byInstitute for Research in Biomedicine (IRB Barcelona)

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Bladder cancer is one of the most common cancers worldwide. Men are around four times more likely to develop it than women, and smoking is the main known environmental risk factor. However, the biological mechanisms behind these risk factors remain unclear. Since cancer can take decades to develop, it is important to look at healthy tissues to understand the very first steps of the disease, with the goal of improving risk prediction, prevention, and early diagnosis.

Researchers led by Dr. Núria López-Bigas and Dr. Abel González-Pérez at IRB Barcelona, and Dr. Rosana Risques at the University of Washington, have now shown that smoking and biological sex influence how normal cells evolve in healthy bladder tissue, favoring the expansion of some mutated cells, which may be key in the development of cancer.

Published in the journalNature,the workcould provide insights into why men and smokers are at higher risk of bladder cancer.

"Healthy tissues accumulate manymutationsthroughout life, but what matters is not only how many there are, but which ones manage to outgrow the others and expand into clones, which are copies of the same cell carrying the same mutations," explains Dr. López-Bigas, ICREA researcher and leader of the Biomedical Genomics group at IRB Barcelona, and Co-lead of Team PROMINENT, Cancer Grand Challenges. "We have seen that smoking and biological sex directly influence this process."

Cancer does not arise overnight. Over decades, the cells in a tissue accumulate mutations, and some clones gain an advantage that allows them to grow faster than others. This study shows that such evolutionary differences are already visible in healthy tissue before disease sets in.

To carry out the study, the researchers analyzed bladder samples from 45 donors. Using a novel approach, comparable to looking at DNA with a powerful new microscope, they were able to detect and quantify thousands of mutations that are invisible to current DNA sequencing techniques.

"The universe is full of stars, but you can't see most of them without the right instrument. This approach is like going from a backyard telescope to the James Webb Space Telescope: Suddenly, multiple mutations become visible in healthy bladder tissue, long before a tumor exists," says Dr. Risques, University of Washington, co-senior author of the study.

The researchers observed clear biological differences between men and women. In male donors, certain mutations in cancer-related genes showed anevolutionary advantage, meaning that clones carrying them were more likely to expand even in the healthy bladder.

They also saw a striking effect of smoking. Among donors over the age of 55, those with a history of smoking carried a high frequency of mutations in the TERT promoter, a DNA element that reactivates telomerase and allows cells to avoid aging and continue dividing. Most importantly, the work provides evidence that tobacco is not only a mutagen—causing new mutations—but also a clonal promoter, facilitating the expansion of cells with pre-existing mutations.

This is the first time such effects have been observed directly in healthy bladder tissue, rather than in tumors, shedding light on the earliest stages of cancer development.

By showing that differences in mutation expansion are already present in healthy tissue, the study offers a new way to understand tissue evolution and provides hints of its malignant transformation. This shift in focus—from simply counting mutations to asking which ones thrive—could help explain why men and smokers are more likely to developbladder cancer.

The findings also pave the way for future applications. Measuring expanding clones in the bladder could eventually inform risk prediction and early detection tools, for example, by urine sample analysis. More broadly, the same approach could be extended to other tissues and exposures—including workplace chemicals or chemotherapy—thereby opening up new avenues for cancer prevention.

"This study is only the tip of the iceberg. We analyzed 16 genes in the bladders of 45 people, yet we already found important differences. The same approach can be applied to other tissues andrisk factors," adds Dr. González-Pérez, research associate at IRB Barcelona.

Ferriol Calvet and Raquel Blanco, Ph.D. students at IRB Barcelona's Biomedical Genomics lab, are the first authors of this study.

More information: Sex and smoking bias in the selection of somatic mutations in human bladder, Nature (2025). DOI: 10.1038/s41586-025-09521-x . www.nature.com/articles/s41586-025-09521-x Journal information: Nature

Provided by Institute for Research in Biomedicine (IRB Barcelona)