Glucosuria, in the absence of diabetes mellitus, can occur in the setting of global dysfunction of the proximal tubule (PT), known as Renal Fanconi Syndrome. The latter is typically accompanied by excessive urinary excretion of amino acids, phosphate and bicarbonate. The occurrence of glucosuria in the absence of PT dysfunction and hyperglycemia is known as renal glucosuria or familial renal glucosuria (FRG) as it is recognized as an inherited disorder.
Patients with FRG have decreased renal tubular reabsorption of glucose from the urine in the absence of hyperglycemia or any other signs of tubular dysfunction. Glucosuria can range from 1 – 150 grams/1.73m2 per day. The majority of affected patients does not develop significant clinical problems and individuals are typically picked up during routine clinical tests. However, higher levels of glucosuria lead to osmotic diuresis and could cause volume contraction when access to fluid is limited.
Both, autosomal recessive and dominant pattern of inheritance for FRG have been reported. Mutations in SCL5A2 (encoding SGLT2) account for the majority if not all families with this condition. So far ~44 different mutations in 14 exons have been reported for FRG. Establishing a definite genotype-phenotype correlation has been difficult due to variable expressivity of SGLT2 and other genes that may have an overall impact on renal glucose reabsorption. In general, heterozygous mutations lead likely to mild glucosuria Here’s a nice review.
In summary, FRG is a relatively benign condition nephrologists should be familiar with, especially with the availability of Dapagliflozin, a new drug for treatment of diabetes mellitus. First trials look promising since inhibiting SGLT2 appears effective and relatively safe.