Having a twin sibling diagnosed with cancer increases the risk of developing any form of cancer, according to findings published online in JAMA.

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A new study suggests that if one twin develops cancer, the other has a higher risk of doing so.

Previous studies have provided familial risk and heritability estimates for the common cancers such as breast, prostate and colon; but, so far, such information has not been reliably identified for rarer cancers.

Large twin studies can help to indicate the relative contribution of inherited factors in cancer and to establish familial cancer risk because of genetic relatedness factors.

The current study was led by researchers at the Harvard T.H. Chan School of Public Health in Boston, MA, the University of Southern Denmark and the University of Helsinki in Finland.

The team aimed to provide family risk estimates for a number of cancers and to examine heritability of cancer, or to what extent the variation in cancer risk of populations is due to genetic factors.

They looked at more than 200,000 twins, both identical and fraternal, in Denmark, Finland, Norway and Sweden, from the Nordic Twin Study of Cancer. Participants were followed over an average of 32 years from 1943-2010.

Overall, 1 in 3 participants developed cancer during their lifetime. Among the 23 different types of cancer studied, an increased familial risk was seen for almost all cancers, including common cancers such as breast and prostate cancer, but also for the less common cancers such as testicular cancer, head and neck cancer, melanoma, ovarian and stomach cancer.

In 3,316 pairs of twins, cancer was diagnosed in both twins. The same type of cancer was diagnosed among 38% of the identical twins and 26% of the fraternal twins. When one fraternal twin was diagnosed with any kind of cancer, the other twin’s risk of getting cancer was estimated at 37%; among identical twins, the risk rose to 46%.

Testicular cancer showed a particularly strong familial link: a man’s risk of developing this disease was found to be 12 times higher if his fraternal twin developed it and 28 times higher if his genetically identical twin developed it.

The heritability of cancer overall was estimated at 33%. Significant heritability was noted for certain cancers: 58% for skin melanoma, 57% for prostate cancer, 43% for non-melanoma skin cancer, 39% for ovarian cancer, 38% for kidney cancer, 31% for breast cancer and 27% for uterine cancer.

When both twins developed cancer, each twin often developed a different type, indicating that some families may share an increased risk of any type of cancer.

Since fraternal twins are similar genetically to siblings who are not twins, an increased cancer risk among fraternal twin pairs could help predict the cancer risk for families in which one sibling gets cancer.

Co-senior author Jaakko Kaprio, from the University of Helsinki, says:

Findings from this prospective study may be helpful in patient education and cancer risk counseling.”

Medical News Today recently reported on research suggesting that a gene that suppresses some types of cancer appears instead to promote colorectal cancer.