Renal Fanconi syndrome is characterized by reduced transport capacity of the proximal tubule and is accompanied by a urinary loss of glucose, amino acids, phosphate, and small molecular weight proteins. Genetic investigation of a family with autosomal dominant inherited Fanconi syndrome revealed a mutation in a peroxisomal gene that we named Fanconi Associated Protein (FAP). The N-terminal mutation leads to the formation of a mitochondrial targeting sequence and subsequent mistargeting of FAP. This study aims at investigating mitochondrial respiration in an inducible proximal tubular cell line overexpressing either wildtype FAP or mutated FAP.

To investigate the functional relevance of this mislocalization and to evaluate the activity of the respiratory chain we performed high-resolution respirometry to measure O₂ consumption of these cell lines. These measurements showed reduced oxidative phosphorylation in cells with mutated FAP upon stimulation. This effect was even more pronounced when octanoyl-carnitine, a specific substrate of mitochondrial β-oxidation, was added. This protocol allowed a more accurate evaluation of the mitochondrial function due to the added contribution of reduction equivalents from β-oxidation to mitochondrial respiration. Furthermore, measurement of metabolites in the cell medium showed an increased lactate/pyruvate ratio in cell expressing mutated FAP pointing to an elevated rate of glycolysis instead of oxidative phosphorylation.

In summary, our results support the hypothesis that the mistargeting of mutated FAP into the mitochondria is responsible for the Fanconi phenotype in these patients. Apparently, impaired oxidative phosphorylation leads to reduced ATP production with a subsequent decrease in the reabsorption capacity of the proximal tubule.