Lysosome-related organelles (LROs) are a heterogeneous group of vesicles that share various features with lysosomes, but are distinct in function, morphology, and composition. The biogenesis of LROs employs a common machinery, and genetic defects in this machinery can affect all LROs or only an individual LRO, resulting in a variety of clinical features. In this review, we discuss the main components in LRO biogenesis. We also address the function, composition and resident cell type of the major LROs. Finally, we describe the clinical characteristics of the major human LRO disorders.

Hartnup disorder, an autosomal recessive defect named after an English family described in 1956 (ref. 1), results from impaired transport of neutral amino acids across epithelial cells in renal proximal tubules and intestinal mucosa. Symptoms include transient manifestations of pellagra (rashes), cerebellar ataxia and psychosis 1,2 . Using homozygosity mapping in the original family in whom Hartnup disorder was discovered, we confirmed that the critical region for one causative gene was located on chromosome 5p15 (ref. 3). This region is homologous to the area of mouse chromosome 13 that encodes the sodium-dependent amino acid transporter B 0 AT1 (ref. 4). We isolated the human homolog of B 0 AT1, called SLC6A19, and determined its size and molecular organization. We then identified mutations in SLC6A19 in members of the original family in whom Hartnup disorder was discovered and of three Japanese families. The protein product of SLC6A19, the Hartnup transporter, is expressed primarily in intestine and renal proximal tubule and functions as a neutral amino acid transporter.Despite molecular characterization of other proximal tubule transporters, the neutral amino acid carrier defective in Hartnup disorder (OMIM 2345000) has resisted genetic identification 2 . We carried out homozygosity mapping and fine mapping in ten members of two consanguineous families (the siblings in whom Hartnup disorder was originally discovered 1 ; family A; Fig. 1a) and in siblings from the US 5 (family B; Fig. 1a). We found linkage of Hartnup disorder to 5p15 only in family A, with a maximum combined multipoint lod score of 2.31 at 11.24 cM (P ¼ 0.01). This confirmed our previous results showing linkage to chromosome 5p15 (ref.3). In family B, we obtained a maximum multipoint lod score of À2.40 at 15.81 cM.We simultaneously pursued two mouse monoamine transporterrelated orphan genes, Slc6a18 (also called Xtrp2; ref. 6) and Slc6a19 (encoding B 0 AT1; ref. 4). These members of the SLC6 family of transporters map to the mouse chromosomal region that is homologous to human chromosome 5p15. Both Slc6a18 and Slc6a19 showed abundant expression in mouse kidney, as assessed by real time RT-PCR (Fig. 2a). Immunohistochemistry confirmed expression of mouse B 0 AT1 at the brush border of small intestine (data not shown) and kidney proximal tubule cells (Fig. 2b).The human homolog, B 0 AT1, is encoded by the predicted locus SLC6A19, with a 2,022-bp open reading frame. PCR amplification using human kidney cDNA produced a 1,905-bp product with 100% identity to SLC6A19 sequence. We next determined the genomic organization of SLC6A19, which has a stop codon 28 bases before the ATG in the 5¢ untranslated region. SLC6A19 has 12 coding exons. The B 0 AT1 protein contains 634 amino acids and 12 predicted transmembrane regions (Fig. 1b). In a panel of human cDNAs, we detected robust expression of SLC6A19 in kidney and small intestine, with minimal expression in pancreas (Fig. 2c). SLC6A19 was also expressed in stomach, liver, duodenum and ileocecum (data n...

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.)