A large, new study of identical and fraternal twins in Nordic countries finds that when one twin is diagnosed with any type of cancer, there is a higher than average risk that the other twin will also develop cancer.

This risk of developing cancer was an estimated 14 percent higher in identical twins if one twin had cancer, and 5 percent higher in fraternal twins if one twin had cancer — compared to the average cancer risk of all the people in the study.

The researchers also found that for 20 out of the 23 specific types of cancer studied, if one twin developed one type of cancer, the risk of the second twin also developing that same type of cancer was higher than average. This was true for cancers of the prostate, skin (melanoma), breast, ovary and uterus, according to the study, published today (Jan. 5) in the Journal of the American Medical Association (JAMA).

But the study also showed that among pairs of twins in which both individuals had developed cancer, more than two-thirds of the twins were diagnosed with different types of cancers. This suggests that, in some families, there is a shared increased risk of any type of cancer, the researchers said.  [10 Do's and Don'ts to Reduce Your Risk of Cancer]

The findings provide insights into the relative contribution of genetic and environmental factors in cancer's development, said the study's lead author, Lorelei Mucci, an associate professor of epidemiology at the Harvard T.H. Chan School of Public Health in Boston.

Genetic factors may underlie 30 to 60 percent of the variability in risk for many cancers, including cancers of the breast, uterus, prostate, testicles, kidney, ovaries and skin, Mucci told Live Science.

Estimating to what extent certain cancers are heritable — or, to what degree people's risk of a cancer varies because of genetic differences — may helpresearchers in the search for specific genetic variants that are linked with cancers, she said.

The analysis found a high estimate of heritability for melanoma, at 58 percent, and for prostate cancer, at 57 percent. This means, for example, that genetics accounted for 58 percent of the variability in melanoma risk in the people in the study. The rest of the variation in people's risk is due to environmental or lifestyle factors.

The study also found that 42 percent of the twins' risk for melanoma could be explained by environmental factors that two twins would not necessarily share. This could include their exposure to UV light, number of sunburns and whether they smoked. The researchers did not find evidence that shared environmental factors, such as eating the same diet while growing up, drinking the same water or having the same family income, had any impact on melanoma risk. [Top 10 Cancer-Fighting Foods]

"It was surprising that the estimate of heritability of melanoma was the largest of all of the studied cancers," Mucci said. People often hear about the role of UV exposure and sunburn as risk factors for melanoma, but the new study points to a strong familial risk for this skin cancer, she added.

Twin studies and cancer risk

In the study, the researchers looked at about 80,000 people who were identical twins and about 123,000 same-sex fraternal twins who were part of twin registries in four Nordic countries: Denmark, Finland, Norway and Sweden. The twins were tracked for an average follow-up period of 32 years. The data were collected between 1943 and 2010.

During the study, about 27,000 cases of cancer were diagnosed in about 24,000 of the twins participating in the study. This corresponded to an overall cancer rate in these Nordic twins of about 32 percent.  

Twin studies are important for understanding the genetic component of a particular disease, such as cancer. Such studies can also provide insight into the role of a shared family environment, or lifestyle factors, as well as nonshared environmental or lifestyle factors. 

Identical twins share all of their inherited genes, whereas fraternal twins share about half of their inherited genes, making these twins as genetically alike as other siblings, Mucci said.

This difference in the shared genetics between the two types of twins allows researchers to examine the links between cancer and genetics, disentangled from the links between cancer and a shared environment, she explained.

What the researchers found

The data revealed that lung cancer was one of the cancers that was most closely tied to shared environmental factors, which is likely due to the tendency of twins to have the same smoking habits (with either both smoking, or neither smoking), Mucci said.

Testicular and breast cancers also appeared to be linked to shared environmental factors, which may reflect the hypothesized in-utero origin of these two cancer types, she said.

Although this study was based on twins, Mucci said the findings are relevant to siblings more generally. She pointed out that the cancer risks in this twin population were very similar to the cancer risks in general Nordic populations. In other words, being a twin in these four countries did not put a person at increased risk for cancer compared with the overall cancer risk in the Nordic population.

One limitation of the study is that twins from the Nordic countries are primarily white, so it's unclear to what extent the data can be generalized to multiethnic populations, the researchers said.

The information on familial risk estimates can be helpful in guiding patients who have a family member who develops cancer, Mucci said. For example, if a man has a brother who was diagnosed with testicular cancer, the man's lifetime risk of testicular cancer is substantially elevated, and he may benefit more than the average person from frequent screening exams, she explained.

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