The persistent environmental contaminant methylmercury (MeHg), which humans are exposed to through the consumption of contaminated seafood, has been found to lead to human motor neuron cell death through calcium pathways . This found in a recently published study that used human induced pluripotent stem cells as a model to study MeHg toxicity . One may think of MeHg as a rare metal. However, MeHg is the most toxic form of mercury (Hg) and it is the most common form found in the environment.
MeHg leads to cell death in human motor neurons. Image summary (above) from a study by Colón-Rodríguez, et al., 2020, that show for the first time in a human model of motor neurons, that MeHg leads to cell death after acute exposure to toxicologically relevant concentrations.
MeHg in the environment
Hg exists in the environment through natural (earth crust and volcanic emissions) and anthropogenic sources such as burning coal, waste incineration, gold mining, and wildfires. Hg0 enters the atmosphere as a vapor. Once in the atmosphere, Hg0 can remain in its vaporous state and readily move throughout the atmosphere, or become oxidized to Hg2+ and recycle to the Earth in the rain [2,3]. Accumulated Hg2+ can be methylated by sulfate-reducing bacteria in bodies of water, generating MeHg.
Two major poisoning events gave insights on its toxicity
Studies from two major MeHg poisoning events in Minamata, Japan (1953- 1956) and Iraq (the 1970s) led to the following findings: there is a latent period between the exposure to MeHg and the onset of symptoms; severity of symptoms is dose-dependent; the first and most prevalent symptom is paresthesia, the second is ataxia, followed by muscle weakness, tremor, dysarthria, hearing and visual impairment [2,4,5].
Persistence in the environment, the case for California and Madre de Dios, Peru
In California (CA), there is a known persistent abundance of MeHg due to abandoned gold mines and wildfires (CA-USGS, 2018). In 2002, 48 states in the U.S. issued advisories for 39 chemical contaminants, and the most commonly found was MeHg . In 2007 there was an advisory in Sacramento San Joaquin Delta, CA, recommending limited consumption of certain Delta fish due to its contamination with MeHg. A report this same year described a high rate of fish consumption on low-income, non-white groups (African Americans, Hispanic, Asian, Hmong) in the Delta region of CA and suggest that fish contamination can have a significant impact on these specific populations in this region . More recently, in 2016, a study by the U.S. Geological Survey shows widespread mercury contamination across western North America. MeHg contamination can be found in fish and birds commonly in this part of the nation (USGS, 2016).
MeHg causes both adult and developmental neurotoxicity. Developmental toxicity was observed in the two major poisoning episodes of Minamata and Iraq. However, MeHg exposure occurs over the life span, and adult-onset exposures are an unexplored area. This is especially relevant for populations in widespread contamination regions, such as the case for Madre de Dios in Peru. The increased use of Hg for artisanal gold mining in this region has made it the current largest known source of Hg pollution in the world . Hg vapors are released to the environment while burned off the gold and disposed into water sources, where it is bioconverted into MeHg. It has been reported that gold miners are exposed to Hg dermally and through vapors and MeHg because of their standard dietary practices of fish consumption [8-11]. This is one of the largest known adult Hg, and MeHg exposures, and the neurological effects have not yet been studied. However, it is believed that more than 48,000 people have been affected .
In 2012, it was reported that only 11% of the population at Madre de Dios mining zones had Hg levels above 16 mg/gram of dry hair, a concentration considered as symptomatic by the World Health Organization . However, recent reports indicating if and how these levels have changed are lacking. Based on previous reports, if exposure to MeHg continues in Madre de Dios, Peru, similarly as it has been reported since 2012, it is expected for this population to present neurological signs similar to those observed in the poisoning of Minamata. Madre de Dios, Peru, and California are current examples of the significance of findings on the mechanism of toxicity of MeHg on the nervous system, the need for additional studies and robust regulations.
1. Colón-Rodríguez, et al., 2020
2. Clarkson y Magos, 2006
3. Horowitz et al., 2014
4. Bakir, Damluji, 1973
5. Eto, 1997
6. Anderson et al., 2004
7. Silver et al., 2007
8. Wade, 2013
9. Ashe, 2012
10. Gardner, 2012
11. Fraser, 2016