Sue Skin Cancer Post April 20 2015
Did you know you can still get skin cancer hours after you are out of the sun? The culprit: melanin.

Spring has finally arrived for Canadians and we are eager to bask in the sun. We all know that too much sun exposure increases our risk of getting skin cancer. However, a recent study found that the damaging effect of the sun’s ultraviolet (UV) radiation on our skin continues hours after we are out of the sun (1).

Let me back track and first describe why UV light exposure – from both the sun and tanning beds – is harmful.

Direct exposure to UV light leads to the creation of lesions within the DNA of our skin cells. These lesions are called cyclobutane pyrimidine dimers (CPDs) and form rapidly (trillionths of a second). It is thought that most skin cancers arise from mutations caused by these rapid-forming DNA lesions.

Our body possesses safety mechanisms to reduce the harmful effects of UV light. For instance, melanin (the pigment that makes our hair, eye and skin color) helps protect our DNA by absorbing UV energy. In the skin, only cells called melanocytes can produce melanin. Unfortunately, melanocytes account for a mere 10% of all skin cells! So how are the other skin cells protected? Melanocytes help their neighbours by giving some melanin to them, which in turn acts as an umbrella to cover and protect their DNA.

But there is a twist to the story- melanin can also be our foe.

Researchers from Yale University recently discovered that melanin could actively participate in the formation of CPDs (1). It was found that in melanocytes, UV causes melanin to break down into fragments. These melanin fragments contain lots of energy, which can then get transferred to DNA and cause damage. This creates what is called “dark CPDs” since they are generated more than 3 hours after initial UV exposure. What this means is that not only do we need to protect ourselves from the DNA damage that occurs during direct sun exposure, but also hours after we are out of the sun. The good news is that since dark CPDs takes longer to form, it allows time for intervention such as developing “evening-after” sunscreens to block the formation of dark CPDs.

The moral of the story? Don’t depend too much on your melanin for protection from the sun. Continue to limit your exposure to UV light to reduce your risk of skin cancer. Remember that DNA damage from the sun builds up with each exposure and is permanent. So while you are enjoying warm weather don’t forget to practice sun safety! For sun-safe tips check out these guidelines from the Canadian Cancer Society.


  1. Premi S et al. (2015). Chemiexcitation of melanin derivatives induce DNA photoproducts long after UV exposure. Science. 347(6224):842-7.

This article was written by Sue Li. Sue has a PhD from the University of Toronto and is currently working at Princess Margaret Hospital developing new drugs to treat prostate cancer. To find out more about Sue and her research check out her bio on the Members page.

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