For thousands of years, people living high in the Andes of Argentina have relied on drinking water that could make many people sick.
There, arsenic naturally occurring in the volcanic rocks seeps into the groundwater, contaminating local water supplies with levels of the toxic metalloid that can pose serious health risks to many people.
But for one group in northern Argentina, natural selection may have provided an unusual genetic advantage.
According to DNA analysis of people across western South America, the Andes of Argentina have different genetic variants that may help them use arsenic more safely.
“Adaptation leads to genomic changes; however, the evidence for adaptation in humans is still limited,” wrote a team led by biologists Carina Schlebusch and Lucie Gattepaille of Uppsala University in a 2015 paper.
“Our data show that adaptation to environmental stressors of arsenic may lead to an increase in the AS3MT protective spectrum, providing the first evidence of human adaptation to toxic chemicals.”
Given enough time and exposure to enough danger, life has shown an amazing ability to adapt to all kinds of wild conditions – from extreme heat to complete lack of oxygen to radiation.
However, little is known about how people adapt to toxic chemicals in their environment. Arsenic is highly toxic, linked to cancer, skin lesions, birth defects and early death. It is also widespread, naturally occurring at high levels in groundwater in many places around the world.
The current recommended limit of arsenic in drinking water, set by the World Health Organization, is 10 micrograms per liter.
Until a treatment system was installed in 2012, the remote, high-altitude town of San Antonio de los Cobres, in Argentina’s Puna de Atacama Plateau, had drinking water with about 200 micrograms of arsenic per liter – about 20 times the recommended limit.
However this area has been inhabited for thousands of years – at least 7,000 years, and possibly as long as 11,000.
This apparent ability to lower dangerous levels of arsenic puzzled scientists for decades. In 1995, scientists noted that Andean women in Argentina had a “special ability” to destroy arsenic, as evidenced by metabolites in their urine.
When arsenic enters the body, enzymes convert it into several chemical forms. One of these intermediates, called monomethylated arsenic (MMA), is highly toxic. The latter form, dimethylated arsenic (DMA), is easier for the body to excrete in the urine.
The people of San Antonio de los Cobres have a tendency to develop central and easily excreted toxins, suggesting that their bodies were unusually efficient at processing arsenic.
Intrigued, Schlebusch, Gattepaille, and their colleagues sought to solve the problem at the genetic level.
The team collected DNA from 124 women in San Antonio de los Cobres using cheek swabs, whose urine samples showed the same arsenic metabolite profile as in the 1995 study. They then analyzed millions of genes throughout the genome.
To determine whether genetic variation was unique to the Argentine population, the researchers compared their results with publicly available genome data from Peru and Colombia, taken from the International 1000 Genomes Project.
Previous research has shown that an enzyme called arsenic (+3 oxidation state) methyltransferase (AS3MT) can play an important role in the metabolism of arsenic, so that’s where researchers have focused their efforts.
What they found was a cluster of genetic variants near the AS3MT gene that strongly influenced the way the body dealt with arsenic. These types were more common in the San Antonio de los Cobres population than in genetically similar populations in Peru and Colombia.
The variety appears to make the body more efficient at converting arsenic into forms that can be safely excreted in the urine, reducing the accumulation of toxic compounds in the body – a result that agrees neatly with earlier studies of arsenic metabolites in urine.
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Although arsenic pollution is common throughout the world, very few nations have lived with such high levels of long-term exposure.
In San Antonio de los Cobres, people have lived with arsenic in their groundwater for thousands of years – long enough for natural selection to favor methods that reduce vulnerability to the toxic effects of arsenic.
Recent research suggests that similar genetic markers may also appear in other Andean populations exposed to arsenic for generations, supporting findings that long-term exposure can drive genetic tolerance, and suggesting that adaptation may spread across the region.
“Due to the negative health effects of arsenic in children and adults,” the researchers wrote, “individuals with the arsenic-tolerance haplotype … may have a strong preference for arsenic-rich areas.”
Research published in Molecular Biology and Evolution.
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