[LUM#16] Detox Champions
Every day, they drink water heavily contaminated with arsenic. And yet these residents of the Altiplano in Bolivia do not seem to suffer from it… A mystery that epidemiologist Jacques Gardon has investigated, revealing how they may have adapted to this poison.
Arsenic—a word with an almost romantic ring to it for a substance that enjoyed its heyday at the court of Louis XIV and among the Borgia family. Yet it remains, sadly, a poison of the present day: “The consumption of arsenic-contaminated water is a public health problem affecting approximately 140 million people worldwide,” notes Jacques Gardon. For more than five years, the epidemiologist at the HydroSciences laboratory has been studying the effects of arsenic on the health of Bolivian populations, specifically among the Urus people who live on the shores of Lake Poopó.
There, the researcher found arsenic levels in the water up to 80 times higher than WHO standards.“The limit is set at 10 micrograms per liter. In some wells, we measured levels as high as 800 micrograms per liter,” he explains. But where does this arsenic come from? “It occurs naturally in the Earth’s crust and in minerals in many regions, and it can contaminate groundwater as a result of soil erosion,” explains Jacques Gardon. This natural process is exacerbated in some regions by mining, as is the case in Bolivia, where miners crush arsenopyrite to extract metals and discharge the arsenic, which then contaminates sediments, rivers, groundwater… and wells.
Shallow wells
“Local residents obtain their drinking water from traditional shallow wells or shallow tube wells. Unfortunately, Bolivia does not have maps detailing the chemical quality of its water with regard to arsenic, and residents consume water that sometimes contains alarming levels of it,” the doctor explains.
What are the health consequences? “Chronic arsenic exposure causes cancer, damages the cardiovascular system, harms the kidneys, and can increase the risk of diabetes,” says Jacques Gardon. Arsenic also causes characteristic lesions on the hands and feet: a thickening of the skin, or hyperkeratosis, which is typical of arsenic poisoning and can develop into cancer. Jacques Gardon took a close look at the hands of the Urus people of Lake Poopó. They were surprisingly normal.“They do not show these skin symptoms.”
How can we explain the absence of this very characteristic symptom despite such high levels of arsenic?“A team of Swedish researchers studied a similar situation in certain villages in northern Argentina; they hypothesized that it might be an adaptation to the poison,” the researcher explains. Over generations, the residents may have become increasingly capable of eliminating arsenic from their bodies.
Removing Arsenic
To test this theory, Jacques Gardon, along with Swedish and Bolivian colleagues, measured the levels of arsenic and arsenic compounds in the urine of 200 women living in ten villages scattered around Lake Poopó.“We focused on women because men, who often leave the village to find work elsewhere, are not exposed to arsenic to the same extent,” explains the doctor.
And the researchers did indeed find arsenic in their urine—which came as no surprise. What was surprising, however, was the chemical form of the arsenic identified. “It’s important to note that arsenic exists in several distinct forms (see sidebar), one of which is less toxic and easier to eliminate than the others. In the general population, the least toxic chemical form accounts for an average of 60% of the arsenic found in urine, but among Uru women, this proportion reaches 80%, ”notes Jacques Gardon. This means that their metabolism is particularly effective at eliminating the toxin.
Adaptation to Poison
To explain this extraordinary resilience, researchers examined the inhabitants’ genomes and identified a mutation in a gene that controls arsenic detoxification.“Normally, this mutation is found in about 20% of the population, but we found it in 80% of the people in this Andean community,” the researcher explains. This mutation was likely selected over time, as the Urus became increasingly capable of eliminating arsenic with each generation.“This is the first documented example of human adaptation to a toxin,” says Jacques Gardon enthusiastically.
A researcher’s enthusiasm was quickly tempered by the doctor’s concerns: “Gaining a better understanding of the relationship between the exposome and the genome is fascinating, but what matters most is ensuring that all populations have access to safe drinking water.” Filtration, ion exchange, oxidation, dilution… there are indeed many effective techniques for removing arsenic from water.“Arsenic is colorless, tasteless, and odorless;most of these people drink contaminated water without even knowing it. It’s a real public health problem, and it’s also our role to address it.”
Effective Detoxification
When we drink water containing dissolved arsenic, it is chemically modified in our liver to facilitate its elimination by the kidneys. This modification involves a process called “methylation,” which converts elemental arsenic into either monomethyl arsenic or dimethyl arsenic. While the body has difficulty eliminating the first form, it eliminates the second more easily, and it is therefore less likely to cause severe symptoms. It is precisely this dimethyl arsenic that researchers find in the highest concentrations in the Urus’s urine. The cause: a mutation in the gene encoding the enzyme involved in this methylation process—called arsenic methyltransferase—which promotes the conversion to dimethyl arsenic and may explain the efficiency of the arsenic detoxification process among the Urus.
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