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Women age differently from men when it comes to health – particularly in conditions like cardiovascular disease and neurodegenerative disorders such as dementia and Parkinson’s. A team at the Technical University of Munich (TUM) has proposed a new explanation for this. In aging female mice, genes on the previously silenced second X chromosome become active again. This mechanism might also influence women's health later in life.
Unlike men, who carry one X and one Y chromosome, women have two X chromosomes in each cell. However, one of the two X chromosomes is effectively silenced. It folds into a compact structure known as the Barr body and can no longer be read. Without this mechanism, the genes on the X chromosome would be read twice as often in women as in men.
Scientists have known for some time that some genes can escape inactivation in the Barr body, resulting in higher gene activity in women. These genes are suspected to influence disease. “We have now shown for the first time that with increasing age, more and more genes escape the inactivation of the Barr body,” says Dr. Daniel Andergassen, group leader at the Institute of Pharmacology and Toxicology at TUM. The study was published in the journal Nature Aging.
Inactive X chromosome loosens with age
The researchers examined the major organs of mice at different stages of life. In the older animals, the proportion of genes that had escaped was on average twice as high as in adult animals – six percent instead of three percent of the genes on the X chromosome. In some organs, the numbers were even higher: in the kidneys, for instance, nearly 9 percent. “With aging, epigenetic processes gradually loosen the tightly packed structure of the inactive X chromosome,” explains first author Sarah Hoelzl. “This mainly happens at the ends of the chromosome, allowing for genes located in those regions to be read again.”
Many reactivated genes are linked to disease
Many of the genes that become active again with age are associated with disease. “Our findings are based on mice, but since the X chromosome is very similar in humans, I believe the same may happen in aging women,” says Daniel Andergassen. Whether this is the case and what effects the reactivated genes may have on disease development will need to be investigated in future studies. According to the researchers, this doubled gene activity could have positive effects in some cases and negative effects in others. ACE2, for example – a gene that escapes in the lungs with agecan help limit pulmonary fibrosis. Increased activity of the gene TLR8 in old age, however, may play a role in autoimmune diseases such as late-onset lupus.
A new perspective on sex-based differences in disease
“Sex differences in age-related disease are incredibly complex,” says Andergassen. “So far, scientific explanations have mostly focused on hormonal or lifestyle factors. While the role of the X chromosome and some escape genes have been studied before, the discovery that many genes on the inactive X can reactivate with age opens up entirely new lines of research. This insight could offer an alternative to hormonal explanations and help improve our understanding of sex differences in age-related diseases – perhaps even contribute to answering the fundamental question of why women statistically live longer.”
Publication:
Hoelzl, S., Hasenbein, T.P., Engelhardt, S., Andergassen, D. Aging promotes reactivation of the Barr body at distal chromosome regions. Nat Aging (2025). https://doi.org/10.1038/s43587-025-00856-8
Further information:
• This news item on tum.de: https://www.tum.de/en/news-and-events/all-news/press-releases/details/silent-x-c...
Subject matter expert:
Dr. Daniel Andergassen
Technical University of Munich
Institute of Pharmacology and Toxicology
Tel. +49 89 4140-3298
daniel.andergassen@tum.de
https://www.professoren.tum.de/en/tum-junior-fellows/andergassen-daniel
https://www.andergassenlab.com/
TUM Corporate Communications Center contact:
Paul Hellmich
Media Relations
Tel. +49 (0) 89 289 22731
presse@tum.de
www.tum.de
Dr. Daniel Andergassen
Technical University of Munich
Institute of Pharmacology and Toxicology
Tel. +49 89 4140-3298
daniel.andergassen@tum.de
https://www.professoren.tum.de/en/tum-junior-fellows/andergassen-daniel
https://www.andergassenlab.com/
Hoelzl, S., Hasenbein, T.P., Engelhardt, S., Andergassen, D. Aging promotes reactivation of the Barr body at distal chromosome regions. Nat Aging (2025). https://doi.org/10.1038/s43587-025-00856-8
In female mammals, one of the two X chromosomes is usually inactive and forms the so-called Barr bod ...
Daniel Andergassen
Daniel Andergassen / TUM
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