Monoamine oxidase A gene polymorphisms and enzyme activity associated with risk of gout in Taiwan aborigines. 

DOI 10.1007/s00439-009-0765-z

O R I G I N A L I N V E S T I G A T I O N

Monoamine oxidase A gene polymorphisms and enzyme

activity associated with risk of gout in Taiwan aborigines

Rod A. Lea · Shang-Lun Chiang · Hung-Che Chiang · Tsu-Nai Wang · Meng-Chuan Huang · Tsan-Teng Ou · Gau-Tyan Lin · Ying-Chin Ko

Received: 28 June 2009 / Accepted: 2 November 2009 / Published online: 14 November 2009 © Springer-Verlag 2009

Abstract Taiwanese aborigines have a high prevalence of hyperuricemia and gout. Uric acid levels and urate excre-tion have correlated with dopamine-induced glomerular Wltration response. MAOs represent one of the major renal dopamine metabolic pathways. We aimed to identify the monoamine oxidase A (MAOA, Xp11.3) gene variants and MAO-A enzyme activity associated with gout risk. This study was to investigate the association between gout and the MAOA single-nucleotide polymorphisms (SNPs) rs5953210, rs2283725, and rs1137070 as well as between gout and the COMT SNPs rs4680 Val158Met for 374 gout cases and 604 controls. MAO-A activity was also mea-sured. All three MAOA SNPs were signiWcantly associated with gout. A synonymous MAOA SNP, rs1137070 Asp470Asp, located in exon 14, was associated with the

risk of having gout (P = 4.0 £ 10¡5, adjusted odds ratio 1.46, 95% conWdence intervals [CI]: 1.11–1.91). We also showed that, when compared to individuals with the MAOA GAT haplotype, carriers of the AGC haplotype had a 1.67-fold (95% CI: 1.28–2.17) higher risk of gout. Moreover, we found that MAOA enzyme activity correlated positively with hyperuricemia and gout (P for trend = 2.00 £ 10¡3 vs. normal control). We also found that MAOA enzyme activity by rs1137070 allele was associated with hyperuricemia and gout (P for trend = 1.53 £ 10¡6 vs. wild-type allele). Thus, our results show that some MAOA alleles, which have a higher enzyme activity, predispose to the development of gout.

H.-P. Tu

Graduate Institute of Medicine, College of Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan e-mail: p915013@kmu.edu.tw

H.-P. Tu · H.-C. Chiang · M.-C. Huang · Y.-C. Ko (&) Department of Public Health,

Faculty of Medicine and Environmental Medicine, College of Medicine, Kaohsiung Medical University, 100 Shih-Chuan 1st Road, Kaohsiung, Taiwan e-mail: ycko@kmu.edu.tw

A. M.-S. Ko · S.-J. Wang · S.-L. Chiang · Y.-C. Ko Center of Excellence for Environmental Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan

C.-H. Lee · T.-N. Wang Faculty of Public Health,

Kaohsiung Medical University, Kaohsiung, Taiwan

R. A. Lea

The Institute of Environmental Science and Research, Wellington, New Zealand

H.-C. Chiang · Y.-C. Ko

Division of Environmental Health and Occupational Medicine, National Health Research Institutes, Zhunan, Miaoli, Taiwan

M.-C. Huang

Department of Nutrition,

Kaohsiung Medical University Hospital, Kaohsiung, Taiwan

T.-T. Ou

Division of Rheumatology, Department of Internal Medicine, Kaohsiung Medical University Hospital,

Kaohsiung Medical University, Kaohsiung, Taiwan

G.-T. Lin

Department of Orthopaedic Surgery, Kaohsiung Medical University Hospital, Kaohsiung Medical University, Kaohsiung, Taiwan

Introduction

Gout is characterized by elevated serum urate, which crys-tallizes as monosodium urate in and around the tissues of joints when it is beyond its physiologic solubility limit (Choi et al. 2005; Emmerson 1996; Terkeltaub 2003). The clinical manifestation of gout varies according to its sever-ity from episodic attacks to recurrent painful attacks of acute inXammatory arthritis, tophaceous gout, chronic poly-articular arthritis, uric-acid urolithiasis, complicated with possible sequelae of renal impairment and failure.

Urate is synthesized mainly in the liver and is mainly excreted in the urine. Approximately, two-thirds of daily urate excretion is via the kidney. An estimated 85–90% of gout cases result from poor renal disposal of urate (Pascual and Perdiguero 2006). Urate transport depends on speciWc transporter molecules (URAT1 [urate transporter 1], SLC2A9 [urate voltage-driven eZux transporter 1], organic anion transport [OAT] family OAT1, OAT3) located within the membrane of the renal proximal tubule cells, which account for part of the urate transport system in the kidney (Anzai et al. 2008; Dalbeth and Merriman 2009; Hediger et al. 2005). However, the intrarenal apical-basolateral urate transport pathway remains unclear. High serum uric acid levels are independently associated with increased proximal tubular sodium reabsorption in men (Cappuccio et al. 1993). Familial juvenile hyperuricemic nephropathy disease, characterized by hyperuricemia with underexcre-tion, gout, and chronic renal failure, is believed to be caused by distal salt wasting and a compensatory upregula-tion of proximal tubule resorpupregula-tion of sodium and uric acid (Taniguchi and Kamatani 2008). Furthermore, this intra-renal signaling pathway could be likely explained by the fact that proximal tubular reabsorption of uric acid occurs by an active transport mechanism closely linked to, or identical with, the tubular reabsorption of sodium. Thus, understanding the molecular mechanisms of urate transport in the kidney has potential research and clinical implications.

Dopamine plays a critical role in the regulation of diVer-ent renal functions, including glomerular Wltration, renin production, and sodium excretion (Zeng et al. 2007). Recently it has been shown that uric acid levels and urate excretion correlated with dopamine-induced glomerular Wltration response (Sulikowska et al. 2008). In the proximal tubule, intrarenal dopamine, released within the tubule lumen and the peritubular space, serves as an autocrine/par-acrine factor, locally modulating renal hemodynamic and/ or excretory functions (Bianchi et al. 2003; Vindis et al.

2001). Many investigators have conWrmed that dopamine decreases sodium reabsorption at the basolateral and apical membranes by inhibiting the Na+–K+ ATPase and the Na+– H+ _{exchanger through the activation of speciWc D1- and}

D2-like receptors (Jose et al. 1992; Pestana et al. 2001).

The metabolism of dopamine involves mitochondrial monoamine oxidases (MAOs) and the cytosolic catechol-O-methyltransferase (COMT), since these enzymes degrade dopamine into 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) which can be easily Wltrated through the tubule (Pestana et al. 2001). Two isoenzymes MAO-A and MAO-B are present in renal tubular epithelial cells. However, MAO-A is the predominant isoenzyme in the rat renal cell types involved in the deamination of the natriuretic hormone dopamine (Guimaraes and Soares-da-Silva 1998). Monoamine oxidases A (MAOA) and COMT, are believed to play a major role in regulating the renal dopamine activity (Pestana et al. 2001; Zeng et al. 2007). Although MAOs has been implicated in the development or the maintenance of mental retardation and neurodegenera-tive disorders (Cases et al. 1995), recent evidence has indi-cated that MAOs induces the production of injurious H₂O₂ in proximal tubule cells, which contributes to extracellular-regulated kinases (ERK) and c-Jun N-terminal kinase (JNK) activation and cell apoptosis, thereby promoting tis-sue injury (Kunduzova et al. 2002).

Taiwanese aborigines have a high prevalence of hyper-uricemia and gout (Chang et al. 1997; Wang et al. 2004). In Taiwanese aborigines men 40 years or older, the prevalence of hyperuricemia is 40–60% and the prevalence of gout 5– 20% (Chang et al. 1997). The direct causal mechanisms linking uric acid metabolism to the occurrence of gout in this susceptible population have not been unequivocally determined. At present there is no positive evidence for a direct role for X chromosome genes in most cases of gout. The goal of this study was to determine whether there is a correlation between some MAOA gene variants and gout. To this end, we examined in Taiwan aborigines the geno-type distributions of MAOA gene variants in individuals with gout and in control individuals in a case–control study design and the relationship between these polymorphisms and MAO-A enzyme activity.

Methods Subjects

From 2004 to 2007, a follow-up study of 374 gout partici-pants (290 males and 84 females) was evaluated and a popu-lation-based study of 604 healthy controls (433 males and 171 females) was enrolled. As much as 514 participants (125 male and 41 female gouty patients; 52 male and 76 female hyperuricemia; 130 male and 90 female controls) could complete MAO-A enzyme levels testing. Gouty patients were diagnosed using criteria from the American College of Rheumatology (Wallace et al. 1977). Cases of gout were ascertained to satisfy 6 or more of the 11 criteria

and were conWrmed by rheumatologist and primary care doctors in the local health center. Demographic and sub-stance use information was collected by interview using a standardized questionnaire. Current/past smokers and drinkers (self-reported) were deWned.

This study protocol was approved by Institutional Review Board of Kaohsiung Medical University and National Health Research Institutes. Written informed con-sent was obtained from all participants.

Uric acid levels determination

Hyperuricemia was deWned by serum uric acid level exceeding 7.0 mg/dL and 6.0 mg/dL in men and women, respectively. An aliquot of plasma blood was stored at 4°C for routine blood tests, which included measurements of plasma total cholesterol, triglycerides, creatinine, and uric acid using an automated analyzer (Beckman LX-20, Palo Alto, California).

Genomic DNA extraction

Total genomic DNA was obtained from white blood cells using a genomic DNA extraction kit (PureGene DNA Puri-Wcation Kit; Gentra Systems, Minneapolis, MN), and stored at ¡20°C until genotyping.

Genotype analysis

One missense SNP of COMT was selected (rs4680 Val158Met, located in exon 4). Two MAOA SNPs were selected (rs2283725, located in intron 3; rs1137070 Asp470Asp, located in exon 14) from a public reference database based on the minor allele frequency of >0.10 and the structure of the haplotype block having similar recom-bination in the HapMap Chinese population. To allow determination of the extent of linkage disequilibrium (LD) beyond the boundaries of the gene, one SNP in the 5⬘ inter-genic region was included (rs5953210, located in 5⬘ near gene). DNA samples were genotyped using the TaqMan SNP allelic discrimination using the ABI7900HT (Applied Biosystems, Foster City, California). As much as 10% of the samples were tested in duplicate with no genotyping errors detected.

Amine oxidase activity measurements

The MAO activity was investigated by measuring the production of hydrogen peroxide (H₂O₂)—and therefore resoruWn—from p-tyramine using the Amplex Red MAO assay kit (Molecular Probe, Eugene, Oregon). The MAO-A inhibitor, clorgyline, and the MAO-B inhibitor, pargyline, were included to help conWrm the identity of the enzyme

responsible for the amine oxidase activity. The assay is based on the detection of H₂O₂ in a horseradish peroxi-dase—coupled reaction using 10-acetyl-3,7-dihydroxy-phenoxazine (Amplex Red reagent). The Amplex Red reagent reacts with H₂O₂ in a 1:1 stoichiometry, and the resulting Xuorescence signal is directly proportional to H₂O₂ production and therefore to amine oxidase enzymatic activity. Experiments were carried out according to the manufacturer’s instructions, with a Wnal substrate concen-tration of 2 mM.

Statistical analysis

Statistical analyses were performed with SAS version 9.1.3 (SAS Institute Inc, Cary, NC). The genotype frequency of control in women conWrmed to the Hardy–Weinberg equi-librium (HWE P ¸ 0.05). For haplotype analysis, we esti-mated haplotype frequencies using the Haploview 4.0 program for a subset of SNPs selected on the basis of indi-vidual association with a given trait. Continuous variables, such as MAO-A enzyme activity and plasma triglyceride concentrations that were not normally distributed were log-transformed to achieve normality before using statistical models. In these analyses, the dependent variables were gout and MAO-A enzyme activity. Independent variables were alleles of the individual MAOA SNPs. The association study analyses used gender-speciWc SNPs from a logistic regression model after adjusting for age, gender, log-trans-formed triglycerides, uric acid, creatinine, and smoking cate-gories. Multiple testing-adjusted P-values using the stepdown Bonferroni method of Holm (considering three SNPs) were computed for multiple testing corrections. Men and women were analyzed together, as well as separately, to examine gender-speciWc eVects. MAO-A enzyme activity was calculated using linear regression models with adjustment for age and gender. P values < 0.05 were considered statistically signiWcant.

Results

The baseline characteristics of the 374 gout cases and the 604 controls are presented in Table1. Gout cases were sig-niWcantly higher in males, log-transformed triglycerides, uric acid levels, and higher cigarette use (P < 0.05) than control subjects. Three analyzed MAOA SNPs showed sig-niWcant associations with gout (P · 9.90 £ 10¡4_{; Tables2,}

3). A synonymous MAOA SNP, rs1137070 Asp470Asp, located in exon 14, was the signiWcantly associated with the risk of having gout: the at-risk rs1137070 C allele fre-quency was 71% in gouty males versus 60% in control males (P = 0.002); at-risk C allele was 71% in gouty females versus 59% in control females (P = 0.007). With

gender-speciWc eVects likely, we also showed the combined groups that SNP rs1137070 was the most signiWcant associ-ations with gout after adjusting covariates (P = 4.00 £

10¡5, adjusted odd ratio 1.46, 95% CI: 1.11–1.91). We also showed that COMT rs4680 was not signiWcantly associated with gout.

Table 1 Characteristics of gout

and control subjects in Taiwan aborigines

Variable Gout (n = 374) Control (n = 604) P value*

Age (years) 50.3 (15.3) 51.9 (16.9) 0.142 Gender (%) Male 290 (78) 433 (72) 0.043 Female 84 (22) 171 (28) Total Cholesterol (mg/dL) 183.4 (50.1) 180.5 (48.7) 0.367 Triglycerides (mg/dL) 269 (289) 224.3 (288.4) 0.020 Log (Triglycerides) (mg/dL) 5.3 (0.8) 5.1 (0.8) <0.001 Uric acid (mg/dL) 9.4 (2.4) 7.5 (2.1) <0.001 Creatinine (mg/dL) 1.2 (0.7) 1.0 (0.2) <0.001 BMI (kg/m2) 26.2 (4.4) 26.1 (4.3) 0.989 Alcohol use (%) Nondrinker 99 (26) 153 (25) 0.692 Drinker 275 (74) 451 (75) Cigarette use (%) Nonsmoker 159 (43) 338 (56) <0.001 Smoker 215 (57) 266 (44)

Values are expressed as mean (standard deviation) unless otherwise stated

TC total cholesterol, BMI body

mass index

* The P value was calculated continuous variables by the t test and by the
2 _{for the categorical}

variables

Table 2 Association between SNPs in MAOA and COMT genes and gout risk

A risk allele, a non-risk allele, RAF risk allele frequency

* Allelic P value was calculated by
2 _test

a _{The genotype frequency of control in women conWrmed to the Hardy–Weinberg equilibrium}

SNP Reference/ risk allele Male Allelic P-value* Femalea _Allelic P value Gout (n = 290) RAF Control (n = 433) RAF Gout (n = 84) RAF Control (n = 171) RAF

MAOA A/a A/a AA/Aa/aa AA/Aa/aa

rs1137070 Asp470Asp T/C 206/84 0.71 259/174 0.60 0.002 44/31/9 0.71 57/86/28 0.59 0.007 rs2283725 A/G 180/110 0.62 232/201 0.53 0.024 40/31/13 0.66 45/88/38 0.52 0.003 rs5953210 G/A 181/109 0.62 242/191 0.56 0.081 41/34/9 0.69 45/96/30 0.54 0.002

COMT AA/Aa/aa AA/Aa/aa AA/Aa/aa AA/Aa/aa

rs4680 Val158Met G/A 22/99/168 0.25 18/146/268 0.21 0.125 2/23/58 0.16 4/67/97 0.22 0.164

Table 3 Association between SNPs located in MAOA and gout risk

A risk allele, a non-risk allele, RAF denotes risk allele frequency a

Odds ratio (OR) of SNP was adjusted for age, gender, log-transformed triglycerides, uric acid, creatinine, and cigarette use (yes/no), and the related 95% conWdence intervals (CI)

SNP Gout Control Allelic

P value

Stepdown Bonferroni

OR (95% CI)a

Number RAFa _{Number RAF}

MAOA A/a A/a

rs1137070 325/133 0.71 459/316 0.59 4.00 £ 10¡5 2.00 £ 10¡4 1.46 (1.11–1.91) rs2283725 291/167 0.64 410/365 0.53 2.80 £ 10¡4 6.00 £ 10¡4 1.38 (1.06–1.79) rs5953210 297/161 0.64 428/247 0.55 9.90 £ 10¡4 _{1.00 £ 10}¡3 _{1.34 (1.03–1.74)}

Certain MAOA haplotypes were signiWcantly associated with gout. We identiWed three SNPs (rs5953210 A > G, rs2283725 G > A, and rs1137070 C > T) in the MAOA gene to investigate the haplotypic eVect of the studied SNPs on the risk of gout. Our results showed that, compared to indi-viduals with MAOA GAT haplotype (gouty frequency 0.27 vs. control 0.36, P = 1.60 £ 10¡3), carriers of MAOA AGC haplotype (gouty frequency 0.61 vs. control 0.50, P = 8.00 £ 10¡5) had a 1.67-fold (95% CI: 1.28–2.17) increased risk of developing gout (Table4). The odds ratio of at-risk haplotype was slightly larger than the eVects of the single SNP, implying that MAOA blocks may be inher-ited more frequently in gout cases.

Moreover, we found that MAO-A enzyme activity corre-lated positively with hyperuricemia (8.88 § 0.05 AU [arbi-trary unit], P = 0.084) and gout (9.00 § 0.06 AU, P = 0.003) compared with control individuals (8.72 § 0.07 AU) (Bonferroni post-hoc test after adjustment of covariates; P for trend = 2.00 £ 10¡3). We also observe that MAO-A enzyme activity by rs1137070 C allele corre-lated positively with hyperuricemia (8.94 § 0.05 AU, P = 3.50 £ 10¡3) and gout (9.06 § 0.06 AU, P = 9.25 £ 10¡5) compared with control individuals of wild-type T allele (8.59 § 0.09 AU) (Bonferroni post-hoc test after adjustment of covariates; P for trend = 1.53 £ 10¡6; Fig.1). Our results suggest that variant of the SNP rs1137070 located within MAOA functional domain could aVect enzyme activity in the renal/circulation system. Discussion

The pathogenesis of gout remains obscure. Genetic studies may provide important insights on the etiology of hyperuri-cemia and may help determine the risk of developing gout. With the strong male predominance of gout it is tempting to speculate that a defective gene or genes on the X chromo-some may be involved. In this study, we found that there is a relationship between three common polymorphisms (risk allele frequency 62% to 71% in gout cases) within the MAOA (Xp11.3) gene and gout risk in Taiwanese aborigi-nes. Indeed, we showed that the MAOA SNPs were associ-ated with a 1.34 to 1.46-fold risk of developing gout and a

risk of developing gout was also detected in subjects carry-ing the AGC haplotype (OR = 1.67). These results indicate that certain polymorphisms in the MAOA gene may be cru-cial in the development of gout.

Proximal tubule cells are the major source of renal dopa-mine synthesized from circulating Wltered L-DOPA (L-3,4-dihydroxy-phenylalanine) to decarboxylation of L-DOPA by the cytosolic aromatic L-amino acid decarboxylase (AADC) (Pestana et al. 2001). Interestingly, sodium pro-motes the delivery of L-DOPA to sites of uptake in renal tubules. Newly formed dopamine in the proximal tubule is not stored; therefore, it can leave the cell and activate spe-ciWc D1- and D2-like receptors, thereby leading to inhibi-tion of sodium transport (Fig.2a; Pestana et al. 2001).

Therefore, there are at least two plausible pathway mechanisms that may account for gout occurrence (Fig.2b). The Wrst relates to the dopaminergic system that plays an important role in the regulation of blood pressure, sodium homeostasis, and kidney function. MAOs repre-sents one of the major renal dopamine metabolic pathways (Fernandes and Soares-da-Silva 1994). Several studies sug-gest that reduced renal dopamine production is associated with a decrease in renal function and lack of dopamine may Table 4 Association between

haplotype analysis across MAOA and gout risk

Haplotype Gout Control
2 P value* OR (95% CI)

Frequency Frequency G–A–T 0.27 0.36 11.51 1.60 £ 10¡3 1.00 A–A–T 0.01 0.03 3.16 2.83 £ 10¡1 _{0.48 (0.16–1.45} A–G–T 0.01 0.02 1.31 7.37 £ 10¡1 _{0.76 (0.27–2.14} G–A–C 0.08 0.08 0.00 1.00 £ 101 1.33 (0.84–2.13 A–G–C 0.61 0.50 16.03 8.00 £ 10¡5 1.67 (1.28–2.17

At risk haplotype (AGC) analy-sis indicates 3 SNPs of MAOA (rs5953210 A > G, rs2283725 G > A and rs1137070 C > T) * P value was analyzed after 100,000 permutations

Fig. 1 Association of MAO-A enzyme activity and rs1137070 allele

with hyperuricemia (HU) and gout compared with normal control (Ref) after adjustment for age and gender. Error bars indicate standard errors; AU arbitrary unit

8.94 8.72 8.88 8.59 8.71 8.86 8.81 9.06 9.00 8.30 8.40 8.50 8.60 8.70 8.80 8.90 9.00 9.10 9.20 lo g (Fl u orescence)(AU) P for trend=2.00 10-3 P for trend=1.53 10-6 Ref HU Gout n=144 n=204 n=166

Ref(T) Ref(C) HU(T) HU(C) Gout(T) Gout(C) n=81 n=161 n=91 n=181 n=53 n=154

contribute to the inability to maintain sodium balance and an increase in blood pressure (Pestana et al. 2001). Renal excretion of uric acid is reduced in situations in which renal tubular reabsorption of sodium is increased (Feig et al.

2008). Urinary excretion of free dopamine (DOPAC) and HVA is markedly higher in patients who have recovered graft function than in those with acute tubular necrosis. In patients with recovered graft function, the daily urinary excretion of DOPAC (MAOs catalyzes the formation of DOPAC), but not that of HVA (COMT catalyzes the for-mation of HVA), increases progressively until day 12 and then remains constant (Pestana et al. 2001). Therefore, it has been suggested that MAOs play an important role in determining renal dopamine activity and urate reabsorption with sodium retention.

Our Wndings indicate that MAO-A enzyme activity cor-relates positively with hyperuricemia and gout. Decreased activity of the dopaminergic system may result in increased MAO-A enzyme activity and enhanced metabolism in the renal tubules. Increased MAO-A enzyme activity may pre-serve tubular units or may result in reduced renal blood Xow with increased sodium reabsorption in these residual tubular units. Paradoxically, diuretics induce hyperuricemia by increasing urate reabsorption. It has been noted that hyperuricemia occurs when diuretics produce suYcient salt and water loss that results in volume contraction; this stimu-lates solute reabsorption at the proximal tubule, and this eVect is corrected by administration of Xuids (Pascual and Perdiguero 2006). Thus, the eVects of MAO-A activity is in part related to increased urate reabsorption, which is due to enhanced reabsorption of sodium during dopamine degra-dation.

A second plausible mechanism relates to dopamine degradation by MAOs which generates hydrogen peroxide. Hydrogen peroxide may not be fully scavenged by intracel-lular antioxidants in proximal tubule cells (Pizzinat et al.

1999). A number of observations show that hydrogen per-oxide combines with other reactive oxygen species (ROS), resulting in the activation of various signal transduction processes (serine/threonine phosphorylation; Monteiro and Stern 1996), and activation enzymes (ERK and JNK), and transcription factor (NF-B, nuclear factor-B; Vindis et al. 2001). These pathways may exert proliferative eVects

on proximal tubule cells and promoting apoptosis or cell necrosis (Bianchi et al. 2003; Kunduzova et al. 2002; Pestana et al. 2001; Vindis et al. 2001). Uric acid is also believed to aVect tubular dysfunction and participate in tubulointerstitial damage (Feig et al. 2008).

Interestingly, the polymorphism at MAOA shows long-standing balancing selection (Tajima’s D = 1.86 and Fay and Wu’s H statistic test = 1.49, P < 0.05) which reveals nine persons of Taiwanese aboriginal, potentially acting on MAOA-related phenotypes (Gilad et al. 2002). Uric acid is a potent antioxidant and free radical scavenger (Choi et al.

2005; Johnson et al. 2008). Thus, it is interesting to notice that early hunter–gatherer societies, whether to oVset the metabolic thrift (suYcient sodium retention which leads to Fig. 2 Proposed model of impaired urate excretion and uric acid

lev-els in response to dopamine (DA) metabolism by monoamine oxidase A (MAOA) within renal tubular proximal epithelial cells. a Physiolog-ical model. Renal DA is synthesized intracellularly from decarboxyl-ation of L-DOPA (3,4-dihydroxy-L-phenylalanine) by cytosolic aromatic L-amino acid (AADC) after co-transport with sodium. Newly formed DA can exit the cell by inhibiting speciWc D1 receptor and blockage of sodium channels (Na+_–K+ _{ATPase and Na}+_–H+

exchang-er) that increase sodium excretion. b Urate reabsorption. Metabolism of DA by MAOA leads to increased formation of dihydroxyphenylace-tic acid (DOPAC) and degradation products of hydrogen peroxide (H2O2), indirectly increasing urate reabsorption with sodium retention

since uric acid levels and urate excretion correlate with dopamine-in-duced glomerular Wltration response. Urate can enter into cell by apical route through scaVolding proteins eVectively exchange for intracellular monocarboxylates (MCs) via apical located SMCT1/2 (sodium-dependent monocarboxylates transporter 1/2) and URAT1 (urate transporter 1). OAT1 and OAT3 (organic anion transporters 1/3) contribute to baso-lateral urate uptake

reduced water loss into the urine) in more MAO-A activity delivery correspondingly more ROS generated so the innate ability to counterbalance free radical production became imperative and increased ability to hunt. This could indi-cate preexisting equilibrium between ROS generation and antioxidant protection, where they naturally conserve more uric acid than the normal general population. Its success is reXected in the higher prevalence of susceptible alleles for MAOA with up to 71% (rs1137070 risk allele frequency). Whatever the reason for the maintenance of a balanced var-iant, it is interesting to note that variation of MAOA may Wt a previously proposed hypothesis whereby alleles that con-fer resistance to physical activity and prevent renal damage in ancient settings are now associated with susceptibility to gout disease.

Limitations to our study is that we have no replication of the racial-speciWc diversity data across other races/ethnics. However, the isolation of the Taiwanese aboriginals has increased their genetic homogeneity, thus facilitating the search for gout susceptibility genes.

In conclusion, our results indicate that MAOA gene and MAO-A enzyme activity are associated with increased sus-ceptibility to gout in Taiwanese aborigines. Additional studies are needed to verify this possibility and to elucidate the role between MAOA gene functional variant and the pathophysiology of gout.

Acknowledgments We thank the medical staVs and primary care

doctors at Jianshih and Wufong health center for the precise clinical phenotypes. We thank the study staV (Chun-Lan Hsu, Yu Tai and Chih-Shan Liu) from Division of Environmental Health and Occupa-tional Medicine, NaOccupa-tional Health Research Institutes for data handling. This study was supported by grants from the National Health Research Institutes (NHRI-98A1-PDCO-0307101), Center of Excellence for Environmental Medicine, Kaohsiung Medical University (KMU-EM-98-1-1), and the National Science Council (NSC97-2314-B-037-007 and NSC97-3112-B-400-001).

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