Women's egg cells shield DNA from the effects of aging

For decades, scientists assumed that as women age, their egg cells collect genetic damage, just like the rest of the body. But new research suggests that human eggs might be playing by their own rules.

In a study spanning women aged 20 to 42, researchers discovered that the tiny power plants inside our cells – mitochondria – keep their genetic material remarkably pristine in eggs, even as the years pass.

These same energy makers in blood and saliva do pick up more damage with age. But in eggs? The mutation rate barely budges.

“When we think about age-related mutations, we think about older people having more mutations than younger people,” said Kateryna Makova at Penn State University. “But expectation is not necessarily the truth.”

Women’s eggs store clean DNA

Mitochondria pass exclusively from mother to child. Their DNA is short, circular, and essential for making energy. Damage to it can sometimes trigger devastating disorders, especially in tissues with high energy demands like muscles and the brain.

“The oocyte [egg] provides this stockpile,” said Ruth Lehmann, professor of biology at the Massachusetts Institute of Technology, referring to the generous stash of mitochondria inside each egg cell.

To see how these precious packages fare over time, the team used a cutting-edge method called duplex sequencing. It can spot even the rarest mutations – changes present in less than one percent of a cell’s mitochondrial copies.

This was no small undertaking: they sequenced 80 single eggs, plus blood and saliva samples, from 22 women.

Eggs resist aging mutations in women

The study revealed a clear difference between eggs and other tissues. Blood and saliva from older women contained more mitochondrial mutations than those from younger women, showing the expected age-related increase.

Egg cells, however, showed no such rise. In fact, they consistently had 17 to 24 times fewer mutations than blood or saliva, regardless of the woman’s age.

Within the mitochondrial genome, vulnerability was uneven. In eggs, most mutations appeared in the D-loop, a noncoding control region, while the crucial protein-coding sections remained mostly untouched.

This pattern points to a protective mechanism that directs genetic changes away from regions essential for energy production.

By protecting these critical genes, eggs help preserve the functional integrity of mitochondria, ensuring they can perform their energy-supplying role for decades. This built-in protection could be a key reason eggs maintain reproductive potential well into later stages of a woman’s life.

Natural filter removes harmful DNA

A woman may inherit some mitochondrial DNA variants from her mother, which then appear in both her eggs and other tissues. These shared variants, called inherited heteroplasmies, often exist at lower levels in eggs than in blood or saliva.

This difference points to a natural quality-control system within the reproductive process that reduces potentially harmful mitochondrial changes before they are passed to future generations.

Researchers also measured the “germline bottleneck” – the sharp reduction in the number of mitochondrial DNA copies that are actually transmitted during egg development. This bottleneck acts as a filter, determining which variants survive to the next generation.

For high-frequency variants, the team estimated the bottleneck size to be around 30 mitochondrial DNA units, which is consistent with earlier findings from family-based genetic studies.

This small number helps explain how reproduction can effectively minimize harmful mutations, protecting offspring from potential mitochondrial disorders.

Unlocking fertility’s genetic secret

The study challenges the long-held assumption that egg cells deteriorate genetically in step with the rest of the body. Instead, they appear to have protective systems that guard their mitochondrial DNA well into a woman’s reproductive years.

“I think that we evolved a mechanism to somehow lower our mutation burden, because we can reproduce later in life,” said Makova.

It’s a finding with big implications. If we can understand – and maybe even mimic – this natural preservation system, we might unlock new ways to improve fertility treatments and reduce the risk of passing on mitochondrial disorders.

For now, the message is surprisingly optimistic: while the rest of the body’s cells may bear the marks of time, women’s eggs seem to have found a way to keep part of their genetic story young.

The study is published in the journal Science Advances.

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