Twin study finds that cancer is 33 percent heritable

A study involving more than 200,000 Nordic twins has found that cancer is 33 percent heritable.

Although the study does not allow predictions to be made about an individual patient's risk of developing cancer, it does find that genetic factors are a bigger contributor to certain types of cancer than previous studies had shown.

'Earlier studies were not critically large enough to be able to provide any information about heritability for [some] cancers,' lead author Dr Lorelei Mucci from Boston's Harvard T. H. Chan School of Public Health told STAT. 'Our ability to provide information about the relative contribution of genetic factors for these more rare cancers is really important.'

The study, which was published in JAMA, included twins from Denmark, Finland, Norway and Sweden, who were part of the Nordic Twin Study of Cancer. It found strong evidence of an increased familial risk for 20 out of the 23 cancer types studied.

The most significant heritability was observed for skin melanoma (58 percent), prostate cancer (57 percent), non-melanoma skin cancer (43 percent), ovarian cancer (39 percent), kidney cancer (38 percent) and breast cancer (31 percent).

Heritability reflects how much genetic differences among people in a specific group account for differences in the risk of cancer within that group; it does not indicate whether the risk of cancer is inherited from parents.

The study showed that while the general risk of being diagnosed with melanoma was about one percent, someone with an identical twin with melanoma had a 14 percent chance of being diagnosed with it. Someone with a fraternal twin with melanoma had a six percent chance of being diagnosed with melanoma.

Comparing identical to fraternal twins allows researchers to account for some of the risk that comes from shared environmental risk factors, such as smoking or sun exposure. (Fraternal twins share an environment but only half their genes, while identical twins share both their environment and all of their genes.)

For most cancer types, there were significant familial risks. The overall risk of all cancers was five percent higher for fraternal twins whose twin had been diagnosed with cancer, and 14 percent higher for identical twins whose twin had been diagnosed with cancer. Overall, the heritability of cancer was 33 percent.

Previous studies of cancer-risk heritability have found it to be much smaller. This one followed around 200,000 twins for an average of 32 years. However, the study only included Nordic twins, and the authors caution that the findings may not tell us about cancer heritability in other populations with different genetic backgrounds.

Dr Evan Charney from Duke University, who was not involved in the study, told STAT: 'I think it would be unconscionable and completely unjustified … to translate the results of a finding like this into risk or probability that a physician would tell a patient.'