Saturday, October 31, 2015

How some animals avoid cancer

I found an interesting article on the BBC World website about animals that don't get cancer (or that are at least prone to much less cancer than expected).
About 20% of humans are likely to die from some from of cancer. But what is less well known is that almost all animals, even marine animals, get cancer, in much the same way as humans do. Even sharks, which have gained a mythical reputation for being completely cancer-free, are susceptible to melanoma skin cancer.
Cancers occur when old or damaged cells spiral out of control and keep reproducing instead of being destroyed. So, typically, larger animals and animals that live longer lives trend to be more prone to cancer, simply because this increases the likelihood that one of its cells will succumb to a random cancer-causing mutation to begin the process. Thus, large dogs get more cancers than small dogs, and even taller humans are more cancer-prone than shorter people.
But there are some exceptions to this.
Elephants are clearly much bigger than humans and have a comparable age span, but only a quarter as many elephants die from cancers as do humans. The reason appears to be that they have twenty times more copies of a particular cancer-fighting gene known as p53 than we do. The gene stops a mutated cell from proliferating, giving it the time it needs to repair itself, and, if the cell cannot be fixed, the gene prompts it to commit suicide ("apoptosis").
Bowhead whales are among the largest animals on the planet, and can live up to 200 years, but they very rarely die from cancer. The key here appears to be a poorly understood mutation in their genome that helps prevent DNA from being damaged, thus protecting the whales from cancer to some extent.
At the other end of the scale, naked mole rats, diminutive and strange-looing rodents, regularly live as long as 30 years, much longer than would be expected of an animal of its size, and have never been observed to contract any kind of cancer. In their case, a thick, sugary substance called hyaluronan, found in the spaces between their cells, prevents mutated cells from dividing and reproducing further, thus preventing cancerous tumours. Humans also produce hyaluronan, but a different shorter-chain version, and in much smaller quantities.
Of course, even if any or all of these natural cancer-fighting techniques could be artificially replicated, there is as yet no way of knowing if any of them would be beneficial in humans. In the few tests that have been done on mice to date, the improved ability to prevent cancer comes with other, often fatal, side effects. Through evolution, over the millennia, elephants, bowhead whales and naked mole rats have obviously developed other adaptations to overcome the negative impacts, adaptations that we are only beginning to look into and understand.
As usual, there is no quick fix.

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