Thursday, November 8, 2007

How cancer starts

My recent genetic testing has rejuvenated my wondering about how cancer starts. I'm not a biologist, but I am a physicist and it's natural for me to try understanding the behavior (and misbehavior) of our bodies and to hypothesize about what mechanism has caused my and other cancers.

First, I'm amazed that the human body works at all. It's an enormously complex machine that actually manages to keep itself alive rather well. It can accept a huge variety of food to maintain its structure and chemical balance. It takes all sorts of abuse and repairs damage to itself for years and years. Mechanically it is far more durable than mankind's best automobiles and airplanes. Computationally it performs tasks easily that sixty plus years of exponential growth in electronic computing has barely touched.

This is all accomplished through interacting chemical systems refined over millions of years of evolution. Our bodies are full of countless tiny chemical reactions tuned by feedback mechanisms to keep the whole system functional. There are systems to detect damage, repair it by regrowing lost cells, and stop when the repair is complete.

If we break a bone then a torrent of cells activates to clean up the mess and fill in the gap. Putting more load on our bones and muscles drives them to strengthen. Spending time in bed or as an astronaut in zero gravity spurs our bodies to save energy by diminishing our bones and muscles.

All of these chemical systems repair not only damage to the body as a whole but also microscopic damage to the systems themselves. We are continually bombarded by solar radiation, chemical poisons, mechanical wear, and the gradual disintegration of molecules over time due to the simple vibrations each atom makes a trillion times a second.

Because of all the interactions and redundancy built into us by the driving force of evolution, it takes many simultaneous failures to lead to death of our whole body. It's similar to how accidents work on a larger scale. A traffic fatality requires several things to go wrong: at least one driver disobeys traffic laws or becomes confused by a poorly designed intersection, the other driver doesn't notice the hazard in time, the brakes slip on rain-soaked pavement, the angle of collision bypasses the crumple zones built into the cars, the seat belts and air bags are unused or ineffective, the injuries go beyond what the passengers bodies can repair themselves, and the paramedics are unable to provide life-saving aid in time.

It's rare that a single mistake — taking one's eyes off the road to dial a cell phone or following the car ahead too closely — is enough to result in a fatality. Our bodies have many more systems to catch errors before they kill us. Faced with the danger of spoiled food we can save ourselves by seeing the discoloration, smelling the foul odors, gagging and spitting at the disgusting taste, vomiting, or neutralizing the ingested toxins through the chemical response of our digestive and immune systems.

Cancer is one way our bodies can fail, and it's particularly cruel since it starts as a way by which our bodies succeed: growth and repair. A cancer originates when a cell suffers damage to its genetic code and forgets to stop growing when its task is complete. But our cells have ways to detect genetic changes, so the change won't take effect unless the error correction system fails first. And our bodies already deal with misbehaving cells all the time by isolating and killing them. So there must also be a failure in recognizing the bad cell and stopping its rampage. Cancerous cells have the advantage that they are closely related to our healthy cells so it's harder for the immune system to recognize them as dangerous.

I used to wonder why cancer wasn't far more common. Ultraviolet light shreds our cells and mutates our DNA, but most of us don't develop skin cancer. Some people smoke for decades and fill their bodies with carcinogenic toxins but never get lung cancer. Now I realize that it takes multiple simultaneous failures for cancer to take hold.

A genetic mutation caused by the environment has a chance to be repaired. Or it might occur in a part of our DNA that's not critical — a skin cell changes its pigmentation. Or it could be immediately lethal to the cell and never spread. Or the cell could be recognized and killed by other cells. Or it could grow unrestrained but too slowly to affect overall health.

Environmental damage to our bodies is cumulative because it damages some part of our safety mechanism and makes it more likely that the next bit of damage will go uncorrected. Some of us are born with hereditary changes that didn't happen to have ill effects in our ancestors or that were beneficial under different circumstances.

This explains why it can be hard to determine which substances in modern life contribute to cancer. Maybe a certain artificial color doesn't cause cancer alone but it lowers the bar for the next toxin. Or it depresses one safety mechanism and becomes dangerous when combined with some other factor. Cigarette smoke must be very, very bad that it can so clearly be shown to be dangerous. The impact of the flood of new chemicals in our environment, after our ancestors have adapted though millions of years of savage evolution in a different environment, will be hard to sort out.