Abstract
Gout is a complex disease caused by over-production and/or under-excretion of urate, which causes hyperuricaemia. This leads to urate deposition, and initiates the formation of monosodium urate (MSU) crystals in joints and other parts of the body. Accumulation of these crystals results in acute inflammatory attacks of gout. Gout is a serious health issue in New Zealand, because 6.1% of New Zealand Māori and 7.6% of Pacific Island peoples aged ≥ 20 years are diagnosed with gout, compared to 3.2% of New Zealand Europeans/other ethnic groups. Also, the prevalence of gout worldwide is increasing.
The aim of this thesis was to test for associations between gout and serum urate with the five gene variants of interest, (LRP1 rs753206313, PRPSAP1 rs749392722, ABCC5 rs369277426, SLC22A3 rs565948998 and BCAS3 rs2643103). These have shown, through a re-sequence project, to have a Polynesian-specific association with hyperuricaemia. I also investigated whether the PRPSAP1 and SLC22A3 genes have any expression quantitative trait loci (eQTLs) signals, which contributes to their gene expression level variation, which mediates serum urate control.
The five SNPs of interest were genotyped using TaqMan®. The Polynesian sample sets had 779 gout cases and 919 controls. The sample sets were recruited from Māori and Pacific peoples populations in the Auckland, Canterbury, and Otago regions of New Zealand, no Melanesian or Micronesian samples were included. All gout cases satisfied the 1977 American Rheumatism Association gout classification criteria. The Köttgen et al. (2013) serum urate GWAS, stomach/liver tissue GTEx (eQTL) and Westra et al. (2013) blood cis/trans (eQTL) datasets were used to find PRPSAP1 and SLC22A3 eQTLs involved in gene expression variation that contributes to serum urate modulation.
Associations with gout were found with the PRPSAP1 rs749392722 and SLC22A3 rs565948998 SNP minor alleles in Eastern and Western Polynesian populations, respectively (PRPSAP1 Eastern P adjusted = 0.01, OR = 2.06; SLC22A3 Western P adjusted = 3.13e-04, OR = 3.19). Both SNPs meta-analysis were in the same direction (PRPSAP1 rs749392722 P overall = 3.00e-03, OR = 2.18; SLC22A3 rs565948998 P overall = 5.00e-04, OR = 2.23). The eQTL analysis showed that SLC22A3 SNP rs10455782 and SNP rs7758229 both associated with serum urate control and both also contributed to SLC22A3 expression level variation. PRPSAP1 also has eQTLs signals which influence its expression levels in the stomach, whilst contributing to the regulation of urate levels. These results provide evidence that both the PRPSAP1 and SLC22A3 genes have a Polynesian- specific causal role in gout risk.
PRPSAP1 plays a negative regulatory role in the PRPP synthetase production, which is an enzyme involved in urate production. The PRPSAP1 SNP rs749392722 is an indel variant that induces the termination of the PRPSAP1 protein. In result, the PRPSAP1 protein is fails to bind to the PRPP synthetase enzyme, which allows continued production of PRPP. Subsequently, leads to the over- production of urates.
The SLC22A3 gene is an organic cation transporter, involved in transporting a wide range of toxins out of the body, including urate. Therefore, SNPs of SLC22A3 such as rs565948998 could alter the structure and stability of the SLC22A3 transporter, reducing its function and leading to the under- excretions of toxins and urates. As a result, hyperuricaemia and possibly gout traits developed. Also, this study provides evidence of SLC22A3 and PRPSAP1 eQTL SNPs involvement in serum urate modulation.
Collectively, the findings from this research provide evidence why gout and hyperuricaemia occurrence is more frequent in Polynesian individuals compared to other ancestries such as Europeans. The findings from this study are of importance for Pacific/Polynesian peoples’ health by identifying genes, gene variants and pathways that could be targeted in preventing and treating of gout and hyperuricaemia, which Polynesian people are more susceptible to.