Cancer may be mostly a matter of chance

This is the message of a paper by Tomasetti and Vogelstein, that came out last week. Unlike a lot of papers in Science it is beautifully written, with a simple idea and a clear message. They start with the simple observation that some cancers are much much more common than others. For example, the American figures they quote give a lifetime risk of cancer of the colon of 5%, and risk of bone cancer of the pelvis of 0.0003%. Why the difference?

Roughly speaking, tissues such as our colons and pelvises contain two types of cells: The cells that do the work, such as digesting our food and maintaining our bones, and the cells that can divide and produce more of these worker cells. There is a high degrees of specialisation going on here. It is the cells called stem cells that divide to produce more worker cells, as these worker cells wear out and die. Stem cells are essential to maintain our bodies, but they are dangerous. Mutations in the DNA of the genes that control their division and growth can easily lead to uncontrolled growth, which is what cancer is. The cells that do the food digesting etc are less dangerous as they typically have much less ability to divide and grow. I am not an expert here, but I think that there is at least some evidence that most cancers arise from the stem not the worker cells.

Tomassetti and Godelstein’s big idea is simple, that each time a stem cell anywhere in our body divides there is a roughly constant probability that it will result in cancer. To back this up they plot the lifetime probability of cancer occurring in different tissues, as a function of the estimated number of stem cell divisions in that tissue. There is a pretty good correlation between the two. Indeed our colons are big (about 1.6 m long) and are a tough environment, so our colon stem cells have to do a lot of dividing over our lifetime to replace the huge numbers of cells that die in our colons each day. Life is easier and more sedentary in our pelvis.So much less stem cell division is needed there. And so much less of a chance of cancer, or so Tomassetti and Godelstein believe. The real picture may not be that simple, but it is a very nice idea.