Part 9

Familial Renal Glucosuria: A Genetic Model of SGLT2 Inhibition

Familial Renal Glucosuria Presentation Glucosuria: 1-170 g/day Asymptomatic Blood Normal glucose concentration No hypoglycemia or hypovolemia Kidney / bladder No tubular dysfunction Normal histology and function Complications No increased incidence of Chronic kidney disease Diabetes Urinary tract infection Familial renal glucosuria is a genetic condition that serves as a model for the effects of SGLT2 inhibition. Patients with this condition are asymptomatic but have impaired functioning of their SGLT2 proteins. As a result, they excrete between a few to ~100 g of glucose per day in their urine.1,2 In other respects, these individuals are normal. Blood glucose concentration is neither high nor low, and blood volume remains essentially normal due to sodium reabsorption through other channels. Kidney and bladder function remain unaffected, and this group of patients does not show an increased incidence of kidney disease, diabetes, or urinary tract infections.1,2 Wright EM, Hirayama BA, Loo DF. Active sugar transport in health and disease. J Intern Med. 2007;261:32-43. Santer R, Kinner M, Lassen CL, et al. Molecular analysis of the SGLT2 gene in patients with renal glucosuria. J Am Soc Nephrol. 2003;14:2873-2882. Santer R, et al. J Am Soc Nephrol. 2003;14:2873-2882; Wright EM, et al. J Intern Med. 2007;261:32-43. 3 3

Familial Renal Glucosuria Theoretical Normal Observed Type B Glucose Reabsorption Type A This slide shows a schematic of glucose reabsorption in normal individuals and those with familial renal glucosuria. As mentioned, the glucose transport maximum (TmG) is approximately 11 mmol/L (198 mg/dL), although the actual threshold is less abrupt (dashed yellow line) and starts to taper off at a glucose concentration of about 10 mmol/L (180 mg/dL).1 Familial renal glucosuria can be divided into 2 main types. In type A, the TmG is lower than normal (orange line). These patients have reduced levels of the SGLT2 protein.2 In type B familial renal glucosuria, the SGLT2 protein has a diminished affinity for glucose, which results in an exaggerated splay (or the difference between the theoretical and actual glucose reabsorption threshold) but a normal TmG (green line).2 Ganong WF. Review of Medical Physiology. 19th ed. Stamford, CT: Appleton & Lange; 1999:667-695. Santer R, Kinner M, Lassen CL, et al. Molecular analysis of the SGLT2 gene in patients with renal glucosuria. J Am Soc Nephrol. 2003;14:2873-2882. 5 10 15 Plasma Glucose Concentration (mmol/L) Santer R, et al. J Am Soc Nephrol. 2003;14:2873-2882. 4 4

Analysis of SGLT2 Gene in Patients With Renal Glucosuria 23 families analyzed for mutations In 23 families, 21 different mutations were detected in SGLT2 Cause of glucosuria in other 2 families remains unknown The genetics of familial renal glucosuria have been studied in 23 families with the disorder, and 21 different mutations in the gene for SGLT2 were detected. Fourteen of 21 individuals were homozygous or compound heterozygous and had severe glucosuria of 15 to 200 g/day. Heterozygous family members had either no glucosuria or mild glucosuria of ≤4.4 g/day. In addition, various nonsense mutations, missense mutations, and small deletions were scattered over the SGLT2 coding sequence. The cause of glucosuria in the 2 remaining families remains unknown but may relate to mutations in GLUT2, the glucose transport protein residing on the basolateral membrane; HNF-1, which regulates SGLT2 transcription; or the genes for SGLT1 or SGLT3. Santer R, Kinner M, Lassen CL, et al. Molecular analysis of the SGLT2 gene in patients with renal glucosuria. J Am Soc Nephrol. 2003;14:2873-2882. Santer R, et al. J Am Soc Nephrol. 2003;14:2873-2882. 5 5

Increased Glucose Transporter Proteins and Activity in Type 2 Diabetes SGLT2 GLUT2 AMG Uptake 8 P<0.05 2000 P<0.05 6 1500 Type 2 diabetes is associated with increases in renal glucose transporter expression and activity. Human exfoliated proximal tubular epithelial cells (HEPTECs), which can be isolated from urine, continue to express a variety of proximal tubular markers, including SGLT2 through several subsequent subcultures. In this study, HEPTECs isolated from individuals with NGT and type 2 diabetes were cultured in a hyperglycemic environment. As shown in the left graph, the cells from the type 2 diabetes patients expressed significantly more SGLT2 and GLUT2 proteins than cells from NGT individuals. In addition, renal glucose uptake, measured using the glucose analogue methyl-α-D-[U14C]-glucopyranoside (AMG), was significantly greater in the type 2 diabetes HEPTECs than the NGT cells. Rahmoune H, Thompson PW, Ward JM, Smith CD, Hong G, Brown J. Glucose transporters in human renal proximal tubular cells isolated from the urine of patients with non-insulin-dependent diabetes. Diabetes. 2005;54:3427-3434. Normalized Glucose Transporter Levels CPM 4 1000 P<0.05 2 500 NGT T2DM NGT T2DM NGT T2DM Rahmoune H, et al. Diabetes. 2005;54:3427-3434. 6

Implications An adaptive response to conserve glucose (ie, for energy needs) becomes maladaptive in diabetes Moreover, the ability of the diabetic kidney to conserve glucose may be augmented in absolute terms by an increase in the renal reabsorption of glucose Together, all the findings described on the preceding slides show that the conservation of glucose through the renal system to help meet energy needs between meals—an adaptive response that has evolved over eons—becomes maladaptive in diabetes. In diabetes, renal glucose reabsorption may be augmented in absolute terms by an increase in the renal Tm for glucose. 7 7

SGLT2 Inhibitors for the Treatment of Type 2 Diabetes 8

Effect of SGLT2 Inhibition on Renal Glucose Handling TmG Splay Glucose Reabsorption and Excretion Reabsorption Excretion SGLT2 inhibition may reduce plasma glucose levels by decreasing TmG, increasing the glucose excretion rate, or both. In normal animals, SGLT2 inhibition has no effect on plasma glucose concentration, because the liver increases glucose production to compensate for glycosuria. In diabetic animals, however, administration of an SGLT2 inhibitor produces both dose-dependent glycosuria and a significant reduction in plasma glucose concentrations. In essence, SGLT2 inhibition “resets” the system by lowering the threshold for glucosuria; consequently, plasma glucose levels decrease and glucotoxicity declines. Abdul-Ghani MA. Inhibition of renal glucose absorption: a novel strategy for achieving glucose control in type 2 diabetes mellitus. Endocr Pract. 2008;14:782-790. Actual Threshold Theoretical threshold 5 10 15 Plasma Glucose Concentration (mmol/L) 9 9