Serum paraoxonase activity is associated with variants in the PON gene cluster and risk of Alzheimer disease

Abstract

Previous studies have shown association of single nucleotide polymorphisms (SNPs) in 3 contiguous genes (PON1, PON2, and PON3) encoding paraoxonase with risk of Alzheimer disease (AD). We evaluated the association of serum paraoxonase activity measured by phenyl acetate (PA) and thiobutyl butyrolactone (TBBL) with risk of AD and with 26 SNPs spanning the PON gene cluster in 266 AD cases and 306 sibling controls from the MIRAGE study. The odds of AD (adjusted for age, gender, and ethnicity) increased 20% for each standard deviation decrease in PA or TBBL activity. There were association signals with activity in all 3 genes. Haplotypes including SNPs spanning the PON genes were generally more significant than haplotypes comprising SNPs from 1 gene. Significant interactions were observed between SNP pairs located across the PON cluster with either serum activity measure as the outcome, and between several PON SNPs and PA activity with AD status as the outcome. Our results suggest that low serum paraoxonase activity is a risk factor for AD. Furthermore, multiple variants in PON influence serum paraoxonase activity and their effects may be synergistic.

Figure 1

PON serum activity phenotypes were measured by reacting individual serum samples with either TBBL or PA and monitoring the rate of product accumulation (hydroxy butyric acid for TBBL; phenol for PA). Selected representative time-course plots for each of the two activity phenotypes are shown including the minimum, maximum and quartile-boundary observations; spontaneous hydrolysis (no serum) which was low in both assays is also shown.

Figure 2

Association of PON SNPs and haplotypes with TBBL activity. A. Single SNP tests. Columns represent the negative log10 of the p-value obtained for each SNP. Red horizontal lines indicate significance thresholds. The PON gene cluster is drawn to scale showing the locations of the PON genes and the SNPs genotyped. SNPs 1-9 and SNPs 11-29 are located in separate haplotype blocks (see Supplementary Figure 1). B. Most significant haplotypes among all possible SNP combinations. C. Most significant haplotypes among SNPs 11-29. D. Most significant haplotypes among SNPs 1-9. Panels B-D: Haplotypes with the most significant global and specific p-values in the designated group of SNPs are shown. SpecP=haplotype specific p-value; globP=global test p-value; freq=haplotype frequency; #fams=the number of informative families included in the test.

Figure 3

Association of PON SNPs and haplotypes with PA activity. A. Single SNP tests. B. Most significant haplotypes among all possible SNP combinations. C. Most significant haplotypes among combinations of SNPs 1, 2, 3, 6, 9, 20 and 26. See Figure 2 for additional details.

Figure 4

Allelic patterns of association for SNPs with serum paraoxanase activity phenotypes and AD risk. Alleles associated with lower activity (TBBL or PA) and higher risk of AD (according to test results of individual SNPs) are shown for each SNP. Dark background – regions of consensus; orange font – significantly associated SNPs (p≤1.9×10⁻⁵ for TBBL and PA; p≤0.05 for AD).

Figure 5

Interaction of phenyl acetate (PA) activity in serum and rs3735586 genotype on odds of AD. Odds of AD per unit of PA activity are shown separately for rs3735586 C/T heterozygotes (red curve) and T/T homozygotes (blue curve).