Cancer, Part 2 – How cancer develops

Cancer cells arise from changes, or mutations, in the DNA (genes), in normal cells. Such genetic “mistakes” may occur during the replication process, when there is an error during copying of DNA strands from a parent cell to its two “daughter” cells. Alternatively, DNA may be damaged by environmental exposure, i.e. chemicals, tobacco smoke, or harmful radiation from the sun. Another source lies in genes we inherit from our parents that may predispose to certain types of cancer. Cells with altered, cancer-producing genes are usually eliminated by our immune system, which recognizes them as foreign. Sometimes, unfortunately, these cells escape detection and begin to replicate. As explained in “Cancer: Part 1,” what makes these cells deadly is their non-stop replication, unresponsive to the usual signals that terminate the replication process, and their ability to spread through the bloodstream or lymphatic channels to other parts of the body. 

The cells comprising individual tumors have their own, unique cancer-causing genetic errors. As they continue to replicate, additional genetic errors may occur. The original cancer cells may be susceptible to specific chemo- or immunotherapy, but the accrual of genetic mutations may render them resistant. This leads to cancer recurrence or poor response to treatment.

Chemicals that cause cancer by changing a cell’s DNA are called carcinogens. These may be in the air we breathe, the water we drink, our food, or in our workplace. A comprehensive list of known carcinogens may be found in the US Department of Health and Human Services “15^th Report on Carcinogens.” The degree to which these chemicals are carcinogenic depends on the magnitude of exposure. Tiny amounts of the metal, cadmium, for example, may be harmless, whereas heavy industrial exposure may pose a significant cancer risk.

It has been known that cigarette smoke may cause cancer since the 1940s, as shown by population (epidemiologic) studies. In the 1950s it was confirmed that particles in cigarette smoke cause cancer in mice. In the 1960s it was discovered that specific molecules in smoke, including benzopyrene, cause DNA mutations that can lead to cancer. At present there are more than 70 known carcinogens in cigarette smoke.

A variety of “cancer genes” may be inherited from our parents. Mutations in pro-oncogenes, tumor suppressor genes, and DNA repair genes are known to cause cancer. When DNA repair genes are mutated, for example, they become ineffective at doing their job, allowing abnormal DNA strands to replicate. These may become the origin of cancer cells.

Note: Although I am a physician, the content in this article is not meant to diagnose, treat, cure or prevent illness or disease in the reader – it is for educational purposes only.

References

• 15^th Report on Carcinogens. U.S. Dept. of Health and Human Services, Public Health Service, National Toxicology Program. https://ntp.niehs.nih.gov/go/roc15 
• Llewellyn GC, Rihner MO, Hanlon PR. An evolution of risk assessment for potential carcinogens in food: Scientific session proceedings. Regul Toxicol Pharmacol. 2021 Nov;126:105047
• Ding YS, Ashley DL, Watson CH. Determination of 10 carcinogenic polycyclic aromatic hydrocarbons in mainstream cigarette smoke. J Agric Food Chem. 2007 Jul 25;55(15):5966-73.