Twin study finds “environment has the principal role in causing sporadic cancer”

The debate about the causes of cancer is on-going. Many cases of cancer are attributed to carcinogens such as tobacco, alcohol, radiation, occupational toxins and infections. A widely accepted estimate is that 80 to 90% of human cancer is due to lifestyle and environmental factors. However, recent well-publicised developments in molecular genetics have revealed genetic mechanisms which pre-dispose people to cancers.

One way of distinguishing genetic from environmental traits is the study of twins’ medical histories. Medical researchers recently compiled and analysed information based on twin registries from Denmark, Finland and Sweden, to assess the risks of cancer for the twins of people with cancer. The study included more than 10,000 cancers in a total population of nearly 90,000 Scandinavian twins.

Statistical modelling was used to estimate the relative importance of hereditary and environmental factors in causing cancer at 11 anatomical sites. An increased risk was found among the twins of affected persons for stomach, colorectal, lung, breast, and prostate cancer. Statistically significant effects of heritable factors were observed for prostate cancer (42% of the risk may be explained by heritable factors), colorectal cancer (35%), and breast cancer (27%).

The report, published on July 13th in the New England Journal of Medicine, concludes that inherited genetic factors make a minor contribution to susceptibility to most types of tissue growths. This suggests that environment has the principal role in causing sporadic cancer. However, the relatively large effect of heredity in cancer at a few sites is a reminder that there are major gaps in our knowledge of the genetics of cancer, and much work remains to be done.