is oxidatively methylated to DMAV, which can be reduced to dimethylarsinic acid (DMAIII) (Tsuji et al. 2019; Zhou and Xi 2018). In rodents and humans exposed to extremely high concentrations of arsenic, DMAIII can be further methylated to trimethyl arsenic oxide (TMAVO) and, in rodents, it can be subsequently reduced to volatile trimethylarsine (TMAIII) (Cohen et al. 2013). The reduction reactions are catalyzed by glutathione-S-transferase omega (GSTO); and oxidative methylation is mainly performed by the arsenic-3-methyl transferase enzyme (AS3MT) (Naranmandura et al. 2006; Tsuji et al. 2019; Zhou and Xi 2018).
The clinical features of acute arsenic poisoning include, but are not limited to, nausea, vomiting, diarrhea, severe abdomen pain, skin rash, and seizures (Ratnaike 2003). Depending on the amount ingested, arsenic can induce severe systemic toxicity and death. Although acute exposures to high levels of arsenic are occasionally reported, chronic exposure to low levels over a long period of time is more common and currently of great concern. The long-term effects of arsenic exposure differ between individuals, population groups, and geographical areas and clinical outcomes include diabetes, pulmonary and cardiovascular disease, skin lesions, hyperkeratosis, and, as discussed earlier, skin, urinary bladder, and lung cancers (IARC 2012; States 2015; WHO 2018). The mechanisms by which arsenic causes cancer remains elusive, but some of the proposed mechanisms of action are inhibition of DNA repair, oxidative stress, aneuploidy, aberrant DNA methylation, and miRNA dysregulation (Hughes 2002; Tam et al. 2020).
Because of arsenic’s multisystem toxicity and carcinogenic potential, it is important to prevent further exposure to this substance by providing safe water supplies for drinking, cooking, and the irrigation of food crops. It is also important to monitor high-risk populations for early signs of arsenic toxicity, which is mainly characterized by skin lesions (WHO 2018). Recently, studies have identified circulating miRNAs associated with arsenic exposure and arsenic-induced disease development. In a study conducted in Mexico, serum levels of miR-155 and miR-126 were found, up- and downregulated respectively, in women with high arsenic levels in urine (Ochoa-Martinez et al. 2021; Ruiz-Vera et al. 2019). Downregulation of miR-126 was also observed in the plasma of children exposed to arsenic from the same area, but the levels of miR-155 were unchanged (Perez-Vazquez et al. 2017). In the plasma of a Chinese population exposed to arsenic, 56 upregulated miRNAs (miR-21, -141, -148a, -145, -155, -191, -218 and -491 presenting the largest fold change increase), and 18 downregulated miRNAs (miR-200b, -200 c, -26, and -34 c levels with the most decreased expression) were observed using miRNA microarray (Sun et al. 2017). Further RT-qPCR validation in larger population-based studies in the same area validated the expression levels of miR-21, miR-145, miR-155 and miR-191 (Sun et al. 2017; Xu et al. 2020; Zeng et al. 2019). Increased levels of miR-21 and miR-145 were seen in patients who presented skin alterations and liver damage (Zeng et al. 2019). Additionally, miR-191 was upregulated in patients with skin (Zeng et al. 2019) and renal alterations (Xu et al. 2020; Zeng et al. 2019) . Finally, miR-155 was found to be increased in patients with skin manifestations of arsenic toxicity (Zeng et al. 2019). Therefore circulating levels of these miRNAs could be important for monitoring arsenic-induced skin, liver, and kidney damage. Arsenic-related skin lesions (precancerous and cancerous) were also associated with 202 dysregulated circulating miRNAs in the plasma (199 miRNAs up- and 3 downregulated) of an exposed population in West Bengal, India (Banerjee et al. 2019). RT-qPCR validated microarray data for miR-21, miR-23a, miR-619, miR-126, miR-3613 (upregulated miRNAs) and miR-1282 and miR-4530 (downregulated miRNAs) (Banerjee et al. 2019). Circulating levels of miR-21 in blood were, again, found to be increased in arsenic-exposed individuals in West Bengal (Banerjee et al. 2017). Within the arsenic-exposed population, a much higher miR-21 expression was found in individuals who presented with arsenic-induced skin lesions (malignant squamous cell carcinoma and basal cell carcinoma) than in individuals without skin lesions (Banerjee et al. 2017). In another study, the levels of miR-200c and miR-205 in urine were inversely associated with arsenic exposure (Michailidi et al. 2015). However, the arsenic-exposed and non-exposed cohorts selected for this study were from different populations (Bangladesh versus Baltimore), which could add confounding factors and bias to the analysis. miR-205 is expressed in epithelial tissues and is up- or downregulated in different epithelial cancers (Ferrari and Gandellini 2020). Rager et al. (2014) identified miRNAs in newborn cord blood samples by miRNA array in order to assess prenatal arsenic exposure in Mexico. Increased expression of miRNAs known to have roles in cancer and inflammatory response—for example let-7a, miR-107, miR-126, miR-16, miR-17, miR-195, miR-20a, miR-20b, miR-26b, miR-454, miR-96, and miR-98—was associated with maternal urinary arsenic (Rager et al. 2014). Two studies evaluated the association between circulating miRNAs and arsenic and its metabolites. In a population-based study conducted in Mexico, plasma MMAIII significantly correlated with plasma levels of miR-423-5p, miR-142-5p-2, miR-423-5p+1, miR-320c-1, miR-320c-2, and miR-454-5p, while no associations were found for plasma inorganic arsenic or DMAV (Beck et al. 2018). miR-142-5p is a common circulating miRNA in type 1, type 2, and gestational diabetes (Collares et al. 2013). Blood levels of miR-548c-3p were also negatively correlated with iAs, MMA, and DMA concentrations in the urine of a Chinese cohort (Cheng et al. 2018). Although published research highlights several circulating miRNAs as potential responders to arsenic exposure and disease development, few miRNAs were consistently dysregulated between studies. miR-126 was downregulated in two studies performed in Mexico and miR-21 was constantly found upregulated in arsenic-exposed individuals from China and India. These might represent good candidates for further validation.
Circulating miRNAs Associated with Lead Exposure
Elemental lead (Pb) is found in lead ore deposits that are distributed throughout the world (Abadin et al. 2007). Lead does not degrade in the environment and is dispersed worldwide as a result of anthropogenic activities such as mining and smelting ore, the manufacture of lead-containing products (lead-containing gasoline, paints, batteries, radiation shields, water pipes, ammunition, ceramics and metal containers), the combustion of coal and oil, and waste incineration (Tokar et al. 2013). Lead is listed as one of the topmost toxic substances, and its toxic properties have been recognized for over 2,000 years (Abadin et al. 2007). However, in the last few decades, there has been increased awareness about the detrimental health effects of low-level lead exposure, particularly in children. This awareness has prompted changes in public health policies, and the result the is phasing out of lead from several sources, including gasoline and paints. Nonetheless, exposure to lead still occurs, primarily from lead-containing paint and water pipes present in older housing, from residues along roadways, or in the form of occupational exposure.
For the general population, lead exposure occurs orally for the most part; however, inhalation and dermal contact can also occur, albeit rarely, in occupational settings. The health effects caused by lead exposure are diverse (neurological, cardiovascular, renal, hematological, reproductive, developmental, respiratory, hepatic, endocrine, gastrointestinal, musculoskeletal, ocular, and cancer) and depend on numerous factors including age, nutritional status, and life stage (e.g., in utero).
The blood lead reference value was set at 5 µg/dL in 2012; elevated lead in blood is an indication of excessive exposure (CDC 2012). The most common metric for lead exposure is the concentration of lead in blood, although other measurements of lead in urine, bone, and hair can be used to quantitate exposure (Abadin et al. 2007). Levels of lead in plasma and semen are difficult to measure because lead concentrations in these fluids are often near the lower limits of detection, and lead measurements in saliva and sweat show inconsistent results when compared to blood lead.
Circulating miRNAs associated with lead exposure have been identified in humans (Mitra et al. 2021; Xu et al. 2017, de Araujo et al. 2021). In a large study that analyzed 1,130 Chinese workers with occupational lead exposure, miRNA microarray analyses from plasma identified three miRNAs that were significantly downregulated (miR-520c-3p, miR-148a, and miR-211) and one miRNA that was significantly upregulated (miR-572) in workers chronically exposed to lead who also had high blood lead levels (BLLs) (51.35 ± 6.86 µg/dL; see Xu et al. 2017). However, workers with high BLLs were compared to workers with low BLLs (8.93 ± 1.53 µg/dL), which still contain levels above the blood lead reference values set by the Centers for Disease Control
