Micronucleus Test (MNT)

Abstract

Mercury, one of the most widely diffused and hazardous organ-specific environmental contaminants, exists in a wide variety of physical and chemical states, each of which with unique characteristics of target organ specificity (Aleo et al. 2002). In nature the different forms of mercury include the metallic form, inorganic compounds, as well as alkyl, alkoxy, and aryl mercury compounds. Once introduced into the environment, mercury compounds can undergo a wide variety of transformations. In sediments, inorganic mercury (HgCl₂) may be converted into methyl (CH₃HgCl) and dimethyl (CH₃CH₂HgCl) forms by methanogenic bacteria. This biotransformation constitutes a serious environmental risk, given that CH₃HgCl is the most toxic of the mercury compounds and accumulates in the aquatic food chain, eventually reaching human diets (Tchounwou et al. 2003). CH₃HgCl has been an environmental concern to public health and regulatory agencies for over 50 years because of its neurotoxicity. Its association with nervous system toxicity in adults and infants near Minamata Bay, Japan, in the 1950s initiated environmental health research inquiries that continue to this day (Faustman et al. 2002). The three modern “faces” of mercury are our perceptions of risk from the exposure of billions of people to CH₃HgCl in fish, mercury vapor from amalgam tooth fillings, and CH₃CH₂HgCl in the form of thimerosal added as an antiseptic to widely used vaccines (Clarkson 2002). Mercury genotoxicity has been usually attributed to its ability to react with the sulfhydryl groups of tubulin, impairing spindle function and leading to chromosomal aberrations and polyploidy (De Flora et al. 1994). Another important mechanism of mercury genotoxicity is its ability to produce free radicals that can cause DNA damage (Schurz et al. 2000; Ehrenstein et al. 2002). In vivo studies have demonstrated a clastogenic effect of mercury on people exposed to this element in their working environment or through the consumption of contaminated food or sometimes accidentally. Increased numbers of chromosome alterations and micronuclei have been reported in people who consume contaminated fish (Amorim et al. 2000; Franchi et al. 1994) and in miners and workers of explosive factories (Al-Sabti et al. 1992; Anwar and Gabal 1991). Negative results were also obtained in some cases (Hansteen et al. 1993; Mabille et al. 1984), demonstrating that cytogenetic monitoring of peripheral blood lymphocytes in individuals exposed to mercury from different sources may not be completely specific (De Flora et al. 1994). The effects of CH₃HgCl contamination have been studied in an increasing way since the outbreaks in Japan and Iraq. Many of these studies had their focus on the neurological effects of CH₃HgCl exposure in adult animals and used high doses of this compound (1,900–30,000 ppb = μg/L) to obtain its most severe effects (National Research Council 2000). Most of the in vitro studies with lymphocytes also used high doses (250–6,250 μg/L) of mercury compounds in order to evaluate its clastogenic effects (Ogura et al. 1996; Betti et al. 1993, 1992).