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 The Causes of Cancer: Is There "Too Much Emphasis on Genes, and Not Enough
                            on the Environment?"
                         How to Avoid Some Mistakes

                 John W. Gofman, M.D., Ph.D., CNR Chairman
             Egan O'Connor, CNR Executive Director. April 2001.

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         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
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         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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