Mistargeting of Peroxisomal EHHADH and Inherited Renal Fanconi's Syndrome

Abstract

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 (1H-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. 1H-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.)

Sporadic Isolated Fanconi Syndrome due to a Mutation of EHHADH: A Case Report

“…To the best of our knowledge, this is the first descripenzyme to the mitochondria where it causes mitochondrial dysfunction and RFS [3].…”

“…Linkage analysis identified a significant locus on chromosome 3q27 (LOD score > 3). Following the sequencing of all coding genes in the region of interest, a novel heterozygous mutation in EH-HADH was found to segregate in all affected individuals [3]. All nine affected individuals in this kindred had normal glomerular function for at least the first six decades.…”

“…This dominant negative model is supported by studies concluding that EHHADH knockout mice are viable and fertile, with apparently preserved mitochondrial function sufficient for adequate growth and development. Moreover, no glycosuria, phosphaturia or aminoaciduria was observed in the knockout mice, nor elevated levels of glycolytic intermediates [3,8].…”

“…In the only previous report of RFS due to the same mutation of EHHADH, glomerular function was well preserved. There are therefore grounds for optimism regarding the outlook for renal function in this patient [3]. Furthermore, dominant negative mutations are better therapeutic targets than loss of function mutations, offering the potential for personalized specific gene-therapy [11].…”

“…The clinical picture was dominated by phosphate wasting resulting from homozygous mutations of SLC34A1, the gene coding for NaPi-IIa, the proximal tubular phosphate transporter [2]. In 2014, Klootwijk, et al described a family with autosomal dominant Fanconi Syndrome due to a mutation in EHHADH, an enzyme involved in peroxisomal fatty acid oxidation [3]. We describe the presentation and investigation of a child with sporadic RFS with the same mutation in EHHADH.…”