompanied by accelerated activation of follicle developmental waves. Meanwhile, primordial follicles are dormant and not considered the target of CPM. However, many researchers have found DNA DSBs and apoptosis within primordial oocytes under CPM-induced ovarian damage instead of premature accelerated activation. A stricter surveillance system of DNA damage is also thought to be in primordial oocytes. So far, the apoptotic death mechanism is considered well-proved, but the premature activation theory is controversial and unacceptable. The connection between the upregulation of PTEN/PI3K/AKT pathways and DNA DSBs and apoptosis within primordial oocytes is also unclear. This review aims to highlight the flaw and/or support of the disputed premature activation theory and the apoptosis mechanism to identify the underlying mechanism of CPM's injury on ovarian reserve, which is crucial to facilitate the discovery and development of effective ovarian protectants. Ultimately, this review finds no good evidence for follicle activation and strong consistent evidence for apoptosis.

Molecular cloning and expression analysis of rad51 gene associated with gametogenesis in Chinese soft-shell turtle (Pelodiscus sinensis)

Rad51 is a recA-like recombinase that plays a crucial role in repairing DNA double-strand breaks through homologous recombination during mitosis and meiosis in mammals and other organisms. However, its role in reptiles remains largely unclear. In this study, we aimed to investigate the physiological role of the rad51 gene in reptiles, particularly in Pelodiscus sinensis. Firstly, the cDNA of rad51 gene was cloned and analyzed in P. sinensis. The cloned cDNA contained an open reading frame (ORF) of 1020 bp and encodeed a peptide of 339 amino acids. The multiple alignments and phylogenetic tree analysis of Rad51 showed that P. sinensis shares the high identity with Chelonia mydas (97.95%) and Mus musculus (95.89%). Secondly, reverse transcription-polymerase chain reaction (RT-PCR) and real-time quantitative polymerase chain reaction (RT-qPCR) analysis showed that rad51 mRNA was highly expressed in both ovary and testis, while being weak in the somatic tissues examined in this study. Furthermore, chemical in situ hybridization (CISH) was performed to examine the expression profile of rad51 mRNA in germ cells at different stages. In the testis, rad51 mRNA expression was found to be stronger in the germ cells at early stages, specifically in spermatogonia and spermatocytes, but it was undetectable in spermatids. In the ovary, rad51 mRNA exhibited a uniform distribution in the cytoplasm of oocytes at early stages. The signal intensity of rad51 mRNA was highest in primary oocytes and gradually declined during oogenesis as the oocytes developed. These results suggest that rad51 plays a vital role in the development of germ cells, particularly during the early stages of gametogenesis in P. sinensis. The dynamic expression pattern of rad51 mRNA provides insights into the mechanisms underlying germ cell development and differentiation into gametes in turtles, even in reptiles.

Beyond apoptosis: evidence of other regulated cell death pathways in the ovary throughout development and life

BACKGROUND

Regulated cell death is a fundamental component of numerous physiological processes; spanning from organogenesis in utero, to normal cell turnover during adulthood, as well as the elimination of infected or damaged cells throughout life. Quality control through regulation of cell death pathways is particularly important in the germline, which is responsible for the generation of offspring. Women are born with their entire supply of germ cells, housed in functional units known as follicles. Follicles contain an oocyte, as well as specialized somatic granulosa cells essential for oocyte survival. Follicle loss-via regulated cell death-occurs throughout follicle development and life, and can be accelerated following exposure to various environmental and lifestyle factors. It is thought that the elimination of damaged follicles is necessary to ensure that only the best quality oocytes are available for reproduction.

OBJECTIVE AND RATIONALE

Understanding the precise factors involved in triggering and executing follicle death is crucial to uncovering how follicle endowment is initially determined, as well as how follicle number is maintained throughout puberty, reproductive life, and ovarian ageing in women. Apoptosis is established as essential for ovarian homeostasis at all stages of development and life. However, involvement of other cell death pathways in the ovary is less established. This review aims to summarize the most recent literature on cell death regulators in the ovary, with a particular focus on non-apoptotic pathways and their functions throughout the discrete stages of ovarian development and reproductive life.

SEARCH METHODS

Comprehensive literature searches were carried out using PubMed and Google Scholar for human, animal, and cellular studies published until August 2022 using the following search terms: oogenesis, follicle formation, follicle atresia, oocyte loss, oocyte apoptosis, regulated cell death in the ovary, non-apoptotic cell death in the ovary, premature ovarian insufficiency, primordial follicles, oocyte quality control, granulosa cell death, autophagy in the ovary, autophagy in oocytes, necroptosis in the ovary, necroptosis in oocytes, pyroptosis in the ovary, pyroptosis in oocytes, parthanatos in the ovary, and parthanatos in oocytes.

OUTCOMES

Numerous regulated cell death pathways operate in mammalian cells, including apoptosis, autophagic cell death, necroptosis, and pyroptosis. However, our understanding of the distinct cell death mediators in each ovarian cell type and follicle class across the different stages of life remains the source of ongoing investigation. Here, we highlight recent evidence for the contribution of non-apoptotic pathways to ovarian development and function. In particular, we discuss the involvement of autophagy during follicle formation and the role of autophagic cell death, necroptosis, pyroptosis, and parthanatos during follicle atresia, particularly in response to physiological stressors (e.g. oxidative stress).

WIDER IMPLICATIONS

Improved knowledge of the roles of each regulated cell death pathway in the ovary is vital for understanding ovarian development, as well as maintenance of ovarian function throughout the lifespan. This information is pertinent not only to our understanding of endocrine health, reproductive health, and fertility in women but also to enable identification of novel fertility preservation targets.

The DNA Damage Response in Fully Grown Mammalian Oocytes

DNA damage in cells can occur physiologically or may be induced by exogenous factors. Genotoxic damage may cause cancer, ageing, serious developmental diseases and anomalies. If the damage occurs in the germline, it can potentially lead to infertility or chromosomal and genetic aberrations in the developing embryo. Mammalian oocytes, the female germ cells, are produced before birth, remaining arrested at the prophase stage of meiosis over a long period of time. During this extensive state of arrest the oocyte may be exposed to different DNA-damaging insults for months, years or even decades. Therefore, it is of great importance to understand how these cells respond to DNA damage. In this review, we summarize the most recent developments in the understanding of the DNA damage response mechanisms that function in fully grown mammalian oocytes.

Local DNA synthesis is critical for DNA repair during oocyte maturation

Mammalian oocytes can be very long-lived cells and thereby are very likely to encounter DNA damage during their lifetime. Defective DNA repair may result in oocytes that are developmentally incompetent or give rise to progeny with congenital disorders. During oocyte maturation, damaged DNA is repaired primarily by non-homologous end joining (NHEJ) or homologous recombination (HR). Although these repair pathways have been studied extensively, the associated DNA synthesis is poorly characterized. Here, using porcine oocytes, we demonstrate that the DNA synthesis machinery is present during oocyte maturation and dynamically recruited to sites of DNA damage. DNA polymerase δ is identified as being crucial for oocyte DNA synthesis. Furthermore, inhibiting synthesis causes DNA damage to accumulate and delays the progression of oocyte maturation. Importantly, inhibition of the spindle assembly checkpoint (SAC) bypassed the delay of oocyte maturation caused by DNA synthesis inhibition. Finally, we found that ∼20% of unperturbed oocytes experienced spontaneously arising damage during maturation. Cumulatively, our findings indicate that oocyte maturation requires damage-associated DNA synthesis that is monitored by the SAC. This article has an associated First Person interview with the first author of the paper.

The cyto-protective effects of LH on ovarian reserve and female fertility during exposure to gonadotoxic alkylating agents in an adult mouse model

STUDY QUESTION

Does LH protect mouse oocytes and female fertility from alkylating chemotherapy?

SUMMARY ANSWER

LH treatment before and during chemotherapy prevents detrimental effects on follicles and reproductive lifespan.

WHAT IS KNOWN ALREADY

Chemotherapies can damage the ovary, resulting in premature ovarian failure and reduced fertility in cancer survivors. LH was recently suggested to protect prepubertal mouse follicles from chemotoxic effects of cisplatin treatment.

STUDY DESIGN, SIZE, DURATION

This experimental study investigated LH effects on primordial follicles exposed to chemotherapy. Seven-week-old CD-1 female mice were randomly allocated to four experimental groups: Control (n = 13), chemotherapy (ChT, n = 15), ChT+LH-1x (n = 15), and ChT+LH-5x (n = 8). To induce primary ovarian insufficiency (POI), animals in the ChT and ChT+LH groups were intraperitoneally injected with 120 mg/kg of cyclophosphamide and 12 mg/kg of busulfan, while control mice received vehicle. For LH treatment, the ChT+LH-1x and ChT+LH-5x animals received a 1 or 5 IU LH dose, respectively, before chemotherapy, then a second LH injection administered with chemotherapy 24 h later. Then, two animals/group were euthanized at 12 and 24 h to investigate the early ovarian response to LH, while remaining mice were housed for 30 days to evaluate short- and long-term reproductive outcomes. The effects of LH and chemotherapy on growing-stage follicles were analyzed in a parallel experiment. Seven-week-old NOD-SCID female mice were allocated to control (n = 5), ChT (n = 5), and ChT+LH-1x (n = 6) groups. Animals were treated as described above, but maintained for 7 days before reproductive assessment.

PARTICIPANTS/MATERIALS, SETTING, METHODS

In the first experiment, follicular damage (phosphorylated H2AX histone (γH2AX) staining and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay), apoptotic biomarkers (western blot), and DNA repair pathways (western blot and RT-qPCR) were assessed in ovaries collected at 12 and 24 h to determine early ovarian responses to LH. Thirty days after treatments, remaining mice were stimulated (10 IU of pregnant mare serum gonadotropin (PMSG) and 10 IU of hCG) and mated to collect ovaries, oocytes, and embryos. Histological analysis was performed on ovarian samples to investigate follicular populations and stromal status, and meiotic spindle and chromosome alignment was measured in oocytes by confocal microscopy. Long-term effects were monitored by assessing pregnancy rate and litter size during six consecutive breeding attempts. In the second experiment, mice were stimulated and mated 7 days after treatments and ovaries, oocytes, and embryos were collected. Follicular numbers, follicular protection (DNA damage and apoptosis by H2AX staining and TUNEL assay, respectively), and ovarian stroma were assessed. Oocyte quality was determined by confocal analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

LH treatment was sufficient to preserve ovarian reserve and follicular development, avoid atresia, and restore ovulation and meiotic spindle configuration in mature oocytes exposed at the primordial stage. LH improved the cumulative pregnancy rate and litter size in six consecutive breeding rounds, confirming the potential of LH treatment to preserve fertility. This protective effect appeared to be mediated by an enhanced early DNA repair response, via homologous recombination, and generation of anti-apoptotic signals in the ovary a few hours after injury with chemotherapy. This response ameliorated the chemotherapy-induced increase in DNA-damaged oocytes and apoptotic granulosa cells. LH treatment also protected growing follicles from chemotherapy. LH reversed the chemotherapy-induced depletion of primordial and primary follicular subpopulations, reduced oocyte DNA damage and granulosa cell apoptosis, restored mature oocyte cohort size, and improved meiotic spindle properties.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This was a preliminary study performed with mouse ovarian samples. Therefore, preclinical research with human samples is required for validation.

WIDER IMPLICATIONS OF THE FINDINGS

The current study tested if LH could protect the adult mouse ovarian reserve and reproductive lifespan from alkylating chemotherapy. These findings highlight the therapeutic potential of LH as a complementary non-surgical strategy for preserving fertility in female cancer patients.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by grants from the Regional Valencian Ministry of Education (PROMETEO/2018/137), the Spanish Ministry of Science and Innovation (CP19/00141), and the Spanish Ministry of Education, Culture and Sports (FPU16/05264). The authors declare no conflict of interest.

Expression of Rad51 and the histo-morphological evaluation of testis of the sterile male cattle-yak

Meiotic recombination is key to the repair of DNA double-strand break damage, provide a link between homologs for proper chromosome segregation as well as ensure genetic diversity in organisms. Defects in recombination often lead to sterility. The ubiquitously expressed Rad51 and the meiosis-specific DMC1 are two closely related recombinases that catalyze the key strand invasion and exchange step of meiotic recombination. This study cloned and sequenced the coding region of cattle-yak Rad51 and determined its mRNA and protein expression levels, evaluated its molecular and evolutionary relationship as well as evaluated the histo-morphological structure of testes in the yellow cattle, yak and the sterile cattle-yak hybrid. The Rad51 gene was amplified using PCR, cloned and sequenced using testicular cDNA from yak and cattle-yak. Real-time PCR was used to examine the expression levels of Rad51/DMC1 mRNA in the cattle, yak and cattle-yak testis while western blotting, immunofluorescence and immunohistochemistry were used to assess the protein expression and localization of Rad51/DMC1 protein in the testicular tissue sections. The results revealed that the mRNA and protein expression of Rad51 and DMC1 are extremely low in the male cattle-yak testis with a corresponding higher incidence of germ cell apoptosis. There was also thinning of the germinal epithelium possibly due to the depletion of the germ cells leading to the widening of the lumen area of the cattle-yak seminiferous tubule. Our findings provide support for the hypothesis that the low expression of Rad51 and DMC1 may contribute to the male hybrid sterility in the cattle-yak.

MUC3A promotes non-small cell lung cancer progression via activating the NFκB pathway and attenuates radiosensitivity

Mucin 3A (MUC3A) is highly expressed in non-small cell lung cancer (NSCLC), but its functions and effects on clinical outcomes are not well understood. Tissue microarray of 92 NSCLC samples indicated that high levels of MUC3A were associated with poor prognosis, advanced staging, and low differentiation. MUC3A knockdown significantly suppressed NSCLC cell proliferation and induced G1/S accumulation via downregulating cell cycle checkpoints. MUC3A knockdown also inhibited tumor growth in vivo and had synergistic effects with radiation. MUC3A knockdown increased radiation-induced DNA double strain breaks and γ-H2AX phosphorylation in NSCLC cells. MUC3A downregulation inhibited the BRCA-1/RAD51 pathway and nucleus translocation of P53 and XCRR6, suggesting that MUC3A promoted DNA damage repair and attenuated radiation sensitivity. MUC3A knockdown also resulted in less nucleus translocation of RELA and P53 in vivo. Immunoprecipitation revealed that MUC3A interacted with RELA and activated the NFκB pathway via promoting RELA phosphorylation and interfering the binding of RELA to IκB. Our studies indicated that MUC3A was a potential oncogene and associated with unfavorable clinical outcomes. NSCLC patients with a high MUC3A level, who should be more frequent follow-up and might benefit less from radiotherapy.

Female ageing affects the DNA repair capacity of oocytes in IVF using a controlled model of sperm DNA damage in mice

STUDY QUESTION

Does female ageing have a negative effect on the DNA repair capacity of oocytes fertilised by spermatozoa with controlled levels of DNA damage?

SUMMARY ANSWER

Compared to oocytes from younger females, oocytes from older females have a reduced capacity to repair damaged DNA introduced by spermatozoa.

WHAT IS KNOWN ALREADY

The reproductive lifespan in women declines with age predominantly due to poor oocyte quality. This leads to decreased reproductive outcomes for older women undergoing assisted reproductive technology (ART) treatments, compared to young women. Ageing and oocyte quality have been clearly associated with aneuploidy, but the range of factors that influence this change in oocyte quality with age remains unclear. The DNA repair activity prior to embryonic genomic activation is considered to be of maternal origin, with maternal transcripts and proteins controlling DNA integrity. With increasing maternal age, the number of mRNAs stored in oocytes decreases. This could result in diminished efficiency of DNA repair and/or negative effects on embryo development, especially in the presence of DNA damage.

STUDY DESIGN, SIZE, DURATION

Oocytes from two age groups of 30 super-ovulated female mice (young: 5-8 weeks old, n = 15; old: 42-45 weeks old, n = 15) were inseminated with sperm from five males with three different controlled DNA damage levels; control: ≤10%, 1 Gray (Gy): 11-30%, and 30 Gy: >30%. Inseminated oocytes (young: 125, old: 78) were assessed for the formation of zygotes (per oocyte) and blastocysts (per zygote). Five replicates of five germinal vesicles (GVs) and five MII oocytes from each age group were analysed for gene expression. The DNA damage response (DDR) was assessed in a minimum of three IVF replicates in control and 1 Gy zygotes and two-cell embryos using γH2AX labelling.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Swim-up sperm samples from the cauda epididymidis of C57BL6 mice were divided into control (no irradiation) and 1- and 30-Gy groups. Treated spermatozoa were irradiated at 1 and 30 Gy, respectively, using a linear accelerator Varian 21iX. Following irradiation, samples were used for DNA damage assessment (Halomax) and for insemination. Presumed zygotes were cultured in a time-lapse incubator (MIRI, ESCO). Gene expression of 91 DNA repair genes was assessed using the Fluidigm Biomark HD system. The DNA damage response in zygotes (6-8 h post-fertilisation) and two-cell embryos (22-24 h post-fertilisation) was assessed by immunocytochemical analysis of γH2AX using confocal microscopy (Olympus FV1200) and 3D volumetric analysis using IMARIS software.

MAIN RESULTS AND THE ROLE OF CHANCE

The average sperm DNA damage for the three groups was statistically different (control: 6.1%, 1 Gy: 16.1%, 30 Gy: 53.1%, P < 0.0001), but there were no significant differences in fertilisation rates after IVF within or between the two age groups [(young; control: 86.79%, 1 Gy: 82.75%, 30 Gy: 76.74%) (old; control: 93.1%, 1 Gy: 70.37%, 30 Gy: 68.18%) Fisher's exact]. However, blastocyst rates were significantly different (P < 0.0001) among the groups [(young; control: 86.95%, 1 Gy: 33.33%, 30 Gy: 0.0%) (old; control: 70.37%, 1 Gy: 0.0%, 30 Gy: 0.0%)]. Between the age groups, 1-Gy samples showed a significant decrease in the blastocyst rate in old females compared to young females (P = 0.0166). Gene expression analysis revealed a decrease in relative expression of 21 DNA repair genes in old GV oocytes compared to young GV oocytes (P < 0.05), and similarly, old MII oocytes showed 23 genes with reduced expression compared to young MII oocytes (P < 0.05). The number of genes with decreased expression in older GV and MII oocytes significantly affected pathways such as double strand break (GV: 5; MII: 6), nucleotide excision repair (GV: 8; MII: 5) and DNA damage response (GV: 4; MII: 8). There was a decreased DDR in zygotes and in two-cell embryos from old females compared to young regardless of sperm treatment (P < 0.05). The decrease in DNA repair gene expression of oocytes and decreased DDR in embryos derived from older females suggests that ageing results in a diminished DNA repair capacity.

LARGE-SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Ionising radiation was used only for experimental purposes, aiming at controlled levels of sperm DNA damage; however, it can also damage spermatozoa proteins. The female age groups selected in mice were intended to model effects in young and old women, but clinical studies are required to demonstrate a similar effect.

WIDER IMPLICATIONS OF THE FINDINGS

Fertilisation can occur with sperm populations with medium and high DNA damage, but subsequent embryo growth is affected to a greater extent with aging females, supporting the theory that oocyte DNA repair capacity decreases with age. Assessment of the oocyte DNA repair capacity may be a useful diagnostic tool for infertile couples.

STUDY FUNDING/COMPETING INTEREST(S)

Funded by the Education Program in Reproduction and Development, Department of Obstetrics and Gynaecology, Monash University. None of the authors has any conflict of interest to report.

DNA repair in primordial follicle oocytes following cisplatin treatment

PURPOSE

Genotoxic chemotherapy and radiotherapy can cause DNA double stranded breaks (DSBs) in primordial follicle (PMF) oocytes, which then undergo apoptosis. The development of effective new fertility preservation agents has been hampered, in part, by a limited understanding of DNA repair in PMF oocytes. This study investigated the induction of classical DSB repair pathways in the follicles of wild type (WT) and apoptosis-deficient Puma^-/- mice in response to DSBs caused by the chemotherapy agent cisplatin.

METHODS

Adult C57BL/6 WT and Puma^-/- mice were injected i.p. with saline or cisplatin (5 mg/kg); ovaries were harvested at 8 or 24 h. Follicles were counted, and H2A histone family member (γH2AX) immunofluorescence used to demonstrate DSBs. DNA repair protein RAD51 homolog 1 (RAD51) and DNA-dependent protein kinase, catalytic subunit (DNA-PKcs) immunofluorescence were used to identify DNA repair pathways utilised.

RESULTS

Puma^-/- mice retained 100% of follicles 24 h after cisplatin treatment. Eight hours post-treatment, γH2AX immunofluorescence showed DSBs across follicular stages in Puma^-/- mice; staining returned to control levels in PMFs within 5 days, suggesting repair of PMF oocytes in this window. RAD51 immunofluorescence eight hours post-cisplatin was positive in damaged cell types in both WT and Puma^-/- mice, demonstrating induction of the homologous recombination pathway. In contrast, DNA-PKcs staining were rarely observed in PMFs, indicating non-homologous end joining plays an insignificant role.

CONCLUSION

PMF oocytes are able to conduct high-fidelity repair of DNA damage accumulated during chemotherapy. Therefore, apoptosis inhibition presents a viable strategy for fertility preservation in women undergoing treatment.

Oocytes mount a noncanonical DNA damage response involving APC-Cdh1-mediated proteolysis

In mitotic cells, DNA damage induces temporary G2 arrest via inhibitory Cdk1 phosphorylation. In contrast, fully grown G2-stage oocytes readily enter M phase immediately following chemical induction of DNA damage in vitro, indicating that the canonical immediate-response G2/M DNA damage response (DDR) may be deficient. Senataxin (Setx) is involved in RNA/DNA processing and maintaining genome integrity. Here we find that mouse oocytes deleted of Setx accumulate DNA damage when exposed to oxidative stress in vitro and during aging in vivo, after which, surprisingly, they undergo G2 arrest. Moreover, fully grown wild-type oocytes undergo G2 arrest after chemotherapy-induced in vitro damage if an overnight delay is imposed following damage induction. Unexpectedly, this slow-evolving DDR is not mediated by inhibitory Cdk1 phosphorylation but by APC-Cdh1–mediated proteolysis of the Cdk1 activator, cyclin B1, secondary to increased Cdc14B-dependent APC-Cdh1 activation and reduced Emi1-dependent inhibition. Thus, oocytes are unable to respond immediately to DNA damage, but instead mount a G2/M DDR that evolves slowly and involves a phosphorylation-independent proteolytic pathway.

Emerging roles for noncoding RNAs in female sex steroids and reproductive disease

The "central dogma" of molecular biology, that is, that DNA blueprints encode messenger RNAs which are destined for translation into protein, has been challenged in recent decades. In actuality, a significant portion of the genome encodes transcripts that are transcribed into functional RNA. These noncoding RNAs (ncRNAs), which are not transcribed into protein, play critical roles in a wide variety of biological processes. A growing body of evidence derived from mouse models and human data demonstrates that ncRNAs are dysregulated in various reproductive pathologies, and that their expression is essential for female gametogenesis and fertility. Yet in many instances it is unclear how dysregulation of ncRNA expression leads to a disease process. In this review, we highlight new observations regarding the roles of ncRNAs in the pathogenesis of disordered female steroid hormone production and disease, with an emphasis on long noncoding RNAs (lncRNAs) and microRNAs (miRNAs). We will focus our discussion in the context of three ovarian disorders which are characterized in part by altered steroid hormone biology - diminished ovarian reserve, premature ovarian insufficiency, and polycystic ovary syndrome. We will also discuss the limitations and challenges faced in studying noncoding RNAs and sex steroid hormone production. An enhanced understanding of the role of ncRNAs in sex hormone regulatory networks is essential in order to advance the development of potential diagnostic markers and therapeutic targets for diseases, including those in reproductive health. Our deepened understanding of ncRNAs has the potential to uncover new applications and therapies; however, in many cases, the next steps will involve distinguishing critical ncRNAs from those which are merely changing in response to a particular disease state, or which are altogether unrelated to disease pathophysiology.

Features of DNA Repair in the Early Stages of Mammalian Embryonic Development

Cell repair machinery is responsible for protecting the genome from endogenous and exogenous effects that induce DNA damage. Mutations that occur in somatic cells lead to dysfunction in certain tissues or organs, while a violation of genomic integrity during the embryonic period often leads to death. A mammalian embryo’s ability to respond to damaged DNA and repair it, as well as its sensitivity to specific lesions, is still not well understood. In this review, we combine disparate data on repair processes in the early stages of preimplantation development in mammalian embryos.

Recent advances in understanding primary ovarian insufficiency

Primary ovarian insufficiency (POI) is an uncommon yet devastating occurrence that results from a premature depletion of the ovarian pool of primordial follicles. Our understanding of both putative and plausible mechanisms underlying POI, previously considered to be largely "idiopathic", has been furthered over the past several years, largely due to advances in the field of genetics and through expansion of translational models for experimental research. In this review, our goal is to familiarize the multidisciplinary readers of the F1000 platform with the strides made in the field of reproductive medicine that hold both preventative and therapeutic implications for those women who are at risk for or who have POI.

BRCA-related ATM-mediated DNA double-strand break repair and ovarian aging

BACKGROUND

Oocyte aging has significant clinical consequences, and yet no treatment exists to address the age-related decline in oocyte quality. The lack of progress in the treatment of oocyte aging is due to the fact that the underlying molecular mechanisms are not sufficiently understood. BRCA1 and 2 are involved in homologous DNA recombination and play essential roles in ataxia telangiectasia mutated (ATM)-mediated DNA double-strand break (DSB) repair. A growing body of laboratory, translational and clinical evidence has emerged within the past decade indicating a role for BRCA function and ATM-mediated DNA DSB repair in ovarian aging.

OBJECTIVE AND RATIONALE

Although there are several competing or complementary theories, given the growing evidence tying BRCA function and ATM-mediated DNA DSB repair mechanisms in general to ovarian aging, we performed this review encompassing basic, translational and clinical work to assess the current state of knowledge on the topic. A clear understanding of the mechanisms underlying oocyte aging may result in targeted treatments to preserve ovarian reserve and improve oocyte quality.

SEARCH METHODS

We searched for published articles in the PubMed database containing key words, BRCA, BRCA1, BRCA2, Mutations, Fertility, Ovarian Reserve, Infertility, Mechanisms of Ovarian Aging, Oocyte or Oocyte DNA Repair, in the English-language literature until May 2019. We did not include abstracts or conference proceedings, with the exception of our own.

OUTCOMES

Laboratory studies provided robust and reproducible evidence that BRCA1 function and ATM-mediated DNA DSB repair, in general, weakens with age in oocytes of multiple species including human. In both women with BRCA mutations and BRCA-mutant mice, primordial follicle numbers are reduced and there is accelerated accumulation of DNA DSBs in oocytes. In general, women with BRCA1 mutations have lower ovarian reserves and experience earlier menopause. Laboratory evidence also supports critical role for BRCA1 and other ATM-mediated DNA DSB repair pathway members in meiotic function. When laboratory, translational and clinical evidence is considered together, BRCA-related ATM-mediated DNA DSB repair function emerges as a likely regulator of ovarian aging. Moreover, DNA damage and repair appear to be key features in chemotherapy-induced ovarian aging.

WIDER IMPLICATIONS

The existing data suggest that the BRCA-related ATM-mediated DNA repair pathway is a strong candidate to be a regulator of oocyte aging, and the age-related decline of this pathway likely impairs oocyte health. This knowledge may create an opportunity to develop targeted treatments to reverse or prevent physiological or chemotherapy-induced oocyte aging. On the immediate practical side, women with BRCA or similar mutations may need to be specially counselled for fertility preservation.

Oocytes can efficiently repair DNA double-strand breaks to restore genetic integrity and protect offspring health

Female fertility and offspring health are critically dependent on an adequate supply of high-quality oocytes, the majority of which are maintained in the ovaries in a unique state of meiotic prophase arrest. While mechanisms of DNA repair during meiotic recombination are well characterized, the same is not true for prophase-arrested oocytes. Here we show that prophase-arrested oocytes rapidly respond to γ-irradiation-induced DNA double-strand breaks by activating Ataxia Telangiectasia Mutated, phosphorylating histone H2AX, and localizing RAD51 to the sites of DNA damage. Despite mobilizing the DNA repair response, even very low levels of DNA damage result in the apoptosis of prophase-arrested oocytes. However, we show that, when apoptosis is inhibited, severe DNA damage is corrected via homologous recombination repair. The repair is sufficient to support fertility and maintain health and genetic fidelity in offspring. Thus, despite the preferential induction of apoptosis following exogenously induced genotoxic stress, prophase-arrested oocytes are highly capable of functionally efficient DNA repair. These data implicate DNA repair as a key quality control mechanism in the female germ line and a critical determinant of fertility and genetic integrity.

Crosstalk between PTEN/PI3K/Akt Signalling and DNA Damage in the Oocyte: Implications for Primordial Follicle Activation, Oocyte Quality and Ageing

The preservation of genome integrity in the mammalian female germline from primordial follicle arrest to activation of growth to oocyte maturation is fundamental to ensure reproductive success. As oocytes are formed before birth and may remain dormant for many years, it is essential that defence mechanisms are monitored and well maintained. The phosphatase and tensin homolog of chromosome 10 (PTEN)/phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB, Akt) is a major signalling pathway governing primordial follicle recruitment and growth. This pathway also contributes to cell growth, survival and metabolism, and to the maintenance of genomic integrity. Accelerated primordial follicle activation through this pathway may result in a compromised DNA damage response (DDR). Additionally, the distinct DDR mechanisms in oocytes may become less efficient with ageing. This review considers DNA damage surveillance mechanisms and their links to the PTEN/PI3K/Akt signalling pathway, impacting on the DDR during growth activation of primordial follicles, and in ovarian ageing. Targeting DDR mechanisms within oocytes may be of value in developing techniques to protect ovaries against chemotherapy and in advancing clinical approaches to regulate primordial follicle activation.

The Impact of Chemotherapy on the Ovaries: Molecular Aspects and the Prevention of Ovarian Damage

Cancer treatment, such as chemotherapy, induces early ovarian follicular depletion and subsequent infertility. In order to protect gametes from the gonadotoxic effects of chemotherapy, several fertility preservation techniques—such as oocyte or embryo cryopreservation with or without ovarian stimulation, or cryopreservation of the ovarian cortex—should be considered. However, these methods may be difficult to perform, and the future use of cryopreserved germ cells remains uncertain. Therefore, improving the methods currently available and developing new strategies to preserve fertility represent major challenges in the area of oncofertility. Animal and ovarian culture models have been used to decipher the effects of different cytotoxic agents on ovarian function and several theories regarding chemotherapy gonadotoxicity have been raised. For example, cytotoxic agents might (i) have a direct detrimental effect on the DNA of primordial follicles constituting the ovarian reserve and induce apoptosis; (ii) induce a massive growth of dormant follicles, which are then destroyed; or (ii) induce vascular ovarian damage. Thanks to improvements in the understanding of the mechanisms involved, a large number of studies have been carried out to develop molecules limiting the negative impact of chemotherapy on the ovaries.

The capacity of oocytes for DNA repair

Female fertility and offspring health are critically dependent on the maintenance of an adequate supply of high-quality oocytes. Like somatic cells, oocytes are subject to a variety of different types of DNA damage arising from endogenous cellular processes and exposure to exogenous genotoxic stressors. While the repair of intentionally induced DNA double strand breaks in gametes during meiotic recombination is well characterised, less is known about the ability of oocytes to repair pathological DNA damage and the relative contribution of DNA repair to oocyte quality is not well defined. This review will discuss emerging data suggesting that oocytes are in fact capable of efficient DNA repair and that DNA repair may be an important mechanism for ensuring female fertility, as well as the transmission of high-quality genetic material to subsequent generations.

Novel insights into the pathophysiology of chemotherapy-induced damage to the ovary

Cancer is the second leading cause of death in the USA and is considered a public health issue worldwide. Early diagnosis and advancement of treatment modalities contributed to declining mortality rates. Consequently, survival rates increased, leading to a greater interest in maintaining the quality of life after cancer treatment. Overall survival and disease-free survival rates are improved with the use of adjuvant chemotherapy. However, chemotherapy treatment might cause short and long-term side effects for cancer survivors. A special concern of young women diagnosed with cancer is their reproductive potential after chemotherapy. Chemotherapy drugs act by distinct mechanisms in the ovaries. DNA damage of primordial follicle oocytes, leading to chemotherapy-induced apoptosis, was recognized as the principal mechanism responsible for the irreversible decline of the ovarian reserve. The oocyte first attempts to repair DNA damage via the DNA damage repair pathway mediated by ataxia-telangiectasia mutated. Elimination through apoptosis occurs in cells in which DNA damage could not be repaired. In this review, the clinical impact and the major mechanisms of ovarian damage from chemotherapy treatment will be briefly described.

Dacarbazine depletes the ovarian reserve in mice and depletion is enhanced with age

Dacarbazine is commonly administered for the treatment of cancers prevalent in reproductive age females. However, investigations of off-target effects of dacarbazine on the ovary are limited. We assessed the impact of dacarbazine on the ovarian reserve of primordial follicles, essential for fertility. Eight week and 6 month old C57BL/6 J mice were administered with dacarbazine or saline on day (d)0 and d7, then sacrificed after 12 hours (h), or 14d (n = 4-5/group). Follicle numbers, follicle density, serum AMH and corpora lutea were quantified and estrous cyclicity monitored. In reproductively young mice, dacarbazine did not affect primordial follicle numbers at 12 h, but resulted in a 36% reduction at 14d (p < 0.05). Dacarbazine-mediated primordial follicle depletion was accelerated with age, with a 24% (p < 0.05) and 36% (p < 0.01) reduction at 12 h and 14d. Follicle density remained unchanged between treatment groups at either age. Dacarbazine depleted antral follicles at 14d (p < 0.05), at both ages. Despite partial reduction of antral follicles, serum AMH, estrous cyclicity and corpora lutea (indicative of ovulation) remained unchanged between treatment groups, at both ages. Importantly, diminished ovarian reserve can result in premature ovarian insufficiency and infertility, thus, fertility preservation options should be considered for young female patients prior to dacarbazine treatment.

The importance of DNA repair for maintaining oocyte quality in response to anti-cancer treatments, environmental toxins and maternal ageing

BACKGROUND

Within the ovary, oocytes are stored in long-lived structures called primordial follicles, each comprising a meiotically arrested oocyte, surrounded by somatic granulosa cells. It is essential that their genetic integrity is maintained throughout life to ensure that high quality oocytes are available for ovulation. Of all the possible types of DNA damage, DNA double-strand breaks (DSBs) are considered to be the most severe. Recent studies have shown that DNA DSBs can accumulate in oocytes in primordial follicles during reproductive ageing, and are readily induced by exogenous factors such as γ-irradiation, chemotherapy and environmental toxicants. DSBs can induce oocyte death or, alternatively, activate a program of DNA repair in order to restore genetic integrity and promote survival. The repair of DSBs has been intensively studied in the context of meiotic recombination, and in recent years more detail is becoming available regarding the repair capabilities of primordial follicle oocytes.

OBJECTIVE AND RATIONALE

This review discusses the induction and repair of DNA DSBs in primordial follicle oocytes.

SEARCH METHODS

PubMed (Medline) and Google Scholar searches were performed using the key words: primordial follicle oocyte, DNA repair, double-strand break, DNA damage, chemotherapy, radiotherapy, ageing, environmental toxicant. The literature was restricted to papers in the English language and limited to reports in animals and humans dated from 1964 until 2017. The references within these articles were also manually searched.

OUTCOMES

Recent experiments in animal models and humans have provided compelling evidence that primordial follicle oocytes can efficiently repair DNA DSBs arising from diverse origins, but this capacity may decline with increasing age.

WIDER IMPLICATIONS

Primordial follicle oocytes are vulnerable to DNA DSBs emanating from endogenous and exogenous sources. The ability to repair this damage is essential for female fertility. In the long term, augmenting DNA repair in primordial follicle oocytes has implications for the development of novel fertility preservation agents for female cancer patients and for the management of maternal ageing. However, further work is required to fully characterize the specific proteins involved and to develop strategies to bolster their activity.

Association Between Impairment of DNA Double Strand Break Repair and Decreased Ovarian Reserve in Patients With Endometriosis

Background: Repair of DNA double strand break (DSB) is an important mechanism for maintaining genetic stability during a DNA damage event. Although, a growing body of recent evidence suggests that DNA DSBs and related repair mechanisms may be important in ovarian aging and in various cancers, there are few reports in endometriosis. We, therefore, examined expression levels of genes pertaining to DNA DSB repair in patients with endometriosis to assess the potential effects on ovarian reserves. Materials and methods: A total of 69 women undergoing laparoscopic surgery for endometriosis and other benign conditions was included; endometriosis group (n = 38) vs. controls (n = 31). DNA DSBs in endometrial and ovarian tissues of both groups were compared via immunohistochemistry, aimed at γ-H2AX expression. To gauge genotoxin-induced DNA DSBs in endometrial stromal cells, γ-H2AX expression was determined by western blot after H₂O₂ treatment of cultured endometrial stromal cells (endometriosis group and controls) and Ishikawa cell-line cultures. Endometrial and ovarian tissue levels of BRCA1, BRCA2, Rad51, and ATM (ataxia-telangiectasia mutated) mRNA expression were also compared. Correlations between expression levels of genes of interest and serum anti-müllerian hormone (AMH) levels were assessed as well. Results: Expression of γ-H2AX in immunostained endometrial and ovarian tissue preparations was greater in the endometriosis group, compared with controls. After H₂O₂ treatment, γ-H2AX expression levels were also significantly greater in cultured stromal cells of the endometriosis group and in the Ishikawa cell line than in controls. Endometrial expression of BRCA1 and Rad51 mRNA proved significantly lower in the endometriosis group (vs. controls), as did ovarian expression of BRCA1 and BRCA2 mRNA. Serum AMH concentration showed a significant correlation with ovarian BRCA1 mRNA expression in women with endometriosis (p = 0.03). Conclusions: In women with endometriosis, expression levels of various genes implicated in DSB repair are decreased and ovarian BRCA1 expression correlates with.

The role of Rad51 in safeguarding mitochondrial activity during the meiotic cell cycle in mammalian oocytes

Rad51 is a conserved eukaryotic protein that mediates the homologous recombination repair of DNA double-strand breaks that occur during mitosis and meiosis. In addition, Rad51 promotes mitochondrial DNA synthesis when replication stress is increased. Rad51 also regulates cell cycle progression by preserving the G2/M transition in embryonic stem cells. In this study, we report a novel function of Rad51 in regulating mitochondrial activity during in vitro maturation of mouse oocytes. Suppression of Rad51 by injection of Rad51 dsRNA into germinal vesicle-stage oocytes resulted in arrest of meiosis in metaphase I. Rad51-depleted oocytes showed chromosome misalignment and failures in spindle aggregation, affecting the completion of cytokinesis. We found that Rad51 depletion was accompanied by decreased ATP production and mitochondrial membrane potential and increased DNA degradation. We further demonstrated that the mitochondrial defect activated autophagy in Rad51-depleted oocytes. Taken together, we concluded that Rad51 functions to safeguard mitochondrial integrity during the meiotic maturation of oocytes.

DNA damage response during mouse oocyte maturation

Because low levels of DNA double strand breaks (DSBs) appear not to activate the ATM-mediated prophase I checkpoint in full-grown oocytes, there may exist mechanisms to protect chromosome integrity during meiotic maturation. Using live imaging we demonstrate that low levels of DSBs induced by the radiomimetic drug Neocarzinostatin (NCS) increase the incidence of chromosome fragments and lagging chromosomes but do not lead to APC/C activation and anaphase onset delay. The number of DSBs, represented by γH2AX foci, significantly decreases between prophase I and metaphase II in both control and NCS-treated oocytes. Transient treatment with NCS increases >2-fold the number of DSBs in prophase I oocytes, but less than 30% of these oocytes enter anaphase with segregation errors. MRE11, but not ATM, is essential to detect DSBs in prophase I and is involved in H2AX phosphorylation during metaphase I. Inhibiting MRE11 by mirin during meiotic maturation results in anaphase bridges and also increases the number of γH2AX foci in metaphase II.  Compromised DNA integrity in mirin-treated oocytes indicates a role for MRE11 in chromosome integrity during meiotic maturation.

Comparative proteomic analysis of primordial follicles from ovaries of immature and aged rats

Age related decline in reproductive performance in women is well documented and apoptosis has been considered as one of the reasons for the decline of primordial follicle reserve. Recently we observed a decline in the efficiency of DNA repair ability in aged rat primordial follicles as demonstrated by decreased mRNA levels of DNA repair genes BRCA1 and H2AX. In the present study, a two-dimensional electrophoresis (2DE) proteomic approach was employed to identify differentially expressed proteins in primordial follicles isolated from ovaries of immature (∼20 days) and aged (∼400-450 days) rats. Using MALDI-TOF/TOF MS, we identified 13 differentially expressed proteins (p < 0.05) which included seven up-regulated and six down-regulated proteins in aged primordial follicles. These proteins are involved in a wide range of biological functions including apoptosis, DNA repair, and the immune system. Interestingly, the differentially expressed proteins such as FIGNL1 (DNA repair) and BOK (apoptotic protein) have not been previously reported in the rat primordial follicles and these proteins can be related to some common features of ovarian aging such as loss of follicle reserve and genome integrity. The quantitative differences of two important proteins BOK and FIGNL1 observed by the proteomic analysis were correlated with the transcript levels, as determined by semi-quantitative RT-PCR. Our results improve the current knowledge about protein factors associated with molecular changes in rat primordial follicles as a function of aging and our understanding of the proteomic processes involved in degenerative changes observed in aging primordial follicles.

Optimizing outcomes from ovarian tissue cryopreservation and transplantation; activation versus preservation

Ovarian tissue cryopreservation and transplantation (OTCP) is gaining increasing traction in the field of fertility preservation as a result of accumulated successes. We now have a decade of experience with the technique, with tens of live births and greater than 90% return of ovarian function in graft recipients. Recently, a novel method of OTCP has been described, termed in vitro activated OTCP which proposes significant changes to the standard protocol. This method aims to stimulate activation of dormant follicles within the grafts prior to transplantation and ensure that mature oocytes can be generated in the immediate short term after transplantation. By contrast, conventional OTCP seeks to maintain dormancy and thus preserve the follicle reserve in the graft with the aim of maximizing graft lifespan. This opinion paper will compare the two methods of OTCP, highlighting their respective advantages and disadvantages, and provide suggestions as to when to apply either one of these methods in a clinical setting.

Prevention of chemotherapy-induced ovarian damage: possible roles for hormonal and non-hormonal attenuating agents

BACKGROUND

Current options for female fertility preservation in the face of cytotoxic treatments include embryo, oocyte and ovarian tissue cryopreservation. However these methods are limited by the patient age, status or available timeframe before treatment and they necessitate invasive procedures. Agents which can prevent or attenuate the ovotoxic effects of treatment would provide significant advantages over the existing fertility preservation techniques, and would allow patients to retain their natural fertility without the necessity for costly, invasive and risky procedures. Recent studies have contributed to our understanding of the mechanisms involved in cytotoxicity-induced ovarian follicle loss and highlight a number of agents that may be able to prevent or reduce this loss.

METHODS

This paper reviews the relevant literature (research articles published in English up to December 2013) on the mechanisms of cytotoxic-induced ovarian damage and the implications for fertility preservation. We present a comprehensive discussion of the potential agents that have been shown to preserve the ovarian follicle reserve in the face of cytotoxic treatments, including an analysis of their respective advantages and risks, and mechanisms of action.

RESULTS

Multiple molecular pathways are involved in the cellular response to cytotoxic treatments, and specific cellular reactions depend on variables including the drug class and dose, cell type, and cell stage. A number of agents acting on different elements of these pathways have demonstrated potential for preventing or reducing ovarian follicle loss, although in most cases, the studies are still very preliminary.

CONCLUSIONS

Advances in our understanding of the mechanisms and pathways involved in both cytotoxic ovarian damage and follicle growth and development have opened up new directions for fertility preservation. In order to bring these agents from the lab to the clinic, it will be vital to accurately evaluate the efficacy of each agent and additionally to demonstrate that co-treatment with these agents will not interfere with the anti-cancer activity of the chemotherapy drugs, or produce genetically comprised embryos.

Impairment of BRCA1-related DNA double-strand break repair leads to ovarian aging in mice and humans

The underlying mechanism behind age-induced wastage of the human ovarian follicle reserve is unknown. We identify impaired ATM (ataxia-telangiectasia mutated)-mediated DNA double-strand break (DSB) repair as a cause of aging in mouse and human oocytes. We show that DSBs accumulate in primordial follicles with age. In parallel, expression of key DNA DSB repair genes BRCA1, MRE11, Rad51, and ATM, but not BRCA2, declines in single mouse and human oocytes. In Brca1-deficient mice, reproductive capacity was impaired, primordial follicle counts were lower, and DSBs were increased in remaining follicles with age relative to wild-type mice. Furthermore, oocyte-specific knockdown of Brca1, MRE11, Rad51, and ATM expression increased DSBs and reduced survival, whereas Brca1 overexpression enhanced both parameters. Likewise, ovarian reserve was impaired in young women with germline BRCA1 mutations compared to controls as determined by serum concentrations of anti-Müllerian hormone. These data implicate DNA DSB repair efficiency as an important determinant of oocyte aging in women.

Meiosis progression and donor age affect expression profile of DNA repair genes in bovine oocytes

Several genetic and physiological factors increase the risk of DNA damage in mammalian oocytes. Two critical events are: (i) meiosis progression, from maturation to fertilization, due to extensive chromatin remodelling during genome decondensation; and (ii) aging, which is associated with a progressive oxidative stress. In this work, we studied the transcriptional patterns of three genes, RAD51, APEX-1 and MLH1, involved in DNA repair mechanisms. The analyses were performed by real-time quantitative PCR (RT-qPCR) in immature and in vitro matured oocytes collected from 17 ± 3-month-old heifers and 94 ± 20-month-old cows. Batches of 30-50 oocytes for each group (three replicates) were collected from ovarian follicles of slaughtered animals. The oocytes were freed from cumulus cells at the time of follicle removal, or after in vitro maturation (IVM) carried out in M199 supplemented with 10% fetal calf serum, 10 IU luteinising hormone (LH)/ml, 0.1 IU follicle-stimulating hormone (FSH)/ml and 1 μg 17β-oestradiol/ml. Total RNA was extracted by Trizol method. The expression of bovine GAPDH gene was used as the internal standard, while primers for bovine RAD51, APEX-1 and MLH1 genes were designed from DNA sequences retrieved from GenBank. Results obtained indicate a clear up-regulation of RAD51, APEX-1 and MLH1 genes after IVM, ranging between two- and four-fold compared with germinal vesicle (GV) oocytes. However, only RAD51 showed a significant transcript increase between the immature oocytes collected from young or old individuals. This finding highlights RAD51 as a candidate gene marker for discriminating bovine immature oocytes in relation to the donor age.

Ceramide and its transport protein (CERT) contribute to deterioration of mitochondrial structure and function in aging oocytes

In women as well as in mice, oocytes exhibit decreased developmental potential (oocyte quality) with advanced age. Our current data implicate alterations in the levels of oocyte ceramide and associated changes in mitochondrial function and structure as being prominent elements contributing to reduced oocyte quality. Both ROS levels and ATP content were significantly reduced in aged oocytes. The decreased in ROS levels are of intrigue because it is contrary to what has been previously reported. Lowered levels of both ROS and ATP indicate diminished mitochondrial function that was accompanied by alterations in mitochondrial structure. Interestingly, developmental potential of old oocytes was improved by microinjection of mitochondria isolated from young oocytes. Co-treatment of aged oocytes with ceramide and a cytoplasmic lipid carrier (l-carnitine) improved both mitochondrial morphology and function, and totally rescued spontaneous in vitro fragmentation. In addition, ceramide localization was altered in old oocytes possibly due to downregulation of the ceramide transport protein (CERT). However, knockdown of CERT alone was not sufficient to increase young oocyte's susceptibility to death, because the sequential manipulation of ceramide levels (its chronic decrease, followed by downregulation of CERT, and finally a ceramide spike) were all necessary to replicate the aging phenotype. These results indicate that oocyte aging is due to a multiplicity of events; and that with increasing biological age, changes in levels of both ceramide and its transport protein contribute to deterioration of oocyte mitochondrial structure and function. Hence, those changes may represent potential targets to manipulate when attempting to ameliorate aging phenotypes in germ cells.

DNA damage-induced primordial follicle oocyte apoptosis and loss of fertility require TAp63-mediated induction of Puma and Noxa

Trp63, a transcription factor related to the tumor suppressor p53, is activated by diverse stimuli and can initiate a range of cellular responses. TAp63 is the predominant Trp53 family member in primordial follicle oocyte nuclei and is essential for their apoptosis triggered by DNA damage in vivo. After γ-irradiation, induction of the proapoptotic BH3-only members Puma and Noxa was observed in primordial follicle oocytes from WT and Trp53(-/-) mice but not in those from TAp63-deficient mice. Primordial follicle oocytes from mice lacking Puma or both Puma and Noxa were protected from γ-irradiation-induced apoptosis and, remarkably, could produce healthy offspring. Hence, PUMA and NOXA are critical for DNA damage-induced, TAp63-mediated primordial follicle oocyte apoptosis. Thus, blockade of PUMA may protect fertility during cancer therapy and prevent premature menopause, improving women's health.

Multiple approaches for individualized fertility protective therapy in cancer patients

In the last decade, fertility preservation has risen as a major field of interest, creating new interactions between oncologists and gynecologists. Various options, such as cryopreservation of ovarian tissue, have been developed and are currently routinely proposed in many centers. However, many of the options remain experimental and should be offered to patients only after adequate counseling. This paper addresses the efficiency and the potential of the different fertility preservation approaches.

Chemotherapy-induced late transgenerational effects in mice

To our knowledge, there is no report on long-term reproductive and developmental side effects in the offspring of mothers treated with a widely used chemotherapeutic drug such as doxorubicin (DXR), and neither is there information on transmission of any detrimental effects to several filial generations. Therefore, the purpose of the present paper was to examine the long-term effects of a single intraperitoneal injection of DXR on the reproductive and behavioral performance of adult female mice and their progeny. C57BL/6 female mice (generation zero; G0) were treated with either a single intraperitoneal injection of DXR (G0-DXR) or saline (G0-CON). Data were collected on multiple reproductive parameters and behavioral analysis for anxiety, despair and depression. In addition, the reproductive capacity and health of the subsequent six generations were evaluated. G0-DXR females developed despair-like behaviors; delivery complications; decreased primordial follicle pool; and early lost of reproductive capacity. Surprisingly, the DXR-induced effects in oocytes were transmitted transgenerationally; the most striking effects being observed in G4 and G6, constituting: increased rates of neonatal death; physical malformations; chromosomal abnormalities (particularly deletions on chromosome 10); and death of mothers due to delivery complications. None of these effects were seen in control females of the same generations. Long-term effects of DXR in female mice and their offspring can be attributed to genetic alterations or cell-killing events in oocytes or, presumably, to toxicosis in non-ovarian tissues. Results from the rodent model emphasize the need for retrospective and long-term prospective studies of survivors of cancer treatment and their offspring.

Somatic cell nuclear reprogramming of mouse oocytes endures beyond reproductive decline

The mammalian oocyte has the unique feature of supporting fertilization and normal development, while capable of reprogramming nuclei of somatic cells toward pluripotency, and occasionally even totipotency. While oocyte quality is known to decay with somatic aging, it is not a given that different biological functions decay concurrently. In this study, we tested whether oocyte's reprogramming ability decreases with aging. We show that oocytes isolated from mice aged beyond the usual reproductive age (climacteric) yield ooplasts that retain reprogramming capacity after somatic nuclear transfer (SCNT), giving rise to higher blastocysts rates compared to young donors ooplasts. Despite the differences in transcriptome between climacteric and young ooplasts, gene expression profiles of SCNT blastocysts were very similar. Importantly, embryonic stem cell lines with capacity to differentiate into tissues from all germ layers were derived from SCNT blastocysts obtained from climacteric ooplasts. Although apoptosis-related genes were down-regulated in climacteric ooplasts, and reprogramming by transcription factors (direct-induced pluripotency) benefits from the inhibition of p53-mediated apoptosis, reprogramming capacity of young ooplasts was not improved by blocking p53. However, more outgrowths were derived from SCNT blastocysts developed in the presence of a p53 inhibitor, indicating a beneficial effect on trophectoderm function. Results strongly suggest that oocyte-induced reprogramming outcome is determined by the availability and balance of intrinsic pro- and anti-reprogramming factors tightly regulated and even improved throughout aging, leading to the proposal that oocytes can still be a resource for somatic reprogramming when they cease to be considered safe for sexual reproduction.

DNA double-strand break repair and the evolution of intron density

The density of introns is both an important feature of genome architecture and a highly variable trait across eukaryotes. This heterogeneity has posed an evolutionary puzzle for the last 30 years. Recent evidence is consistent with novel introns being the outcome of the error-prone repair of DNA double-stranded breaks (DSBs) via non-homologous end joining (NHEJ). Here we suggest that deletion of pre-existing introns could occur via the same pathway. We propose a novel framework in which species-specific differences in the activity of NHEJ and homologous recombination (HR) during the repair of DSBs underlie changes in intron density.