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: 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.)

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.

The present study describes two novel compound heterozygous mutations, c.410C>T(p.T137M) (T137M) on the maternal and g.4225_50del on the paternal allele of SLC34A3, in a previously reported male with hereditary hypophosphatemic rickets with hypercalciuria (HHRH) and recurrent kidney stones (Chen C, Carpenter T, Steg N, Baron R, Anast C. Pediatrics 84: 276-280, 1989). For functional analysis in vitro, we generated expression plasmids encoding enhanced green fluorescence protein (EGFP) concatenated to the NH2 terminus of wild-type or mutant human type IIc Na-Pi cotransporter (NaPi-IIc), i.e., EGFP-hNaPi-IIc, EGFP-[M137]hNaPi-IIc, or EGFP-[Stop446]hNaPi-IIc. The V446Stop mutant showed complete loss of expression and function when assayed for apical patch expression in opossum kidney (OK) cells and sodium-dependent 33P uptake into Xenopus laevis oocytes. Conversely, EGFP-[M137]hNaPi-IIc was inserted into apical patches of OK cells and into oocyte membranes. However, when quantified by confocal microscopy, surface fluorescence was reduced to 40% compared with wild-type. After correction for surface expression, the rate of 33P uptake by oocytes mediated by EGFP-[M137]hNaPi-IIc was decreased by an additional 60%. The resulting overall reduction of function of this NaPi-IIc mutant to 16%, taken together with complete loss of expression and function of g.4225_50del(V446Stop), thus appears to be sufficient to explain the phenotype in our patient. Furthermore, the stoichiometric ratio of 22Na and 33P uptake was increased to 7.1 +/- 3.65 for EGFP-[M137]hNaPi-IIc compared with wild-type. Two-electrode studies indicate that EGFP-[M137]hNaPi-IIc is nonelectrogenic but displayed a significant phosphate-independent inward-rectified sodium current, which appears to be insensitive to phosphonoformic acid. M137 thus may uncouple sodium-phosphate cotransport, suggesting that this amino acid residue has an important functional role in human NaPi-IIc.

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. 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. 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. analysis showed that the particular 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. GATM aggregates-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. In this novel genetic disorder, fully penetrant heterozygous missense mutations in 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.

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.

Objective To determine if there was a genetic contribution to our patient’s unusual clinical presentation of nephrolithiasis and nonhealing stress fracture. Methods 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. Results Prior to referral to 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 one allele he has a previously identified missense mutation in exon 7 c.575C>T (p.S192L). The other allele carries a novel nonsense mutation in exon 3 c.145C>T (p.Q49*). 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. Conclusion The patient is a compound heterozygote for mutations in the SLC34A3 gene including a novel nonsense mutation in exon 3. His atypical clinical presentation illustrates that both environmental and genetic factors potentially affect phenotypic expression of SLC34A3 mutation.

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.