Superfund Research Program

Paraoxonases: Biomarkers of Susceptibility to Environmentally-Induced Diseases

Project Leader: Lucio G. Costa
Grant Number: P42ES004696
Funding Period: 2000 - 2009

Project-Specific Links

Final Progress Reports

Year:   2008  2005 

The paraoxonases, PON1, PON2, and PON3, play important roles in gene/environment interactions, drug metabolism and susceptibility to vascular and infectious disease. PON2 is expressed ubiquitously and has antioxidant properties. PON1 and PON3 are expressed in the liver and are secreted into the serum, where they are associated with high density lipoprotein particles.  PON1 hydrolyzes several organophosphorus (OP) insecticides, oxidized lipids, and other substrates. Many polymorphisms have been identified in PON1, including one that causes an amino acid substitution at Q192R, altering its catalytic efficiency.  PON1 levels are highly variable among adults and very low in infants. Dr. Costa and his research team coined the term “PON1 Status” to describe a method for simultaneous determination of both PON1 levels and the amino acid present at position 192.

To examine the relevance of the Q192R polymorphism for OP exposures in vivo, the project researchers used transgenic mice expressing human PON1Q192 or PON1R192 in the absence of mouse PON1. Dose response and time course studies showed that mice expressing PON1Q192 were much more sensitive to chlorpyrifos and chlorpyrifos oxon (CPO) than were mice expressing PON1R192. These data are being used by Charles Timchalk, the project’s collaborator at the Pacific Northwest National Laboratories, to refine pharmacokinetic and pharmacodynamic models of OP exposure.  Related experiments involving chronic exposure of mice to low levels of CPO during postnatal development showed that mice expressing PON1Q192 were also more sensitive to neocortical histopathology and changes in neocortical gene expression than were mice expressing PON1R192. Thus, individuals expressing PON1Q192 are particularly susceptible to chlorpyrifos toxicity, especially if combined with low PON1 levels. This is of concern because 50% of the population is homozygous for PON1Q192.

The PON genes are candidates for association with Parkinson’s Disease (PD) because PON1 modulates pesticide toxicity and PON2 is expressed in the brain and has antioxidant properties. Previously project researchers had examined polymorphisms in PON1 and PON2 in a group of PD patients and controls, and found no associations with PON genotype. In the case of carotid artery disease, PON1 genotype alone did not predict disease, but PON1 Status established low PON1 levels as a risk factor. Thus, the researchers measured PON1 Status in about 200 new samples from PD patients and controls from Project 5 (Checkoway), and have submitted the data for analysis.  To measure PON2 levels in lymphocytes/monocytes, they generated anti-PON2 antibodies, which can also be used for immunohistochemical localization. They also generated active recombinant PON2, which should allow them to develop spectral assays for determining “PON2 Status”.

Another goal of this project was to identify novel PON1 substrates. A most exciting result was the project’s finding that PON1 (and also PON2 and PON3) degrades acyl-homoserine lactone (acyl-HSL), a quorum-sensing signal that the pathogenic bacterium Pseudomonas aeruginosa uses to regulate genes controlling virulence and biofilm formation. A number of non-toxic substrates are being tested to replace the compounds currently used for assessing PON1 Status. An example which provides reasonably good resolution of the PON-Q192R phenotypes uses differential rates of phenylacetate  and chlorophenylacetate hydrolysis at different salt concentrations.  Other newly-discovered substrates for PON1 include tricresyl phosphate, (a widely used additive in lubricants), retinyl esters, triacetin, p-nitrophenyl phosphorylcholine, p-nitrophenyl acetate, beta naphthyl acetate, 4-nitrophenyl valerate, and coumaphos-oxon.