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John W. Gofman, M.D., Ph.D., CNR Chairman
Egan O'Connor, CNR Executive Director. April 2001.
A Scandinavian study of twins has explored the chance that twins would develop the same kind of cancer (July 13, 2000, New England Journal of Medicine Vol.343: 78-85). When all types of cancer are considered, the study finds that even identical twins seldom develop the same kind of cancer. Because identical twins start out with identical genes at conception, the study's authors conclude "that the overwhelming contributor to the causation of cancer in the populations of twins that we studied was the environment."
The study received widespread attention in the mass media and in newsletters. For instance, Newsweek featured the study in an article entitled "Stop Blaming Your Genes," in which Newsweek wrote, "It's time to grow out of our love affair with all things genetic and figure out what is really killing the 1,500 Americans who will die of cancer every day this year." Some environmental and health newsletters picked up on that theme, suggesting that too much emphasis had been placed on "the genetic causes of cancer" and on the human genome project, and too little on toxic agents. Such comments appear to reflect the mistaken assumption that "genetic" always means "inherited."
Genetic Causes of Cancer: Inherited and Acquired
Genetic abnormalities can be divided into two classes: Inherited and acquired. Inherited genetic abnormalities are present in the genetic molecules (the chromosomes) of each of the estimated 50 trillion body-cells of a human adult, because the genetic abnormalities were present in the fertilized egg.
By contrast, acquired genetic abnormalities (mutations in the chromosomes) are acquired by individual cells during a person's lifespan, and are present only in the cells which are descendants of those particular cells. Acquired genetic abnormalities can range from very small chromosomal changes to complex alterations in chromosomal structure.
Several lines of evidence convince most cancer biologists that cancer starts its development from one genetically abnormal cell. Indeed, in order to become a malignancy, that one cell almost certainly must accumulate (acquire) several predisposing genetic abnormalities in addition to any predisposing genetic abnormalities which are present from heredity. The Scandinavian twin-study does not disparage the importance of acquired genetic abnormalities in the causation and behavior of cancer cells.
Unlucky individuals inherit genetic abnormalities which can predispose every cell to become malignant, but the full set of genetic abnormalites required for malignancy is very rarely (if ever) inherited. Instead, the full set is accumulated in a particular cell over time. And the probability of accumulation in the same cell is determined by the cell's "environment" --- by its exposure to agents which can cause alterations in that cell's genetic molecules. Because the importance of acquired genetic abnormalities, in causing cancer, is very widely acknowledged, no discontinuity really exists between "genetic causes" and "environmental causes" of cancer.
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