Abnormal expression and dysfunction of novel SGLT2 mutations identified in familial renal glucosuria patients

The Impact of Genetic Polymorphisms on the Anti-Hyperglycemic Effect of Dapagliflozin

Background

The influence of genetic variants on the glucose-lowering effects of dapagliflozin remains unclear. This study aims to investigate the impact of polymorphisms in solute carrier family 5 member 2 (SLC5A2), uridine diphosphate glucuronosyltransferase 1A9 (UGT1A9), solute carrier family 2 member 2 (SLC2A2) and member 4 (SLC2A4) on the anti-hyperglycemic effect of dapagliflozin in patients with type-2 diabetes mellitus (T2DM).

Methods

A total of 141 patients with T2DM were included in this prospective cohort study. Twenty-nine single nucleotide polymorphisms (SNPs) were selected and genotyped using the Sequenom MassArray platform or Sanger sequencing. Glycated hemoglobin (HbA1c) and fasting blood glucose (FBG) levels were compared before and after the treatment with dapagliflozin.

Results

Among the 29 SNPs selected, 27 were successfully analyzed. After three months of dapagliflozin treatment, FBG levels were significantly reduced (8.00 mmol/L (5.45-10.71) mmol/L vs 6.40 mmol/L (5.45-9.20) mmol/L, p = 0.003) in patients with T2DM. However, there was no significant change in HbA1c levels (8.10% (6.88-10.00)% vs 8.10% (6.83-10.00)%, p = 0.452). Analysis of covariance showed that patients with the minor allele homozygote or heterozygote of rs12471030 (CT/TT), rs12988520 (AC/CC) or rs2602381 (TC/CC) had higher FBG levels compared to those with the major allele homozygote (p = 0.014, p = 0.024, and p = 0.044, respectively). After adjusting for baseline FBG level, age, gender, body mass index, use of insulin and use of metformin, three SNPs-rs12471030, rs12988520 and rs2602381-were associated with the anti-hyperglycemic effect of dapagliflozin. However, using a stringent significance threshold (p < 0.002 with Bonferroni correction), none of these selected SNPs were significantly associated with FBG and HbA1c levels after dapagliflozin treatment.

Conclusion

After adjusting for confounding variables, polymorphisms in SLC5A2, UGT1A9, SLC2A2 and SLC2A4 genes were not associated with the anti-hyperglycemic effect of dapagliflozin in the Chinese population.

Clinical Trial Registration Number

ChiCTR2200059645.

Role of SLC5A2 polymorphisms and effects of genetic polymorphism on sodium glucose cotransporter 2 inhibitorsinhibitor response

Type 2 diabetes mellitus (T2DM) is a complex metabolic disease characterized by hyperglycaemia. T2DM is a highly heterogeneous polygenic disease. Due to genetic variation, variations in lifestyle and other environmental exposures, there are certain variations in the phenotype of T2DM patients. Sodium glucose cotransporter 2 (SGLT2) inhibitors are novel hypoglycaemic agents that increase urinary glucose excretion by inhibiting glucose reabsorption in the proximal tubules of the kidney. For glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors, studies have confirmed a variety of gene variants that may modify their effects. For SGLT2 inhibitors, research has focused on the SLC5A2 gene encoding SGLT2 and UGT1A9 gene polymorphisms affecting SGLT2 inhibitor metabolism. The SLC5A2 polymorphism rs9934336 have been associated with decreased HbA1c during the oral glucose tolerance test. Common variants of the SLC5A2 gene are related to blood glucose and insulin concentrations, but not glucagon concentrations. SLC5A2 rs9934336 and rs3116150 are related to a lower risk of heart failure. SGLT2 inhibitor exposure of UGT1A9*3 carriers is commonly higher than that of noncarriers, while these effects commonly have no obvious clinical significance on SGLT2 inhibitor pharmacokinetics. In terms of efficacy, general SLC5A2 variants show no significant effect on the response to the SGLT2 inhibitor empagliflozin. At present, research on the relationship between genetic polymorphisms and the efficacy of SGLT2 inhibitors is limited. The main purpose of this review is to elucidate the general effects of SGLT2 polymorphisms and the association between polymorphisms and the treatment response to SGLT2 inhibitors.

Excretion of glucose analogue with SGLT2 affinity predicts response effectiveness to sodium glucose transporter 2 inhibitors in patients with type 2 diabetes mellitus

PURPOSE

Sodium-glucose cotransporter 2 inhibitor (SGLT2i) regulation, developed as treatment for patients with type 2 diabetes, can be imaged with the glucose analogue alpha-methyl-4-deoxy-4-[18F]fluoro-D-glucopyranoside (Me4FDG), a positron emission tomography (PET) tracer with a high affinity for SGLT1 and SGLT2 proteins. With regard to therapy effectiveness, we aimed to investigate whether clinical parameters or Me4FDG excretion could predict response to SGLT2i in patients with type 2 diabetes.

METHODS

In a longitudinal, prospective study, 19 patients with type 2 diabetes underwent Me4FDG combined PET and magnetic resonance imaging (PET/MRI) scans at baseline and 2 weeks after initiation of therapy with SGLT2i, accompanied by the collection of blood and urine samples. Me4FDG-excretion was determined from the Me4FDG uptake in the bladder. Long-term response was determined by HbA1c level after 3 months; a strong response to the therapy was defined as a reduction of HbA1c by at least 10% from baseline.

RESULTS

SGLT2i resulted in significantly increased Me4FDG excretion (4.8 vs. 45.0, P < 0.001) and urine glucose (56 vs. 2806 mg/dl, P < 0.001). Baseline urine glucose and baseline Me4FDG excretion correlated both with long-term decline in HbA1c with r = 0.55 (P < 0.05). However, only Me4FDG excretion was a predictor of a strong response to SGLT2i (P = 0.005, OR 1.9).

CONCLUSIONS

Using Me4FDG-PET, we demonstrated for the first time renal SGLT2-related excretion before and after short-term SGLT2i treatment. In contrary to other clinical parameters, SGLT2-related excretion before treatment was a robust predictor of long-term HbA1c response in patients with type 2 diabetes, suggesting that therapy effectiveness is only dependent of endogenous SGLT2 processes.

Associations of SGLT2 genetic polymorphisms with salt sensitivity, blood pressure changes and hypertension incidence in Chinese adults

Sodium-glucose cotransporter 2 (SGLT2) inhibitors lowers blood pressure (BP) and exert a salutary effect on the salt sensitivity of BP. This study aimed to examine the associations of SGLT2 genetic variants with salt sensitivity, longitudinal BP changes and the risk of incident hypertension in Baoji Salt-Sensitive Study. A total of 514 participants were recruited when the cohort was established in 2004, and 333 participants received a dietary intervention that consisted of a 3-day usual diet followed sequentially by a 7-day low-salt diet and a 7-day high-salt diet. The cohort was then followed up for 14 years to evaluate the longitudinal BP changes and development of hypertension. We found that SGLT2 SNP rs3813007 was significantly associated with the systolic BP (SBP) responses to the low-salt diet. Over the 14 years of follow-up, SNPs rs3116149 and rs3813008 were significantly associated with the longitudinal SBP changes, and SNPs rs3116149, rs3813008, rs3813007 in SGLT2 were significantly associated with incidence of hypertension. Furthermore, gene-based analyses revealed that SGLT2 was significantly associated with hypertension incidence. Our study suggests that SGLT2 genetic polymorphisms may be involved in salt sensitivity and development of hypertension.

Dysuricemia-A New Concept Encompassing Hyperuricemia and Hypouricemia

The importance of uric acid, the final metabolite of purines excreted by the kidneys and intestines, was not previously recognized, except for its role in forming crystals in the joints and causing gout. However, recent evidence implies that uric acid is not a biologically inactive substance and may exert a wide range of effects, including antioxidant, neurostimulatory, proinflammatory, and innate immune activities. Notably, uric acid has two contradictory properties: antioxidant and oxidative ones. In this review, we present the concept of "dysuricemia", a condition in which deviation from the appropriate range of uric acid in the living body results in disease. This concept encompasses both hyperuricemia and hypouricemia. This review draws comparisons between the biologically biphasic positive and negative effects of uric acid and discusses the impact of such effects on various diseases.

Review on Molecular Mechanism of Hypertensive Nephropathy

Hypertension, a prevalent chronic ailment, has the potential to impair kidney function, and thereby resulting in hypertensive nephropathy. The escalating incidence of hypertensive nephropathy attributed to the aging population in urban areas, has emerged as a prominent cause of end-stage renal disease. Nevertheless, the intricate pathogenesis of hypertensive nephropathy poses considerable obstacles in terms of precise clinical diagnosis and treatment. This paper aims to consolidate the research findings on the pathogenesis of hypertensive nephropathy by focusing on the perspective of molecular biology.

Hypouricemia and Urate Transporters

Hypouricemia is recognized as a rare disorder, defined as a serum uric acid level of 2.0 mg/dL or less. Hypouricemia is divided into an overexcretion type and an underproduction type. The former typical disease is xanthinuria, and the latter is renal hypouricemia (RHUC). The frequency of nephrogenic hypouricemia due to a deficiency of URAT1 is high in Japan, accounting for most asymptomatic and persistent cases of hypouricemia. RHUC results in a high risk of exercise-induced acute kidney injury and urolithiasis. It is vital to promote research on RHUC, as this will lead not only to the elucidation of its pathophysiology but also to the development of new treatments for gout and hyperuricemia.

Clinical features of pediatric renal glucosuria cases due to SLC5A2 gene variants

BACKGROUND

Familial renal glycosuria (FRG) is a rare renal tubular disorder characterized by a variable loss of glucose in the urine despite normal blood glucose levels, which is seen in a condition in which other tubular functions are preserved. In this study, the molecular and clinical characteristics of pediatric FRG cases due to SLC5A2 gene variants were defined.

METHODS

Demographic features, diagnostic tests, and molecular analyses of patients with a diagnosis of FRG cases due to SLC5A2 gene variants were retrospectively analyzed between 2016 and 2019.

RESULTS

The data of 16 patients who were clinically and genetically diagnosed with FRG in a 4-year period were analyzed. Seven (44%) of the cases were female and 9 (56%) were male. The median age at diagnosis was 6 years old (2 months old to 17 years old). Neuromotor development was found to be appropriate for the age in each case. Systemic blood pressure was evaluated as normal. A homozygous pathogenic variant in the SLC5A2 gene was detected in 14 patients in the genetic examination. A heterozygous variant was detected in one patient. In the other patient, two different heterozygous pathological variants were found in the SLC5A2 gene.

CONCLUSIONS

It was revealed that growth and development were normal in children with glucosuria due to variations in the SCL5A2 gene. Renal function tests and urinary amino acid excretion were also within normal values. In our case series, the most common genetic variation in the SCL5A2 gene was the A219T (c.655G>A) variant.

Treatment Response to SGLT2 Inhibitors: From Clinical Characteristics to Genetic Variations

SGLT2 (sodium-glucose cotransporter 2) inhibitors are a new class of antihyperglycaemic drugs that act on the proximal tubules of the kidney. They have shown efficacy in the management of diabetes mellitus type 2 and their cardiovascular and renal safety have been extensively investigated and confirmed in clinical trials. However, inter-individual differences in response to treatment with SGLT2 inhibitors may present in everyday clinical practice, and good predictors of glycemic response and the risk for adverse events in an individual patient are lacking. As genetic variability of SGLT2 may influence the treatment response, pharmacogenetic information could support the choice of the most beneficial treatment strategy in an individual patient. This review focuses on the clinical and genetic factors that may influence the treatment response to SGLT2 inhibitors in type 2 diabetes patients with comorbid conditions.

High urinary glucose is associated with improved renal prognosis in patients with diabetes mellitus

AIMS/INTRODUCTION

The relationship between renal function and urinary glucose is poorly understood in diabetes patients who are not using sodium-glucose cotransporter 2 inhibitors. This study aimed to investigate the association of urinary glucose excretion with renal function prognosis in such patients.

MATERIALS AND METHODS

This retrospective cohort study included 1,172 patients with type 1 or 2 diabetes mellitus. Patients were recruited and data were collected between 1 January 2007 and 31 December 2011; follow-up data were collected until 30 June 2015. The primary outcome was set as a 30% decline in estimated glomerular filtration rate relative to baseline. The relationship between this outcome and urinary glucose was investigated using Cox proportional hazards model. For analysis, patients were categorized into two groups: urinary glucose <5 g/day or ≥5 g/day. Interaction terms were analyzed.

RESULTS

Multivariate analysis showed that the prognosis of renal function was significantly better in patients with high urinary glucose (≥5 g/day; adjusted hazard ratio 0.58, 95% confidence interval 0.35-0.96; P = 0.034). Significant interactions were observed between high urinary glucose and male sex (hazard ratio 0.33, 95% confidence interval 0.14-0.74; P = 0.007), and between high urinary glucose and longer duration of diabetes (≥10 years; hazard ratio 0.25, 95% confidence interval 0.11-0.58; P = 0.001).

CONCLUSIONS

The present study suggests that high urinary glucose is associated with prognosis in diabetes patients not taking sodium-glucose cotransporter 2 inhibitors. Measurement of 24-h urinary glucose excretion might have clinical utility for predicting renal prognosis.

Impact of an SGLT2-loss of function mutation on renal architecture, histology, and glucose homeostasis

Inhibitors of sodium/glucose co-transporter 2 (SGLT2) are currently in clinical use for type 2 diabetes (T2D) treatment due to their anti-hyperglycemic effect exerted by the inhibition of glucose reabsorption in the kidney. Inhibition of SGLT2 is associated with improvement of renal outcomes in chronic kidney disease associated with T2D. Our study aimed to describe the renal-specific phenotypic consequences of the SGLT2-loss of function "Jimbee" mutation within the Slc5a2 mouse gene in a non-diabetic/non-obese background. The Jimbee mice displayed reduced body weight, glucosuria, polyuria, polydipsia, and hyperphagia but were normoglycemic, with no signs of baseline insulin resistance or renal dysfunction. Histomorphological analysis of the kidneys revealed a normal architecture and morphology of the renal cortex, but shrinkage of the glomerular and tubular apparatus, including Bowman's space, glomerular tuft, mesangial matrix fraction, and proximal convoluted tubule (PCT). Immunofluorescent analysis of renal sections showed that SGLT2 was absent from the apical membrane of PCT of the Jimbee mice but remnant positive vesicles were detected within the cytosol or at the perinuclear interface. Renal localization and abundance of GLUT1, GLUT2, and SGLT1 were unchanged in the Jimbee genotype. Intriguingly, the mutation did not induce hepatic gluconeogenic gene expression in overnight fasted mice despite a high glucose excretion rate. The Jimbee phenotype is remarkably similar to humans with SLC5A2 mutations and provides a useful model for the study of SGLT2-loss of function effects on renal architecture and physiology, as well as for identifying possible novel roles for the kidneys in glucose homeostasis and metabolic reprogramming.

Six Exonic Variants in the SLC5A2 Gene Cause Exon Skipping in a Minigene Assay

Background

Familial renal glucosuria is a rare renal tubular disorder caused by SLC5A2 gene variants. Most of them are exonic variants and have been classified as missense variants. However, there is growing evidence that some of these variants can be detrimental by affecting the pre-mRNA splicing process. Therefore, we hypothesize that a certain proportion of SLC5A2 exonic variants can result in disease via interfering with the normal splicing process of the pre-mRNA.

Methods

We used bioinformatics programs to analyze 77 previously described presumed SLC5A2 missense variants and identified candidate variants that may alter the splicing of pre-mRNA through minigene assays.

Results

Our study indicated six of 7 candidate variants induced splicing alterations. Variants c.216C > A, c.294C > A, c.886G > C, c.932A > G and c.962A > G may disrupt splicing enhancer motifs and generate splicing silencer sequences resulting in the skipping of exon 3. Variants c.305C > T and c.1129G > A probably disturb splice sites leading to exon skipping.

Conclusion

To our knowledge, we report, for the first time, SLC5A2 exonic variants that produce alterations in pre-mRNA. Our research reinforces the importance of assessing the consequences for putative point variants at the mRNA level. Additionally, we propose that minigenes function analysis may be valuable to evaluate the impact of SLC5A2 exonic variants on pre-mRNA splicing without patients’ RNA samples.

Sodium-coupled glucose transport, the SLC5 family, and therapeutically relevant inhibitors: from molecular discovery to clinical application

Sodium glucose transporters (SGLTs) belong to the mammalian solute carrier family SLC5. This family includes 12 different members in human that mediate the transport of sugars, vitamins, amino acids, or smaller organic ions such as choline. The SLC5 family belongs to the sodium symporter family (SSS), which encompasses transporters from all kingdoms of life. It furthermore shares similarity to the structural fold of the APC (amino acid-polyamine-organocation) transporter family. Three decades after the first molecular identification of the intestinal Na+-glucose cotransporter SGLT1 by expression cloning, many new discoveries have evolved, from mechanistic analysis to molecular genetics, structural biology, drug discovery, and clinical applications. All of these advances have greatly influenced physiology and medicine. While SGLT1 is essential for fast absorption of glucose and galactose in the intestine, the expression of SGLT2 is largely confined to the early part of the kidney proximal tubules, where it reabsorbs the bulk part of filtered glucose. SGLT2 has been successfully exploited by the pharmaceutical industry to develop effective new drugs for the treatment of diabetic patients. These SGLT2 inhibitors, termed gliflozins, also exhibit favorable nephroprotective effects and likely also cardioprotective effects. In addition, given the recent finding that SGLT2 is also expressed in tumors of pancreas and prostate and in glioblastoma, this opens the door to potential new therapeutic strategies for cancer treatment by specifically targeting SGLT2. Likewise, further discoveries related to the functional association of other SGLTs of the SLC5 family to human pathologies will open the door to potential new therapeutic strategies. We furthermore hope that the herein summarized information about the physiological roles of SGLTs and the therapeutic benefits of the gliflozins will be useful for our readers to better understand the molecular basis of the beneficial effects of these inhibitors, also in the context of the tubuloglomerular feedback (TGF), and the renin-angiotensin system (RAS). The detailed mechanisms underlying the clinical benefits of SGLT2 inhibition by gliflozins still warrant further investigation that may serve as a basis for future drug development.

Inherited Renal Tubulopathies-Challenges and Controversies

Electrolyte homeostasis is maintained by the kidney through a complex transport function mostly performed by specialized proteins distributed along the renal tubules. Pathogenic variants in the genes encoding these proteins impair this function and have consequences on the whole organism. Establishing a genetic diagnosis in patients with renal tubular dysfunction is a challenging task given the genetic and phenotypic heterogeneity, functional characteristics of the genes involved and the number of yet unknown causes. Part of these difficulties can be overcome by gathering large patient cohorts and applying high-throughput sequencing techniques combined with experimental work to prove functional impact. This approach has led to the identification of a number of genes but also generated controversies about proper interpretation of variants. In this article, we will highlight these challenges and controversies.

SLC5A2 mutations, including two novel mutations, responsible for renal glucosuria in Chinese families

Background

Familial renal glucosuria (FRG) is characterized by persistent glucosuria without other impairments of tubular function in the presence of normal serum glucose. SGLT2, which is almost exclusively expressed in the kidney, accounts for most of the glucose reabsorption. Recently, some studies have confirmed that SLC5A2 mutations are responsible for the pathogenesis of familial renal glucosuria, but FRG cases are still rare. Furthermore, there are a few reports about splice-site mutations in previous studies, but the effect of these variants at the mRNA level has hardly been verified.

Methods

Ten patients were recruited in our renal division because of persistent glucosuria, and clinical data of the patients and their family members were recorded as much as possible. The entire coding region and adjacent intronic segments of SLC5A2 were sequenced in FRG patients and their relatives. Permanent growing lymphoblastoid cell lines from FRG patients were established to better preserve genetic information.

Results

A total of nine different mutations were identified: IVS1-16C > A, c.305C > T/p.(A102V), c.395G > A/p.(R132H), c.736C > T/p.(P246S), c.886(−10_-31)delGCAAGCGGGCAGCTGAACGCCC, c.1152_1163delGGTCATGCTGGC/p.(Val385_Ala388del), c.1222G > T/p.(D408Y), c.1496G > A/p.(R499H) and c.1540C > T/p.(P514S); two novel mutations in SLC5A2, c.1222G > T/p.(D408Y) and c.1496G > A/p.(R499H), were identified in the Chinese FRG pedigrees. Ten individuals with heterozygous or compound heterozygous variants had glucosuria in the range of 3.1 to 37.6 g/d.

Conclusion

We screened ten additional Chinese FRG pedigrees for mutations in the SLC5A2 gene and found nine mutations, including two novel mutations. Most variants were private, but IVS1-16C > A and c.886(−10_-31) del may be high frequency splice-site mutations that could be preferentially screened when variants cannot be found in the SLC5A2 exon. Furthermore, we successfully established a permanent growing lymphoblastoid cell line from patients with FRG, which could facilitate further studies of the SLC5A2 gene. The current study provides a valuable clue for further research on the molecular mechanism of SGLT2.

Exome sequencing revealed DNA variants in NCOR1, IGF2BP1, SGLT2 and NEK11 as potential novel causes of ketotic hypoglycemia in children

Unexplained or idiopathic ketotic hypoglycemia (KH) is the most common type of hypoglycemia in children. The diagnosis is based on the exclusion of routine hormonal and metabolic causes of hypoglycemia. We aimed to identify novel genes that cause KH, as this may lead to a more targeted treatment. Deep phenotyping of ten preschool age at onset KH patients (boys, n = 5; girls, n = 5) was performed followed by trio exome sequencing and comprehensive bioinformatics analysis. Data analysis revealed four novel candidate genes: (1) NCOR1 in a patient with KH, iron deficiency and loose stools; (2) IGF2BP1 in a proband with KH, short stature and delayed bone age; (3) SLC5A2 in a proband with KH, intermittent glucosuria and extremely elevated p-GLP-1; and (4) NEK11 in a proband with ketotic hypoglycemia and liver affliction. These genes are associated with different metabolic processes, such as gluconeogenesis, translational regulation, and glucose transport. In conclusion, WES identified DNA variants in four different genes as potential novel causes of IKH, suggesting that IKH is a heterogeneous disorder that can be split into several novel diseases: NCOR1-KH, IGF2BP1-KH, SGLT2-KH or familial renal glucosuria KH, and NEK11-KH. Precision medicine treatment based on exome sequencing may lead to advances in the management of IKH.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

A novel compound heterozygous mutation in SLC5A2 contributes to familial renal glucosuria in a Chinese family, and a review of the relevant literature

Familial renal glucosuria (FRG) is a rare condition that involves isolated glucosuria despite normal blood glucose levels. Mutations in the solute carrier family 5 member 2 (SLC5A2) gene, which encodes sodium-glucose cotransporter 2 (SGLT2), have been reported to be responsible for the disease. Genetic testing of the SLC5A2 gene was conducted in a Chinese family with FRG. A number of online tools were used to predict the potential effect of the identified mutations on SGLT2 function. Additionally, the SLC5A2 mutations previously reported in PubMed were summarized. A novel compound heterozygous mutation (c.514T>C, p.W172R; c.1540C>T, p.P514S) of the SLC5A2 gene in a Chinese child with FRG was identified. In total, 86 mutations of the SLC5A2 gene have been reported to be associated with FRG. The novel compound heterozygous mutation (c.514T>C, p.W172R; c.1540C>T, p.P514S) of the SLC5A2 gene may be responsible for the onset of FRG. The present study provides a starting point for further investigation of the molecular pathogenesis of the SLC5A2 gene mutation in patients with FRG.

The roles of sodium-glucose cotransporter 2 inhibitors in preventing kidney injury in diabetes

Diabetic nephropathy (DN) is the leading cause of end stage renal disease (ESRD) worldwide. The early effective treatment of high plasma glucose could delay or prevent the onset of DN. Sodium-glucose cotransporter 2 (SGLT2) inhibitors are new target treatments for ameliorating high plasma glucose and help to maintain glucose homeostasis in diabetic patients. Reduced renal glucose reabsorption by SGLT2 inhibition seems to have high potential to improve glycemic control in diabetes mellitus (DM) not only through glucose lowering but also through glucose-independent effects such as blood pressure-lowering and direct renal effects in diabetes. Of note, the important events in the pathogenesis of glucose-induced renal injury and DN including oxidative stress, inflammation, fibrosis and apoptosis conditions have shown to be ameliorate after the treatment with SGLT2 inhibitors. Interestingly, SGLT2 inhibitors have been reported to reduce albuminuria in DM via an activation of renal tubuloglomerular feedback by increased macula densa sodium and chloride delivery, leading to afferent vasoconstriction and attenuated diabetes-induced renal hyperfiltration. These effects also help to conserve glomerular integrity. Thus, the treatment of diabetes mellitus using SGLT2 inhibitors could be one of the effective approach for the management of diabetic-associated kidney disease like DN. This review summarizes the up to date information and discusses the bidirectional relationship between the SGLT2 inhibitor treatments and the renal functions that are available from both basic research and clinical reports. The details of renal outcomes of SGLT2 inhibitors in DN are also provide in this review.

Renal glucosuria is not associated with atherosclerotic cardiovascular disease outcome in a general Japanese community

BACKGROUND AND AIMS

Renal glucosuria is defined as the excretion of detectable amounts of glucose in the urine without diabetes or hyperglycemia. Few data exist regarding the prevalence of renal glucosuria and its clinical impact on atherosclerotic cardiovascular diseases.

METHODS

This study included 47,842 subjects (16,913 men, 35.4%) aged ≥40 years who underwent the Japanese specific health checkup in Kanazawa City during 2014. We defined renal glucosuria as fulfillment of all of the following three criteria: 1) detectable glucosuria; 2) the absence of diabetes; 3) normal blood glucose (<110 mg/dl fasting, and <140 mg/dl non-fasting). The presence of renal glucosuria and of factors associated with atherosclerotic cardiovascular diseases, including coronary artery disease and stroke, was assessed.

RESULTS

The criteria for renal glucosuria were met by 665 (1.4%) subjects. Significantly higher proportions of subjects with renal glucosuria exhibited coronary artery disease, stroke, or either outcome than those without (14.9% vs. 12.1%, p = 0.0305; 9.9% vs. 6.9%, p = 0.00255; 22.3% vs. 17.0%, p = 4.0 × 10^-4, respectively), but multivariate logistic regression analyses revealed that renal glucosuria was not associated with coronary artery disease (odds ratio [OR] = 0.940, 95% confidence interval [CI] = 0.748-1.171, not significant), stroke (OR = 1.122, 95% CI = 0.853-1.453, not significant), or atherosclerotic cardiovascular diseases (OR = 1.122, 95% CI = 0.853-1.453, not significant).

CONCLUSIONS

These results indicate that the prevalence of renal glucosuria in the Japanese general population was 1.4%, and that renal glucosuria was not associated with atherosclerotic cardiovascular diseases per se.

A recurrent deletion in the SLC5A2 gene including the intron 7 branch site responsible for familial renal glucosuria

Familial renal glycosuria (FRG) is caused by mutations in the SLC5A2 gene, which codes for Na⁺-glucose co-transporters 2 (SGLT2). The aim of this study was to analyze and identify the mutations in 16 patients from 8 families with FRG. All coding regions, including intron-exon boundaries, were analyzed using PCR followed by direct sequence analysis. Six mutations in SLC5A2 gene were identified, including five missense mutations (c.393G > C, p.K131N; c.1003A > G, p.S335G; c.1343A > G, p.Q448R; c.1420G > C, p.A474P; c.1739G > A, p.G580D) and a 22-bp deletion in intron 7 (c.886(-10_-31)del) removing the putative branch point sequence. By the minigene studies using the pSPL3 plasmids, we confirmed that the deletion c.886(-10_-31)del acts as a splicing mutation. Furthermore, we found that this deletion causes exclusion of exon 8 in the SCL5A2 transcript in patients. The mutation c.886(-10_-31)del was present in 5 (62.5%) of 8 families, and accounts for about 37.5% of the total alleles (6/16). In conclusion, six mutations resulting in FRG were found, and the c.886(-10_-31)del may be the high frequency mutation that can be screened in FRG patients with uniallelic or negative SLC5A2 mutations.

Novel SLC5A2 mutation contributes to familial renal glucosuria: Abnormal expression in renal tissues

Familial renal glucosuria (FRG) is characterized by persistent glucosuria in the presence of normal serum glucose concentrations, while other impairments of tubular function are absent. Mutations in the sodium-glucose co-transporter 2 (SLC5A2) gene have been found to be responsible for FRG. However, direct evidence for the presence of SLC5A2 mutant in renal tissues is very rare. In previous studies, a non-sense mutation (c.1320 G>A:p.W440X) that would cause premature termination of the protein was found. However, the effects in the renal tissues were not reported. In the current study, a patient with FRG and a urinary glucose excretion rate of 8.3 g/day is described, for whom a novel missense mutation (c.1319G>A:p.W440X) was revealed by sequencing. Furthermore, in the immunofluorescence examination of a renal biopsy specimen, SLC5A2 was detected in the apical side of the proximal convoluted tubule, discontinuously decreased in comparison with that in normal and disease controls. The results imply that both wild-type SLC5A2 and mutant SLC5A2 with abnormal distribution were expressed in the renal tissues, and that the reduction of SLC5A2 expression and function were due to the c.1319G>A:p.W440X mutation. The current study provides valuable clues regarding the SLC5A2 molecule from genotype to phenotype in families affected by FRG.

Decreased expression and function of sodium-glucose co-transporter 2 from a novel C-terminal mutation: a case report

Background

Familial renal glucosuria (FRG) is characterized by persistent glucosuria in the presence of normal serum glucose concentrations, and the absence of other impairments of tubular function. Mutations in the sodium–glucose co-transporter 2 (SGLT2) gene (SLC5A2) are causative of FRG the long-term outcome of which is well know. In the search for potential new drug targets for SGLT2 inhibitors with which to treat the diabetes, expressional and functional studies of SGLT2 have been the focus of attention, but reports of these are rare. Furthermore, it is well known that the alleles in the C-terminal are very important for the expression and function in some genes. However, little is known about the effect of mutation in SLC5A2 C- terminal.

Case presentation

Here, we identified a FRG patient with urine glucose excretion 7.56 g/day and a novel SLC5A2 missense mutation, c.1891G > A/p.(E631K), by DNA sequencing. Expression and function of the mutant SGLT2 (631 K) fused to green fluorescent protein (GFP) were verified by western blotting, confocal laser microscopy, and transport activity assays in cultured HEK293 cells. Although wild-type SGLT2–GFP and 631 K mutant–GFP fusion proteins were properly expressed in a punctate pattern in the cell membrane, and co-localized with the cell membrane marker DiIC18(3), the expression of the mutant fusion protein was obviously decreased (24 %). Moreover, the uptake activity of the mutant SGLT2 631 K–GFP fusion protein was significantly decreased compared with wild-type (3629 ± 1082 vs. 7926 ± 1153, P < 0.001).

Conclusion

These results suggest that the SLC5A2 C-terminal is very important for protein expression. We speculate that the observed reduced expression of the mutant transporter led to a decrease in transport of the glucose analog 2-(N-(7-nitrobenz-2-oxa-1,3- diazol-4-yl)amino)-2-deoxyglucose. The current study provides a starting point for further investigations of the SGLT2 molecular mechanism in FRG families, and offers functional insights into the development of anti-diabetes drugs.

Influence of Familial Renal Glycosuria Due to Mutations in the SLC5A2 Gene on Changes in Glucose Tolerance over Time

Familial renal glycosuria is an inherited disorder resulting in glucose excretion in the urine despite normal blood glucose concentrations. It is most commonly due to mutations in the SLC5A2 gene coding for the glucose transporter SGLT2 in the proximal tubule. Several drugs have been introduced as means to lower glucose in patients with type 2 diabetes targeting SGLT2 resulting in renal glycosuria, but no studies have addressed the potential effects of decreased renal glucose reabsorption and chronic glycosuria on the prevention of glucose intolerance. Here we present data on a large pedigree with renal glycosuria due to two mutations (c.300-303+2del and p.A343V) in the SLC5A2 gene. The mutations, which in vitro affected glucose transport in a cell line model, and the ensuing glycosuria were not associated with better glycemic control during a follow-up period of more than 10 years. One individual, who was compound heterozygous for mutations in the SLC5A2 gene suffered from severe urogenital candida infections and postprandial hypoglycemia. In conclusion, in this family with familial glycosuria we did not find any evidence that chronic loss of glucose in the urine would protect from deterioration of the glucose tolerance over time.

Renal sodium-glucose cotransporter inhibition in the management of type 2 diabetes mellitus

Hyperglycemia is the primary factor responsible for the microvascular, and to a lesser extent macrovascular, complications of diabetes. Despite this well-established relationship, approximately half of all type 2 diabetic patients in the US have a hemoglobin A1c (HbA1c) ≥7.0%. This is associated in part with the side effects, i.e., weight gain and hypoglycemia, of currently available antidiabetic agents and in part with the failure to utilize medications that reverse the basic pathophysiological defects present in patients with type 2 diabetes. The kidney has been shown to play a central role in the development of hyperglycemia by excessive production of glucose throughout the sleeping hours and enhanced reabsorption of filtered glucose by the renal tubules secondary to an increase in the threshold at which glucose spills into the urine. Recently, a new class of antidiabetic agents, the sodium-glucose cotransporter 2 (SGLT2) inhibitors, has been developed and approved for the treatment of patients with type 2 diabetes. In this review, we examine their mechanism of action, efficacy, safety, and place in the therapeutic armamentarium. Since the SGLT2 inhibitors have a unique mode of action that differs from all other oral and injectable antidiabetic agents, they can be used at all stages of the disease and in combination with all other antidiabetic medications.

Transport of sugars

Soluble sugars serve five main purposes in multicellular organisms: as sources of carbon skeletons, osmolytes, signals, and transient energy storage and as transport molecules. Most sugars are derived from photosynthetic organisms, particularly plants. In multicellular organisms, some cells specialize in providing sugars to other cells (e.g., intestinal and liver cells in animals, photosynthetic cells in plants), whereas others depend completely on an external supply (e.g., brain cells, roots and seeds). This cellular exchange of sugars requires transport proteins to mediate uptake or release from cells or subcellular compartments. Thus, not surprisingly, sugar transport is critical for plants, animals, and humans. At present, three classes of eukaryotic sugar transporters have been characterized, namely the glucose transporters (GLUTs), sodium-glucose symporters (SGLTs), and SWEETs. This review presents the history and state of the art of sugar transporter research, covering genetics, biochemistry, and physiology-from their identification and characterization to their structure, function, and physiology. In humans, understanding sugar transport has therapeutic importance (e.g., addressing diabetes or limiting access of cancer cells to sugars), and in plants, these transporters are critical for crop yield and pathogen susceptibility.

Patient profiling in diabetes and role of canagliflozin

Background

Physicians attempt to achieve glycemic goals in patients with type 2 diabetes mellitus (T2DM) through various means, including glucose-lowering medications. There is interindividual variability in response to medications, which can be partially explained by the presence of genetic polymorphisms that affect drug metabolism. Pharmacogenomics studies the hereditary basis of interpatient variations in drug response and aims to identify subgroups of patients whose drug management could be tailored accordingly. The aim of this review is to explore patient profiling in the management of T2DM with a focus on the sodium glucose transporter inhibitor canagliflozin.

Methods

The PubMed database was searched using the terms “pharmacogenomics” and “diabetes” through May 31, 2014. Published articles and abstracts presented at national/international meetings were considered.

Results and conclusion

Genome-wide association studies have opened the door for patient profiling and research into genetic variants in multifactorial T2DM. Clinically, it may be possible to tailor the type of medication used based on the presence or absence of the various genetic variants. However, the polymorphisms studied may only explain some of the variability in response to T2DM drugs and needs further validation to ensure its authenticity. There are still unidentified factors which appear to play a role in the interindividual variability seen in clinical practice. The potential exists for pharmacogenomics to promote efficacious, safe, and cost-effective individualized diabetes management. Pharmacogenomics is still in its early stages, and the idea of defining patients genetically to predict individual responses to drugs and obtain safe and effective T2DM management is promising, in spite of existing barriers. Currently, clinical profiling of patients with T2DM and using an individualized approach with most drugs, including canagliflozin, based on comorbid conditions still remains the most accepted approach for the management of T2DM.

Use of systems pharmacology modeling to elucidate the operating characteristics of SGLT1 and SGLT2 in renal glucose reabsorption in humans

In the kidney, glucose in glomerular filtrate is reabsorbed primarily by sodium-glucose cotransporters 1 (SGLT1) and 2 (SGLT2) along the proximal tubules. SGLT2 has been characterized as a high capacity, low affinity pathway responsible for reabsorption of the majority of filtered glucose in the early part of proximal tubules, and SGLT1 reabsorbs the residual glucose in the distal part. Inhibition of SGLT2 is a viable mechanism for removing glucose from the body and improving glycemic control in patients with diabetes. Despite demonstrating high levels (in excess of 80%) of inhibition of glucose transport by SGLT2 in vitro, potent SGLT2 inhibitors, e.g., dapagliflozin and canagliflozin, inhibit renal glucose reabsorption by only 30-50% in clinical studies. Hypotheses for this apparent paradox are mostly focused on the compensatory effect of SGLT1. The paradox has been explained and the role of SGLT1 demonstrated in the mouse, but direct data in humans are lacking. To further explore the roles of SGLT1/2 in renal glucose reabsorption in humans, we developed a systems pharmacology model with emphasis on SGLT1/2 mediated glucose reabsorption and the effects of SGLT2 inhibition. The model was calibrated using robust clinical data in the absence or presence of dapagliflozin (DeFronzo et al., 2013), and evaluated against clinical data from the literature (Mogensen, 1971; Wolf et al., 2009; Polidori et al., 2013). The model adequately described all four data sets. Simulations using the model clarified the operating characteristics of SGLT1/2 in humans in the healthy and diabetic state with or without SGLT2 inhibition. The modeling and simulations support our proposition that the apparent moderate, 30-50% inhibition of renal glucose reabsorption observed with potent SGLT2 inhibitors is a combined result of two physiological determinants: SGLT1 compensation and residual SGLT2 activity. This model will enable in silico inferences and predictions related to SGLT1/2 modulation.

Novel SLC5A2 variants contribute to renal glucosuria in Chinese families: abnormal expression and dysfunction of variant SLC5A2

Familial renal glucosuria (FRG) is characterized by persistent glucosuria despite normal serum glucose and the absence of overt tubular dysfunction. Variants in solute carrier family 5 (sodium-glucose cotransporter), member 2 (SLC5A2) have been reported in FRG patients. However, the functional and expression-related consequences of such variants have been scarcely investigated. In the current study, we studied five FRG families and identified six missense mutations, including four novel variants (c.1051T>C/.(C351R), c.1400T>C/p.(V467A), c.1420G>C/p.(A474P), c.1691G>A/p.(R564Q); RNA not analyzed) and two variants that had been previously reported (c.294C>A/p.(F98L), c.736C>T/p.(P246S); RNA not analyzed). The probands were either heterozygous or compound heterozygous for SLC5A2 variants and had glucosuria of 5.9%-19.6 g/day. Human 293 cells were transfected with plasmid constructs to study the expression and function of SLC5A2 variants in vitro. Western blotting revealed that the expression levels of SLC5A2-351R-GFP, SLC5A2-467A-GFP, SLC5A2-474P-GFP, and SLC5A2-564Q-GFP were significantly decreased compared with wild-type SLC5A2-GFP (37%-55%). Confocal microscopy revealed that three variants (c.1400T>C, c.1420G>C, c.1691G>A) resulted in a loss of the punctate membrane pattern typical of wild-type SLC5A2. All variants had a significantly lower transport capacity in than the wild-type control. The current study provides a starting point to further investigate the molecular mechanism of SLC5A2 in FRG families and provides functional clues for antidiabetes drugs.

SGLT2 inhibitor lowers serum uric acid through alteration of uric acid transport activity in renal tubule by increased glycosuria

Sodium glucose cotransporter 2 (SGLT2) inhibitors have been reported to lower the serum uric acid (SUA) level. To elucidate the mechanism responsible for this reduction, SUA and the urinary excretion rate of uric acid (UE(UA)) were analysed after the oral administration of luseogliflozin, a SGLT2 inhibitor, to healthy subjects. After dosing, SUA decreased, and a negative correlation was observed between the SUA level and the UE(UA), suggesting that SUA decreased as a result of the increase in the UE(UA). The increase in UE(UA) was correlated with an increase in urinary D-glucose excretion, but not with the plasma luseogliflozin concentration. Additionally, in vitro transport experiments showed that luseogliflozin had no direct effect on the transporters involved in renal UA reabsorption. To explain that the increase in UE(UA) is likely due to glycosuria, the study focused on the facilitative glucose transporter 9 isoform 2 (GLUT9ΔN, SLC2A9b), which is expressed at the apical membrane of the kidney tubular cells and transports both UA and D-glucose. It was observed that the efflux of [(14) C]UA in Xenopus oocytes expressing the GLUT9 isoform 2 was trans-stimulated by 10 mm D-glucose, a high concentration of glucose that existed under SGLT2 inhibition. On the other hand, the uptake of [(14) C]UA by oocytes was cis-inhibited by 100 mm D-glucose, a concentration assumed to exist in collecting ducts. In conclusion, it was demonstrated that the UE(UA) could potentially be increased by luseogliflozin-induced glycosuria, with alterations of UA transport activity because of urinary glucose.

SGLT2: a potential target for the pharmacogenetics of Type 2 diabetes?

The kidney has attracted the attention of diabetologists as an organ involved in the regulation of glucose homeostasis not only by gluconeogenesis, but also by renal glucose excretion. Sodium–glucose cotransporters (SGLTs), particularly SGLT2, are responsible for reabsorption of up to 99% of the filtered glucose. SGLT2 is coded by the SLC5A2 gene, which maps on chromosome 16. Pharmacological reduction of tubular glucose reabsorption results in improved glycemic control in Type 2 diabetic patients. Since the SGLTs reabsorb most of the filtered glucose (90%), it is not surprising that mutations in SLC5A2 cause familial renal glucosuria. A recent study pointed out a possible role of common genetic variation in SLC5A2 in the control of glucose homeostasis. SLC5A2 polymorphisms might therefore represent potential candidates for pharmacogenomic studies targeting the impact of these variants on the efficacy of antidiabetic treatment that is based on inhibition of SGLT2 activity.

A novel SGLT is expressed in the human kidney

Selective inhibitors of sodium-glucose cotransporter 2 (SGLT2)-mediated reabsorption of glucose in the proximal tubule of the kidney are being developed for the treatment of diabetes. SGLT2 shares high degree of homology with SGLT3; however, very little is known about the expression and functional role of SGLT3 in the human kidney. Indeed, the SGLT2 inhibitors that are currently in clinical trials might affect the expression and/or the activity of SGLT3. Therefore, the present study examined the expression of SGLT3 mRNA and protein in human kidney and in a human proximal tubule HK-2 cell line. The results indicated that human SGLT3 (hSGLT3) message and protein are expressed both in vivo and in vitro. We also studied the activity of hSGLT3 protein following its over-expression in mammalian kidney-derived COS-7 cells and in HK-2 cells treated with the imino sugar deoxynojirimycin (DNJ), a potent agonist of hSGLT3. Over-expression of hSGLT3 in COS-7 cells increased intracellular sodium concentration by 3-fold without affecting glucose transport. Activation of hSGLT3 with DNJ (50μM) increased sodium uptake in HK-2 cells by 5.5 fold and this effect could be completely blocked with SGLT inhibitor phlorizin (50μM). These results suggest that SGLT3 is expressed in human proximal tubular cells where it serves as a novel sodium transporter. Up-regulation of the expression of SGLT3 in the proximal tubule in diabetic patients may contribute to the elevated sodium transport in this segment of the nephron that has been postulated to promote hyperfiltration and renal injury.

Familial renal glucosuria: a clinicogenetic study of 23 additional cases

BACKGROUND

Familial renal glucosuria (FRG) is an inherited renal tubular disorder characterized by persistent isolated glucosuria in the absence of hyperglycemia that is caused by mutations in the sodium-glucose cotransporter SGLT2 coding gene, SLC5A2.

OBJECTIVE

We conducted molecular and phenotype analyses of a cohort of 23 unrelated Korean children with FRG.

METHODS

Mutational analysis of the SLC5A2 gene was conducted in this multicenter study organized by the Korean Society of Pediatric Nephrology.

RESULTS

A total of 21 different SLC5A2 mutations were detected, including 19 novel mutations. All patients had at least one mutated allele; ten patients had homozygous or compound heterozygous mutations and 13 patients had a single heterozygous mutation. Most mutations were private. Patients with two mutations were diagnosed earlier with larger amounts of urinary glucose excretion than patients with single mutations. Pedigree analysis data were consistent with the inheritance of a codominant trait with incomplete penetrance.

CONCLUSIONS

These findings extend the allelic heterogeneity in FRG and confirm previous observations of inheritance and genotype–phenotype correlation in patients with this disease.