The Evolution of Disease in a Rapidly Changing World
September 13, 2010 Leave a comment
As humans evolve and the world they live in changes, the types and prevalence of disease they get changes as well. And while both environment and genes can be responsible for different diseases, a new study is shedding light on the crossroads between them. New research out of Stanford Medical School shows that as humans’ environments change quickly and drastically (which often happens), genes can become selected which simultaneously make them more fit and more susceptible to a certain disease. The old Darwinian mantra of positive benefit, positive selection becomes complicated when environment changes at an unprecedented pace.
The study, published in PloS One, looked at several common diseases that are complex, meaning that their genetic basis is associated with several variants, called single nucleotide polymorphisms (SNPs). In calculating disease risk, the researchers looked at many SNPs in a person’s genes; some of these SNPs correlate to greater susceptibility to the disease, while others protect against it. By looking at the SNPs, the researchers could determine whether humans were positively selecting for a certain disease, meaning they had more SNPs that favored the development of the disease than those that warded against it.
Seven diseases were examined for their genetic selection: type-1 and type-2 diabetes, rheumatoid arthritis, hypertension, Crohn’s disease, coronary artery disease and bipolar disorder. Type-1 diabetes and rheumatoid arthritis were both shown to be positively selected for. With type-1 diabetes, 80 SNPs were increasing in prevalence, and of the 80, 58 were linked to increased risk of developing the disease. For rheumatoid arthritis, the results were less dramatic, but still showed a positive selection for alleles that increased the risk for developing the disease. Of the other diseases researched, Crohn’s, bipolar disorder, and coronary artery disease were all being selected against, with more SNPs protecting against the disease than causing it. Hypertension showed about an equal selection for both types of SNPs.
The evidence for Crohn’s, bipolar disorder, and coronary artery disease, isn’t so surprising. It makes sense, at least according to natural selection, that nature would select against those genes which make an organism diseased and less likely to reproduce. So what to make of type-1 diabetes and rheumatoid arthritis, two diseases prevalent in today’s society that are currently undergoing positive selection?
Atul Butte, the head researcher behind the study, said that the positive selection for these diseases could be due to an ancestral benefit that today’s environment is reversing. Another possible explanation is that the genetic basis for the disease also confers a hidden benefit to its carrier.
With regards to type-1 diabetes, current research supports the latter idea. It has been shown that type-1 diabetes is also linked to a variant in the gene FIH1 that creates a heightened response to enterovirus infection, which can cause deadly abdominal stress. Butte remarked that today’s effective treatments for type-1 diabetes have possibly caused the positive selection: “While T1D is rapidly fatal without insulin therapy, there was likely a net selective pressure favoring intense immune responses to enterovirus, even with T1D as an occasional consequence.”
The story behind rheumatoid arthritis is one of ancient benefit and recent environmental change. Rheumatoid arthritis has been shown to be inversely concurrent with tuberculosis, suggesting that the genetic grouping that wards against TB also causes rheumatoid arthritis. As TB has caused millions of deaths in recent history, it makes sense why humans would be selecting genetically for a variation that wards against the disease.
Rheumatoid arthritis’s history gets even more complicated, however: the disease was not seen outside America until Columbus’s voyage there. This suggests that there is an environmental cause or pathogen that triggers the disease that was spread from the New World.
In Europe, there was no beneficial aspect to selecting against rheumatoid arthritis until the mid 1800’s (since the RA trigger didn’t exist there), but there was the positive benefit of warding off TB. The fact that rheumatoid arthritis is still positively selected for is potentially a remnant of this old world benefit. Columbus’s voyage changed the nature of global interactions in only a few years, and it seems that our genes just haven’t kept up.
Further reading: Western Diseases, An Evolutionary Perspective, by Tessa M. Pollard