There are more than 100 kinds of cancer, but according to Robert Weinberg, cancer researcher at MIT's Whitehead Institute in Cambridge, they all have one thing in common. All are caused by mutations to several genes on the DNA of a single cell. And most of the damage is done to genes that regulate cell growth. Most cells only make copies of themselves every once in a while, usually to replace similar cells that have gotten old or damaged and have died off. And they only reproduce when molecules called growth factors land on special receptors on the outside of the cell membrane. The growth factor sends a message to a chain of proteins inside the cell, setting off an intracellular game of Post Office. Each protein interprets the message and either passes it along, changes it a little, or doesn't pass it along at all. Only the right message can finally get to the DNA and make the cell reproduce.
The blueprint for each of those proteins is in the genes on our DNA. When something mutates the genes that make proteins which control cell growth, those proteins are defective. They don't work right. The message in the game of Post Office gets changed. Growth regulation breaks down and cancer starts.
"Cancer," says Weinberg, "is a disease of cells that are proliferating out of control."
Fortunately, the cell has so many growth-controlling systems that several mutations have to occur, all in the same cell, before it becomes cancerous. And the chances of all that happening to just a few critical sections of one strand of DNA are relatively low.
So what causes these mutations? Sometimes they occur naturally, when a cell makes a copy of itself and its DNA, and doesn't get things exactly right. Once every million or ten million times, the DNA in the daughter cell isn't a precise copy of the DNA in the mother cell. If that mutation damages a gene that controls cell growth, it's the first step toward what could become cancer.
Here are a couple of examples. --The estrogen cycle in women causes monthly bursts of cell growth. Since women are reaching menstruation at an earlier age, they are experiencing more menstrual cycles, more cell growth, and more natural mutation. Scientists think that may be contributing to higher rates of breast cancer.
--Heavy alcohol consumption kills cells in the mouth and throat. That triggers the remaining cells to reproduce and the rate of natural mutation goes up.
Another source of natural mutation comes from chemicals called free radicals our cells produce as part of their energy cycle. Cells can protect against most, but not all, of the internal damage caused by free radicals.
Genetics plays a role in cancer. A person who inherits a damaged growth-regulating gene may never develop cancer, but their chances are much greater because every cell in their body is already one step down that road.
Weinberg says external factors are the biggest cause of mutations to our genes. Life style choices like tobacco smoke, diet (too much fat/too few fruits and vegetables), or too much exposure to the sun, expose us to carcinogens.
We're also exposed to external carcinogens like radon, motor vehicle exhaust, and biological agents like viruses.
The scientific war on cancer has produced incredible insights into how cancer gets going in a cell, and how it spreads throughout our bodies. Those insights make scientists like Weinberg hopeful that cures may not be far away. Next week: How cancer spreads.
