Junichiro Nishiyama

Common Defects of ABCG2, a High-Capacity Urate Exporter, Cause Gout: A Function-Based Genetic Analysis in a Japanese Population

Dysfunctional genotype combinations of polymorphic adenosine 5′-triphosphate–binding cassette transporter gene ABCG2/BCRP, which encodes a high-capacity urate secretion transporter in human gut and kidney, are major causes of gout. Gout, the “Disease of Kings” as it is often known, is a painful medical condition characterized by sharp acute pain in bone joints, due to the high deposition of uric acid crystals from the blood serum into the surrounding cartilage. It affects approximately 1% of the U.S. population and remains a significant public health concern. The prevalence of gout is much higher in certain Asian ethnic groups, and is also reportedly rising in African Americans. Current medical treatments are aimed at ameliorating pain severity, but as the underlying genetic etiology of the disease unfolds, new targets for future therapies are likely to be found. Although genome-wide association studies (GWAS) have enabled the calculation of risk predispositions for a wide variety of complex diseases, the relation of gene function to the causality of disease-related mutations has remained largely unclear. A recent U.S. population–based study supported an association between urate levels and gout in individuals carrying variants in a multifunctional transporter gene, ABCG2. This study identified Q141K as a high-risk variant in nearly 10% of gout cases in Caucasians. Now, a team led by Hirotaka Matsuo report that in a Japanese population, another risk variant in ABCG2, namely the Q126X nonfunctional mutation, confers an even higher risk associated with an increase in uric acid deposition in the blood and may cause gout in Asians. Because this gene is responsible for giving rise to a protein that transports harmful waste products and metabolites out of the kidney and gut, they extensively validate the biological activity of ABCG2 using functional assays in vitro that effectively recapitulate human data obtained from Japanese individuals afflicted with the disease. These findings lend weight to previously reported GWAS; moreover, these newly identified specific high-risk variants that block urate secretion may serve as potential intervention points for quelling the disease. Gout based on hyperuricemia is a common disease with a genetic predisposition, which causes acute arthritis. The ABCG2/BCRP gene, located in a gout-susceptibility locus on chromosome 4q, has been identified by recent genome-wide association studies of serum uric acid concentrations and gout. Urate transport assays demonstrated that ABCG2 is a high-capacity urate secretion transporter. Sequencing of the ABCG2 gene in 90 hyperuricemia patients revealed several nonfunctional ABCG2 mutations, including Q126X. Quantitative trait locus analysis of 739 individuals showed that a common dysfunctional variant of ABCG2, Q141K, increases serum uric acid. Q126X is assigned to the different disease haplotype from Q141K and increases gout risk, conferring an odds ratio of 5.97. Furthermore, 10% of gout patients (16 out of 159 cases) had genotype combinations resulting in more than 75% reduction of ABCG2 function (odds ratio, 25.8). Our findings indicate that nonfunctional variants of ABCG2 essentially block gut and renal urate excretion and cause gout.

Up-Regulation of Galectin-3 in Acute Renal Failure of the Rat

Galectin-3, a multifunctional beta-galactoside-binding lectin, is known to participate in development, oncogenesis, cell-to-cell attachment, and inflammation. We studied to determine whether galectin-3 is associated with cell injury and regeneration in two types of acute renal failure (ARF), namely ischemic and toxic ARF. In ischemia/reperfusion renal injury in rats (bilateral renal pedicles clamped for 40 minutes), galectin-3 mRNA began to increase at 2 hours and extended by 6.2-fold at 48 hours (P: < 0.01 versus normal control rats), and then decreased by 28 days after injury. In addition, a significant negative correlation between galectin-3 mRNA expression and serum reciprocal creatinine was shown at 48 hours after injury (n = 13, r = -0.94, P: < 0.0001). In folic acid-induced ARF, galectin-3 mRNA was found to be up-regulated at 2 hours after injury and increased levels continued until at least 7 days post-injury. In immunohistochemistry, at 2 hours following reperfusion, galectin-3 began to develop in proximal convoluted tubules. From 6 hours up to 48 hours, galectin-3 was also found in proximal straight tubules, distal tubules, thick ascending limbs, and collecting ducts. In later stages of regeneration, galectin-3 expressions were found in macrophages. In conclusion, we demonstrated that galectin-3 expressions were markedly up-regulated in both ischemic and toxic types of ARF. Galectin-3 may play an important role in acute tubular injury and the following regeneration stage.

Glomerular proliferating cell kinetics in acute post-streptococcal glomerulonephritis (APSGN)

To investigate the time sequence of glomerular cell proliferation in acute human glomerulonephritis, renal biopsy tissues were examined from 15 acute post‐streptococcal glomerulonephritis (APSGN) patients (who were biopsied 1–31 days after onset), using an immunoperoxidase technique with monoclonal antibodies against proliferating cell nuclear antigen (PCNA) and various cell surface markers. Few, if any, PCNA+ cells were observed in normal glomeruli, but many cells were positive for PCNA in the acute phase of APSGN. Glomerular PCNA+ cells were observed either within glomerular tufts, or lining Bowmans capsule (parietal epithelial cells); the number of positive cells tended to decrease exponentially as the disease duration increased (r=−0·91, P<0·0001). PCNA+ cells within glomerular tufts were further identified by double immunostaining. PCNA was not found in PMN or T cells, but a small proportion of macrophages were PCNA+. Most of the remaining PCNA+ cells were resident glomerular cells; the proportion of PCNA+ endothelial cells (CD31+) was over 80 per cent in the early phase, but as the disease continued the proportion of mesangial cells (α‐smooth muscle actin+) increased to about half of the total PCNA+ cells within the tuft. These data indicate that the hypercellular glomeruli in APSGN are due not only to immune cell infiltration, but also to resident glomerular cell proliferation, probably induced by locally produced growth factors.

Pathogenic GLUT9 mutations causing renal hypouricemia type 2 (RHUC2).

Renal hypouricemia (MIM 220150) is an inherited disorder characterized by low serum uric acid levels and has severe complications such as exercise-induced acute renal failure and urolithiasis. We have previously reported that URAT1/SLC22A12 encodes a renal urate-anion exchanger and that its mutations cause renal hypouricemia type 1 (RHUC1). With the large health-examination database of the Japan Maritime Self-Defense Force, we found two missense mutations (R198C and R380W) of GLUT9/SLC2A9 in hypouricemia patients. R198C and R380W occur in highly conserved amino acid motifs in the “sugar transport proteins signatures” that are observed in GLUT family transporters. The corresponding mutations in GLUT1 (R153C and R333W) are known to cause GLUT1 deficiency syndrome because arginine residues in this motif are reportedly important as the determinants of the membrane topology of human GLUT1. Therefore, on the basis of membrane topology, the same may be true of GLUT9. GLUT9 mutants showed markedly reduced urate transport in oocyte expression studies, which would be the result of the loss of positive charges in those conserved amino acid motifs. Together with previous reports on GLUT9 localization, our findings suggest that these GLUT9 mutations cause renal hypouricemia type 2 (RHUC2) by their decreased urate reabsorption on both sides of the renal proximal tubule cells. However, a previously reported GLUT9 mutation, P412R, was unlikely to be pathogenic. These findings also enable us to propose a physiological model of the renal urate reabsorption via GLUT9 and URAT1 and can lead to a promising therapeutic target for gout and related cardiovascular diseases.

Juxtaglomerular cell tumor with retroperitoneal fibrosis and secondary immune-complex glomerulonephritis: A possible contribution of the renin angiotensin system to renal fibrosis

We present a case of a 25-year-old woman with a renin-secreting juxtaglomerular cell tumor, retroperitoneal fibrosis associated with glomerular hypertrophy, glomerulonephritis, and marked tubulointerstitial alterations. Myofibroblasts, as shown by positive immunostaining for alpha-smooth muscle actin, were found along with transforming growth factor-beta (TGF-beta) in the interstitium of the tumor-free kidney. Regarding the pathogenesis of renal fibrosis and glomerular hypertrophy, this case may provide evidence not only experimentally but also clinically that the renin-angiotensin system plays an important role because angiotensin II is known to induce renal fibrosis associated with increased TGF-beta and the appearance of myofibroblasts.