Mistargeting of peroxisomal EHHADH and inherited renal Fanconi's syndrome.

BACKGROUND: In renal Fanconi's syndrome, dysfunction in proximal tubular cells leads to renal losses of water, electrolytes, and low-molecular-weight nutrients. For most types of isolated Fanconi's syndrome, the genetic cause and underlying defect remain unknown. METHODS: We clinically and genetically characterized members of a five-generation black family with isolated autosomal dominant Fanconi's syndrome. We performed genomewide linkage analysis, gene sequencing, biochemical and cell-biologic investigations of renal proximal tubular cells, studies in knockout mice, and functional evaluations of mitochondria. Urine was studied with the use of proton nuclear magnetic resonance ((1)H-NMR) spectroscopy. RESULTS: We linked the phenotype of this family's Fanconi's syndrome to a single locus on chromosome 3q27, where a heterozygous missense mutation in EHHADH segregated with the disease. The p.E3K mutation created a new mitochondrial targeting motif in the N-terminal portion of EHHADH, an enzyme that is involved in peroxisomal oxidation of fatty acids and is expressed in the proximal tubule. Immunocytofluorescence studies showed mistargeting of the mutant EHHADH to mitochondria. Studies of proximal tubular cells revealed impaired mitochondrial oxidative phosphorylation and defects in the transport of fluids and a glucose analogue across the epithelium. (1)H-NMR spectroscopy showed elevated levels of mitochondrial metabolites in urine from affected family members. Ehhadh knockout mice showed no abnormalities in renal tubular cells, a finding that indicates a dominant negative nature of the mutation rather than haploinsufficiency. CONCLUSIONS: Mistargeting of peroxisomal EHHADH disrupts mitochondrial metabolism and leads to renal Fanconi's syndrome; this indicates a central role of mitochondria in proximal tubular function. The dominant negative effect of the mistargeted protein adds to the spectrum of monogenic mechanisms of Fanconi's syndrome. (Funded by the European Commission Seventh Framework Programme and others.).

Mutation analysis of the GLUT2 gene in three unrelated Egyptian families with Fanconi-Bickel syndrome: revisited gene atlas for renumbering.

BACKGROUND: Fanconi-Bickel syndrome (FBS) is an autosomal recessive disorder caused by defects in the facilitative glucose transporter 2 (GLUT2 or SLC2A2) gene which codes for the glucose transporter protein 2 expressed in hepatocytes and renal tubular cells causing a defect in carbohydrate metabolism, hepatomegaly, severe hypophosphatemic rickets and failure to thrive. SUBJECTS AND METHODS: Among 17 unrelated Egyptian families with heritable renal tubular acidosis, three families clinically suspected as FBS were enrolled for this study after providing written informed consent. The three families had positive consanguinity and index cases with characteristic clinical features of FBS (hepatorenal glycogen accumulation, glucose and galactose intolerance, fasting hypoglycemia, a characteristic tubular nephropathy). Laboratory work-up included urinalysis, renal and liver function tests, fasting and postprandial blood sugar, serum calcium, phosphorus, alkaline phosphatase, sodium and potassium, lipid profile and arterial blood gas analysis. Imaging studies included bone survey and abdominal ultrasound. Liver biopsy was performed to confirm pathological diagnosis of the liver enlargement. Molecular analysis was performed for all family members-polymerase chain reaction followed by direct sequencing of the coding segments as well as the flanking introns. RESULTS: Three different mutations were detected, one specific for each family, including two new mutations. In the first family, exon 3, two bases (GA) were deleted (c.253_254delGA causing a frameshift mutation (p. Glu85fs); the patient presented with early symptoms but unfortunately died despite adequate treatment. In the second family, a mutation was found in exon 6, in the splicing acceptor site with intron 5 (c.776-1G>C or IVS5-1G>A). The third family showed a missense mutation C-to-T substitution at c.1250 (c.1250C>T) causing change of codon 417 (CCG) for proline to CTG for leucine (p. P417L); this is a well-known mutation in the Arab population previously localized in exon 9; however, it is currently renumbered to exon 10. CONCLUSION: Neither the new mutations nor the reported one were particularly more frequent; however, the third mutation (c.1250C>T) needs more attention in survey studies especially if performed in Arab patients as it has been renumbered because of the 'change' of gene structure since the initial reports.

Two cases of Fanconi-Bickel syndrome: first report from China with novel mutations of SLC2A2 gene.

Fanconi-Bickel syndrome (FBS) is a rare inherited disease caused by mutations in the glucose transporter 2 gene, SLC2A2. We reported the first two Chinese cases of FBS. Both cases presented typical clinical features of hepatomegaly, hypophosphatemic rickets, severely stunted growth, fasting hypoglycemia along with postprandial hyperglycemia, and proximal renal tubular dysfunction with disproportionately severe glucosuria. Genetic analysis of SLC2A2 gene revealed novel compound heterozygous mutations in both patients. The characteristics of being born as small for gestational age and apparent liver dysfunction in our cases have been seldom discussed in the literature. It seems FBS patients in general have lower birth weight than normal, but further data collection is still needed. Symptomatic treatments were effective, but the serum transaminase of patient 2 remained moderately increased, and he patient needed further follow-up. The present study will supplement the up-to-date clinical characteristic spectrum for FBS.