Superfund Research Program
Heme Pathway Polymorphisms in Mercury Neurotoxicity in Adults and Children
Project Leader: James S. Woods
Grant Number: P42ES004696
Funding Period: 1995 - 2009
Final Progress Reports
In a major methodological advance, Dr. Woods’ research group demonstrated the utility of genotype-specific cell lines created from Epstein Barr viral (EBV)-transformed human lymphocytes as an experimental model for studying the effect of genotype on chemical-mediated changes in cell functions. Specifically, the researchers acquired blood samples from human subjects previously identified as wildtype (WT), heterozygous (Het) or full mutant (Mut) for the A814C polymorphism of the coproporphyrinogen oxidase (CPOX) gene and established transformed cell lines of each to study the consequences of the CPOX mutation on cell viability and the effects of Hg2+ on this process.
These findings demonstrate that genotype modifies the effect of Hg2+ on cell viability in a genotype- and Hg2+ concentration-dependent manner, with CPOX Mut cells undergoing 40% loss of viability compared with 27% and 21% of Het and WT, respectively, at 5 µM Hg2+. Additional studies suggest that cellular heme serves as a component of pro-survival cell signaling processes and that modulation of cell heme content by CPOX genotype and Hg2+ underlies the decreased viability observed. These results suggest that the A814C CPOX polymorphism increases the susceptibility of cells to Hg2+ toxicity and that this effect is mediated by heme deficiency. Additionally, these studies demonstrate the utility of EBV-transformed cells as an alternative to transgenic animal models for studying the effects of genetic variation on chemical-mediated changes in cell functions.
Another major highlight was the discovery of a gene-dose effect of the A814C polymorphism of CPOX on measures of neurological functions in children with mercury exposure. Z-scores comparing tests of children receiving mercury amalgam dental fillings with those receiving composite resin showed statistically significant negative gene-dose trends for measures of attention, visuomotor function, memory and nerve conduction, with children carrying the full CPOX double allelic mutation (CPOX4) most negatively affected compared to WT. These findings are consistent with the group’s previous observations of increased susceptibility to impaired neurobehavioral functions associated with CPOX genotype among mercury-exposed dental professionals. The researchers have also observed suggestions of gene-dose effects on specific neurological functions associated with polymorphisms of catechol-o-methyltransferase (COMT), brain derived neurotropic factor (BDNF) and the serotonin transporter gene promoter (5-HTTLPR) in mercury-exposed children.