Normal skin cells contain an unexpectedly large amount of cancer-associated DNA changes, said researchers at the Wellcome Trust Sanger Institute in Cambridge.
Healthy facial skin cells carried thousands of changes in their DNA known as mutations, which are caused by exposure to sunlight.
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Dr Phil Jones, a Cancer Research UK expert in pre-cancer and author on the study, said: "It was quite astonishing to find so many genetic changes in cells in normal skin. It seems that, even though so many cells have DNA damaged by the sun's UV rays, the skin keeps working as normal."
The researchers used advanced DNA analysis technology for the study on 234 biopsies.
Samples were collected from four patients aged between 55 and 73 who had variable histories of skin exposure and getting cosmetic surgery to tighten their eyelids.
DNA analysis revealed 3,760 different mutations from all 234 biopsies, equivalent to more than 100 cancer-associated mutations per square centimeter of skin.
Some cells with mutated DNA formed clusters, known as clones. While some clones had grown to be twice the size of normal group cells, but not cancerous.
"These first cancer-associated mutations give cells a boost compared to their normal neighbors," explained study author Dr Peter Campbell.
The researchers believe the study offers important insight into the first step for a normal cell to become cancerous. The researchers hope to study further what exactly pushes these cells over the edge.
Sarah Williams, Cancer Research UK's health information manager, said, "The findings reinforce the need to stay safe in the sun. Although we all need some sun, people can avoid sunburn and skin damage when the sun is strong by spending time in the shade, covering up with clothing and using plenty of sunscreen with at least SPF15 and 4 or more stars."
The study is published in Science