Sunday, September 22, 2013

What is cancer?

PZ Myers has written a post called What are oncogenes? and I want to reblog the first part of that, because it's such a clear, easy-to-understand description of cancer:
Cancer is not a creative, original disease; it has not been honed by ages of evolution to craft novel lines of attack on your body. Instead, it’s an opportunistic thief. Cancer misuses and perverts existing processes in your cells to send them out of control. Everything cancer does is simply the same thing your cells normally do, only amplified and unconstrained, driven by damage to the genes that would normally regulate their behavior.

Here’s a metaphor, a car with a dangerous defect. It has acquired a glitch in the accelerator so that every time you start it up, it immediately roars up to full speed, as if you’d floored the pedal. The problem hasn’t created anything new in the car, it’s just taken something you normally need to do, that is, regulate the speed of the machine, and stripped you of all ability to control it. That’s what an oncogene does; it is a gene that is normally involved in controlling the rate of cell proliferation, for instance, and a mutation has broken it in such a way that it now tells the cell to divide as rapidly as possible.

Now if you were driving down the freeway and suddenly your accelerator were stuck and you couldn’t slow it down, you’d have alternative strategies to stop (and so does the cell). You could hit the brakes or shift gears or turn off the ignition key. Cancers acquire another set of mutations that destroy the ability to shut off cell processes, analogous to breaking the brake pedal or snapping off the gear shift handle. These genes that can block the effects of out-of-control cell regulators are called tumor suppressors, and I’ll write about those at another time. Today I focus on oncogenes, regulators of the cell that must be damaged by mutation to produce an excessive response.

The first concern that comes to everyone’s mind is that you don’t want to have your cells running amuck — no one wants cancer. Just as you can do your best to maintain your car, you can also live sensibly — eat in moderation, avoid carcinogens or other behaviors that expose you to radiation, and get regular checkups — to reduce the likelihood of deleterious mutations. But they can happen anyway, through no fault of your own. Every time your cells divide, there is a very small chance of an error in replication that inserts a mutation into an oncogene. Just existing, even while doing everything exactly right to maximize your health, brings with it a base chance for a mutation. Given normal rates of cell division, every single one of you reading this is going to acquire about 20,000 DNA lesions today and every day. Almost every one of them will be patched up by DNA repair mechanisms (you have no idea how important DNA repair is to your continued health), but even so, one will occasionally slip through — over your lifetime, your cells will acquire an estimated 10,000 mutations. Live long enough, playing these odds, and cancer is essentially inevitable.

So cancer is fundamentally a chance process. There is no reason people get cancer, no purpose behind it, and everyone is susceptible. Some behaviors can increase the odds — smoking, failing to use sunblock — and you can also inherit genetic predispositions that increase the likelihood of acquiring a full set of mutations that lead to cancer, but ultimately, no one is at fault for cancer.

Fascinating, isn't it? Myers continues with details about oncogenes, if you're interested. (Well, he is a biology professor.) But when I read this, all I could think was,... is it any wonder that evolution is the foundation of modern biology?

Keep in mind that he's not talking about sex cells here, not specifically. Cancer isn't hereditary (though the susceptibility to certain cancers can be), because most of these mutations are to other parts of the body.

But it's a numbers game: 20,000 DNA lesions every day! Only a tiny fraction of those slip through your body's repair mechanisms, but those add up to around 10,000 unrepaired mutations in your lifetime (obviously, depending on how long you live, among other things). Given the numbers - and the reality of 7 billion people on the planet - even long odds will give results that are relatively common (just like the fact that we see many lottery winners, even though our own odds of winning any lottery are pathetic).

Evolution occurs for similar reasons: regular mutations (but these in cells where they do affect reproduction) over long periods of time (sometimes, millions of generations). Most of those have no effect, and most of the remainder are deleterious. But given high enough numbers, even a very small chance can produce an advantageous result eventually.

Of course, results are only beneficial given the circumstances. There's a lot more to evolution than just mutations! A mutation has to be beneficial to that specific organism in those specific circumstances - and even then, it will just increase its odds of survival, its odds of successful reproduction, not guarantee anything.

But in an isolated population, such a mutation will have a chance to spread. Meanwhile, mutations continue. Evolution doesn't happen like magic, as Creationists claim (and, ironically, as their god supposedly created everything), but building on what has come before - in each separate group of each separate species.

Well, I'm getting off the subject here, aren't I? (Not too unusual, for me.) And I'm not a biologist myself, so I'm hardly qualified to explain such things. But I certainly liked this description of cancer.

And, you know, I really have to think that our susceptibility to lottery scams is based on that same difficulty with odds and large numbers which makes so many people doubt evolution, too.

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