Eli5 why does having more than fifty moles on your body make you more prone to skin cancer?
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One of the ways to understand it is viewing moles as a sort of benign tumour (there’s arguments for and against this, but for this explanation it works). Something has happened to the skin’s dna in that area to make a cluster of melanocytes (the pigmented cells) to grow.
One of the risk factors for developing multiple moles across an area is sun exposure. The radiation from it damages dna, altering the code of the dna. And, by just chance, this damage can affect the genes that control division of the cells, leading to tumours. If the damage affects other gems it may of other affects (eg dying) or no affect at all.
Cancer is when the areas of dna damage cause the cells to divide in an unregulated way and allowing them to spread from the original site.
Back to moles. So you’ve already got cells that have something abnormal about them that made them divide more than normal. Further damage to other genes can then further affect the behaviour of those dividing cells and causes them to be able to divide faster and in a more uncontrolled manner, and spread from the original site.
If you’ve got more than 50 moles, especially on sun exposed areas, it reflects that there has been or there still is excessive exposure to sunlight and it’s dna damaging radiation. So more dna is damaged, and therefore closer to becoming cancerous. One random mutation and one area could become cancerous.
The other thing that could be happening is that someone is born already with a dna mutation that makes them prone to this semi-uncontrolled division that leads to moles. And the excessive number of moles reflects those dna variants already present. So the person is already one step closer to cancer development before the sun exposure. So they just need the wrong genes to be damaged to turn this areas into a cancer.
So if someone has multiple moles, slapping on the high spf sun screen and avoiding excessive sun exposure reduces the chances of getting that dna damage that causes cancer.
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