In brief: Certain age-related DNA mutations are less common in human eggs

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UNIVERSITY PARK, Pa. — Unlike other tissues in the human body, human eggs don’t pick up as many mutations in some of their DNA as they age. The findings from a new study, published in [Science Advances](https://www.science.org/doi/10.1126/sciadv.adw4954) and led by researchers at Penn State, suggests that human eggs — the reproductive cells that, when fertilized with sperm, provide half of the genetic instructions for embryos — may have developed a protective mechanism to guard against a certain kind of age-related mutation. The researchers said this finding could provide insights to understand reproductive aging and health.

Mitochondria are cellular components that supply energy to the body’s cells and have their own DNA. With age, mitochondrial DNA can mutate; most changes are generally harmless, but in some cases, these mutations can lead to disease. Since mitochondrial DNA is passed down only from mother to offspring, scientists want to understand what happens to mitochondrial DNA in egg cells over time, explained Kateryna Makova, professor of biology at Penn State and senior author of the study.

“If there are more mutations in egg cells of older mothers, then there is a higher risk of transmitting these mutations to their offspring,” Makova said.

The international research team set out to examine DNA found in mitochondria and how it changes with age in human eggs compared to other tissues in the body. They collected blood and saliva samples and egg cells from 22 women between the ages of 22 and 42. Using a high-resolution sequencing method to analyze the samples for mutations in mitochondrial DNA, they found that:

Across all age groups, there were 17-to-24-fold fewer mutations in the mitochondrial DNA in egg cells compared to blood or saliva samples.

The frequency of mutations in mitochondrial DNA doesn’t increase with age in human egg cells. In contrast, the frequency of mitochondrial mutations does increase with age in blood and salivary cells.

The mitochondrial mutations that do occur in egg cells are less prevalent in functionally important portions of the DNA that carry instructions to build proteins, compared to regions that do not carry such instructions, suggesting that natural selection works to remove most of the functionally important mutations.

“Human egg cells seem to be protected against some age-related mutations,” Makova said. “This finding could have important implications for our understanding of how mutations accumulate in reproductive cells as humans tend to reproduce later in life, but more research is needed.”

For the full list of authors and funding sources, see the paper.

Media highlights:

“Human eggs don’t accumulate as many mutations with age as we thought,” The New Scientist “Human eggs have special protection against certain types of aging, study hints,” Live Science “Women’s eggs have evolved to mutate less as we age, researchers say,” Stylist