The largest study of its kind to date reveals some answers to an age old question: nature versus nurture.
Harvard University scientists pored over data on 35 million Americans – including thousands of pairs of twins – followed over 24 years to work out which diseases are driven by genetics and which are more influenced by environment.
In general, most diseases are the result of how these two component interact. The environment can change how genes express themselves and genes can influence how our bodies respond to the environment.
But there is wide variation in this interplay based on the disease, and the Harvard scientists set out to hone in on which factors have the most effect for which diseases.
The new study found that cognitive issues were the most closely connected to the genetic code, while eye diseases are the most influenced by environment.
Both nature and nurture – or DNA and environment influence disease risks, and Harvard University scientists have begun to parse out just what role each plays in 560 disorders
For the over 6,000 genetically inherited disorders there is a clear cause and effect: a bit of the genetic code that is flawed missing.
Detecting these was part of the goal of the massive, international undertaking, The Human Genome Project, which was completed in 2003.
Mapping the genome was a monumental accomplishment, but it only tells part of the story.
Humans – and all animals – are much more than the sum of our DNA. Each persons’s DNA is unique, and their environments are even more unique, as well as constantly shifting.
This makes parsing out which is responsible for what a daunting task.
Twins – best of all, identical ones – have long been the pet subjects of geneticists because they offer a rare DNA opportunity to see what happens to the same DNA in different environments.
Researchers at Harvard collected data on 45 million people through a database of insurance information.
They compared genetic data as well as diagnostic and vital medical statistics (such s height and weight) to subjects’ zip codes, which allowed them to surmise environmental influences like socioeconomic status.
Though all of the information was important to their analyses, the 56,000 pairs of twins included in their study offered some of the most crucial data points.
By no means does this large story solve the complicated question of what is nature and what is nurture, but it does shed light on a wide range of disease risk factors.
Almost 40 percent of the diseases the researches examined – including musculoskeletal, cognitive, eye and respiratory and reproductive disorders, among others – had a genetic component.
Environment at least partially drove one quarter of the 560 diseases the team researched.
Genetics had little to do with the development of eye disorders. Conversely, 27 out of 42 eye diseases were environmentally driven.
Diseases of the connective tissues, such at rheumatoid arthritis, were least influenced by DNA, while reproductive disorders were least affected by a person’s environment.
The opposite was true for cognitive diseases, four out of five of which were genetically inherited.
Genetic predisposition also predicted about 60 percent of how much people would spend on healthcare from month to month as well.
So, those genetic components may even alter an environmental factor (socioeconomic status) in such a way to further change disease risks.
Second to eye diseases, respiratory diseases were the most closely linked to environmental factors, with 34 out of 48 such conditions being highly environmentally determined.
Overall, environmental factors were weaker predictors of diseases than genetics, according to the data on the twin pairs.
But some of the results on which environmental factors had the greatest effects on disease risks were nothing short of alarming.
We have long known that poorer Americans are sicker, and the Harvard study, published in Nature Genetics, bore that out, showing that socioeconomic status affected 145 different diseases, with the strongest link to obesity.
‘This finding opens up a whole slew of questions, including whether and how a change in socioeconomic status and lifestyle might compare against genetic predisposition to obesity,’ said senior study author Charig Patel, an assistant professor of biomedical informatics at Harvard’s medical school.
But the study’s results also indicated that climate change is already having and will continue to have dramatic effects on our health.
Changes in temperature changed the risks of 117 different diseases – even more than were altered by poor air quality.
Continuing to hone the roles genes as a wide variety of seemingly subtle environmental factors play in diseases will help open the doors to better preventative measures and treatments, beyond simply gaining understanding, the study authors hope.
‘The nurture-versus-nature question is very much at the heart of our study. We foresee the value of this type of large-scale analysis will be in shining light on the relative contribution of genes versus shared environment in a multitude of diseases,” said Dr Patel.
First study author Chirag Lakhani added: ‘Our findings can provide signposts that inform subsequent research efforts and helps scientists narrowly focus their pursuits.’