Environmental Factors Greatly Influence Cancer Risk (dateline July 26, 2000)
Factors such as diet, smoking, alcohol, exposure to chemicals and radiation, and lack of exercise can significantly influence a person’s likelihood of developing many types of cancer, according to a study of nearly 45,000 pairs of twins in three countries. However, the study also found that heritable factors play a role in over one-fourth of breast cancer cases, suggesting that scientists still lack an understanding of many of the genetic components involved with breast cancer.
In studying the risk of cancer at 28 sites on the body, the researchers concluded that on average, environmental factors cause approximately twice as many cancers as genetics. There is a sense that if your brother or mother has cancer, you are destined to have it too, said Robert N. Hoover of the National Cancer Institute. However, this study shows that controllable environmental and behavioral factors such as diet and exercise can influence cancer development. It is important to note that all cancers are due to changes in genes, though many of these genetic changes are caused by environmental or behavioral factors.
To reach their conclusions, the researchers examined government records of 44,788 pairs of twins listed in the Swedish, Danish, and Finnish twin registries. Identical twins have the same genetic makeup while fraternal (non-identical) twins share 50% of the other’s genes.
While environmental factors strongly influenced cancer development, the researchers did find a strong genetic component in breast, prostate, and colorectal cancers. Approximately 27% of breast cancer cases are due to heritable factors according to the study, while 42% of prostate cancers and 35% of colorectal cancers are caused by heritable factors. In the study, heritable factors played no role in cervical or uterine cancers.
The large genetic component seen in twins with breast cancer in the study reveals that significant research is still needed in this area. Researchers have previously found that women who have genetic mutations of the BRCA1 (breast cancer gene1) or BRCA2 (breast cancer gene 2) genes are at a higher risk of developing breast cancer than women without these genetic mutations. However, BRCA gene mutations only account for 5% to 10% of all breast cancer cases. This study suggests that genetic components other than the BRCA genes may also be involved with breast cancer.
While 27% of the breast cancer cases in the study were due to genetic factors, many of the remaining cases could be due to environmental and behavioral factors. Researchers are yet to determine why some women develop breast cancer while others do not, but they have identified a number of risk factors , other than genetics, that can contribute to breast cancer incidence. These factors include:
- Delayed childbirth (having a first child after age 30)
- Not having children
- Alcohol and smoking
- Diet and weight
- Previous radiation therapy (especially at a young age)
- Hormone replacement therapy (HRT) (controversial; most experts agree that breast cancer risk is not significant if a woman is on HRT for less than five years)
While further research may examine which specific environmental factors cause the majority of sporadic (non-inherited) cancer, the twin study shows that lifestyle choices can greatly increase or decrease the likelihood of cancer.
- The study discussed in this article, “Environmental and Heritable Factors in the Causation of Cancer – Analyses of Cohorts of Twins from Sweden, Denmark, and England,” is published in the July 13, 2000 issue of the New England Journal of Medicine. An abstract of the study is available at http://www.nejm.org/content/2000/0343/0002/0078.asp
- To learn more about the risk factors for breast cancer, please visit http://www.imaginis.com/breasthealth/bc_risks.asp
- To learn more about breast cancer and genetics, please visit http://www.imaginis.com/breasthealth/genetic_risks.asp