[LUM#16] Detox Champions
Every day, they drink water heavily contaminated with arsenic. And yet these inhabitants of Bolivia’s Altiplano don’t 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 once enjoyed its heyday, both at the court of Louis XIV and among the Borgia family. Yet it remains a poison that is sadly still relevant today: “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 living 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 is naturally present 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. A natural process exacerbated in certain regions by mining, as is the case in Bolivia, where miners grind 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 lacks data on the chemical quality of its water with regard to arsenic, and residents consume water that sometimes contains alarming levels of it,” explains the doctor.
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 exhibit these skin symptoms.”
How can we explain the absence of this 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 have reportedly become increasingly capable of eliminating arsenic.
Remove 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 seek work elsewhere, are not exposed to arsenic to the same extent,” explains the doctor.
And, unsurprisingly, the researchers did indeed find arsenic in their urine. 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 box), 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. A mutation likely selected over time, as the Urus have become increasingly capable of eliminating arsenic from generation to generation.“This is the first documented example of human adaptation to a toxin,” enthuses Jacques Gardon.
A researcher’s enthusiasm quickly tempered by a 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 has no color, taste, or smell;most of these people drink contaminated water without knowing it. It’s a real public health problem, and our role is also 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 first form is poorly eliminated by the body, the second is eliminated more easily and is therefore less associated with severe symptoms. It is precisely this dimethyl arsenic that researchers find predominantly 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 in the Urus.
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