Are Cancers Simply Bad Luck?
Worldwide, more than 8 million people die each year from cancer and its related complications. Approximately 14 million people receive a new diagnosis of cancer every year. The World Health Organization estimates that these figures will rise by about 70 percent over the next 20 years. Because it is one of the leading causes of human morbidity and mortality, scientists have been crunching numbers in an effort to understand cancers and design strategies to prevent them.
For more than a century now, it has been well understood that some tissues in the human body are much more prone to giving rise to cancer than others. Scientists have concluded that these differences are on account of the number of stem cell divisions that occur in each type of tissue. A strong correlation has been found between the lifetime incidence of cancer in a tissue and the number of stem cell divisions that normally take place in that tissue. In other words, random errors in DNA replication during stem cell division are, to a large degree, responsible for the development of cancer.
It is estimated that inherited predispositions and environmental factors contribute to only one-third of the variation we see in cancer risk among different tissues. The majority of the increase in risk is due to accidental mutations in normal adult stem cells undergoing division. This would suggest that cancers are simply bad luck.
There are contradictory claims to this theory, however. Some researchers believe that bad luck does play a role, but is not the primary contributing factor in the development of cancer.
All cancers are the result of mutations in the DNA when cells are growing and dividing. These errors lead to an uncontrolled growth and division of the cells, resulting in cancerous growth of the tissue. Lifestyle factors such as sun exposure and cigarette smoking are well-known causes of some types of cancers. Other cancers are the result of inherited DNA mutations that are passed on from a parent to a child. However, environmental and genetic factors cannot fully explain the varying risk of cancer in different tissues.
Now, new research at the University Medical Center in Utrecht in The Netherlands suggests that the rate of acquired random DNA mutations in various organs remains stable regardless of the patient’s age or the tissue that gave rise to the stem cells. The study led by Dr. Ruben van Boxtel assessed normal adult stem cells from the small intestine, colon, and liver to measure the rate and pattern of DNA mutations in these tissues. The stem cells were obtained from human donors between 3 and 87 years old. The results, published in Nature, found that an average of 40 mutations a year accumulate in stem cells and that this number remains steady in people of all ages and in cells from different organs. The researchers concluded that while bad luck is certainly part of the story, it can only partially explain cancers.