Gene-Environment Interactions: Paraoxonase (PON1) and Sensitivity to Organophosphate Toxicity

Lucio G. Costa; Toby B. Cole; Clement E. Furlong

Disclosures

Lab Med. 2006;37(2):109-114. 

In This Article

Functional Genomics of PON1: The Determination of PON1 Status

Most studies investigating genetic variability of PON1 rely on measurement of nucleotide polymorphisms Q192R, L55M, and C-108T with polymerase chain reaction-based assays. However, even if an individual were genotyped for all known PON1 polymorphisms, this analysis would not provide the level of plasma PON1 activity, nor the phase of polymorphisms (ie, which polymorphisms are on each of an individual's 2 chromosomes). A functional genomic analysis provides a much more informative approach, as measurement of an individual's PON1 function (plasma activity) takes into account all polymorphisms that may affect activity. It should be noted that in a given population, plasma PON1 activity can vary by more than 40-fold,[16,23,35] and differences in PON1 protein levels up to 13-fold are also present within a single PON192 genotype.[36]

Determination of PON1 status for an individual is accomplished through the use of a 2-substrate enzyme assay, utilizing PO and DZO (Figure 1). For this assay, which has been described in detail elsewhere,[37] plasma is isolated from heparin-collected blood. Citrate plasma (or serum) can also be used, though consistency is necessary among samples. EDTA plasma is not useful, as PON1 activity is calcium-dependent, and EDTA irreversibly inhibits PON1. Initial rates of DZO and of PO hydrolysis are measured by monitoring the formation of 2-isopropyl-4-methyl-6-hydroxypirimidine and p-nitrophenol, respectively. This method separates individuals into the 3 phenotypes of PON1192 activity, (ie, PON1192QQ, PON1192QR and PON1192RR). In addition, PON activities within a genotype provide information on the levels of PON1 in the plasma of a given individual within that genotype group. The PON1 genotype can be accurately inferred by this analysis, and can be verified by genotyping the individuals DNA.[37] However, with very few exceptions, this functional genomics analysis is very accurate, and does not require verification.[12] An example of possible discordance is an individual who genotypes as a PON1QR heterozygote, but whose PON1 status functional analysis indicates a PON1192Q or PON1192R homozygote; this would indicate that one of the alleles is nonfunctional, because of additional mutations (eg, W194X; 34), and underlines the advantage of the PON1 status analysis over straight genotyping. It is also important to note that several recent studies investigating the role of PON1 in coronary heart disease, have indeed provided evidence that PON1 status (encompassing genotype and activity levels) is a much better predictor of disease than PON1 genotype alone.[12,13]

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