A new study published in JAMA studied the effect of genetics heredity on overall cancer incidence, as well as for specific cancer types, and found a substantial genetic contribution to cancer's toll.
The Nordic Twin Study of Cancer collaboration collected health data on cancer incidence among over 200,000 twins from Scandinavia (Denmark, Norway, Finland and Sweden), with a mean follow-up of 32 years, beginning in 1943. The study subjects included 80,000-plus monozygotic (identical) and 123,000-plus dizygotic (fraternal but same sex) twin individuals, among whom 27,000-plus cancers were diagnosed, a cumulative incidence of 32 percent. Thirty-eight percent of the identical twins and 26 percent of the fraternal twins had the same type of cancer.
The international multicenter study group was led by Lorelei Mucci, Sc.D., MPH, of the Harvard T.H.Chan School of Public Health, with co-authors based in Iceland, Sweden, Norway, Denmark, Finland, and Singapore. The article was entitled "Familial Risk and Heritability of Cancer Among Twins in Nordic Countries." They found that there was a 33 percent increased risk of cancer attributable to genetic factors, with the strongest links found to melanoma (58 percent), prostate (57 percent), non-melanoma skin cancers (43 percent), ovary (39 percent), kidney (38 percent), breast (31 percent), and uterus (27 percent).
There was little heritable link found for lung cancer, nor for colorectal, cervical or oropharyngeal cancers (the former is highly related to smoking, while the latter two have been shown to be related to HPV infection).
Twins, of course, are no more likely to get cancer than anyone else. But the study does seem to show that if a close relative (fraternal twins have the same genetic link as any siblings), especially a genetically-identical one (an identical twin), gets cancer, the odds of getting cancer are increased in the non-affected relative/sibling. When both twins developed cancer, the specific types most commonly found included breast cancer and testicular cancer.
The actual link found is to "heritability," a concept which reflects how much genetic differences among people in a specific group account for differences in the risk of cancer within that group. Heritability is a population-level property, which can vary, even if the individuals in the population stay exactly the same.
These results are based on individuals in Nordic countries, and authors caution that they cannot be generalized to other populations. Their actual conclusion: "In this long-term follow-up study among Nordic twins, there was significant excess familial risk for cancer overall and for specific types of cancer, including prostate, melanoma, breast, ovary, and uterus. This information about hereditary risks of cancers may be helpful in patient education and cancer risk counseling."