As I was considering actually studying for my Genetics and Developmental Biology exams (considering studying as opposed to actually studying), I figured not everyone knows the prevailing theory about how cancer comes about, so I thought I’d try to explain it, minus the medical mumbo-jumbo, for the good of humanity. As well as the good of my procrastination.
Always best to start at the beginning: what’s cancer? A cancer is a tumor. And what’s a tumor? A tumor is a mass of cells that have decided to grow because of gene mutations, disobeying the body’s signals and rules about when cells can divide (multiply) and grow. Cancer is a malignant tumor–that is, it keeps growing and growing, multiplying, and potentially invading other parts of your body. A benign tumor is one that grows, but generally respects its boundaries still (ie: isn’t able to leave its part of the body and spread to other parts of the body and start new tumors).
The prevailing theory is that cancer comes about due to mutations in DNA. Mutations in your DNA are due to deletions of part of your genes (areas of your DNA that make your body’s proteins). Mutations come about if you’re exposed to radiation, certain chemicals, or can even occur due to your body’s own errors when making new DNA.
But your body’s prepared. Evolution has told it that cells some times get DNA wrong, or DNA gets mutated. There are an enormous number of genes and proteins whose only job is to keep you from getting mutations (and therefore, keep you from getting cancer). There are more of these genes, called “tumor-suppressor genes,” being discovered every day. They act in a lot of different ways:
* Some of them go through your DNA and read every single part of it, one at a time, to make sure there are no errors. If it finds an error, it calls in a bunch of other proteins to cut out the mutation (plus some of the good DNA on each side, to make sure it’s gotten all of it), and re-make the correct sequence of DNA.
* Some proteins tell your cells to quit growing or dividing, so they don’t start getting out of control.
* Other proteins actually tell some of your cells to commit suicide (known as apoptosis). If one of these proteins starts to detect that the cell may be starting to turn into a tumor, it can force the cell to die.
And there’s a lot of overkill with these and other preventative functions. There are a bunch of apoptosis proteins, a bunch that repair your DNA, etc. So your body knows things can go wrong, and it does its best to watch for them.
But, so the theory goes, every so often, in some people, your body’s going to miss one of the mutations in your DNA. It’s not perfect. It has an error rate of 1 error per billion, which is pretty damn incredible, but isn’t always good enough. And, every so often, in some people, that mutation that it doesn’t catch is going to be in some really important gene that is helping you from getting cancer.
So, say, that cell keeps growing, even though it’s been told not to (maybe it no longer understands the signals it’s being sent, doesn’t know what to do with the signals, or just ignores the signals completely). So it grows, and multiples. One cell to two, two to four, four to eight, and so on. And here’s how the theory works: if one random mutation gets a cell to start growing quickly, that cell’s progeny will have that initial growth mutation, and because it’s growing so quickly, one of its progeny might develop a second mutation (maybe it starts ignoring cell signals to commit cell suicide). So from that first cell, you get 500 cells. But of those 500, maybe one gets that second mutation. So that new cell with two mutations grows even faster. Understand why cancer can be so dangerous yet? All it takes is one rotten mutation to create some very rapid, uncontrolled cell growth.
It’s the exponential growth and rapid mutation that really makes cancer so bad. It’s basically like a micro-version of evolution: the cells that can grow the fasted and most unrestrained divide and create progeny that are more likely to survive. For cancer cells, it’s almost like the more mutations, the better. Cancers generally can’t grow much larger than a couple millimeters in size without a blood source. So, unsurprisingly, most cancers secrete proteins that cause blood vessels to grow and develop around them. And many that escape their initial location and take up residence in some other place in the body (the process of metastasis) have to secrete proteins that let them break down membranes that keep them in a certain part of the body, as well as other proteins to let them invade other tissues, too.
This is all why chemotherapy (drugs given to patients with cancer) is so toxic. Many of the drugs specifically try to create more mutations, in hopes that they’ll get into cancer cells and be so toxic to them that they end up being deadly to the cancer cells. Some of the side effects, however, are that these mutations end up killing many of the body’s normal cells, too. It’s the same reason radiation is used: if you can irradiate cancer cells, you can mutate them, and hopefully kill them off. And since cancer cells are so great at growing and so dangerous, it’s why many cancer patients get such strong doses of both radiation and chemotherapy: if you leave even one cancer cell, you can potentially allow it to create a whole new cancer sometime in the future.
And finally, why some people are more likely to get cancer than others, and how some cancers can run in families. Some people develop cancer without any family history. But others carry a gene that has some sort of initial or half-mutation so that it makes it much easier for them to get cancer. We have two copies of our DNA in our cells–one set from mom, one set from dad. If a cell normally needs two mutations to lose one of its cancer-prevention functions, and it already has one from a mutation mom gave us, the odds are much more likely that a person will develop cancer compared to a person that needs two mutations. This is known as the “two-hit hypothesis,” if you were wondering.
Hope that helps somebody at some point. If you have questions or want anything more specific, just ask.