Performance of the ACMG-AMP criteria in a large familial renal glucosuria cohort with identified SLC5A2 sequence variants.

Familial Renal Glucosuria (FRG) is a co-dominantly inherited trait characterized by orthoglycaemic glucosuria. From 2003 to 2015 we have reported several cohorts validating SLC5A2 (16p11.2), encoding SGLT2 (Na+/glucose cotransporter family member 2), as the gene responsible for FRG. The aim of this work was to validate the variants identified in our extended FRG cohort of published, as well more recent unreported cases, according to the ACMG-AMP 2015 criteria. Forty-six variants were evaluated, including 16 novel alleles first described in this study. All are rare, ultra-rare or absent from population databases and most are missense changes. According to the ACMG-AMP standards, only 74% of the variants were classified as P/LP. The lack of descriptions of unrelated patients with similar variants or failing to test additional affected family members, averted a conclusion for pathogenicity in the alleles that scored VUS, highlighting the importance of both family testing and variant reporting. Finally, the cryo-EM structure of the hSGLT2-MAP17 complex in the empagliflozin-bound state improved the ACMG-AMP pathogenicity score by identifying critical/functional protein domains.

Cysteine as a Multifaceted Player in Kidney, the Cysteine-Related Thiolome and Its Implications for Precision Medicine.

In this review encouraged by original data, we first provided in vivo evidence that the kidney, comparative to the liver or brain, is an organ particularly rich in cysteine. In the kidney, the total availability of cysteine was higher in cortex tissue than in the medulla and distributed in free reduced, free oxidized and protein-bound fractions (in descending order). Next, we provided a comprehensive integrated review on the evidence that supports the reliance on cysteine of the kidney beyond cysteine antioxidant properties, highlighting the relevance of cysteine and its renal metabolism in the control of cysteine excess in the body as a pivotal source of metabolites to kidney biomass and bioenergetics and a promoter of adaptive responses to stressors. This view might translate into novel perspectives on the mechanisms of kidney function and blood pressure regulation and on clinical implications of the cysteine-related thiolome as a tool in precision medicine.

Zebrafish Model as a Screen to Prevent Cyst Inflation in Autosomal Dominant Polycystic Kidney Disease.

In autosomal dominant polycystic kidney disease (ADPKD), kidney cyst growth requires the recruitment of CFTR (cystic fibrosis transmembrane conductance regulator), the chloride channel that is defective in cystic fibrosis. We have been studying cyst inflation using the zebrafish Kupffer's vesicle (KV) as model system because we previously demonstrated that knocking down polycystin 2 (PC2) induced a CFTR-mediated enlargement of the organ. We have now quantified the PC2 knockdown by showing that it causes a 73% reduction in the number of KV cilia expressing PC2. According to the literature, this is an essential event in kidney cystogenesis in ADPKD mice. Additionally, we demonstrated that the PC2 knockdown leads to a significant accumulation of CFTR-GFP at the apical region of the KV cells. Furthermore, we determined that KV enlargement is rescued by the injection of Xenopus pkd2 mRNA and by 100 µM tolvaptan treatment, the unique and approved pharmacologic approach for ADPKD management. We expected vasopressin V2 receptor antagonist to lower the cAMP levels of KV-lining cells and, thus, to inactivate CFTR. These findings further support the use of the KV as an in vivo model for screening compounds that may prevent cyst enlargement in this ciliopathy, through CFTR inhibition.

MAP17 Is a Necessary Activator of Renal Na+/Glucose Cotransporter SGLT2.

The renal proximal tubule reabsorbs 90% of the filtered glucose load through the Na+-coupled glucose transporter SGLT2, and specific inhibitors of SGLT2 are now available to patients with diabetes to increase urinary glucose excretion. Using expression cloning, we identified an accessory protein, 17 kDa membrane-associated protein (MAP17), that increased SGLT2 activity in RNA-injected Xenopus oocytes by two orders of magnitude. Significant stimulation of SGLT2 activity also occurred in opossum kidney cells cotransfected with SGLT2 and MAP17. Notably, transfection with MAP17 did not change the quantity of SGLT2 protein at the cell surface in either cell type. To confirm the physiologic relevance of the MAP17-SGLT2 interaction, we studied a cohort of 60 individuals with familial renal glucosuria. One patient without any identifiable mutation in the SGLT2 coding gene (SLC5A2) displayed homozygosity for a splicing mutation (c.176+1G>A) in the MAP17 coding gene (PDZK1IP1). In the proximal tubule and in other tissues, MAP17 is known to interact with PDZK1, a scaffolding protein linked to other transporters, including Na+/H+ exchanger 3, and to signaling pathways, such as the A-kinase anchor protein 2/protein kinase A pathway. Thus, these results provide the basis for a more thorough characterization of SGLT2 which would include the possible effects of its inhibition on colocalized renal transporters.

Oxalate (dys)Metabolism: Person-to-Person Variability, Kidney and Cardiometabolic Toxicity.

Oxalate is a metabolic end-product whose systemic concentrations are highly variable among individuals. Genetic (primary hyperoxaluria) and non-genetic (e.g., diet, microbiota, renal and metabolic disease) reasons underlie elevated plasma concentrations and tissue accumulation of oxalate, which is toxic to the body. A classic example is the triad of primary hyperoxaluria, nephrolithiasis, and kidney injury. Lessons learned from this example suggest further investigation of other putative factors associated with oxalate dysmetabolism, namely the identification of precursors (glyoxylate, aromatic amino acids, glyoxal and vitamin C), the regulation of the endogenous pathways that produce oxalate, or the microbiota's contribution to oxalate systemic availability. The association between secondary nephrolithiasis and cardiovascular and metabolic diseases (hypertension, type 2 diabetes, and obesity) inspired the authors to perform this comprehensive review about oxalate dysmetabolism and its relation to cardiometabolic toxicity. This perspective may offer something substantial that helps advance understanding of effective management and draws attention to the novel class of treatments available in clinical practice.

Effect of kidney disease on glucose handling (including genetic defects).

Reabsorption of glucose in the proximal renal tubule involves the Na(+)-coupled glucose cotransporter (SGLT) and the facilitative glucose transport (GLUT) multigene glucose transport families. Mutations in SLC5A2, the SGLT2 coding gene, are responsible for familial renal glucosuria (FRG), a genetic disorder characterized by glucosuria in the absence of both hyperglycemia and generalized proximal tubular dysfunction. In this paper we focus on FRG and describe other inherited and acquired clinical conditions associated with glucosuria. In addition, a brief review on the regulation of renal glucose transport in diabetes is provided.

Genetic and Clinical Predictors of Age of ESKD in Individuals With Autosomal Dominant Tubulointerstitial Kidney Disease Due to UMOD Mutations.

INTRODUCTION: Autosomal dominant tubulo-interstitial kidney disease due to UMOD mutations (ADTKD-UMOD) is a rare condition associated with high variability in the age of end-stage kidney disease (ESKD). The minor allele of rs4293393, located in the promoter of the UMOD gene, is present in 19% of the population and downregulates uromodulin production by approximately 50% and might affect the age of ESKD. The goal of this study was to better understand the genetic and clinical characteristics of ADTKD-UMOD and to perform a Mendelian randomization study to determine if the minor allele of rs4293393 was associated with better kidney survival. METHODS: An international group of collaborators collected clinical and genetic data on 722 affected individuals from 249 families with 125 mutations, including 28 new mutations. The median age of ESKD was 47 years. Men were at a much higher risk of progression to ESKD (hazard ratio 1.78, P < 0.001). RESULTS: The allele frequency of the minor rs4293393 allele was only 11.6% versus the 19% expected (P < 0.01), resulting in Hardy-Weinberg disequilibrium and precluding a Mendelian randomization experiment. An in vitro score reflecting the severity of the trafficking defect of uromodulin mutants was found to be a promising predictor of the age of ESKD. CONCLUSION: We report the clinical characteristics associated with 125 UMOD mutations. Male gender and a new in vitro score predict age of ESKD.

An Atypical Presentation of Thrombotic Microangiopathy After Lung Transplant: A Case Report.

Thrombotic microangiopathy (TMA) is a pathologic condition characterized by microangiopathic hemolytic anemia, thrombocytopenia, and organ injury due to microvascular endothelial lesions and thrombosis. It occurs in a variety of diseases and, unless recognized and treated, leads to severe morbidity and mortality. We present the case of a 48-year-old woman who underwent lung transplantation, initially under tacrolimus, mycophenolate mofetil (MMF), and prednisolone. Several complications emerged in the following months, including abdominal aortic and left renal artery thrombosis and cutaneous infections, although her renal function remained normal. Six months after transplant, her renal function began to deteriorate, which was assumed to be due to elevated tacrolimus levels and doses were adjusted. Due to leukopenia, MMF was changed to everolimus. One year after, she was admitted with fatigue, anemia, and renal dysfunction. Complementary exams revealed only iron deficiency, leukopenia, normal platelets, and elevated lactate dehydrogenase; her renal ultrasound was normal. A renal biopsy was performed and thrombotic microangiopathy was subsequently identified as the main cause of the renal dysfunction. Tacrolimus was therefore discontinued and MMF restarted with slow improvement of renal function. Only when everolimus was stopped did the patient's renal function show incremental improvement. TMA may be a serious complication after lung transplantation and the risk is higher when a combination of tacrolimus and everolimus is used. Renal biopsy findings are essential to confirm the final diagnosis of TMA, allowing for a change in immunosuppression to prevent permanent and severe renal damage.

Effect of empagliflozin beyond glycemic control: Cardiovascular benefit in patients with type 2 diabetes and established cardiovascular disease. / Efeito da empagliflozina para além do controlo glicémico: benefício cardiovascular em doentes com DMT2 e doença cardiovascular estabelecida.

The prevalence of type 2 diabetes (T2D) continues to increase, and its association with cardiovascular (CV) disease has led to the inclusion of CV endpoints in clinical trials on the treatment of T2D. This article explores the various trials already performed and under development in this field, with particular focus on the EMPA-REG OUTCOME trial. In this trial, empagliflozin, a sodium-glucose co-transporter 2 inhibitor, demonstrated a reduction in CV risk in patients with T2D and established CV disease, in addition to CV safety and a decrease in glycated hemoglobin. This represents a paradigm shift that has led to changes in the international guidelines for the treatment of T2D. These results were maintained in subsequent subgroup analysis for heart failure, chronic kidney disease and peripheral arterial disease, although there are many questions concerning the mechanisms involved in these effects, including whether they are hemodynamic, metabolic or due to decreased myocardial cytoplasmic sodium concentrations. With this reduction in risk for major CV events in patients with T2D, the EMPA-REG OUTCOME trial demonstrated CV protection from a hypoglycemic drug for the first time, and opened a new era in the treatment and management of T2D. This study has led to the development of ongoing trials that will establish which patients will benefit most from this therapy, particularly with regard to comorbidities.

Twenty-one additional cases of familial renal glucosuria: absence of genetic heterogeneity, high prevalence of private mutations and further evidence of volume depletion.

INTRODUCTION: Familial renal glucosuria (FRG) is a rare renal tubular disorder caused by mutations within the SLC5A2 gene. It is characterized by persistent glucosuria in the absence of hyperglycaemia and any other signs of generalized tubular dysfunction. In small series of patients previously reported, the molecular and phenotypic findings in FRG families, including first hints of extracellular volume depletion and activation of the renin-angiotensin-aldosterone system induced by natriuresis, have been described. We have now extended this analysis to another 21 consecutive cases from 17 pedigrees, including 11 cases with severe glucose excretion. METHODS: Mutation analysis was performed by direct sequencing of the genomic coding segments of the SLC5A2 gene. In two cases with severe glucosuria, basal plasma renin activity and serum aldosterone concentrations were determined. RESULTS: Within the 17 pedigrees, we have identified a total of 20 different SLC5A2 mutations. Fifteen have not been previously reported. In all glucosuric individuals tested, at least one SLC5A2 mutation could be identified. Heterozygous individuals were found to have only mild glucose excretion whereas homozygous or compound heterozygous patients had severe glucosuria, ranging from 10 to 86.5 g/1.73 m(2)/24 h. In two patients of the latter group, basal plasma renin activity and serum aldosterone concentration were determined and found to be raised to an average of 4.6-fold and 3.1-fold of the upper limit of the normal range, respectively. Discussion. The identification of at least one mutated allele in every affected individual in this cohort of 17 consecutively investigated families strongly suggests that genetic heterogeneity is not prevalent in FRG. Although 5 of the detected alleles have been described previously, 15 are novel, confirming that most mutations in FRG are private. Our finding of an activation of compensatory mechanisms for salt loss may warrant more detailed studies of long-term hormonal and metabolic imbalances in patients with FRG.

The Na+ -coupled glucose transporter SGLT2 interacts with its accessory unit MAP17 in vitro and their expressions overlap in the renal proximal tubule.

Na+ -glucose cotransporter 2 is the renal Na+ -coupled glucose transporter responsible for the tubular glucose reabsorption, while MAP17 was recently identified as its accessory unit. Mutations in either of the proteins' coding genes, SLC5A2 and PDZK1IP1, lead to urinary glucose excretion. To investigate whether MAP17 interacts with SGLT2 in vitro, we engineered a V5-tagged SGLT2 construct and evaluated HEK293T cells coexpressing it together with a HA tagged MAP17 construct. MAP17 is shown to colocalize and coimmunoprecipitate with SGLT2. Also, in human kidney sections, the expression of both proteins overlaps at the apical surface of tubular epithelia. This interaction provides the rationale behind SGLT2 activation by MAP17 as well the similarity of the SLC5A2 and PDZK1IP1 glucosuric phenotypes.

Atypical adult-onset methylmalonic acidemia and homocystinuria presenting as hemolytic uremic syndrome.

Thrombotic microangiopathy (TMA) syndromes can be secondary to a multitude of different diseases. Most can be identified with a systematic approach and, when excluded, TMA is generally attributed to a dysregulation in the activity of the complement alternative pathways-atypical hemolytic uremic syndrome (aHUS). We present a challenging case of a 19-year-old woman who presented with thrombotic microangiopathy, which was found to be caused by methylmalonic acidemia and homocystinuria, a rare vitamin B12 metabolism deficiency. To our knowledge, this is the first time that an adult-onset methylmalonic acidemia and homocystinuria presents as TMA preceding CNS involvement.

A novel heterozygous missense mutation in the UMOD gene responsible for Familial Juvenile Hyperuricemic Nephropathy.

BACKGROUND: Familial Juvenile Hyperuricemic Nephropathy is an autosomal dominant nephropathy, characterized by decreased urate excretion and progressive interstitial nephritis. Mutations in the uromodulin coding UMOD gene have been found responsible for the disease in some families. CASE PRESENTATION: We here describe a novel heterozygous p.K307T mutation in an affected female with hyperuricemia, renal cysts and renal failure. The proband's only son is also affected and the mutation was found to segregate with the disease. CONCLUSIONS: This mutation is the fourth reported in exon 5. Initial studies identified a mutation clustering in exon 4 and it has been recommended that sequencing this exon alone should be the first diagnostic test in patients with chronic interstitial nephritis with gout or hyperuricemia. However, regarding the increasing number of mutations being reported in exon 5, we now suggest that sequencing exon 5 should also be performed.

Adult presentation of Bartter syndrome type IV with erythrocytosis.

Bartter syndrome comprises a group of rare autosomal-recessive salt-losing disorders with distinct phenotypes, but one unifying pathophysiology consisting of severe reductions of sodium reabsorption caused by mutations in five genes expressed in the thick ascending limb of Henle, coupled with increased urinary excretion of potassium and hydrogen, which leads to hypokalemic alkalosis. Bartter syndrome type IV, caused by loss-of-function mutations in barttin, a subunit of chloride channel CLC-Kb expressed in the kidney and inner ear, usually occurs in the antenatal-neonatal period. We report an unusual case of late onset presentation of Bartter syndrome IV and mild phenotype in a 20 years-old man who had hypokalemia, deafness, secondary hyperparathyroidism and erythrocytosis.

Determination of the renal threshold for glucose excretion in Familial Renal Glucosuria.

BACKGROUND/AIMS: Familial Renal Glucosuria (FRG) is characterized by the presence of persistent isolated glucosuria in the absence of hyperglycemia. Mutations in SLC5A2, the gene coding for the sodium-glucose co-transporter 2 (SGLT2), are responsible for FRG. Phenotype/genotype correlations in FRG have mostly relied on the quantification of Urinary Glucose Excretion (UGE), which is dependent on both the filtered glucose load and the renal glucose reabsorptive capacity. In the current work, the renal threshold for glucose excretion (RTG) was determined in an FRG cohort, with the purpose of characterizing the impact of SGLT2 mutations on renal glucose transport. METHODS: From January to December of 2013, eight FRG individuals with identified SLC5A2 mutations were enrolled. Patients were given a Mixed-Meal Tolerance Test during which blood glucose and UGE were measured over a 4 h period and the data was used to calculate RTG, according to a recently validated protocol. RESULTS: In patients with homozygous mutations, RTG values were very low, with a mean (SD) of 0.95 (1.17) mmol/l, compared to commonly reported values of approximately 10-11.1 mmol/l in healthy subjects. In subjects with heterozygous mutations, mean (SD) RTG values were 4.91 (1.23) mmol/l, which are approximately one-half of the values in subjects without mutations. CONCLUSIONS: In FRG, mutations in SLC5A2 lead to reductions in RTG and increases in UGE. Because determination of RTG is not influenced by the filtered glucose load, the calculated RTG values provide a more refined measure of the impact of mutations on renal glucose transport than can be obtained from UGE alone.

Hypouricaemia and hyperuricosuria in familial renal glucosuria.

Familial renal glucosuria is a rare co-dominantly inherited benign phenotype characterized by the presence of glucose in the urine. It is caused by mutations in the SLC5A2 gene that encodes SGLT2, the Na(+)-glucose cotransporter responsible for the reabsorption of the bulk of glucose in the proximal tubule. We report a case of FRG displaying both severe glucosuria and renal hypouricaemia. We hypothesize that glucosuria can disrupt urate reabsorption in the proximal tubule, directly causing hyperuricosuria.

Familial renal glucosuria and SGLT2: from a mendelian trait to a therapeutic target.

Four members of two glucose transporter families, SGLT1, SGLT2, GLUT1, and GLUT2, are differentially expressed in the kidney, and three of them have been shown to be necessary for normal glucose resorption from the glomerular filtrate. Mutations in SGLT1 are associated with glucose-galactose malabsorption, SGLT2 with familial renal glucosuria (FRG), and GLUT2 with Fanconi-Bickel syndrome. Patients with FRG have decreased renal tubular resorption of glucose from the urine in the absence of hyperglycemia and any other signs of tubular dysfunction. Glucosuria in these patients can range from <1 to >150 g/1.73 m(2) per d. The majority of patients do not seem to develop significant clinical problems over time, and further description of specific disease sequelae in these individuals is reviewed. SGLT2, a critical transporter in tubular glucose resorption, is located in the S1 segment of the proximal tubule, and, as such, recent attention has been given to SGLT2 inhibitors and their utility in patients with type 2 diabetes, who might benefit from the glucose-lowering effect of such compounds. A natural analogy is made of SGLT2 inhibition to observations with inactivating mutations of SGLT2 in patients with FRG, the hereditary condition that results in benign glucosuria. This review provides an overview of renal glucose transport physiology, FRG and its clinical course, and the potential of SGLT2 inhibition as a therapeutic target in type 2 diabetes.

BI-10773, a sodium-glucose cotransporter 2 inhibitor for the potential oral treatment of type 2 diabetes mellitus.

BI-10773, being developed by Boehringer Ingelheim Corp, is a sodium-glucose cotransporter (SGLT)2 inhibitor for the oral treatment of type 2 diabetes mellitus (T2DM). Preclinical and clinical research has demonstrated that inhibition of SGLT2, the major pathway of renal glucose reabsorption, leads to increased urinary glucose excretion with concomitant reductions in fasting and postprandial plasma glucose levels, HbA1c levels and body mass. In phase I clinical trials in patients with T2DM, once-daily BI-10773 increased urinary glucose excretion resulting in dose-proportional reductions in fasting plasma glucose and mean daily glucose levels. BI-10773 was not associated with significant hypoglycemic episodes or other clinically important adverse events. Because of its mechanism of action, BI-10773 may be combined with other oral antidiabetic agents; indeed, the results of small trials suggested that coadministration of BI-10773 and metformin was safe and well tolerated. In animal studies, BI-10773 correlated with an increase in urinary volume and a reduction in body fat but not water content. This may represent an additional benefit of BI-10773 for the control of T2DM. At the time of publication, phase III clinical trials of BI-10773 were underway.

Secondary amyloidosis in a patient with long duration Crohn's disease treated with infliximab.

Systemic AA amyloidosis is a serious complication of many chronic inflammatory disorders. Its association with Crohn's disease implies that the inflammatory burden is high enough for amyloid fibrils to form deposits in tissues. A case is presented in which this complication occurred while the patient was clinically well, with biological and endoscopic markers showing an inactive or mildly active disease under anti-tumor necrosis factor alpha therapy.

Adult presentation of Bartter syndrome type IV with erythrocytosis / Apresentação tardia de síndrome de Bartter tipo IV com eritrocitose

Abstract Bartter syndrome comprises a group of rare autosomal-recessive salt-losing disorders with distinct phenotypes, but one unifying pathophysiology consisting of severe reductions of sodium reabsorption caused by mutations in five genes expressed in the thick ascending limb of Henle, coupled with increased urinary excretion of potassium and hydrogen, which leads to hypokalemic alkalosis. Bartter syndrome type IV, caused by loss-of-function mutations in barttin, a subunit of chloride channel CLC-Kb expressed in the kidney and inner ear, usually occurs in the antenatal-neonatal period. We report an unusual case of late onset presentation of Bartter syndrome IV and mild phenotype in a 20 years-old man who had hypokalemia, deafness, secondary hyperparathyroidism and erythrocytosis.

Resumo A síndrome de Bartter compreende um grupo raro de doenças autossômicas recessivas perdedoras de sal, decorrentes de mutações em genes expressos na porção ascendente espessa da alça de Henle, com fenótipos distintos, porém fisiopatogenia única, que consiste em redução severa da reabsorção de sódio, e aumento da excreção urinária de hidrogênio e potássio, levando à alcalose hipocalêmica. A síndrome de Bartter tipo IV, causada por mutações com perda de função da bartina, uma subunidade do canal de cloro CLC-Kb expressa no rim e ouvido interno, geralmente se apresenta nos períodos ante e neonatal. No presente relato, descreve-se um caso não usual de síndrome de Bartter tipo IV com apresentação tardia e fenótipo atenuado, diagnosticado por análise molecular, em um homem adulto de 20 anos que se apresentava com hipocalemia, surdez, hiperparatireoidismo secundário e eritrocitose.