Internal clock influences skin cancer risk

Repair of UV damage in the cells varies during the day

Daytime variations in DNA repair and replication © University of North Carolina
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How harmful UV rays are to our skin is also determined by our internal clock. Evidence has been discovered by US researchers in mice. The ability of skin cells to repair UV damage fluctuates throughout the day, the researchers report in the journal Proceedings of the National Academy of Sciences. The risk of skin cancer developing from such UV damage is therefore higher at certain times of the day - even when the UV radiation is the same. "Also, humans are likely to show such a circadian rhythm in their susceptibility to UV-induced skin cancer, " study coordinator Aziz Sancar from the University of North Carolina at Chapel Hill and his colleagues write.

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For their study, the researchers expose mice to the same dose of UV radiation at different times of the day. Both groups developed skin cancer over several weeks. The animals that were irradiated in the morning, however, got up to 500 percent more tumors than animals that were exposed to radiation only in the afternoon. By analyzing the skin cells, the scientists also found out why: "Mice in the skin are copied in the morning more DNA molecules, but there are fewer repairs than in the evening, " say the researchers. As a result, the cells are more susceptible to DNA damage in the morning, as caused by UV light.

Inner clock is different in humans and mice

The scientists also believe that humans have such a day cycle of DNA repair. Compared to the nocturnal mice, however, the daytime running of the internal clock is offset by twelve hours. The DNA repair is therefore the most effective in humans in the morning and the weakest in the evening, the researchers conclude. "Our findings suggest that reducing the risk of skin cancer in humans would be to just sunbathe or visit a tanning salon in the morning, " says study leader Aziz Sancar of the University of North Carolina at Chapel Hill.

So far, there have been only recommendations to avoid the sun during the particularly UV-rich noon. In the morning and in the evening, the UV levels of sunlight fall and are then about the same low. Now you know that the skin even with the same radiation exposure at different times of the day also react differently sensitively. display

Repair protein ensures the repair of UV damage

Responsible for the daily differences in DNA repair is mainly a repair protein, the so-called XPA, the researchers say. This usually ensures that damaged areas are cut out of the genome molecule and replaced. To find out how active this protein is at different times of the day, the researchers kept mice in each of twelve hours of light and twelve hours of darkness. Each hour they took skin samples from the animals and analyzed the content of the protein XPA and the effectiveness of the DNA repair. The researchers performed the same test on mice whose internal clock was disabled by a gene mutation.

Daily differences only with functioning inner clock

For mice with a normal working clock, the highest levels of XPA were found in the early evening, the researchers report. In the morning, on the other hand, the salary is very low. By contrast, the values ​​for the "no-time" mice always remained the same. This proves that the amount of this protein in the mice and, most likely, in humans is also controlled by the internal clock, the researchers say. In order to find out how much the influence of the internal clock on human DNA repair is, they now want to measure this on human skin samples. (Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073 / pnas.1115249108)

(PNAS / University of North Carolina, 10/25/2011 - NPO)