Masayuki Sakiyama

ABCG2 dysfunction causes hyperuricemia due to both renal urate underexcretion and renal urate overload

Gout is a common disease which results from hyperuricemia. We have reported that the dysfunction of urate exporter ABCG2 is the major cause of renal overload (ROL) hyperuricemia, but its involvement in renal underexcretion (RUE) hyperuricemia, the most prevalent subtype, is not clearly explained so far. In this study, the association analysis with 644 hyperuricemia patients and 1,623 controls in male Japanese revealed that ABCG2 dysfunction significantly increased the risk of RUE hyperuricemia as well as overall and ROL hyperuricemia, according to the severity of impairment. ABCG2 dysfunction caused renal urate underexcretion and induced hyperuricemia even if the renal urate overload was not remarkable. These results show that ABCG2 plays physiologically important roles in both renal and extra-renal urate excretion mechanisms. Our findings indicate the importance of ABCG2 as a promising therapeutic and screening target of hyperuricemia and gout.

Common dysfunctional variants in ABCG2 are a major cause of early-onset gout

Gout is a common disease which mostly occurs after middle age, but more people nowadays develop it before the age of thirty. We investigated whether common dysfunction of ABCG2, a high-capacity urate transporter which regulates serum uric acid levels, causes early-onset gout. 705 Japanese male gout cases with onset age data and 1,887 male controls were genotyped, and the ABCG2 functions which are estimated by its genotype combination were determined. The onset age was 6.5 years earlier with severe ABCG2 dysfunction than with normal ABCG2 function (P = 6.14 × 10−3). Patients with mild to severe ABCG2 dysfunction accounted for 88.2% of early-onset cases (twenties or younger). Severe ABCG2 dysfunction particularly increased the risk of early-onset gout (odds ratio 22.2, P = 4.66 × 10−6). Our finding that common dysfunction of ABCG2 is a major cause of early-onset gout will serve to improve earlier prevention and therapy for high-risk individuals.

Genome-wide association study of clinically defined gout identifies multiple risk loci and its association with clinical subtypes

Objective Gout, caused by hyperuricaemia, is a multifactorial disease. Although genome-wide association studies (GWASs) of gout have been reported, they included self-reported gout cases in which clinical information was insufficient. Therefore, the relationship between genetic variation and clinical subtypes of gout remains unclear. Here, we first performed a GWAS of clinically defined gout cases only. Methods A GWAS was conducted with 945 patients with clinically defined gout and 1213 controls in a Japanese male population, followed by replication study of 1048 clinically defined cases and 1334 controls. Results Five gout susceptibility loci were identified at the genome-wide significance level (p<5.0×10−8), which contained well-known urate transporter genes (ABCG2 and SLC2A9) and additional genes: rs1260326 (p=1.9×10−12; OR=1.36) of GCKR (a gene for glucose and lipid metabolism), rs2188380 (p=1.6×10−23; OR=1.75) of MYL2-CUX2 (genes associated with cholesterol and diabetes mellitus) and rs4073582 (p=6.4×10−9; OR=1.66) of CNIH-2 (a gene for regulation of glutamate signalling). The latter two are identified as novel gout loci. Furthermore, among the identified single-nucleotide polymorphisms (SNPs), we demonstrated that the SNPs of ABCG2 and SLC2A9 were differentially associated with types of gout and clinical parameters underlying specific subtypes (renal underexcretion type and renal overload type). The effect of the risk allele of each SNP on clinical parameters showed significant linear relationships with the ratio of the case–control ORs for two distinct types of gout (r=0.96 [p=4.8×10−4] for urate clearance and r=0.96 [p=5.0×10−4] for urinary urate excretion). Conclusions Our findings provide clues to better understand the pathogenesis of gout and will be useful for development of companion diagnostics.

GWAS of clinically defined gout and subtypes identifies multiple susceptibility loci that include urate transporter genes

Objective A genome-wide association study (GWAS) of gout and its subtypes was performed to identify novel gout loci, including those that are subtype-specific. Methods Putative causal association signals from a GWAS of 945 clinically defined gout cases and 1213 controls from Japanese males were replicated with 1396 cases and 1268 controls using a custom chip of 1961 single nucleotide polymorphisms (SNPs). We also first conducted GWASs of gout subtypes. Replication with Caucasian and New Zealand Polynesian samples was done to further validate the loci identified in this study. Results In addition to the five loci we reported previously, further susceptibility loci were identified at a genome-wide significance level (p<5.0×10−8): urate transporter genes (SLC22A12 and SLC17A1) and HIST1H2BF-HIST1H4E for all gout cases, and NIPAL1 and FAM35A for the renal underexcretion gout subtype. While NIPAL1 encodes a magnesium transporter, functional analysis did not detect urate transport via NIPAL1, suggesting an indirect association with urate handling. Localisation analysis in the human kidney revealed expression of NIPAL1 and FAM35A mainly in the distal tubules, which suggests the involvement of the distal nephron in urate handling in humans. Clinically ascertained male patients with gout and controls of Caucasian and Polynesian ancestries were also genotyped, and FAM35A was associated with gout in all cases. A meta-analysis of the three populations revealed FAM35A to be associated with gout at a genome-wide level of significance (pmeta=3.58×10−8). Conclusions Our findings including novel gout risk loci provide further understanding of the molecular pathogenesis of gout and lead to a novel concept for the therapeutic target of gout/hyperuricaemia.

NPT1/SLC17A1 is a renal urate exporter in humans and its common gain-of-function variant decreases the risk of renal underexcretion gout

Serum uric acid (SUA) levels in humans are mainly regulated by urate transporters. Recent genome‐wide association studies suggested that common variants of the human sodium‐dependent phosphate cotransporter type 1 gene (NPT1/SLC17A1) influence SUA. NPT1 has been reported to mediate urate transport, but its physiologic role in regulating SUA in humans remains unclear. Furthermore, the findings of replication studies of the relationship between NPT1 variants and gout have been inconsistent. The aims of this study were to investigate the effect of NPT1 on gout and to determine its physiologic role.

ABCG2 Dysfunction Increases the Risk of Renal Overload Hyperuricemia

ATP-binding cassette transporter, sub-family G, member 2 (ABCG2/BCRP) is identified as a high-capacity urate exporter, and its dysfunction has an association with serum uric acid levels and gout/hyperuricemia risk. Generally, hyperuricemia has been classified into urate “overproduction type,” “underexcretion type,” and “combined type” based on only renal urate excretion, without considering an extra-renal pathway such as gut excretion. In this study, we investigated the effects of ABCG2 dysfunction on human urate handling and the mechanism of hyperuricemia. Clinical parameters for urate handling including urinary urate excretion (UUE) were examined in 644 Japanese male outpatients with hyperuricemia. The severity of their ABCG2 dysfunction was estimated by genotype combination of two common ABCG2 variants, nonfunctional Q126X (rs72552713) and half-functional Q141K (rs2231142). Contrary to the general understanding that ABCG2 dysfunction leads to decreased renal urate excretion, UUE was significantly increased by ABCG2 dysfunction (P = 3.60 × 10−10). Mild, moderate, and severe ABCG2 dysfunctions significantly raised the risk of “overproduction” hyperuricemia including overproduction type and combined type, conferring risk ratios of 1.36, 1.66, and 2.35, respectively. The present results suggest that common dysfunctional variants of ABCG2 decrease extra-renal urate excretion including gut excretion and cause hyperuricemia. Thus, “overproduction type” in the current concept of hyperuricemia should be renamed “renal overload type,” which is caused by two different mechanisms, “extra-renal urate underexcretion” and genuine “urate overproduction.” Our new concept will lead to a more accurate diagnosis and more effective therapeutic strategy for hyperuricemia and gout.

Common dysfunctional variants of ABCG2 have stronger impact on hyperuricemia progression than typical environmental risk factors

Gout/hyperuricemia is a common multifactorial disease having typical environmental risks. Recently, common dysfunctional variants of ABCG2, a urate exporter gene also known as BCRP, are revealed to be a major cause of gout/hyperuricemia. Here, we compared the influence of ABCG2 dysfunction on serum uric acid (SUA) levels with other typical risk factors in a cohort of 5,005 Japanese participants. ABCG2 dysfunction was observed in 53.3% of the population investigated, and its population-attributable risk percent (PAR%) for hyperuricemia was 29.2%, much higher than those of the other typical environmental risks, i.e. overweight/obesity (BMI ≥ 25.0; PAR% = 18.7%), heavy drinking (>196 g/week (male) or >98 g/week (female) of pure alcohol; PAR% = 15.4%), and aging (≥60 years old; PAR% = 5.74%). SUA significantly increased as the ABCG2 function decreased (P = 5.99 × 10−19). A regression analysis revealed that ABCG2 dysfunction had a stronger effect than other factors; a 25% decrease in ABCG2 function was equivalent to “an increase of BMI by 1.97-point” or “552.1 g/week alcohol intake as pure ethanol” in terms of ability to increase SUA. Therefore, ABCG2 dysfunction originating from common genetic variants has a much stronger impact on the progression of hyperuricemia than other familiar risks. Our study provides a better understanding of common genetic factors for common diseases.

Bullous pyoderma gangrenosum: a case report and review of the published work.

Pyoderma gangrenosum (PG) is an ulcerative skin disorder characterized by neutrophilic infiltrations. PG is generally classified into four types: (i) ulcerative; (ii) pustular; (iii) bullous; and (iv) vegetative. Among them, bullous PG is known as a rare type. Herein, we report a case of bullous PG together with a summary of the 12 PG cases treated in our department over the previous 15 years, and we review 38 well‐documented bullous PG cases (65.8% female; aged 18–80 years [mean ± standard deviation, 51.6 ± 16.8]) in the published work, including the present case, from 1972–2011. Although the disease most frequently associated with PG is inflammatory bowel disease, bullous PG is most commonly associated with hematological disorders (25/38, 65.8%), which indicates the characteristic pathophysiology specific to bullous PG.

The effects of URAT1/SLC22A12 nonfunctional variants,R90H and W258X, on serum uric acid levels and gout/hyperuricemia progression

Urate transporter 1 (URAT1/SLC22A12), a urate transporter gene, is a causative gene for renal hypouricemia type 1. Among several reported nonsynonymous URAT1 variants, R90H (rs121907896) and W258X (rs121907892) are frequent causative mutations for renal hypouricemia. However, no case-control study has evaluated the relationship between gout and these two variants. Additionally, the effect size of these two variants on serum uric acid (SUA) levels remains to be clarified. Here, 1,993 primary gout patients and 4,902 health examination participants (3,305 males and 1,597 females) were genotyped with R90H and W258X. These URAT1 variants were not observed in any gout cases, while 174 subjects had the URAT1 variant in 2,499 health examination participants, respectively (P = 8.3 × 10−46). Moreover, in 4,902 health examination participants, the URAT1 nonfunctional variants significantly reduce the risk of hyperuricemia (P = 6.7 × 10−19; risk ratio = 0.036 in males). Males, having 1 or 2 nonfunctional variants of URAT1, show a marked decrease of 2.19 or 5.42 mg/dl SUA, respectively. Similarly, females, having 1 or 2 nonfunctional variants, also evidence a decrease of 1.08 or 3.89 mg/dl SUA, respectively. We show that URAT1 nonfunctional variants are protective genetic factors for gout/hyperuricemia, and also demonstrated the sex-dependent effect size of these URAT1 variants on SUA (P for interaction = 1.5 × 10−12).

NRF2 Is a Key Target for Prevention of Noise-Induced Hearing Loss by Reducing Oxidative Damage of Cochlea

Noise-induced hearing loss (NIHL) is one of the most common sensorineural hearing deficits. Recent studies have demonstrated that the pathogenesis of NIHL is closely related to ischemia-reperfusion injury of cochlea, which is caused by blood flow decrease and free radical production due to excessive noise. This suggests that protecting the cochlea from oxidative stress is an effective therapeutic approach for NIHL. NRF2 is a transcriptional activator playing an essential role in the defense mechanism against oxidative stress. To clarify the contribution of NRF2 to cochlear protection, we examined Nrf2–/– mice for susceptibility to NIHL. Threshold shifts of the auditory brainstem response at 7 days post-exposure were significantly larger in Nrf2–/– mice than wild-type mice. Treatment with CDDO-Im, a potent NRF2-activating drug, before but not after the noise exposure preserved the integrity of hair cells and improved post-exposure hearing levels in wild-type mice, but not in Nrf2–/– mice. Therefore, NRF2 activation is effective for NIHL prevention. Consistently, a human NRF2 SNP was significantly associated with impaired sensorineural hearing levels in a cohort subjected to occupational noise exposure. Thus, high NRF2 activity is advantageous for cochlear protection from noise-induced injury, and NRF2 is a promising target for NIHL prevention.