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BACKGROUND: The endocytic reabsorption of proteins in the proximal tubule requires a complex machinery and defects can lead to tubular proteinuria. The precise mechanisms of endocytosis and processing of receptors and cargo are incompletely understood. EHD1 belongs to a family of proteins presumably involved in the scission of intracellular vesicles and in ciliogenesis. However, the relevance of EHD1 in human tissues, in particular in the kidney, was unknown.METHODS: Genetic techniques were used in patients with tubular proteinuria and deafness to identify the disease-causing gene. Diagnostic and functional studies were performed in patients and disease models to investigate the pathophysiology.RESULTS: We identified six individuals (5-33 years) with proteinuria and a high-frequency hearing deficit associated with the homozygous missense variant c.1192C>T (p.R398W) in EHD1. Proteinuria (0.7-2.1 g/d) consisted predominantly of low molecular weight proteins, reflecting impaired renal proximal tubular endocytosis of filtered proteins. Ehd1 knockout and Ehd1R398W/R398W knockin mice also showed a high-frequency hearing deficit and impaired receptor-mediated endocytosis in proximal tubules, and a zebrafish model showed impaired ability to reabsorb low molecular weight dextran. Interestingly, ciliogenesis appeared unaffected in patients and mouse models. In silico structural analysis predicted a destabilizing effect of the R398W variant and possible inference with nucleotide binding leading to impaired EHD1 oligomerization and membrane remodeling ability.CONCLUSIONS: A homozygous missense variant of EHD1 causes a previously unrecognized autosomal recessive disorder characterized by sensorineural deafness and tubular proteinuria. Recessive EHD1 variants should be considered in individuals with hearing impairment, especially if tubular proteinuria is noted.

Background For many patients with kidney failure, the cause and underlying defect remain unknown. Here, we describe a novel mechanism of a genetic order characterized by renal Fanconi syndrome and kidney failure. Methods We clinically and genetically characterized members of five families with autosomal dominant renal Fanconi syndrome and kidney failure. We performed genome-wide linkage analysis, sequencing, and expression studies in kidney biopsy specimens and renal cells along with knockout mouse studies and evaluations of mitochondrial morphology and function. Structural studies examined the effects of recognized mutations. Results The renal disease in these patients resulted from monoallelic mutations in the gene encoding glycine amidinotransferase (GATM), a renal proximal tubular enzyme in the creatine biosynthetic pathway that is otherwise associated with a recessive disorder of creatine deficiency. In silico analysis showed that the particular GATM mutations, identified in 28 members of the five families, create an additional interaction interface within the GATM protein and likely cause the linear aggregation of GATM observed in patient biopsy specimens and cultured proximal tubule cells. GATMaggregates-containing mitochondria were elongated and associated with increased ROS production, activation of the NLRP3 inflammasome, enhanced expression of the profibrotic cytokine IL-18, and increased cell death. Conclusions In this novel genetic disorder, fully penetrant heterozygous missense mutations in GATM trigger intramitochondrial fibrillary deposition of GATM and lead to elongated and abnormal mitochondria. We speculate that this renal proximal tubular mitochondrial pathology initiates a response from the inflammasome, with subsequent development of kidney fibrosis.

Steroid-sensitive nephrotic syndrome is the most common form of nephrotic syndrome in childhood, defined by the response to treatment with glucocorticoids with consequent remission. While most children eventually experience spontaneous resolution of the disease, some have a difficult course with frequent relapses or steroid dependence nephrotic syndrome (FRSDNS). The consequent steroid toxicity often prompts administration of other immunosuppressive drugs, traditionally cyclophosphamide. Recently, rituximab has been reported as effective in this disorder, but long-term experience is lacking. Retrospective note review of all children with FRSDNS treated with a first course of cyclophosphamide and/or rituximab in our center between December 2006 and April 2015. We reviewed time to first relapse after treatment, co-medications, and side effects. A total of 102 children were treated with cyclophosphamide (79) and/or rituximab (42). Of these, 34 received cyclophosphamide prior to rituximab. Median time to first relapse was 7 months after cyclophosphamide and 14 months after rituximab. Documented side effects of cyclophosphamide included neutropenia, hair loss, and hemorrhagic cystitis (1). Rituximab was associated with an allergic reaction at infusion in two patients. Rituximab was used in children with the most difficult to treat FRSDNS, yet was associated with longer remission time and less side effects than cyclophosphamide. A randomized controlled trial is needed to directly compare these drugs.

Mutations in HNF1B, the gene encoding hepatocyte nuclear factor 1β are the most commonly identified genetic cause of renal malformations. HNF1B was first identified as a disease gene for diabetes (MODY5) in 1997, and its involvement in renal disease was subsequently noted through clinical observations in pedigrees affected by MODY5. Since then, a whole spectrum of associated phenotypes have been reported, including genital malformations, autism, epilepsy, gout, hypomagnesaemia, primary hyperparathyroidism, liver and intestinal abnormalities and a rare form of kidney cancer. The most commonly identified mutation, in approximately 50 % of patients, is an entire gene deletion occurring in the context of a 17q12 chromosomal microdeletion that also includes several other genes. Some of the associated phenotypes, especially the neurologic ones, appear to occur only in the context of this microdeletion and thus may not be directly linked to HNF1B. Here we review the spectrum of associated phenotypes and discuss potential implications for clinical management.

Background/Aims: Calcium homeostasis requires regulated cellular and interstitial systems interacting to modulate the activity and movement of this ion. Disruption of these systems in the kidney results in nephrocalcinosis and nephrolithiasis, important medical problems whose pathogenesis is incompletely understood. Methods: We investigated 25 patients from 16 families with unexplained nephrocalcinosis and characteristic dental defects (amelogenesis imperfecta, gingival hyperplasia, impaired tooth eruption). To identify the causative gene, we performed genome-wide linkage analysis, exome capture, next-generation sequencing, and Sanger sequencing. Results: All patients had bi-allelic FAM20A mutations segregating with the disease; 20 different mutations were identified. Conclusions: This au-tosomal recessive disorder, also known as enamel renal syndrome, of FAM20A causes nephrocalcinosis and amelogenesis imperfecta. We speculate that all individuals with biallelic FAM20A mutations will eventually show nephrocalcinosis.

Renal fibromuscular dysplasia (FMD) can cause hypertension, and previous reports suggest that FMD is familial. We hypothesized that, in families containing an individual with proven FMD, relatives of index cases would have an increased risk of hypertension. ACTA2 mutations cause a spectrum of extra-renal arteriopathy, leading to our second hypothesis that mutations are implicated in FMD. The blood pressure of first-degree relatives was measured using standard devices and, when indicated, with 24-h ambulatory monitoring. Leucocyte DNA was obtained from FMD index cases and ACTA2 sequenced. Thirteen unrelated index cases, aged 2-32 (median 15) years, were recruited. Blood pressure was assessed in 40 first-degree relatives, comprising 22 parents aged 28-58 (median 44) years and 18 siblings aged 3-30 (median 13) years. Hypertension was evident in six (27%) parents but in none of the eight adult siblings. Of the ten screened siblings aged less than 18 years, one teenager was pre-hypertensive (90th-95th centile), the remainder being normotensive. No ACTA2 mutations were found in 13 index cases. Hypertension was evident in 20% of all assessed adult first-degree relatives and is therefore not increased relative to 25% of the adult population. Although hypertensive parents did not undergo angiography to assign FMD status, this observation, together with the lack of hypertension in 18 siblings, indicates that FMD is unlikely to confer an excess hypertension risk in first-degree relatives up to middle-age. Furthermore, in our cohort, FMD was not caused by ACTA2 mutations.

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is characterized by rickets, hyperphosphaturia, hypophosphatemia, elevated 1,25-dihydroxyvitamin-D, increased gastrointestinal calcium absorption and hypercalciuria. Serum calcium, 25-hydroxyvitamin-D and PTH levels are normal. Here we describe a boy with HHRH, nephrolithiasis, and compound heterozygosity for one previously described mutation (g.4225_50del) and a novel splice mutation (g.1226G>A) in SLC34A3, the gene encoding the renal sodium-phosphate co-transporter NaPi-IIc. The patient’s mother and grandmother are carriers of g.4225_50del, and both have a history of nephrolithiasis associated with hypercalciuria and elevated 1,25-dihydroxyvitamin-D. His three siblings (2–6 years old), who are also carriers of g.4225_50del, have hypercalciuria but so far their renal ultrasounds are normal. Thus, SLC34A3/NaPi-IIc mutations appear to be associated with variable phenotypic changes at presentation, which can include recurrent nephrolithiasis.

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.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.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.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.).

To determine if there was a genetic contribution to our patient's unusual clinical presentation of nephrolithiasis and nonhealing stress fracture.We describe a 31-year-old man who had rickets as a child and developed a femur insufficiency fracture and recurrent nephrolithiasis as an adult after moving to the United States from India. The patient's clinical course and results from radiographic and biochemical analyses are described. Analysis of the SLC34A3 gene was performed using genomic DNA samples from the patient and his family members.Before referral to the Yale Bone Center, the patient was treated with calcitriol, ergocalciferol, and phosphate. Changing therapy to phosphate alone led to clinical improvement. Genetic analysis revealed that the patient is a compound heterozygote for mutations in the SLC34A3 gene. On 1 allele, he has a previously described missense mutation in exon 7: c.575CT (p.Ser192Leu). The other allele carries a novel nonsense mutation in exon 3: c.145CT (p.Gln49X). One unaffected sibling is a carrier of the missense mutation and 1 sister with a history of flank pain is a carrier of the novel mutation.Hereditary hypophosphatemic rickets with hypercalciuria is a rare metabolic disorder associated with mutations in SLC34A3, the gene that encodes the renal sodium phosphate cotransporter NaPi-IIc. Although hypercalciuria is a distinguishing feature of the disease, nephrolithiasis is rarely described. The patient's atypical clinical presentation illustrates that both environmental and genetic factors potentially affect phenotypic expression of SLC34A3 mutations.

A 5-year-old boy was referred for treatment due to a longstanding history of polyuria/polydipsia of more than 3 liters per day and failure-to-thrive. He woke up three to four times during the night to drink. The mother reported feeding problems, including vomiting since he was 5 months old. The results of biochemical tests at 13 months of age had been normal, with plasma sodium of 143 mmol/l and creatinine of 36 μmol/l. His family history was remarkable for a 1-year-old brother having similar symptoms. In addition, a maternal uncle had a longstanding history of polyuria. Physical examination was unremarkable. The boy had a healthy appearance, his height and weight were at the 2nd percentile and blood pressure was 74/52 mmHg. Repeat renal function biochemistry results were all in the normal range, with sodium at 142 mmol/l and creatinine at 43 μmol/l. Urine albumin was

FUNCTIONAL ANALYSIS OF HUMAN MUTATIONS IN NAPIIIC REVEALS IMPORTANT RESIDUES FOR SURFACE EXPRESSION AND SODIUM-PHOSPHATE CO-TRANSPORT Clemens Bergwitz1 Graciana Jaureguiberry1, Thomas O Carpenter3, Stuart Forman2, Harald Juppner1 1Endocrine Unit and 2Dept. of Anesthesiology, Massachusetts General Hospital, Boston, MA, United States; 3 Dept. of Pediatrics and Endocrine Unit, Yale School of Medicine, New Haven, CT, United States. We recently reported homozygous and compound heterozygous lossof-function mutations in SLC34A3/NaPi-IIc as the cause for hereditary hypophosphatemic rickets with hypercalciuria (HHRH). To better understand how some of the point mutations reported by us and others (T137M, S138F, S192L, G196R, R468W, del527L, R353L, D237N and A413E) lead to renal phosphate wasting, we generated expression plasmids encoding enhanced green-fluorescence protein (EGFP) concatenated to the N-terminus of wild-type or mutant human NaPi-IIc. After transient transfection in Opossum kidney cells G196R and R468W were not expressed in apical patches (the renal brush border membrane equivalent), nor were these mutants present in plasma membranes of Xenopus oocytes following cRNA injection (50 ng/oocyte). For S138F, S192L, R353L, D237N and A413E oocyte membrane fluorescence was reduced to