Student Research: Wan-Fen Li
PhD, Toxicology (Tox), 1999
Faculty Advisor: Lucio G. Costa
Development of a Mouse Model to Study the Role of Paraoxonase (PON1) in Organophosphate Detoxication
Organophosphorus (OP) insecticides are used worldwide and pose a threat to public health. Paraoxonase (PON1) is a HDL-associated enzyme that exhibits a genetic polymorphism in humans. PON1 hydrolyzes the toxic metabolites of several OP insecticides, such as parathion, diazinon and chlorpyrifos, as well as the nerve agents soman and sarin. Injection of purified PON1 protects rodents from OP toxicity. This thesis project was conducted to develop a mouse model for studying the role of PON1 in OP detoxication in vivo. The first step was to characterize the mouse PON1 gene. The PON1 gene is highly conserved between humans and mice, with an 81% identity in deduced protein sequence and a syntenic chromosomal location. The developmental expression of mouse PON1 was also determined. Serum PON1 activity was very low at birth and increased with time, reaching adult levels at 3 weeks of age. The mRNA levels in liver showed a parallel age-related increase, indicating a regulatory mechanism of PON1 gene expression during development. The increase of serum PON1 activity in developing animals correlates well with their increased resistance to OP toxicity. Mouse PON1 levels can also be modulated by various environmental factors. Phenobarbital treatment resulted in a temporary decrease in serum PON1 activity, but an increase in liver PON1 activity and mRNA levels. The acute phase response, mimicked by LPS injection, caused a decrease in serum PON1 activity and liver PON1 mRNA levels. The role of PON1 in OP detoxication is clearly demonstrated in PON1-knockout mice. PON1-knockout mice were extremely sensitive to chlorpyrifos-oxon and diazoxon, and slightly more sensitive to the parent compounds. Surprisingly, PON1-knockout mice did not show increased sensitivity to paraoxon. Kinetic studies revealed that mouse PON1 is very efficient in hydrolyzing chlorpyrifos-oxon and diazoxon but not paraoxon, which may explain the observations in the toxicology studies. The attempt to express human PON1 cDNA in transgenic mice was not successful, indicating that the introduction of the whole PON1 gene may be essential for effective expression in mice. Overall, this study has established a mouse model in which to assess the functions of PON1 in detoxication of certain OP insecticide in vivo.