Testing the Vitamin D3 metabolic gene variants for association with gout in the New Zealand case-control sample-sets

Abstract

Gout is a common disease caused by immune response to monosodium urate (MSU) crystal deposition in articular or periarticular tissues and in the renal tract after chronic hyperuricemia (uric acid levels exceeding 7mg/dL). Gout generally progresses in three clinical stages: asymptomatic hyperuricemia (elevated serum urate levels but no evidence of gout), recurrent episodes of acute gout attacks with interspersed intercritical periods and chronic tophaceous gout (nodular masses of uric acid crystals). Epidemiological evidence have shown the prevalence of gout to be increasing in the New Zealand (NZ) population. In particular Māori (13.9%) and Pacific Island (14.9%) people experience an earlier age of onset and higher frequency of multiple tophi and polyarticular gout. The significant increase of susceptibility to gout in Māori and Pacific Island people in the 21st century has become a public health epidemic. Furthermore inpatients with gout often have a wide variety of co-morbidities including type II diabetes mellitus, congestive heart failure and metabolic syndrome. Vitamin D3 is a secosteroid (steroid molecule with a ‘broken’ ring) hormone and is produced in large quantities in humans upon exposure of skin to sunlight. However most populations are considered to be Vitamin D3 deficient through a combination of environmental, behavioral and genetic factors. Sufficient levels of Vitamin D3 were found to protect against a wide range of diseases including type II diabetes mellitus, cancers and cardiovascular problems. In recent genome wide studies, Vitamin D3 metabolic variants have found to be associated with immune–mediated diseases that are characterized by an imbalance in helper T-cell development. Therefore the aim of this study was to test for associations between Vitamin D3 genetic variants with gout in the NZ case-control sample-sets (Caucasian, Māori, East Polynesian and West Polynesian). Nine single nucleotide polymorphisms (SNPs) from 6 genes were successfully genotyped in the NZ gout cohorts. Case-control analysis was performed for each SNP to test for association of the genomic marker to gout in the NZ gout cohorts. Genotyping data obtained from the NZ gout Caucasian cohort was combined with the Framingham Heart Study (FHS) dataset to provide a more accurate estimate of the overall significance of the SNP to the disease. Although there was some evidence for association with several of the SNPs with gout in the Māori and Eastern Polynesian case-control sample-sets, true association may be distorted by the presence of population stratification within these cohorts. The STRUCTURE and STRAT program was utilized to circumvent the presence of population stratification in this study. However, due to limited number of genomic markers available, population stratification still may not adequately control for the extent of Caucasian ancestry within the Māori and Polynesian case-control sample-sets. The Vitamin D binding protein (VDBP) gene variants, rs7041 and rs4588, showed evidence for association and a trend towards association with gout in the NZ Caucasian case-control sample-set respectively (rs7041: p= 0.02, rs4588: p= 0.05). However both of these variants also conferred strong susceptibility to Vitamin D3 deficiency in the FHS dataset (rs7041: p= 5.4x10-8, rs4588: p= 7.6x10-11). The VDBP gene variants were also tested for association with Rheumatoid arthritis (WTCCC sample-set), presence of tophus (NZ ‘suspected-tophus’ case-control sample-sets) and gender influences with gout presentation (NZ gout case-control sample-sets). These findings suggest that VDBP gene variants may protect an individual against the onset of acute gout. However people with these VDBP gene variants may paradoxically be at a greater risk of developing chronic gout via suppression of Tregulatory cell responses. A further aim of this study was to investigate VDBP gene variants, rs7041 and rs4588 with differences in Vitamin D supplementation and levels of serum 25(OH)D in a separate NZ supplementation sample-set. There was no evidence for differences in binding affinity of the VDBP gene variants with Vitamin D2 (rs7041: p= 0.412, rs4588: p= 0.202) and Vitamin D3 (rs7041: p= 0.408, rs4588: p= 0.432) supplementation and serum 25(OH)D concentrations in the pre-assigned NZ supplement groups. These results suggest that the Vitamin D3 genetic variants may have a profound role in the development of gout. However to obtain any real meaningful etiological effect with gout, Vitamin D3 genetic variants must be genotyped in larger sample-sizes and stratified with increased number of genomic markers to account for Caucasian admixture.