Of moles and men – how sunlight and moles affect the risk of melanoma

Having lots of moles increases the risk of skin cancer but staying safe in the sun is still important.

An article in the Sunday Times yesterday claimed that “sunshine is not the main cause of the most dangerous form of skin cancer” and that “warnings about the perils of sunbathing are scaring people unnecessarily.”

The Sunday Times piece was based on a new study published in Nature Genetics that looked for variations in people’s genes that increase their risk of developing melanoma. The study found two different genetic variants that affect the number of moles that people have. They also showed that these variants are also linked to a higher risk of melanoma.

These results are interesting – we know that people with lots of moles have a higher risk of melanoma, so it makes sense that genetic factors which increase the risk of having lots of moles also affect the risk of this cancer.

But these results are barely touched upon in the Sunday Times piece, which uses it as a basis for attacking advice to stay safe in the sun. But to say that that moles are more important than sunlight in increasing the risk of melanoma is confusing and contradictory. Let’s look at why.

Moles, sunlight and melanoma

Our skin gets its colour because melanocytes – specialised cells found in our skin – produce a dark pigment called melanin. When our skin is exposed to sunlight, these cells make more melanin and our skin darkens and tans.

Moles (or to give them their medical name, ‘nevi’) are concentrated clumps of melanocytes, which is why they look dark in colour.

A melanoma is a type of cancer that develops from melanocytes; it’s what happens when these cells develop mutations – changes in their DNA – that make them grow out of control.

This is why people with more moles (clusters of melanocytes) have a higher risk of melanoma. In fact, one study found that melanoma is almost 7 times more common in people with over 100 moles, compared to those with fewer than 15 moles.

But the importance of moles doesn’t negate the importance of staying safe in the sun. Overexposure to ultraviolet radiation from the sun can damage DNA in skin cells, causing mutations that could lead to melanocytes dividing out of control. So having lots of moles puts people one step closer on the road to melanoma, and getting too much sun exposure (especially to the point of burning) nudges our skin along that road.

In fact, there’s solid evidence that sun exposure can play a role in the development of moles in the first place (as reviewed in this paper)! And some trials have shown that staying safe in the sun can reduce the number of new moles developing on children’s skin.

And it’s important to remember that not all melanomas develop from pre-existing moles.

Sun exposure and other melanoma risks

In addition to moles, other things can increase your risk of melanoma, the most serious type of skin cancer. As we mention on our SunSmart site, these include:

• fair skin that burns easily
• a history of sunburn
• red or fair hair
• light-coloured eyes
• a personal or family history of skin cancer

And most of these traits are again related to sun exposure. People with fair skin have a higher risk of melanoma because they are more likely to burn in the sun and accrue the sort of DNA damage that can turn a healthy cell into a cancerous one.

Even in people who inherit a higher risk of melanoma because of faulty genes (such as the CDKN2A gene), sun exposure can still have a positive impact on their risk, according to research published in peer-reviewed scientific journals. In one study, people with faulty versions of CDKN2A were 26 times more likely to develop melanoma if they had been sunburnt in the past.

Another study found that in families carrying a CDKN2A mutation, the risk of developing melanoma by age 80 was 91 per cent in Australia but only 58 per cent in Europe. This suggests that greater UV exposure exacerbates the effects of CDKN2A mutations. The scientific evidence strongly suggests that sun exposure is a crucial factor in determining melanoma risk, even in people whose genes predispose them to the disease.

Checking moles vs. sun safety

The Sunday Times piece says that “health warnings would be more useful if they focused on people who have more than 100 moles, and taught them to check regularly the moles for changes in shape, size or colour.”

That’s certainly true in terms of detecting melanoma early, and again, we provide advice on checking moles on our SunSmart website. Encouraging people to spot early signs of melanoma is important because if the disease is caught in its earliest stages, it is much easier to treat successfully.

However, there’s a crucial distinction to be made here. Checking your skin helps detect melanomas that have already developed. But advising people to stay safe in the sun aims to prevent melanoma in the first place.

Are rates of melanoma going up?

Melanoma is the fastest rising type of cancer in the UK. Rates have quadrupled since the 1970s and now, more than 10,000 cases are diagnosed every year. Our statisticians estimate that within 15 years, there will be around 15,500 cases per year. Similar trends have been found in other countries around the world.

The reasons behind these rises are controversial. It seems reasonable that a rise in tan-seeking behaviour, foreign holidays to sunny places, or sunbed use may be contributing to the rise. However, many scientists have suggested that the rise simply reflects more diagnoses, with skin-checks “picking up borderline changes in the skin that were unlikely to cause harm.”

But that idea was contradicted by a US study published earlier this year, which showed that incidence rates were going up for melanomas of all types and sizes. Even larger tumours are becoming more common. Incidence rates have also increased in all socioeconomic groups.

If increased detection was behind the rise in melanoma rates, you would expect the rise to only apply to smaller tumours that were being diagnosed earlier. That’s not the case. You would also expect incidence rates to be more level in more deprived populations that have more limited access to screening and health checks. Again, that’s not the case.

The bottom line

There is no question that having lots of moles puts you at a significantly higher risk of developing melanoma. But does this mean that you can ignore recommendations to stay safe in the sun?

Not at all. In fact, the opposite is true. Overexposure to ultraviolet radiation from the sun is the major cause of melanoma.

People who have lots of moles (or other physical traits like fair skin that put them at higher risk) need to be especially careful in the sun and take steps to avoid sunburn. They also need to keep an eye on their skin and report any unusual changes to moles or any other patch of skin to their doctor, as soon as possible.

Ed

Reference:

Falchi, M. et al (2009). Genome-wide association study identifies variants at 9p21 and 22q13 associated with development of cutaneous nevi Nature Genetics DOI: 10.1038/ng.410