Biometals 2010

A significant relationship between mercury exposure from dental amalgams and urinary porphyrins: a further assessment of the Casa Pia children's dental amalgam trial.

ABSTRACT:

Previous studies noted specific changes in urinary porphyrin excretion patterns associated with exposure to mercury (Hg) in animals and humans. In our study, urinary porphyrin concentrations were examined in normal children 8–18 years-old from a reanalysis of data provided from a randomized, prospective clinical trial that was designed to evaluate the potential health consequences of prolonged exposure to Hg from dental amalgam fillings (the parent study).

Our analysis examined dose-dependent correlations between increasing Hg exposure from dental amalgams and urinary porphyrins utilizing statistical models with adjustments for the baseline level (i.e. study year 1) of the following variables: urinary Hg, each urinary porphyrin measure, gender, race, and the level of lead (Pb) in each subject's blood. Significant dose-dependent correlations between cumulative exposure to Hg from dental amalgams and urinary porphyrins associated with Hg body-burden (pentacarboxyporphyrin, precoproporphyrin, and coproporphyrin) were observed.

Overall, 5–10% increases in Hg-associated porphyrins for subjects receiving an average number of dental amalgam fillings in comparison to subjects receiving only composite fillings were observed over the 8-year course of the study. In contrast, no significant correlations were observed between cumulative exposure to Hg from dental amalgams and urinary porphyrins not associated with Hg body-burden (uroporphyrin, heptacarboxyporphyrin, and hexacarboxyporphyrin).

In conclusion, our study, in contrast to the no-effect results published from the parent study, further establishes the sensitivity and specificity of specific urinary porphyrins as a biomarker for low-level Hg body-burden, and also reveals that dental amalgams are a significant chronic contributor to Hg body-burden.

INTRODUCTION:

Porphyrins are formed as derivatives of the heme synthesis pathway, an essential biochemical pathway, as shown in Fig. 1. Heme biosynthesis occurs in nearly all eukaryotic tissues. In humans and other mammals, porphyrins with eight, seven, six, five, and four carboxyl groups are commonly formed in excess of that required for heme biosynthesis and the excess amounts are excreted through the urine in a wellestablished pattern (Bowers et al. 1992; Woods et al. 1993).

Porphyrins can be utilized to afford a measure of xenobiotic exposure (Brewster 1988). In previous studies specific changes in urinary porphyrin excretion patterns (porphyrin profiles) associated with prolonged exposure of both animals and humans to mercury (Hg)and other metals were described (Woods and Fowler 1977, 1978; Fowler et al. 1987). The changes in urinary porphyrin excretion patterns were shown to involve both metal-induced inhibition of specific heme pathway enzymes in target tissues and metal-facilitated oxidation of reduced porphyrins that accumulate in tissue cells because of impaired porphyrin metabolism (Woods 1996). Changes in porphyrin excretion patterns are largely metal specific, correlate with metal concentrations in tissue cells, and occur prior to the onset of target tissue injury. Thus, investigators have found that urinary porphyrin profile measurements can be utilized as biomarkers of metal exposure and toxicity in human subjects (Woods 1996).

Previous studies have shown that urinary porphyrins can be particularly useful in measuring Hg exposure (Gonzalez-Ramirez et al. 1995; Geier and Geier 2006, 2007; Geier et al. 2009a, b; Kern et al. 2010a,b; Woods, 1996; Pingree et al. 2001).

Urinary porphyrins are not a direct measure of Hg in the urine, but a measure of presence of Hg in the body (or Hg body-burden) by the level of disruption of the heme synthesis pathway that Hg causes. The presence of Hg inhibits specific enzymes that are necessary for the heme synthesis pathway to progress. This inhibition or interference results in a ''backlog'' and an increase urinary excretion of specific porphyrins. The level of increase in these ''backlogged'' metabolites measured in the urine correlates with the level of disruption of this pathway and indicates the extent of Hg tissue burden. There is a high degree of statistical correlation between renal Hg burden and the urinary excretion of specific porphyrins (Pingree et al. 2001).

 This correlation is consistently observed in animal models of Hg exposure; and the heme pathway is highly conserved across species (Gonzalez- Ramirez et al. 1995). Further, studies in humans have found similar findings, i.e., that specific patterns of urinary porphyrins suggest the presence of Hg and the extent of the burden (Woods et al. 1993). 

Specifically, Hg body-burden has been demonstrated  to be associated with elevations in urinary coproporphyrin (cP), pentacarboxyporphyrin (5cxP), and by the expression of an atypical porphyrin– precoproporphyrin (PrcP) (also known as keto-isocoproporphyrin) not found in the urine of unexposed controls. Several studies showed this  characteristic pattern of porphyrinuria with Hg intoxication and  also showed that elevated levels of Hg associated urinary porphyrins can be normalized with Hg-specific chelation therapy (Gonzalez-Ramirez et al. 1995; Geier and Geier 2006, 2007; Woods et al. 1993;  Woods 1996; Pingree et al. 2001; Nataf et al. 2006). Woods (1996) noted that these distinct changes in urinary porphyrin concentrations were observed as early as 1–2 weeks after initiation of Hg exposure, and that they increased in a dose- and time-related fashion with the concentration of Hg in the kidney, a principal target organ of Hg compounds. 

In addition, Hg-associated urinary porphyrin profiles not only correlate significantly with Hg body-burden, but also with specific neurobehavioral deficits associated  with low-level Hg exposure. Echeverria et al. (1995), for example, examined the behavioral effects of low-level exposure to Hg vapor among dentists. These investigators observed that urinary porphyrins were as sensitive as urinary Hg levels for predicting adverse effects of Hg on cognitive and motor testing. Several studies have been completed recently that used urinary porphyrins to examine Hg toxicity in children, both in neurotypical children and in children with neurodevelopmental disorders (Geier and Geier 2006, 2007; Kern et al. 2010b; Nataf et al. 2006; Austin and Shandley, 2008; Young et al. 2010). Several of these studies examined the relationship between the severity of the neurological impairment and urinary porphyrins, and found significant positive correlations with Hg-associated urinary porphyrins and the severity of the neurological impairment (Nataf et al. 2006;Geier et al. 2009a, b; Kern et al. 2010a). In addition, one recent study examined two groups of neurotypical children with different levels of environmental Hg exposure and found that the level of Hg exposure was reflected in the Hg-associated urinary porphyrin levels (Kern et al. 2010c). Importantly, Woods et al. (2009) recently published a study supporting the utility of specific urinary porphyrins as biological indicators of subclinical Hg exposure in children.

In our study, urinary porphyrin concentrations were examined in children 8–18 years-old, with and without dental amalgam fillings, from a reanalysis of data collected from a completed clinical trial (the parent study) that was designed to evaluate the potential health consequences of prolonged exposure to Hg from dental amalgam fillings (DeRouen et al. 2006). Our study is designed to determine whether there was a significant dose-dependent correlation between increasing Hg exposure from dental amalgams and specific urinary porphyrins associated with Hg body-burden.

CONCLUSION:

Our study, in contrast to the published results of the parent study, found that the characteristic pattern of porphyria associated with Hg body- burden, specifically, elevated 5cxP, PrcP, and cP were significantly correlated with dental amalgam exposure in a dose dependent fashion. The higher level of Hg amalgams exposure resulted in higher levels of Hg-associated porphyrins. Further, consistent with previous studies that examined Hg exposure using porphyrins, the non- Hg-associated porphyrins showed no statistically significant relationship with amalgam exposure. As a result, this study helps to further establish the utility of urinary porphyrins as a biomarker of low-level Hg body-burden.

In addition, the relative 5–10% increases in the Hg-associated porphyrins observed over the 8 year course of the present study, suggest that dental amalgams for the average individual do not cause a significant acute exposure to Hg, but instead represent a significant life-long source of chronic exposure to Hg, with a continuing impact on increasing Hg body-burden.

It is of theoretical concern from extrapolation of our results that over the course of a life- time of 70 years, the contribution to Hg bodyburden from dental amalgams may eventually result in elevations in urinary porphyrins similar to those observed in individuals with diagnosed neurological conditions associated with Hg intoxication, and hence result in Hg body-burden levels associated with Hg toxicity. As a result, in any study of dental amalgam safety, a follow-up period of decades would be advisable in order to determine accurately the actual long-term pathological adverse effects amalgam may induce, even in initially healthy individuals.