Newly Found Oocyte Protein OMC1 Keeps Egg Chromosomes Cohesive for Life

In a discovery that could reshape how scientists think about reproductive aging, researchers in Japan have identified a previously uncharacterized protein that acts as a molecular guardian of egg cell chromosomes across a female’s entire reproductive lifespan. The protein, named OMC1 for oocyte-specific meiotic chromosome protein 1, is required to keep sister chromatids held together from fetal development all the way through ovulation decades later. When the gene encoding it is deleted in mice, chromosome cohesion deteriorates prematurely, and the eggs accumulate the same segregation errors that normally appear only in naturally aged oocytes.

The study, published in Nature Aging by Sakie Iisaka, Shuhei Yoshida, Tomoya S. Kitajima and Kei-ichiro Ishiguro of Kumamoto University together with colleagues at Chiba University and RIKEN, addresses one of the most striking quirks of mammalian biology. Oocytes enter meiosis during fetal development and then stall for years, sometimes decades, in a suspended state called the dictyate arrest. During this prolonged pause, the sister chromatids of every chromosome remain tethered by ring-shaped cohesin complexes built around the meiosis-specific subunit REC8. Crucially, because REC8 cohesin is loaded only around the time of meiotic entry and is not replenished after the premeiotic S phase, the egg must preserve the very same cohesin molecules for its entire lifetime. As females age, this cohesion gradually wears down, predisposing eggs to chromosome segregation errors. Egg aneuploidy, the resulting wrong number of chromosomes, is a major cause of infertility, miscarriage and congenital disorders such as Down syndrome.

To find factors that counteract this deterioration, the team mined single-cell RNA sequencing data comparing fetal oocytes with juvenile testicular germ cells, both of which pass through meiotic prophase. Of roughly 12,000 genes expressed in fetal oocytes, only about 1,800 were female-specific. The researchers then narrowed the search using a mouse strain carrying a mutation in the RB-binding site of STRA8, a factor needed for proper meiotic entry; females with this mutation fail to reach dictyate arrest, so genes essential for long-term oocyte survival should be underexpressed in them. That filter pointed to Ccdc201, a gene previously annotated in databases as encoding a noncoding RNA. Homology searches revealed it actually encodes a protein conserved across vertebrates, and the team renamed it Omc1.

The expression pattern of Omc1 is strikingly specific. Its messenger RNA was detected in embryonic ovaries and, using a technique called photoisolation chemistry that enriches transcripts from primordial oocytes, in oocytes of postnatal ovaries as well. Sequencing data from isolated adult germinal vesicle-stage oocytes showed abundant Omc1 expression there, with little or none in the surrounding cumulus cells. Immunostaining placed the OMC1 protein in the nuclei of oocytes beginning around embryonic day 17.5, and it was absent from pachytene spermatocytes in testes, confirming its oocyte-specific identity.

Where OMC1 sits on the chromosomes proved to be the key clue to its function. At the pachytene stage it lined up along the lateral elements of the synaptonemal complex, precisely where REC8 cohesin resides, a positioning confirmed by immunogold electron microscopy. The protein persisted through dictyate arrest, remaining detectable on chromosomes of primordial follicle oocytes in adult ovaries, and stayed associated with sister chromatid axes during meiosis I in mature eggs, with residual signal at the inner centromere by meiosis II. Its localization depended on the cohesin subunit SMC1β, and a proximity ligation assay showed OMC1 and REC8 sitting within nanometers of each other on chromosome spreads. Intriguingly, freshly synthesized OMC1, introduced as GFP-tagged RNA into fully grown oocytes, could still be recruited to chromosomes during adult meiotic maturation, unlike REC8 itself.

To test whether the protein matters functionally, the researchers generated knockout mice lacking Omc1 using CRISPR-Cas9. The knockout females looked superficially normal: their ovaries had comparable primordial follicle reserves, and their oocytes completed meiotic prophase and resumed meiosis in culture without obvious problems. Yet the defects emerged on closer inspection. Knockout females produced progressively smaller litters than age-matched controls as they aged, producing almost no pups after 30 weeks, and embryos derived from in vitro fertilized knockout oocytes showed less efficient preimplantation development and developmental delay as early as 8 weeks of age.

The cytological picture explained these fertility problems. In fetal knockout oocytes, REC8 loaded onto chromosomes normally and the synaptonemal complex assembled largely intact, showing that OMC1 is dispensable for the initial establishment of cohesion. But during the long dictyate arrest, chromosome-bound REC8 began to disappear prematurely: levels were normal shortly after the ovarian reserve formed, but significantly reduced in knockout oocytes at 8 and 22 weeks of age. Mature knockout eggs showed reduced chromosomal REC8 at metaphase I, widened gaps between sister kinetochores indicating weakened centromeric cohesion, and frequent terminalized chiasmata, all hallmarks of naturally aged oocytes. Notably, total REC8 protein levels were unchanged, suggesting the cohesin was slipping off chromosomes rather than being degraded, and centromere identity markers such as CENPA remained intact, indicating OMC1 acts specifically on cohesion maintenance rather than broadly preserving all chromosome components.

Live imaging then revealed how the weakened cohesion translates into aneuploidy. At 8 weeks of age, knockout eggs segregated chromosomes normally during meiosis I but then showed severe precocious separation of sister chromatids, known as PSSC, at metaphase II: roughly 78 percent contained between two and eight single chromatids. Because the total kinetochore count stayed normal, these errors arose after entry into meiosis II rather than from mis-segregation in meiosis I, recapitulating a predominant route to aneuploidy in early oocyte aging. By 35 weeks, knockout oocytes also began showing premature bivalent separation into univalents at metaphase I, a defect seen in severely aged mouse and human oocytes. Strikingly, the PSSC frequency in 8-week-old knockout eggs already matched that of 35-week-old wild-type eggs, meaning loss of OMC1 effectively accelerated oocyte aging. Partial experimental depletion of REC8 in young oocytes reproduced the phenotype, supporting the conclusion that the chromosome defects stem largely from failed cohesin maintenance.

The team also probed whether OMC1 is needed acutely during meiotic maturation. Injecting Omc1 mRNA into germinal vesicle-stage knockout oocytes raised chromosomal REC8 levels at metaphase I and markedly suppressed PSSC at metaphase II, though sister kinetochore distances remained wider than in wild type, so the rescue was only partial. Conversely, acutely removing OMC1 from wild-type eggs with TRIM-Away caused cohesion defects that mRNA injection could reverse. Together these experiments indicate OMC1 protects cohesion both during the long dictyate arrest and during the transition from the germinal vesicle stage to metaphase I, a previously underappreciated vulnerable window, but newly supplied protein cannot reverse damage already accumulated during the preceding arrest.

The findings carry an intriguing human connection. A genome-wide association study has linked loss-of-function stop-gain mutations in CCDC201, the presumed human ortholog of Omc1, to early menopause, and women homozygous for such a variant reach menopause earlier and have fewer children. The researchers showed that a corresponding mouse R151 stop-gain mutant failed to localize to chromosomes and could not rescue the PSSC phenotype in knockout oocytes, demonstrating that the human menopause-associated truncation abolishes the chromosome-associated function of the protein. The authors caution that much remains unknown, including how OMC1 physically maintains REC8 on chromosomes and whether altered OMC1 function contributes to variation in human reproductive aging. Restoring OMC1 expression in eggs before in vitro maturation may, they suggest, offer a conceptual framework for future strategies to mitigate age-associated reproductive decline, though such applications remain distant and will require substantially more research.

Subject of Research: An oocyte-specific protein, OMC1, that maintains chromosome cohesion and suppresses age-related egg aneuploidy in mice

Article Title: Oocyte-specific OMC1 is required for maintaining chromosome cohesion through reproductive life

Article References: Iisaka, S., Yoshida, S., Shimada, R., Mishina, T., Fujimura, S., Koshiguchi, M., Kimura, R., Oki, S., Araki, K., Kitajima, T. S., & Ishiguro, K.-I. (2026). Oocyte-specific OMC1 is required for maintaining chromosome cohesion through reproductive life. Nature Aging. https://doi.org/10.1038/s43587-026-01245-5

Image Credits: AI Generated

DOI: 10.1038/s43587-026-01245-5

Keywords: OMC1, oocytes, chromosome cohesion, REC8 cohesin, reproductive aging, aneuploidy, meiosis, dictyate arrest, knockout mice, early menopause, CCDC201, fertility