Study shows higher cancer risk among twins, siblings

Twins share the same genes, and when one gets cancer, the other faces a higher risk of getting sick too, according to a new study that included 200,000 people.

Having an identical twin diagnosed with cancer increases the other twin’s risk of developing not just that type but any form of cancer, said lead researcher Lorelei Mucci, an associate professor of epidemiology at the Harvard T.H. Chan School of Public Health in Boston.

The study, which was published in the Journal of the American Medical Association on Tuesday, suggests that some families carry a shared increased risk for any type of cancer, based on their genes, according to the AFP.

“Different cancers may share an inherited susceptibility based on genetic factors,” Mucci said. “This is an area that we’re just learning about.”

However, the influence of genetics varies widely depending on the type of cancer, Mucci added.

For example, testicular, skin and prostate cancers were shown to be influenced strongly by genetics, while lung, colon and rectal cancers appeared to be driven largely by environmental factors, she said.

But just because one twin falls ill does not mean that the other is certain to get the same cancer – or any cancer at all, according the report.

In fact, the amount of increased risk of cancer was just 14 per cent higher in identical pairs in which one twin was diagnosed with cancer, the AFP reported, adding that identical twins develop from the same egg and share the exact same genetic material.

Among fraternal twins, which develop from two eggs and are as genetically similar as typical biological siblings, the risk of cancer in a twin whose co-twin was infected was five per cent higher.

The twins in the study hailed from Denmark, Finland, Sweden and Norway – all countries that maintain detailed health registries – and were followed between 1943 and 2010.

When researchers looked at the group as a whole, they found that about one in three individuals developed cancer (32 per cent).

Therefore, the risk of cancer in an identical twin whose twin was diagnosed was calculated to be 46 per cent.

In fraternal twins it amounted to a 37 per cent risk of developing cancer if a co-twin was diagnosed.

The exact same cancer was diagnosed in 38 per cent of identical twins and 26 per cent of fraternal pairs.

The cancers that were most likely to be shared among twins were skin melanoma (58 per cent), prostate (57 per cent), non-melanoma skin (43 per cent), ovary (39 per cent), kidney (38 per cent), breast (31 per cent), uterine cancer (27 per cent).

“Because of this study’s size and long follow-up, we can now see key genetic effects for many cancers,” said Jacob Hjelmborg, from the University of Southern Denmark and co-lead author of the study.

Researchers said the findings may help patients and doctors understand more about the hereditary risks of cancer, a disease that kills eight million people around the world each year.

Specifically, the researchers studied 80,309 monozygotic and 123,382 same-sex dizygotic twin individuals (203,691) within the population-based registers of Denmark, Finland, Norway, and Sweden.

Twins were followed up for a median of 32 years between 1943 and 2010. There were 50,990 individuals who died of any cause, and 3,804 who emigrated and were lost to follow-up.

“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,” the researchers said.

“This information about hereditary risks of cancers may be helpful in patient education and cancer risk counseling.”