Fanconi-Bickel syndrome: GLUT2 mutations associated with a mild phenotype

Fanconi-Bickel syndrome (FBS, OMIM #227810), a congenital disorder of carbohydrate metabolism, is caused by mutations in GLUT2 (SLC2A2), the gene encoding the glucose transporter protein-2. The typical clinical picture is characterized by hepatorenal glycogen accumulation resulting in hepato- and nephromegaly, impaired utilization of glucose and galactose, proximal tubular nephropathy, rickets, and severe short stature. We report on two siblings with FBS and an unusually mild clinical course. A 9.5-year-old boy with failure to thrive was diagnosed at the age of 9 months, his younger sister (4.5 years) was investigated in the first months of life and also diagnosed with FBS. Both patients were found to be compound heterozygous for the novel GLUT2 (SLC2A2) mutations c.457_462delCTTATA (p.153_4delLI) and c.1250C>G (p.P417R). On a diet restricted in free glucose and galactose, both children showed normal growth. Hepatomegaly, nephromegaly and hypophosphatemic rickets have never been observed. Glucosuria and tubular proteinuria were only mild compared to previously reported patients with FBS. This report describes an unusually mild phenotype of FBS expanding the spectrum of this disease. Some clinical signs that have been considered hallmarks of FBS like hepatomegaly and short stature may be absent in this condition. As a consequence, clinicians will have to look for GLUT2 mutations even in patients with isolated glucosuria.