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

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.

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.

Mouse Models with SGLT2 Mutations: Toward Understanding the Role of SGLT2 beyond Glucose Reabsorption

The sodium–glucose cotransporter 2 (SGLT2) mainly carries out glucose reabsorption in the kidney. Familial renal glycosuria, which is a mutation of SGLT2, is known to excrete glucose in the urine, but blood glucose levels are almost normal. Therefore, SGLT2 inhibitors are attracting attention as a new therapeutic drug for diabetes, which is increasing worldwide. In fact, SGLT2 inhibitors not only suppress hyperglycemia but also reduce renal, heart, and cardiovascular diseases. However, whether long-term SGLT2 inhibition is completely harmless requires further investigation. In this context, mice with mutations in SGLT2 have been generated and detailed studies are being conducted, e.g., the SGLT2−/− mouse, Sweet Pee mouse, Jimbee mouse, and SAMP10-ΔSglt2 mouse. Biological changes associated with SGLT2 mutations have been reported in these model mice, suggesting that SGLT2 is not only responsible for sugar reabsorption but is also related to other functions, such as bone metabolism, longevity, and cognitive functions. In this review, we present the characteristics of these mutant mice. Moreover, because the relationship between diabetes and Alzheimer’s disease has been discussed, we examined the relationship between changes in glucose homeostasis and the amyloid precursor protein in SGLT2 mutant mice.

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.

Factors Associated with Poor Glycemic Control Amongst Rural Residents with Diabetes in Korea

(1) Background: Glycemic control is an effective way to reduce the cardiovascular complications of diabetes. The purpose of this study was to identify the factors associated with poor glycemic control amongst rural residents with diabetes in Korea. (2) Methods: This cross-sectional analysis was conducted amongst a total of 522 participants who had completed baseline health examinations for the Korean Genome and Epidemiology Study (KoGES) Rural Cohort from 2005 to 2011. The subjects were divided into two groups: the good glycemic control group (GCG) (glycosylated hemoglobin (HbA1C) < 7%) and the poor GCG (HbA1C ≥ 7%). Logistic regression was used to examine the role of sociodemographics, health-related behavior, comorbidity and diabetes-related and clinical factors in poor glycemic control amongst rural residents with diabetes. (3) Results: In total, 48.1% of participants were in the poor GCG. Poor GCG was significantly associated with drinking (odds ratio (OR) = 0.42, 95% CI = 0.24-0.71), lack of regular physical activity (OR = 1.68, 95% CI = 1.03-2.76), fasting blood glucose (FBG) > 130 mg/dL (OR = 7.80, 95% CI = 4.35-13.98), diabetes for > 7 years (OR = 1.79, 95% CI = 1.08-2.98), cholesterol ≥ 200 mg/dL (OR = 1.73, 95% CI = 1.05-2.84) and positive urine glucose (OR = 6.24, 95% CI = 1.32-29.44). (4) Conclusion: Intensive glucose control interventions should target individuals amongst rural residents with diabetes who do not engage in regular physical activity, have been diagnosed with diabetes for more than seven years and who have high fasting-blood glucose, high cholesterol levels and glucose-positive urine.

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.

Identification of ten novel SLC5A2 mutations and determination of the renal threshold for glucose excretion in Chinese patients with familial renal glucosuria

BACKGROUND

Familial renal glucosuria (FRG) is a rare renal tubular disorder characterized by isolated persistent glucosuria without both abnormal glucose metabolism and any signs of proximal tubular dysfunction. SLC5A2 gene mutations are responsible for most FRG cases.

METHODS

Quantitative test for 24-hour urine glucose and RT_G were determined in 9 families (totaling 25 subjects). All coding regions, including intron-exon boundaries, were analyzed with PCR followed by direct sequence analysis.

RESULTS

Ten novel mutations were identified (c.331 T > C, p.W111R; c.374T>C, p.M125T; c.394C>T, p.R132C; c.612G>C, p.Q204H; c.829C>T, p.P277S; c.880G>A, p.D294N; c.1129G>A, p.G377S; c.1194C>A, p.F398L; c.1540C > T, p.P514S and c.1573C>T, p.H525Y). c.886(-10_-31)del that is specific to Chinese population was found in 5 out of 9 families, with a mutation rate of 28% (5/18). The compound heterozygotes presented with much lower RT_G values (1.28 ± 0.10 mmol/L), compared with the carriers of heterozygous variants (5.14 ± 0.77 mmol/L) (p<0.01); c.886(-10_-31)del heterozygotes had significant lower RT_G values than others (4.43 ± 0.37 vs 5.7 ± 0.51 mmol/L; p<0.01).

CONCLUSIONS

Ten novel SLC5A2 mutations are found and c.886(-10-31)del may be a hot spot mutation in Chinese population. Compound heterozygotes had much lower RT_G values than simple heterozygotes. Mixed-meal tolerance test is a simple method for determining RT_G in FRG patients.

The Relationship between Increases in Morning Spot Urinary Glucose Excretion and Decreases in HbA1C in Patients with Type 2 Diabetes After Taking an SGLT2 Inhibitor: A Retrospective, Longitudinal Study

INTRODUCTION

Sodium glucose co-transporter 2 (SGLT2) inhibitors increase urinary glucose excretion (UGE) by reducing the renal threshold for glucose excretion, which results in decreased serum glucose concentrations in patients with type 2 diabetes mellitus (T2D). However, no study to date has determined whether larger increases in UGE after SGLT2 inhibitor treatment correspond to larger reductions in glycated hemoglobin (HbA_1C).

METHODS

We enrolled participants who were newly prescribed an SGLT2 inhibitor (dapagliflozin 10 mg or ipragliflozin 50 mg, once daily) as an add-on therapy. Patients were tested for HbA_1C and first morning spot urinary-creatinine and -glucose concentrations immediately prior to administration of the SGLT2 inhibitor and at a 12-week follow-up appointment. We investigated the relationship between increases in morning spot UGE and decreases in HbA_1C.

RESULTS

A total of 101 participants with T2D were enrolled. The median age and diabetes duration were 61.0 and 12.8 years, respectively, and the median HbA_1C was 8.10%. SGLT2 inhibitors significantly lowered the HbA_1C level, with a median change from baseline to week 12 of -0.60% (p < 0.001). Robust increases from baseline were seen for the morning spot urinary glucose-to-creatinine ratio (UGCR), with a median change at week 12 of 47.3 mg/mg. In the correlation analysis, the ∆HbA_1C level showed a significant positive correlation with ∆morning spot UGCR (r = 0.395, p < 0.001). In other words, a greater reduction in HbA_1C was correlated with a smaller increase in UGE. After adjusting for confounding variables, ∆HbA_1C was significantly associated with ∆morning spot UGCR.

CONCLUSIONS

Although SGLT2 inhibitor treatment leads to a reduced HbA_1C level by augmenting UGE, larger increases in UGE do not correlate to larger reductions in HbA_1C. This suggests that the increase in UGE might not be an indicator of the degree of reductions in blood glucose.

Morning Spot Urine Glucose-to-Creatinine Ratios Predict Overnight Urinary Glucose Excretion in Patients With Type 2 Diabetes

BACKGROUND

With the advent of sodium glucose co-transporter 2 inhibitors to control glucose and treat diabetes, laboratory data aided by either timed or spot glucose levels in the urine could be used as an alternative marker of drug response. The aim of this study was to assess the agreement between overnight urinary glucose excretion (UGE) and morning spot urinary glucose-to-creatinine ratio (UGCR).

METHODS

In this prospective cross-sectional study, we enrolled a total of 215 participants with either normal glucose tolerance (NGT), pre-diabetes, or type 2 diabetes mellitus (T2DM). To exclude external factors such as food intake and physical activity, urine samples collected overnight at an 8-hr interval and the first-voided morning spot urine were collected and compared.

RESULTS

The median values of overnight 8-hr UGE in participants with NGT (N=14), pre-diabetes (N=41), and T2DM (N=160) were 35.0 mg, 35.6 mg, and 653.4 mg, respectively. In participants with T2DM, the median values of overnight 8-hr UGCR and first-voided morning spot UGCR (M-UGCR) were 1.37 mg/mg and 0.16 mg/mg, respectively. Quantitative analyses using an intraclass correlation coefficient (ICC) demonstrated a good reliability of measurement of the overnight 8-hr UGCR and M-UGCR (ICC=0.943, P<0.001). The M-UGCR was also significantly related to the overnight 8-hr UGE (r=0.828, P<0.001).

CONCLUSIONS

M-UGCR and overnight 8-hr UGCR showed good agreement, suggesting that M-UGCR be used as a simple index for estimating overnight amounts of UGE in patients with T2DM.