Determinants of PON1 variation in Mexican-American children and mothers

The paraoxonase (PON1) enzyme has antioxidant properties and detoxifies oxon derivatives of organophosphate pesticides (OPs). Since there is high interindividual variability of PON1 phenotype within populations, we hypothesize that PON1 genotype and enzyme activity in young children and pregnant mothers confer differential susceptibility to OPs and oxidative stress. We aimed to: (1) validate PON1 enzymatic assays, considered more informative measures than PON1 genotype alone, for use in longitudinal studies, (2) identify functional PON1 polymorphisms in Mexican-Americans, (3) compare the role of genetic variability on control of PON1 phenotype in mothers and children, and (4) determine the ontogeny of PON1 in young children. First, we validated four PON1 enzymatic assays for use in longitudinal studies and large pediatric cohorts and identified potential sources of technical variability (e.g. assay temperature, specimen type, storage duration) that should be taken into account for reliable data interpretation. To explore genetic variability, we resequenced samples from the CHAMACOS cohort and identified 6 novel single nucleotide polymorphisms (SNPs), 1 insertion, and 2 deletions among 94 common PON1 variants. Next, we demonstrated the functional significance of common haplotypes, genotypes, insertions, and deletions with PON1 enzyme activity in 339 children and 361 mothers. The genetic contribution of PON1 SNPs on phenotypic variation was lower in newborns compared to seven-year olds (1.6-fold, paraoxonase assay) and also 2-fold lower in pregnant mothers compared to non-pregnant mothers (arylesterase assay). Thus, depending on age and physiological condition, the relative influence of factors, other than PON1 genetics, (epigenetics, genetic networks) may differ in the same individual. Contrary to previous reports that PON1 activities plateau at age two, PON1 enzyme activities increased with age (p<0.001) in children (n=458) followed longitudinally from birth through age seven. These increases were significantly modified by PON1 genotype, rendering some children more susceptible (>64-fold) than others. Even at age seven, PON1 levels and activities remained lower in children than mothers. Our study provides the first quantitative data in a large cohort that lower levels of the protective PON1 enzyme persist in young children well into school-age and underscores the need for more stringent policies protecting young children from pesticide exposures.