DNA Repair Protein APEX1 Emerges as Key Guardian of Egg Cell Quality

A single protein that has long been known for patching damaged DNA may hold the key to understanding why some women’s ovaries age faster than others. In a new study published in the Journal of Ovarian Research, researchers in Beijing report that a molecule called apurinic/apyrimidinic endonuclease 1, or APEX1, acts as a molecular guardian inside the cells that nurture developing eggs. When levels of this protein fall, a cascade of cellular damage unfolds: mitochondria go uncleaned, iron metabolism spirals out of control, and cells succumb to a destructive form of cell death known as ferroptosis. The findings offer a fresh mechanistic explanation for diminished ovarian reserve, a condition in which both the number and the quality of a woman’s oocytes decline, often leaving few options for patients hoping to conceive.

Diminished ovarian reserve, commonly abbreviated DOR, is one of the most frustrating diagnoses in reproductive medicine. Women with DOR produce fewer eggs during stimulation cycles for in vitro fertilization, and the eggs they do produce are more likely to carry chromosomal abnormalities or fail to develop into viable embryos. The causes are heterogeneous, ranging from genetic predisposition to chemotherapy exposure, and treatment options remain limited largely to donor eggs or aggressive stimulation protocols. What has become increasingly clear in recent years is that oxidative stress, the accumulation of reactive oxygen species that damage lipids, proteins, and DNA, sits at the center of the pathology. The ovary is an exceptionally oxygen-hungry organ during folliculogenesis, and granulosa cells, the somatic cells that surround and feed each growing oocyte, are particularly vulnerable to oxidative assault.

APEX1 caught the researchers’ attention because it sits at the intersection of several stress-response pathways. Also known as Ref-1, the protein performs a dual role in the cell. In the nucleus, it functions as the primary enzyme of base excision repair, snipping out damaged apurinic and apyrimidinic sites in DNA that arise from oxidation and other insults. In the cytoplasm and through its redox-signaling domain, it helps maintain transcription factors in their active, reduced states, thereby shaping how cells respond to stress. Given that genomic integrity and redox homeostasis are both compromised in aging ovaries, the team led by Cong Wang, Jingyu Li, and senior authors Ying Fang and Xiaokui Yang of Beijing Obstetrics and Gynecology Hospital, Capital Medical University, hypothesized that APEX1 loss might be a driving event rather than a bystander in DOR.

The investigation began with human samples. The researchers isolated granulosa cells from follicular fluid collected during oocyte retrieval and quantified APEX1 expression. The results were striking: APEX1 was significantly downregulated in the granulosa cells of patients with diminished ovarian reserve. Moreover, the degree of reduction correlated positively with levels of anti-Müllerian hormone, the standard clinical marker of ovarian reserve, and negatively with patient age. In other words, the less APEX1 a patient’s granulosa cells expressed, the more depleted her ovarian reserve appeared to be. While correlation alone cannot establish causation, the pattern provided a compelling rationale for the functional experiments that followed.

To test whether APEX1 loss directly damages ovarian cell function, the team turned to KGN cells, a human ovarian granulosa-like tumor cell line widely used as a model for granulosa cell biology. Using small interfering RNA, they silenced APEX1 expression and then measured a battery of cellular outcomes. The consequences were broad and consistent. Proliferation, assessed with the CCK-8 assay, dropped. Apoptosis, detected by Annexin V-FITC/PI flow cytometry, rose. Mitochondrial membrane potential, measured with JC-1 staining, collapsed, while reactive oxygen species, visualized with DCFH-DA, accumulated. Western blotting revealed the molecular fingerprints behind these changes: the pro-apoptotic protein Bax increased, the anti-apoptotic Bcl-2 decreased, and the mitochondrial dynamics proteins MFN1 and DRP1 shifted in ways consistent with a fragmented, dysfunctional mitochondrial network.

Two interlinked processes emerged as the crux of the damage. The first was mitophagy, the specialized autophagic program that identifies and degrades damaged mitochondria before they leak reactive oxygen species and trigger cell death. In APEX1-deficient cells, mitophagic flux was impaired, as shown by disruptions in the PINK1/Parkin pathway and altered processing of the autophagy marker LC3. Without functional mitophagy, damaged mitochondria linger in the cell, spewing mitochondrial reactive oxygen species and further compounding oxidative stress. The second process was ferroptosis, an iron-dependent form of regulated cell death driven by lipid peroxidation. The researchers found that APEX1 knockdown activated ferroptotic signaling, disturbing iron homeostasis through changes in iron regulatory proteins IRP1 and IRP2, the iron exporter ferroportin 1, ferritin heavy and light chains, the iron importer DMT1, and the ferroptosis executor glutathione peroxidase 4, alongside the iron-recycling cofactor NCOA4. The picture that emerges is a vicious cycle: failing mitophagy permits mitochondrial damage, mitochondrial damage fuels oxidative stress, oxidative stress promotes lipid peroxidation, and iron overload converts that peroxidation into lethal ferroptosis.

Demonstrating the reverse effect in living animals was the study’s most clinically significant step. The team delivered an APEX1-overexpressing construct directly into mouse ovaries by in situ microinjection of adeno-associated virus, then exposed the animals to cyclophosphamide, a chemotherapy drug well known to deplete ovarian follicles and induce ovarian injury. The results were clear across multiple readouts. Histopathological staining with hematoxylin and eosin showed reduced follicular atresia, the degenerative process by which follicles are lost. Follicle counting confirmed a better-preserved follicular pool. Superovulation assays yielded improved oocyte output and quality, and live-cell fluorescence imaging indicated restored iron homeostasis within the ovarian tissue. At the molecular level, APEX1 overexpression reinvigorated mitophagic activity, suppressed ferroptotic markers, and preserved mitochondrial membrane potential, effectively reversing the damage signature that the in vitro experiments had established.

The study’s authors conclude that APEX1 exerts its protective effects by coordinating mitophagy and inhibiting ferroptosis, thereby sustaining cellular homeostasis and oocyte quality. Framed more broadly, the work positions APEX1 as a potential diagnostic biomarker and therapeutic target for diminished ovarian reserve. If APEX1 expression in granulosa cells reliably tracks ovarian reserve status, it could complement anti-Müllerian hormone measurements and perhaps offer a more mechanistic window into ovarian health. More ambitiously, strategies to boost APEX1 activity or mimic its downstream protective functions, for example by enhancing mitophagy or buffering iron-mediated lipid peroxidation, could one day help preserve ovarian function in women facing gonadotoxic chemotherapy or age-related fertility decline. Such applications remain distant; adeno-associated virus delivery to the human ovary is not an established clinical approach, and the safety and efficacy of any APEX1-targeted intervention would require extensive preclinical and clinical validation.

Nevertheless, the study adds an important piece to a rapidly evolving puzzle in reproductive biology. Over the past decade, ferroptosis has been implicated in a growing list of degenerative conditions, and mitophagy has become recognized as a central determinant of oocyte quality, with poor mitophagic clearance linked to aneuploidy and developmental failure in eggs. By connecting a redox-sensitive DNA repair protein to both of these pathways in the ovary, the Beijing team has drawn a line from a single molecule to a clinically devastating phenotype. The research also underscores a broader lesson about the ovary: it is not merely a passive reservoir of eggs that dwindles with time, but an active tissue whose somatic support cells constantly fight oxidative battles on behalf of the gametes they nurture. When that fight is lost, whether through chemotherapy, aging, or genetic vulnerability, the quality of the eggs suffers with it. Identifying the molecular sentinels, like APEX1, that keep those battles winnable may ultimately reshape how clinicians assess, monitor, and protect female fertility. For now, the findings provide mechanistic insight into DOR and a concrete starting point for the next generation of ovarian-protective therapies.

Subject of Research: The role of APEX1 in regulating mitophagy and ferroptosis in diminished ovarian reserve and oocyte quality.

Article Title: APEX1 deficiency impairs oocytes quality by disrupting mitophagy and activating ferroptosis in diminished ovarian reserve

Article References: APEX1 deficiency impairs oocytes quality by disrupting mitophagy and activating ferroptosis in diminished ovarian reserve. (n.d.). https://doi.org/10.1186/s13048-026-02272-x

Image Credits: AI Generated

DOI: 10.1186/s13048-026-02272-x

Keywords: APEX1, diminished ovarian reserve, oocyte quality, mitophagy, ferroptosis, oxidative stress, granulosa cells, ovarian reserve, mitochondrial dysfunction, fertility, DNA repair, Journal of Ovarian Research