Our Science Surgery series answers your cancer science questions.
Jenny asked: “How is skin cancer related to sun exposure?”
“When we talk about skin cancer, we’re actually talking about a number of different types,” says Professor Richard Marais, a skin cancer expert and director of the Cancer Research UK Manchester Institute. “And for almost all of these types, it’s very clear that the environmental carcinogen is sunlight.”
More specifically, it’s the ultraviolet (UV) radiation given out by the sun that causes these cancers. This type of radiation can penetrate the skin and damage the DNA inside cells.
It’s this damage that can also cause skin to change colour.
“UV light causes DNA damage which means certain cells in your skin distribute a pigment called melanin to other cells. This causes the skin to darken and is what people call a tan. You only get the tan if you have the DNA damage,” says Marais.
Sun signatures and skin cancer
The link between UV radiation and skin cancer was discovered in stages. First, UV radiation was linked to particular types of DNA damage.
“Scientists would expose cells in a test tube to UV light and then look at the DNA for the types of changes that occurred,” says Marais. “And they were able to see that UV light causes a specific type of change in the DNA.”
This type of DNA damage (mutation) was different to that caused by other carcinogens, such as tobacco. In other words, UV radiation left a unique mark on the DNA, often called a ‘UV signature’.
It was this signature that led scientists to connect sun exposure and skin cancer.
“When scientists were able to sequence massive numbers of tumours, they found that the exact same mutations that had been defined in the lab were found in skin cancers,” says Marais.
Marais says that each skin cancer has on average around 30,000 DNA faults caused by exposure to sunlight.
“Now of course, not all of these are what’s driving the cancer, but the more mutations you have the more chance there is that you’ll hit the wrong combinations of genes and you will get a cancer.”
Is it all skin cancers?
While most skin cancers are tied to sun exposure, there are some that don’t seem to share this link. One of these is a rare type of cancer that develops on skin without hair, called acral skin.
“It’s the skin on the palms of the hands, the soles of the feet and underneath your nail beds where you can develop acral melanomas,” says Marais. “And around half of acral melanomas don’t seem to have a UV signature, which means they aren’t caused by UV light.”
Marais says there are also a small number of cases of the more common skin cancers where the UV signature in the tumour seems to be very low.
“The same is true of other carcinogens like tobacco – around 10% of lung cancers aren’t caused by smoking,” he adds.
Scientists are looking at other factors that could contribute to skin cancer risk, including our genes.
Is it just sunlight?
“Some people have a higher predisposition to skin cancer, people with blonde hair and fairer skin for example,” says Marais.
People with paler skin produce less of the skin pigment eumelanin. This pigment in particular can help protect cells from getting DNA damage by absorbing UV radiation.
But according to Marais, it’s not a direct relationship between skin tone and hair colour and melanin production. “It’s also about the rest of your genetic makeup,” he says
This could include DNA variants that affect how effectively someone’s cells are able to repair DNA damage. Marais says that people whose cells are less able to repair DNA damage have a higher risk of developing skin cancer.
“It’s likely to be a combination of many different things.”
How much sunlight?
When it comes to knowing how much sun is too much, Marais says there isn’t a clear answer.
“What we do know is that in lab studies, one shot of exposure is enough to increase the chances of getting skin cancer,” says Marais. “And from large population studies, we know that the more often you burn the higher your chances are.”
Marais says it may be that each person has an individual threshold of damage.
“It’s a numbers game really, you have millions and millions of melanocytes in your skin,” says Marais. “And the more you expose yourself the more likely it is that one of those cells will pick up the wrong constellation of damage in the wrong constellation of genes, and unfortunately that will lead to melanoma.”
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