Nearly 40 percent of 560 different diseases have a genetic component, while 25 percent are driven by environmental factors shared by twins who are growing up in the same household, the researchers reported.
Brain disorders were most strongly influenced by genetics, the investigators found, with four of five cognitive diseases having a genetic component.
On the other hand, eye diseases and respiratory disorders were most likely to be influenced by the environment in which twins were raised, the results showed.
This report could serve as a roadmap for people interested in studying the causes of any of the 560 diseases considered by the investigators, said lead researcher Chirag Lakhani, a postdoctoral research fellow at Harvard Medical School.
It can be costly and time-consuming to perform studies that feature a full genetic analysis (“genotype”) of participants, Lakhani said. Scientists might skip this step for diseases that are clearly not influenced by genetics.
“Maybe this disease isn’t worth our investment to genotype if you see within a twin study the genetic role is low,” Lakhani said. “Maybe it’s not worth looking into for a specific population.”
For this study, Lakhani and his colleagues used an insurance claims database from Aetna that included nearly 45 million patient records.
The researchers identified more than 56,000 twins and more than 724,000 pairs of siblings. All patients had been part of the insurance database for at least three years, and the twin pairs ranged in age from newborns to 24 years old.
The team tracked the health of the siblings using insurance records, focusing on a set of 560 diseases that are not rare and affect both men and women, Lakhani said.
Twin studies are valuable because identical twins share 100 percent of their genetics, while fraternal twins and siblings share about half their genetics, on average, said senior researcher Chirag Patel, an assistant professor of biomedical informatics at Harvard Medical School.
Diseases that tend to strike identical twin pairs at a higher rate than fraternal twins or siblings are probably influenced by genetics, Lakhani said. Diseases that happen in sibling pairs, regardless of whether they are twins, tend to be strongly influenced by environmental factors.
Genetics played at least some role in about 40 percent of the diseases investigated, the researchers determined.
The researchers also used ZIP codes in the database to estimate the influence of environmental factors, like socioeconomic status, climate conditions and air quality on disease in the siblings.
The findings indicated that about 25 percent of diseases were influenced by socioeconomic status, 20 percent were affected by changes in temperature and 6 percent were influenced by air quality.