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by Dr. Barry Starr, Stanford University

If we live long enough, would everyone eventually develop some form of cancer?

-A curious adult from California

I love this question! I think the answer is probably maybe (very satisfying, huh?). To understand why I'm waffling, I need to explain some current theories on cancer and aging and how the two might be related.

One of the most important ideas in cancer research is the two hit hypothesis. The underlying idea is that cancer is caused by changes in our DNA.

These DNA changes affect certain genes called oncogenes. When these genes get DNA changes in them, cancer develops.

But you need changes in both copies of a gene for cancer to develop—this is where the two hits come in. (Remember, we have two copies of most of our genes—one from mom and one from dad.) Normally, one mutation isn't enough.

Unless you start out with one already. This is what happens to people who have certain cancers that run in their family. Let's say breast cancer runs in your family. Then one of two copies of a gene like BRCA1 or BRCA2 may already have a DNA change.

People with a mutation or DNA change in one copy of their BRCA1 gene only need to have the other copy get a DNA change to get breast cancer. And getting a DNA change in only one copy of a gene is much more likely than getting it in both copies.

In fact, this reasoning is what led to the two hit hypothesis. A man named Alfred Knudson compared the rates of cancers that ran in a family vs. cancers that appear out of nowhere.

The rates fit perfectly with the idea that if a cancer runs in the family, then one copy of an oncogene already has a mutation. If it doesn't run in the family, then you need to develop two hits—one in each copy of the same gene.

These DNA changes occur randomly and are a consequence of being alive. Things like UV light from the sun, chemicals in the environment, or even eating can cause mutations. Also, when you copy DNA over and over to get from a single cell to 50 trillion cells, you're bound to mess up every now and then.

So if we assume some sort of constant rate of DNA change, then if we live longer, we'll get more mutations. And eventually everyone will get cancer. As long as whatever we've done doesn't affect how quickly we build up DNA changes, that is.

And this gets us into the theories of aging. Why do we get old anyway? There are two competing theories.

DNA damage caused
the mouse on the left
to age too soon.

In one, we are programmed to age and die by a certain time. This makes some kind of sense when we think about, for example, mice and people.

Mice live a couple of years; we live 75 or so. Why is this? Some people believe the programming found in our genes is responsible.

In the second theory, we age because our DNA gets beat up by our environment over time. Important genes are damaged leading to aging. Eventually, we have too much damage and die.

Let's say the second model is the right one. If so, then living longer wouldn't necessarily lead to more cancers. This is because to live longer, we would need to decrease our rate of DNA change.

And if DNA damage leads to cancer, then there wouldn't be more cancers. The current cancer rates would just be spread out over a longer lifespan.

Now if we are all programmed to die at a certain time, then if we are living longer we somehow changed that programming. If this were the case, then we would get DNA mutations at our current rate. Which would mean we'd all probably end up with cancer if we lived long enough.

Since reality is probably a combination of these two models, my guess is that in our first stage of living longer, we won't develop any more cancers. This is because it will be easier to decrease the chance of DNA damage than to reprogram our cells.

It is much easier to check for radon in the basement, stop smoking, stay out of the sun and, maybe, eat fewer calories, than it is to reprogram all the cells of our body. Once we can tell our cells to last a bit longer, then the percentage of people who get cancer will probably go up.

And if we live long enough, we'll probably all get cancer. Nice to end on a rosy note, huh? Remember, they'll be working on trying to treat and cure cancers better as well so all won't be lost!

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