A study conducted by Harvard scientists in collaboration with Danish and Finnish researchers found that at least 22 types of cancer are genetic diseases that are passed from generation to generation in families. According to the results of this large study, about one-third of cancers are inherited. Those with the highest familial inherited risk include prostate, breast, lung and bowel cancers. The study, conducted by Harvard scientists in collaboration with Danish and Finnish researchers, found that at least 22 cancers are genetic diseases that are passed from one generation to the next in families. According to the results of this large study, about one-third of cancers are inherited. Those with the highest familial inherited risk include prostate, breast, lung and bowel cancers. Scientists found that if a sibling gets these cancers, the risk of other siblings getting the same cancer increases by 33 percent. This is the largest and longest-running study ever to examine the relationship between genetics and cancer. The study found that the genetic risk varied for different cancers. Melanoma and testicular cancer were among the most strongly associated with genetics. The overall findings are not surprising, but rather further confirm earlier findings that about 1/3 of cancer cases are caused by genetic defects, with most of the rest attributed to so-called lifestyle factors such as smoking, diet and exercise. Genetic risk varies by type of cancer A team of researchers led by Lorelei Mucci of the Harvard School of Public Health reported in the Journal of the American Medical Association that about 38 percent of kidney cancers, 31 percent of breast cancers, 27 percent of uterine cancers, 58 percent of melanomas, 57 percent of prostate cancers and 39 percent of ovarian cancers have now been found to be genetically linked. Researchers say that more than 8 percent of children with cancer have unexpected genetic mutations in their families. Memphis, of Jude Children’s Research Hospital, believes the findings not only use new ways to detect the presence of cancer risk in children and their families, but also may lead to better ways to treat them. The study followed more than 200,000 twins in Sweden, Norway, Denmark and Finland from 1943 to 2010, tracking each individual for an average of 32 years. This large-scale twin study allowed scientists to assess the importance of genetic factors in cancer and to assess the risk of cancer in families by measuring the genetic similarities and differences between identical and fraternal twins. Identical twins are genetically identical, while heterozygotic twins are only genetically similar, so the difference in cancer risk between the two points clearly to genetic factors. Mucci’s group also found that 38 percent of identical twins and 26 percent of heterozygous twins were diagnosed with cancer. When one of the twins was diagnosed with cancer, the risk of the other developing cancer increased significantly, from 37 percent for heterozygotic twins to 46 percent for identical twins. Jacob Hjelmborg of the University of Southern Denmark said, “Because of the large size of this study and the long follow-up period, we can see the impact of key genes on many types of cancer.” The genetic risk for testicular cancer is high. A person’s risk of developing testicular cancer increases 12-fold if his or her identical twin brother has testicular cancer. And for identical twins, one person with testicular cancer has a 28-fold increased risk of the other person developing testicular cancer. The team also found that about 1 percent got melanoma, which is the deadliest form of skin cancer. If one of the identical twins had melanoma, the risk of the other person getting melanoma was 6 percent, while for identical twins, the risk of the other person getting melanoma was 20 percent. The risk of prostate and breast cancers may be due to the fact that they shared a womb as children, and both are strongly influenced by hormones, including those that affect fetal growth. Lung cancer is more influenced by the environment, possibly due to the fact that twins have mostly the same smoking habits. It is important to note that cancer was the number one cause of death in the Nordic countries that participated in the study. In the United States and other developed countries, cancer was the second leading cause of death, after heart disease.