Researchers at the Karolinska Institutet and Uppsala University in Sweden led by professor Karin Broberg have shown for the first time that humans have genetically adapted to survive in a polluted environment. Adaptation is a very gradual process and it drives genomic changes. Until now, there have been very limited evidences of adaptation in humans, like adaptation to live in high altitude and to the malarial parasite.
People living in the Argentinean Andes have been exposed to high levels of arsenic in drinking water for thousands of years. The present study shows that people who live in this region have uniquely adapted to tolerate the toxic chemical arsenic. They showed higher frequency of a gene variant AS3MT (arsenic [+3 oxidation state] methyltransferase), which enables the body to efficiently metabolize arsenic by methylating and excreting a less-toxic arsenic metabolite; in comparison, to people who lacked the protective gene variant, and produced a more toxic arsenic metabolite if they were exposed to arsenic.
The study did a genome wide survey from a group of 124 Andean women and they were screened for their ability to metabolize arsenic, measured by the metabolite levels in the urine. They genotyped these women for single nucleotide polymorphisms (SNPs) and found a strong association between the AS3MT gene and mono- and dimethylated arsenic in urine, suggesting that AS3MT is the major gene responsible for arsenic metabolism in humans.
Also, they narrowed down certain key nucleotide variants in the AS3MT gene in the Andean population which were present in much higher frequencies, compared to control populations from Columbia and Peru which had less environmental arsenic. Thus, this Andean population is a striking example of how humans have been able to adapt to harmful environmental conditions.
The researchers also estimated that the increase in frequency of these variants occurred recently, about 7,000 years ago, based on the age of a recently excavated mummy that was found to have high arsenic levels in its hair.
“Our study shows that there are not only extra-susceptible individuals, but also individuals who are particularly tolerant to environmental toxicants. This phenomenon is probably not unique to arsenic, but also applies to other toxicants in food and the environment, to which humans have been exposed for a long time. The results also highlight the necessity to be observant and not base health risk assessments for chemicals on data from people who may have strong genetic tolerance to the particular chemical”, says Karin Broberg, researcher at the Institute of Environmental Medicine at Karolinska Institutet.
The presence of these protective gene variants in some regions of the Andes could explain how people in these regions lived longer and had more children. Only a few such examples of adaptation have previously been described in man.
The original publication can be accessed here.