Having a twin sibling diagnosed with cancer increases the risk for also developing the disease, according to a new study published Tuesday in JAMA. This large-scale study, based on more than 200,000 same-sex twins, sheds light on the role genetic and environmental factors play in contributing to the risk for the disease, suggesting family history is significant to cancer risk.
The study looked at familial risk, which is a person’s lifetime risk for cancer given that their sibling develops the disease. But it also analyzed the data for cancer heritability, a measure of the variation in a population that is due to genetic differences that may contribute to cancer risk. To calculate heritability, the researchers compared the familial risk for cancer in fraternal and identical twins.
“Twins are no more likely to get cancer than non-twins,” says Lorelei Mucci, associate professor of epidemiology at Harvard University’s Chan School of Public Health. “But what we saw is the risk for cancer increased if you have a relative who developed cancer.”
Mucci and her fellow researchers analyzed data from the Nordic Twin Study of Cancer conducted between 1943 and 2010. The cohort was made up of 80,309 identical twins and 123,382 fraternal twins from Denmark, Finland, Sweden and Norway, who were an average age of 32. Identical twins originate from the same ovum and share 100 percent of their genetics. Fraternal twins come from two separate fertilized eggs, and share approximately 50 percent of genetics, according to Mucci.
They found that overall, one in three people in the study developed some type of cancer over their lifetime. The risk was 5 percent higher for fraternal twins and 14 percent higher for identical twins if their sibling was diagnosed with cancer. Among all the sets of twins where both siblings developed cancer, 38 percent of identical and 26 percent of fraternal twin sets were diagnosed with the same type of cancer.
This trend was found in all 23 types of cancer studied. These included common cancers, such as breast and prostate and rarer types, including testicular, head and neck, melanoma, ovarian and stomach cancer. The trend was strongest in testicular cancer: A man’s risk for testicular cancer was 12 times higher if his fraternal twin developed it, and as much as 28 times higher if he had an identical twin who had also been diagnosed with testicular cancer.
Overall heritability for cancer was 33 percent among the entire study population, and notably higher for certain types of cancers. Significant heritability was found in 58 percent of diagnosed skin melanomas, 57 percent of prostate cancers, 43 percent of non-melanoma skin cancers, 39 percent of ovarian cancers, 38 percent of kidney cancers, 31 percent of breast cancers and 27 percent of uterine cancers.
But shared environmental and lifestyle factors such as socioeconomic status, diet, occupation and health care access screening also impact cancer risk and heritability, so researchers accounted for those as well.
“The reason that cancers might co-occur in a family is shared genetic factors or shared environmental factors,” says Mucci. “When one sibling smokes, both siblings are more likely to smoke.”
The findings provide critical information that may explain how genetics and epigenetics play a role in overall cancer risk. Researchers already know that it isn’t necessarily the site of the cancer that is significant but rather the cancer’s genetic profile. For example, mutations to the BRCA1 or BRCA2 genes can increase a woman’s lifetime risk for breast cancer by as much as 80 percent and her lifetime risk for developing ovarian cancer by up to approximately 70 percent, according to the American Cancer Society. And recent research has found these specific mutations may also play a role in some prostate and pancreatic cancers. For patients, the implication of each of these cancers are wildly different, but looking at their similar genetic profiles is critical for scientists and physicians hoping to improve treatments and survival rates.
Mucci says she hopes to further study the genetics of the cancers to see if twins share some of the same mutations. In one ongoing project, she is analyzing tissue samples of tumors from twin men who develop testicular cancer. “We’re interested in the genetic factors that determine what the tumor looks like,” she says.