Localization of a gene for autosomal recessive distal renal tubular acidosis with normal hearing (rdRTA2) to 7q33-34

Monogenic features of urolithiasis: A comprehensive review

Objective

Urolithiasis formation has been attributed to environmental and dietary factors. However, evidence is accumulating that genetic background can contribute to urolithiasis formation. Advancements in the identification of monogenic causes using high-throughput sequencing technologies have shown that urolithiasis has a strong heritable component.

Methods

This review describes monogenic factors implicated in a genetic predisposition to urolithiasis. Peer-reviewed journals were evaluated by a PubMed search until July 2023 to summarize disorders associated with monogenic traits, and discuss clinical implications of identification of patients genetically susceptible to urolithiasis formation.

Results

Given that more than 80% of urolithiases cases are associated with calcium accumulation, studies have focused mainly on monogenetic contributors to hypercalciuric urolithiases, leading to the identification of receptors, channels, and transporters involved in the regulation of calcium renal tubular reabsorption. Nevertheless, available candidate genes and linkage methods have a low resolution for evaluation of the effects of genetic components versus those of environmental, dietary, and hormonal factors, and genotypes remain undetermined in the majority of urolithiasis formers.

Conclusion

The pathophysiology underlying urolithiasis formation is complex and multifactorial, but evidence strongly suggests the existence of numerous monogenic causes of urolithiasis in humans.

Shared features in ear and kidney development - implications for oto-renal syndromes

The association between ear and kidney anomalies has long been recognized. However, little is known about the underlying mechanisms. In the last two decades, embryonic development of the inner ear and kidney has been studied extensively. Here, we describe the developmental pathways shared between both organs with particular emphasis on the genes that regulate signalling cross talk and the specification of progenitor cells and specialised cell types. We relate this to the clinical features of oto-renal syndromes and explore links to developmental mechanisms.

Hearing Loss Related to Gene Mutations in Distal Renal Tubular Acidosis

INTRODUCTION

Distal renal tubular acidosis (dRTA) is a disease that may develop either primarily or secondarily, resulting from urinary acidification defects in distal tubules. Hearing loss may accompany primary forms of dRTA. This study aims to determine the characteristics of hearing loss due to different gene mutations in patients with dRTA.

METHODS

Behavioral and electrophysiological audiological evaluations were performed after otolaryngology examination in 21 patients with clinically diagnosed dRTA. Radiological imaging of the inner ear (n = 9) was conducted and results of genetic analyses using next-generation sequencing method (n = 16) were included.

RESULTS

Twenty-one patients with dRTA from 20 unrelated families, aged between 8 months and 33 years (median = 12, interquartile range = 20), participated. All patients with ATP6V1B1 mutations (n = 9) had different degrees of hearing loss. There was one patient with hearing loss in patients with ATP6V0A4 mutations (n = 6). One patient with the WDR72 mutation had normal hearing. Large vestibular aqueduct syndrome (LVAS) was detected in 6 (67%) of 9 patients whose radiological evaluation results were available.

CONCLUSIONS

LVAS is common in patients with dRTA and may influence the type and severity of hearing loss in these patients. The possibility of both congenital and late-onset and progressive hearing loss should be considered in dRTA patients. A regular audiological follow-up is essential for the early detection of a possible late-onset or progressive hearing loss in these patients.

Genetics of kidney stone disease

Kidney stone disease (nephrolithiasis) is a common problem that can be associated with alterations in urinary solute composition including hypercalciuria. Studies suggest that the prevalence of monogenic kidney stone disorders, including renal tubular acidosis with deafness, Bartter syndrome, primary hyperoxaluria and cystinuria, in patients attending kidney stone clinics is ∼15%. However, for the majority of individuals, nephrolithiasis has a multifactorial aetiology involving genetic and environmental factors. Nonetheless, the genetic influence on stone formation in these idiopathic stone formers remains considerable and twin studies estimate a heritability of >45% for nephrolithiasis and >50% for hypercalciuria. The contribution of polygenic influences from multiple loci have been investigated by genome-wide association and candidate gene studies, which indicate that a number of genes and molecular pathways contribute to the risk of stone formation. Genetic approaches, studying both monogenic and polygenic factors in nephrolithiasis, have revealed that the following have important roles in the aetiology of kidney stones: transporters and channels; ions, protons and amino acids; the calcium-sensing receptor (a G protein-coupled receptor) signalling pathway; and the metabolic pathways for vitamin D, oxalate, cysteine, purines and uric acid. These advances, which have increased our understanding of the pathogenesis of nephrolithiasis, will hopefully facilitate the future development of targeted therapies for precision medicine approaches in patients with nephrolithiasis.

Molecular Pathophysiology of Acid-Base Disorders

Acid-base balance is critical for normal life. Acute and chronic disturbances impact cellular energy metabolism, endocrine signaling, ion channel activity, neuronal activity, and cardiovascular functions such as cardiac contractility and vascular blood flow. Maintenance and adaptation of acid-base homeostasis are mostly controlled by respiration and kidney. The kidney contributes to acid-base balance by reabsorbing filtered bicarbonate, regenerating bicarbonate through ammoniagenesis and generation of protons, and by excreting acid. This review focuses on acid-base disorders caused by renal processes, both inherited and acquired. Distinct rare inherited monogenic diseases affecting acid-base handling in the proximal tubule and collecting duct have been identified. In the proximal tubule, mutations of solute carrier 4A4 (SLC4A4) (electrogenic Na⁺/HCO₃^--cotransporter Na⁺/bicarbonate cotransporter e1 [NBCe1]) and other genes such as CLCN5 (Cl^-/H⁺-antiporter), SLC2A2 (GLUT2 glucose transporter), or EHHADH (enoyl-CoA, hydratase/3-hydroxyacyl CoA dehydrogenase) causing more generalized proximal tubule dysfunction can cause proximal renal tubular acidosis resulting from bicarbonate wasting and reduced ammoniagenesis. Mutations in adenosine triphosphate ATP6V1 (B1 H⁺-ATPase subunit), ATPV0A4 (a4 H⁺-ATPase subunit), SLC4A1 (anion exchanger 1), and FOXI1 (forkhead transcription factor) cause distal renal tubular acidosis type I. Carbonic anhydrase II mutations affect several nephron segments and give rise to a mixed proximal and distal phenotype. Finally, mutations in genes affecting aldosterone synthesis, signaling, or downstream targets can lead to hyperkalemic variants of renal tubular acidosis (type IV). More common forms of renal acidosis are found in patients with advanced stages of chronic kidney disease and are owing, at least in part, to a reduced capacity for ammoniagenesis.

Whole exome sequencing identified ATP6V1C2 as a novel candidate gene for recessive distal renal tubular acidosis

Distal renal tubular acidosis is a rare renal tubular disorder characterized by hyperchloremic metabolic acidosis and impaired urinary acidification. Mutations in three genes (ATP6V0A4, ATP6V1B1 and SLC4A1) constitute a monogenic causation in 58-70% of familial cases of distal renal tubular acidosis. Recently, mutations in FOXI1 have been identified as an additional cause. Therefore, we hypothesized that further monogenic causes of distal renal tubular acidosis remain to be discovered. Panel sequencing and/or whole exome sequencing was performed in a cohort of 17 families with 19 affected individuals with pediatric onset distal renal tubular acidosis. A causative mutation was detected in one of the three "classical" known distal renal tubular acidosis genes in 10 of 17 families. The seven unsolved families were then subjected to candidate whole exome sequencing analysis. Potential disease causing mutations in three genes were detected: ATP6V1C2, which encodes another kidney specific subunit of the V-type proton ATPase (1 family); WDR72 (2 families), previously implicated in V-ATPase trafficking in cells; and SLC4A2 (1 family), a paralog of the known distal renal tubular acidosis gene SLC4A1. Two of these mutations were assessed for deleteriousness through functional studies. Yeast growth assays for ATP6V1C2 revealed loss-of-function for the patient mutation, strongly supporting ATP6V1C2 as a novel distal renal tubular acidosis gene. Thus, we provided a molecular diagnosis in a known distal renal tubular acidosis gene in 10 of 17 families (59%) with this disease, identified mutations in ATP6V1C2 as a novel human candidate gene, and provided further evidence for phenotypic expansion in WDR72 mutations from amelogenesis imperfecta to distal renal tubular acidosis.

Distal renal tubular acidosis: genetic causes and management

BACKGROUND

Distal renal tubular acidosis (dRTA) is a kidney tubulopathy that causes a state of normal anion gap metabolic acidosis due to impairment of urine acidification. This review aims to summarize the etiology, pathophysiology, clinical findings, diagnosis and therapeutic approach of dRTA, with emphasis on genetic causes of dRTA.

DATA SOURCES

Literature reviews and original research articles from databases, including PubMed and Google Scholar. Manual searching was performed to identify additional studies about dRTA.

RESULTS

dRTA is characterized as the dysfunction of the distal urinary acidification, leading to metabolic acidosis. In pediatric patients, the most frequent etiology of dRTA is the genetic alteration of genes responsible for the codification of distal tubule channels, whereas, in adult patients, dRTA is more commonly secondary to autoimmune diseases, use of medications and uropathies. Patients with dRTA exhibit failure to thrive and important laboratory alterations, which are used to define the diagnosis. The oral alkali and potassium supplementation can correct the biochemical defects, improve clinical manifestations and avoid nephrolithiasis and nephrocalcinosis.

CONCLUSIONS

dRTA is a multifactorial disease leading to several clinical manifestations. Clinical and laboratory alterations can be corrected by alkali replacement therapy.

Renal Tubular Acidosis

Renal tubular acidosis should be suspected in poorly thriving young children with hyperchloremic and hypokalemic normal anion gap metabolic acidosis, with/without syndromic features. Further workup is needed to determine the type of renal tubular acidosis and the presumed etiopathogenesis, which informs treatment choices and prognosis. The risk of nephrolithiasis and calcinosis is linked to the presence (proximal renal tubular acidosis, negligible stone risk) or absence (distal renal tubular acidosis, high stone risk) of urine citrate excretion. New formulations of slow-release alkali and potassium combination supplements are being tested that are expected to simplify treatment and lead to sustained acidosis correction.

Hypokalemic Distal Renal Tubular Acidosis

Distal renal tubular acidosis (DRTA) is defined as hyperchloremic, non-anion gap metabolic acidosis with impaired urinary acid excretion in the presence of a normal or moderately reduced glomerular filtration rate. Failure in urinary acid excretion results from reduced H⁺ secretion by intercalated cells in the distal nephron. This results in decreased excretion of NH₄⁺ and other acids collectively referred as titratable acids while urine pH is typically above 5.5 in the face of systemic acidosis. The clinical phenotype in patients with DRTA is characterized by stunted growth with bone abnormalities in children as well as nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria, hypocitraturia, and relatively alkaline urine. Hypokalemia is a striking finding that accounts for muscle weakness and requires continued treatment together with alkali-based therapies. This review will focus on the mechanisms responsible for impaired acid excretion and urinary potassium wastage, the clinical features, and diagnostic approaches of hypokalemic DRTA, both inherited and acquired.

The vacuolar-ATPase complex and assembly factors, TMEM199 and CCDC115, control HIF1α prolyl hydroxylation by regulating cellular iron levels

Hypoxia Inducible transcription Factors (HIFs) are principally regulated by the 2-oxoglutarate and Iron(II) prolyl hydroxylase (PHD) enzymes, which hydroxylate the HIFα subunit, facilitating its proteasome-mediated degradation. Observations that HIFα hydroxylation can be impaired even when oxygen is sufficient emphasise the importance of understanding the complex nature of PHD regulation. Here, we use an unbiased genome-wide genetic screen in near-haploid human cells to uncover cellular processes that regulate HIF1α. We identify that genetic disruption of the Vacuolar H+ ATPase (V-ATPase), the key proton pump for endo-lysosomal acidification, and two previously uncharacterised V-ATPase assembly factors, TMEM199 and CCDC115, stabilise HIF1α in aerobic conditions. Rather than preventing the lysosomal degradation of HIF1α, disrupting the V-ATPase results in intracellular iron depletion, thereby impairing PHD activity and leading to HIF activation. Iron supplementation directly restores PHD catalytic activity following V-ATPase inhibition, revealing important links between the V-ATPase, iron metabolism and HIFs.

Functional coupling of V-ATPase and CLC-5

Dent’s disease is an X-linked renal tubulopathy characterized by low molecular weight proteinuria, hypercalciuria and progressive renal failure. Disease aetiology is associated with mutations in the CLCN5 gene coding for the electrogenic 2Cl^-/H⁺ antiporter chloride channel 5 (CLC-5), which is expressed in the apical endosomes of renal proximal tubules with the vacuolar type H⁺-ATPase (V-ATPase). Initially identified as a member of the CLC family of Cl^- channels, CLC-5 was presumed to provide Cl^- shunt into the endosomal lumen to dissipate H⁺ accumulation by V-ATPase, thereby facilitating efficient endosomal acidification. However, recent findings showing that CLC-5 is in fact not a Cl^- channel but a 2Cl^-/H⁺ antiporter challenged this classical shunt model, leading to a renewed and intense debate on its physiological roles. Cl^- accumulation via CLC-5 is predicted to play a critical role in endocytosis, as illustrated in mice carrying an artificial Cl^- channel mutation E211A that developed defective endocytosis but normal endosomal acidification. Conversely, a recent functional analysis of a newly identified disease-causing Cl^- channel mutation E211Q in a patient with typical Dent’s disease confirmed the functional coupling between V-ATPase and CLC-5 in endosomal acidification, lending support to the classical shunt model. In this editorial, we will address the current recognition of the physiological role of CLC-5 with a specific focus on the functional coupling of V-ATPase and CLC-5.

Disorders of lysosomal acidification-The emerging role of v-ATPase in aging and neurodegenerative disease

Autophagy and endocytosis deliver unneeded cellular materials to lysosomes for degradation. Beyond processing cellular waste, lysosomes release metabolites and ions that serve signaling and nutrient sensing roles, linking the functions of the lysosome to various pathways for intracellular metabolism and nutrient homeostasis. Each of these lysosomal behaviors is influenced by the intraluminal pH of the lysosome, which is maintained in the low acidic range by a proton pump, the vacuolar ATPase (v-ATPase). New reports implicate altered v-ATPase activity and lysosomal pH dysregulation in cellular aging, longevity, and adult-onset neurodegenerative diseases, including forms of Parkinson disease and Alzheimer disease. Genetic defects of subunits composing the v-ATPase or v-ATPase-related proteins occur in an increasingly recognized group of familial neurodegenerative diseases. Here, we review the expanding roles of the v-ATPase complex as a platform regulating lysosomal hydrolysis and cellular homeostasis. We discuss the unique vulnerability of neurons to persistent low level lysosomal dysfunction and review recent clinical and experimental studies that link dysfunction of the v-ATPase complex to neurodegenerative diseases across the age spectrum.

A single nucleotide polymorphism in kidney anion exchanger 1 gene is associated with incomplete type 1 renal tubular acidosis

Various conditions including distal renal tubular acidosis (dRTA) can induce stone formation in the kidney. dRTA is characterized by an impairment of urine acidification in the distal nephron. dRTA is caused by variations in genes functioning in intercalated cells including SLC4A1/AE1/Band3 transcribing two kinds of mRNAs encoding the Cl⁻/HCO3⁻ exchanger in erythrocytes and that expressed in α-intercalated cells (kAE1). With the acid-loading test, 25% of urolithiasis patients were diagnosed with incomplete dRTA. In erythroid intron 3 containing the promoter region of kAE1, rs999716 SNP showed a significantly higher minor allele A frequency in incomplete dRTA compared with non-dRTA patients. The promoter regions of the kAE1 gene with the minor allele A at rs999716 downstream of the TATA box showed reduced promoter activities compared that with the major allele G. Patients with the A allele at rs999716 may express less kAE1 mRNA and protein in the intercalated cells, developing incomplete dRTA.

Acidosis and Urinary Calcium Excretion: Insights from Genetic Disorders

Metabolic acidosis is associated with increased urinary calcium excretion and related sequelae, including nephrocalcinosis and nephrolithiasis. The increased urinary calcium excretion induced by metabolic acidosis predominantly results from increased mobilization of calcium out of bone and inhibition of calcium transport processes within the renal tubule. The mechanisms whereby acid alters the integrity and stability of bone have been examined extensively in the published literature. Here, after briefly reviewing this literature, we consider the effects of acid on calcium transport in the renal tubule and then discuss why not all gene defects that cause renal tubular acidosis are associated with hypercalciuria and nephrocalcinosis.

Proteomic changes in response to crystal formation in Drosophila Malpighian tubules

Kidney stone disease is a major health burden with a complex and poorly understood pathophysiology. Drosophila Malpighian tubules have been shown to resemble human renal tubules in their physiological function. Herein, we have used Drosophila as a model to study the proteomic response to crystal formation induced by dietary manipulation in Malpighian tubules. Wild-type male flies were reared in parallel groups on standard medium supplemented with lithogenic agents: control, Sodium Oxalate (NaOx) and Ethylene Glycol (EG). Malpighian tubules were dissected after 2 weeks to visualize crystals with polarized light microscopy. The parallel group was dissected for protein extraction. A new method of Gel Assisted Sample Preparation (GASP) was used for protein extraction. Differentially abundant proteins (p<0.05) were identified by label-free quantitative proteomic analysis in flies fed with NaOx and EG diet compared with control. Their molecular functions were further screened for transmembrane ion transporter, calcium or zinc ion binder. Among these, 11 candidate proteins were shortlisted in NaOx diet and 16 proteins in EG diet. We concluded that GASP is a proteomic sample preparation method that can be applied to individual Drosophila Malpighian tubules. Our results may further increase the understanding of the pathophysiology of human kidney stone disease.

Band 3, the human red cell chloride/bicarbonate anion exchanger (AE1, SLC4A1), in a structural context

The crystal structure of the dimeric membrane domain of human Band 3(1), the red cell chloride/bicarbonate anion exchanger 1 (AE1, SLC4A1), provides a structural context for over four decades of studies into this historic and important membrane glycoprotein. In this review, we highlight the key structural features responsible for anion binding and translocation and have integrated the following topological markers within the Band 3 structure: blood group antigens, N-glycosylation site, protease cleavage sites, inhibitor and chemical labeling sites, and the results of scanning cysteine and N-glycosylation mutagenesis. Locations of mutations linked to human disease, including those responsible for Southeast Asian ovalocytosis, hereditary stomatocytosis, hereditary spherocytosis, and distal renal tubular acidosis, provide molecular insights into their effect on Band 3 folding. Finally, molecular dynamics simulations of phosphatidylcholine self-assembled around Band 3 provide a view of this membrane protein within a lipid bilayer.

Mutations in ATP6V1B1 and ATP6V0A4 genes cause recessive distal renal tubular acidosis in Mexican families

Background

Autosomal recessive distal renal tubular acidosis (dRTA) is a rare disease characterized by a hyperchloremic metabolic acidosis with normal anion gap, hypokalemia, hypercalciuria, hypocitraturia, nephrocalcinosis, and conserved glomerular filtration rate. In some cases, neurosensorial deafness is associated. dRTA is developed during the first months of life and the main manifestations are failure to thrive, vomiting, dehydration, and anorexia.

Methods

Nine unrelated families were studied: seven children, a teenager, and an adult with dRTA. Hearing was preserved in four children. Coding regions of the genes responsible for recessive dRTA were analysed by Sanger sequencing.

Results

Molecular defects were found in the genes ATP6V1B1 and ATP6V0A4. We identified three homozygous variants in ATP6V1B: a frameshift mutation (p.Ile386Hisfs*56), a nucleotide substitution in exon 10 (p.Pro346Arg), and a new splicing mutation in intron 5. Three patients were homozygous for one novel (p.Arg743Trp) and one known (p.Asp411Tyr) missense mutations in the ATP6V0A4 gene. Three patients were compound heterozygous: one proband displayed two novel mutations, the frameshift mutation p.Val52Metfs*25, and a large deletion of exons 18–21; two probands showed the missense mutation p.Asp411Tyr and as a second mutation, p.Arg194Ter and c.1691+2dup, respectively.

Conclusion

ATP6V0A4 and ATP6V1B1 genes were involved in recessive dRTA of Mexican families. All ATP6V1B1 mutations detected were homozygous and all patients developed sensorineural hearing loss (SNHL) early in infancy. ATP6V0A4 mutations were found in one infant and three children without SNHL, and in one teenager and one adult with SNHL confirming the phenotypic variability in this trait. The mutation p.Asp411Tyr detected in four Mexican families was due to a founder effect. Screening of these mutations could provide a rapid and valuable tool for diagnosis of dRTA in this population.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Autosomal dominant distal renal tubular acidosis caused by a mutation in the anion exchanger 1 gene in a Japanese family

Autosomal dominant distal renal tubular acidosis (dRTA) is a rare disorder caused by a mutation in the AE1 gene encoding the chloride-bicarbonate (Cl^-/HCO₃^-) anion exchanger 1 (AE1). Most patients with this disorder present with clinical symptoms in adulthood and their phenotype is milder than that of those with autosomal recessive dRTA. In this report, we describe a Japanese family with autosomal dominant dRTA in which the mother and her daughter presented with severe symptoms caused by hypokalemia at 2 years of age. The heterozygous AE1 mutation G609R, which is a known causative mutation of dRTA, was identified in both patients. To our knowledge, this is the first report of a Japanese family with autosomal dominant type dRTA caused by an AE1 mutation. We, therefore, propose that alterations of AE1 should be considered causative of autosomal dominant dRTA even if typical symptoms appear during early childhood and the clinical features are severe.

Collecting duct intercalated cell function and regulation

Intercalated cells are kidney tubule epithelial cells with important roles in the regulation of acid-base homeostasis. However, in recent years the understanding of the function of the intercalated cell has become greatly enhanced and has shaped a new model for how the distal segments of the kidney tubule integrate salt and water reabsorption, potassium homeostasis, and acid-base status. These cells appear in the late distal convoluted tubule or in the connecting segment, depending on the species. They are most abundant in the collecting duct, where they can be detected all the way from the cortex to the initial part of the inner medulla. Intercalated cells are interspersed among the more numerous segment-specific principal cells. There are three types of intercalated cells, each having distinct structures and expressing different ensembles of transport proteins that translate into very different functions in the processing of the urine. This review includes recent findings on how intercalated cells regulate their intracellular milieu and contribute to acid-base regulation and sodium, chloride, and potassium homeostasis, thus highlighting their potential role as targets for the treatment of hypertension. Their novel regulation by paracrine signals in the collecting duct is also discussed. Finally, this article addresses their role as part of the innate immune system of the kidney tubule.

Mutations in exons 3 and 7 resulting in truncated expression of human ATP6V1B1 gene showing structural variations contributing to poor substrate binding-causative reason for distal renal tubular acidosis with sensorineural deafness

Distal renal tubular acidosis (dRTA) is an autosomal recessive syndrome results defect in either proximal tubule bicarbonate reabsorption or in distal tubule H(+) secretion and is characterized by severe hyperchloraemic metabolic acidosis in childhood. dRTA is associated with functional variations in the ATP6V1B1 gene encoding β1 subunit of H(+)-ATPase, key membrane transporters for net acid excretion of α-intercalated cells of medullary collecting ducts. In the present study, a 13-year-old male patient suffering with nephropathy and sensorineural deafness was reported in the Department of Nephrology. We predicted improper functioning of ATP6V1B1 gene could be the reason for diseased condition. Therefore, exons 3, 4, and 7 contributing active site of ATP6V1B1 gene was amplified and sequenced (Accession numbers: KF571726, KM222653). The obtained sequences were BLAST searched against the wild type ATP6V1B1 gene which showed novel mutations c.307 A > G, c.308 C > A, c.310 C > G, c.704 T > C, c.705 G > T, c.709 A > G, c.710 A > G, c.714 G > A, c.716 C > A, c.717delC, c.722 C > G, c.728insG, c.741insT, c.753G > C. These mutations resulted in the expression of truncated protein terminating at Lys 209. The mutated ATP6V1B1structure superimposed with wild type showed extensive variations with RMSD 1.336 Å and could not bind to substrate ADP leading to non-functional ATPase. These results conclusively explain these mutations in ATP6V1B1 gene resulted in structural changes causing accumulation of H(+) ions contributing to dRTA with sensorineural deafness.

Renal acid-base regulation: new insights from animal models

Because majority of biological processes are dependent on pH, maintaining systemic acid-base balance is critical. The kidney contributes to systemic acid-base regulation, by reabsorbing HCO3 (-) (both filtered by glomeruli and generated within a nephron) and acidifying urine. Abnormalities in those processes will eventually lead to a disruption in systemic acid-base balance and provoke metabolic acid-base disorders. Research over the past 30 years advanced our understanding on cellular and molecular mechanisms responsible for those processes. In particular, a variety of transgenic animal models, where target genes are deleted either globally or conditionally, provided significant insights into how specific transporters are contributing to the renal acid-base regulation. Here, we broadly overview the mechanisms of renal ion transport participating to acid-base regulation, with emphasis on data obtained from transgenic mice models.

Investigation of ATP6V1B1 and ATP6V0A4 genes causing hereditary hearing loss associated with distal renal tubular acidosis in Iranian families

BACKGROUND

Hearing defects are the most common sensory disorders, affecting 1 out of every 500 newborns. ATP6V1B mutations are associated with early sensorineural hearing loss, whereas ATP6V0A4 mutations are classically associated with either late-onset sensorineural hearing loss or normal hearing. ATP6V1B1 and ATP6V0A4 genetic mutations cause recessive forms of distal renal tubular acidosis.

METHOD

Ten unrelated deaf Iranian families with distal renal tubular acidosis were referred to the Genetics Research Centre, University of Social Welfare and Rehabilitation Sciences, Tehran. All exons of the ATP6V1B1 and ATP6V0A4 genes were sequenced in affected family members.

RESULTS

We identified a previously reported ATP6V1B1 frameshift mutation (P385fsX441) in two families and a nucleotide substitution in exon 10 (P346R) in three families. In addition, one patient was homozygous for a novel nucleotide substitution in exon 3.

CONCLUSION

ATP6V1B1 genetic mutations were detected in more than half of the families studied. Mutations in this gene therefore seem to be the most common causative factors in hearing loss associated with distal renal tubular acidosis in these families.

Bicaudal-C spatially controls translation of vertebrate maternal mRNAs

The Xenopus Cripto-1 protein is confined to the cells of the animal hemisphere during early embryogenesis where it regulates the formation of anterior structures. Cripto-1 protein accumulates only in animal cells because cripto-1 mRNA in cells of the vegetal hemisphere is translationally repressed. Here, we show that the RNA binding protein, Bicaudal-C (Bic-C), functioned directly in this vegetal cell-specific repression. While Bic-C protein is normally confined to vegetal cells, ectopic expression of Bic-C in animal cells repressed a cripto-1 mRNA reporter and associated with endogenous cripto-1 mRNA. Repression by Bic-C required its N-terminal domain, comprised of multiple KH motifs, for specific binding to relevant control elements within the cripto-1 mRNA and a functionally separable C-terminal translation repression domain. Bic-C-mediated repression required the 5' CAP and translation initiation factors, but not a poly(A) tail or the conserved SAM domain within Bic-C. Bic-C-directed immunoprecipitation followed by deep sequencing of associated mRNAs identified multiple Bic-C-regulated mRNA targets, including cripto-1 mRNA, providing new insights and tools for understanding the role of Bic-C in vertebrate development.

Genetic causes and mechanisms of distal renal tubular acidosis

The primary or hereditary forms of distal renal tubular acidosis (dRTA) have received increased attention because of advances in the understanding of the molecular mechanism, whereby mutations in the main proteins involved in acid-base transport result in impaired acid excretion. Dysfunction of intercalated cells in the collecting tubules accounts for all the known genetic causes of dRTA. These cells secrete protons into the tubular lumen through H(+)-ATPases functionally coupled to the basolateral anion exchanger 1 (AE1). The substrate for both transporters is provided by the catalytic activity of the cytosolic carbonic anhydrase II (CA II), an enzyme which is also present in the proximal tubular cells and osteoclasts. Mutations in ATP6V1B1, encoding the B-subtype unit of the apical H(+) ATPase, and ATP6V0A4, encoding the a-subtype unit, lead to the loss of function of the apical H(+) ATPase and are usually responsible for patients with autosomal recessive dRTA often associated with early or late sensorineural deafness. Mutations in the gene encoding the cytosolic CA II are associated with the autosomal recessive syndrome of osteopetrosis, mixed distal and proximal RTA and cerebral calcification. Mutations in the AE1, the gene that encodes the Cl(-)/HCO(3)(-) exchanger, usually present as dominant dRTA, but a recessive pattern has been recently described. Several studies have shown trafficking defects in the mutant protein rather than the lack of function as the major mechanism underlying the pathogenesis of dRTA from AE1 mutations.

Tropical distal renal tubular acidosis: clinical and epidemiological studies in 78 patients

BACKGROUND

Distal renal tubular acidosis (dRTA) caused by mutations of the SLC4A1 gene encoding the erythroid and kidney isoforms of anion exchanger 1 (AE1 or band 3) has a high prevalence in some tropical countries, particularly Thailand, Malaysia, the Philippines and Papua New Guinea (PNG). Here the disease is almost invariably recessive and can result from either homozygous or compound heterozygous SLC4A1 mutations.

METHODS

We have collected and reviewed our own and published data on tropical dRTA to provide a comprehensive series of clinical and epidemiological studies in 78 patients.

RESULTS

Eight responsible SLC4A1 mutations have been described so far, four of them affecting multiple unrelated families. With the exception of the mutation causing South-East Asian ovalocytosis (SAO), none of these mutations has been reported outside the tropics, where dRTA caused by SLC4A1 mutations is much rarer and almost always dominant, resulting from mutations that are quite different from those found in the tropics. SLC4A1 mutations, including those causing dRTA, may cause morphological red cell changes, often with excess haemolysis. In dRTA, these red cell changes are usually clinically recessive and not present in heterozygotes. The high tropical prevalence of dRTA caused by SLC4A1 mutations is currently unexplained.

CONCLUSION

A hypothesis suggesting that changes in red cell metabolism caused by these mutations might protect against malaria is put forward to explain the phenomenon, and a possible mechanism for this effect is proposed.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Genetic determinants of urolithiasis

Urolithiasis affects approximately 10% of individuals in Western societies by the seventh decade of life. The most common form, idiopathic calcium oxalate urolithiasis, results from the interaction of multiple genes and their interplay with dietary and environmental factors. To date, considerable progress has been made in identifying the metabolic risk factors that predispose to this complex trait, among which hypercalciuria predominates. The specific genetic and epigenetic factors involved in urolithiasis have remained less clear, partly owing to the candidate gene and linkage methods that have been available until now, being inherently low in their power of resolution and in assessing modest effects in complex traits. However, together with investigations of rare, Mendelian forms of urolithiasis associated with various metabolic risk factors, these methods have afforded insights into biological pathways that seem to underlie the development of stones in the urinary tract. Monogenic diseases account for a greater proportion of stone formers in children and adolescents than in adults. Early diagnosis of monogenic forms of urolithiasis is of importance owing to associated renal injury and other potentially treatable disease manifestations, but diagnosis is often delayed because of a lack of familiarity with these rare disorders. In this Review, we will discuss advances in the understanding of the genetics underlying polygenic and monogenic forms of urolithiasis.

A novel SLC4A1 variant in an autosomal dominant distal renal tubular acidosis family with a severe phenotype

Mutations in SLC4A1, encoding the chloride-bicarbonate exchanger AE1, cause distal renal tubular acidosis (dRTA), a disease of defective urinary acidification by the distal nephron. We searched for SLC4A1 gene mutations in six patients from a Chinese family with a severe phenotype of dRTA (growth impairment, severe metabolic acidosis, with/or without gross nephrocalcinosis and renal impairment). All coding regions of kidney isoform of AE1, including intron-exon boundaries, were analyzed using PCR followed by direct sequence analysis. A novel 1-bp duplication at nucleotide 2713 (c.2713dupG, band 3 Qingdao) in exon 20 of SLC4A1 in this family was identified by direct sequencing analysis. This duplication alters the encoded protein through codon 905, and results in a reading frame for 15 extra condons (instead of 8) before the new stop condon at position 919 (p.Asp905Glyfs15). We suggest that RTA should be considered as a diagnostic possibility in adult subjects with nephrocalcinosis and chronic renal insufficiency, and family survey should be carefully performed.

The kidney and ear: emerging parallel functions

The association between renal dysplasia and minor malformations of the external ear is weak. However, there is a remarkable list of syndromes that link the kidney to the inner ear. To organize these seemingly disparate syndromes, we cluster representative examples into three groups: (a) syndromes that share pathways regulating development; (b) syndromes involving dysfunction of the primary cilium, which normally provides critical information to epithelial cells about the fluid in which they are bathed; (c) syndromes arising from dysfunction of specialized proteins that transport ions and drugs in and out of the extracellular fluid or provide structural support.

The calcium-sensing receptor promotes urinary acidification to prevent nephrolithiasis

Hypercalciuria increases the risk for urolithiasis, but renal adaptive mechanisms reduce this risk. For example, transient receptor potential vanilloid 5 knockout (TPRV5(-/-)) mice lack kidney stones despite urinary calcium (Ca(2+)) wasting and hyperphosphaturia, perhaps as a result of their significant polyuria and urinary acidification. Here, we investigated the mechanisms linking hypercalciuria with these adaptive mechanisms. Exposure of dissected mouse outer medullary collecting ducts to high (5.0 mM) extracellular Ca(2+) stimulated H(+)-ATPase activity. In TRPV5(-/-) mice, activation of the renal Ca(2+)-sensing receptor promoted H(+)-ATPase-mediated H(+) excretion and downregulation of aquaporin 2, leading to urinary acidification and polyuria, respectively. Gene ablation of the collecting duct-specific B1 subunit of H(+)-ATPase in TRPV5(-/-) mice abolished the enhanced urinary acidification, which resulted in severe tubular precipitations of Ca(2+)-phosphate in the renal medulla. In conclusion, activation of Ca(2+)-sensing receptor by increased luminal Ca(2+) leads to urinary acidification and polyuria. These beneficial adaptations facilitate the excretion of large amounts of soluble Ca(2+), which is crucial to prevent the formation of kidney stones.

Vacuolar H+-ATPase meets glycosylation in patients with cutis laxa

Glycosylation of proteins is one of the most important post-translational modifications. Defects in the glycan biosynthesis result in congenital malformation syndromes, also known as congenital disorders of glycosylation (CDG). Based on the iso-electric focusing patterns of plasma transferrin and apolipoprotein C-III a combined defect in N- and O-glycosylation was identified in patients with autosomal recessive cutis laxa type II (ARCL II). Disease-causing mutations were identified in the ATP6V0A2 gene, encoding the a2 subunit of the vacuolar H(+)-ATPase (V-ATPase). The V-ATPases are multi-subunit, ATP-dependent proton pumps located in membranes of cells and organels. In this article, we describe the structure, function and regulation of the V-ATPase and the phenotypes currently known to result from V-ATPase mutations. A clinical overview of cutis laxa syndromes is presented with a focus on ARCL II. Finally, the relationship between ATP6V0A2 mutations, the glycosylation defect and the ARCLII phenotype is discussed.

The effect of kidney transplantation on distal tubular vacuolar H+-ATPase

BACKGROUND

The presence of vacuolar (v)H+-ATPase in distal tubule alpha-intercalated cells is essential for hydrogen excretion and maintenance of acid-base homeostasis. Loss of vH+-ATPase after kidney transplantation could cause posttransplant distal renal tubular acidosis (dRTA).

METHOD

Immunostaining of the kidney specific vH+-ATPase of cortical collecting duct cells (CCT) was performed in 37 kidney biopsies taken immediately prior to transplantation and after engraftment (median [range]: 10 [1-181] months). Apical or intracytoplasmatic staining intensity was classified as grade 0 (absent), grade 1 (weak), or grade 2 (strong), and positive cells expressed as percentage of all CCT cells. In addition, kidney biopsies were scored for damage by the Banff schema. Serum and urinary pH, anion gap, and serum potassium were obtained for the diagnoses of dRTA.

RESULTS

Fourteen transplant recipients had dRTA type I, 5 had rate-limited RTA, six had type IV dRTA, and 12 had no RTA. In pretransplant biopsies, 40% [3-77%] of CCT cells were positive for vH+-ATPase but only 17% [0-39%] after transplantation (P<0.0001). The loss of vH+-ATPase expression was similar in patients with dRTA type I (-21%), type IV (-25%), rate limited RTA (-21%), or no RTA (-29%). The decrease affected predominantly the apical proton pump expression. The individual loss of vH+-ATPase expression was not related to the time elapsed since transplantation, immunosuppressive drugs, acute transplant rejection, or tubulointerstitial changes.

CONCLUSION

Kidney transplantation leads to a general decrease of distal tubular vH+-ATPase expression. Loss of proton pump activity occurs unrelated to immunosuppressive therapy or transplant related histologic changes.

Inherited renal acidoses

Inherited acidosis may result from a primary renal defect in acid-base handling, emphasizing the central role of the kidney in control of body pH; as a secondary phenomenon resulting from abnormal renal electrolyte handling; or from excess production of acid elsewhere in the body. Here, we review our current understanding of the inherited renal acidoses at a genetic and molecular level.

Distal renal tubular acidosis and the potassium enigma

Severe hypokalemia is a central feature of the classic type of distal renal tubular acidosis (RTA), both in hereditary and acquired forms. In the past decade, many of the genetic defects associated with the hereditary types of distal RTA have been identified and have been the subject of a number of reviews. These genetic advances have expanded our understanding of the molecular mechanisms that lead to distal RTA. In this article, we review data published in the literature on plasma potassium from patients with inherited forms of distal RTA. The degree of hypokalemia varies depending on whether the disease is autosomal autosomal-recessive or dominant, but, interestingly, it occurs in defects caused by mutations in genes encoding the AE-1 exchanger, the carbonic anhydrase II gene, and genes encoding different subunits of the H+ adenosine triphosphatase. This shows that a unique defect involving the H+/K+-adenosine triphosphatase leading to renal potassium wastage cannot explain the hypokalemia seen in virtually all types of classic distal RTA.

V1 and V0 domains of the human H+-ATPase are linked by an interaction between the G and a subunits

The specialized H(+)-ATPases found in the inner ear and acid-handling cells in the renal collecting duct differ from those at other sites, as they contain tissue-specific subunits, such as a4 and B1, and in the kidney, C2, d2, and G3 as well. These subunits replace the ubiquitously expressed forms. Previously, we have shown that, in major organs of both mouse and man, G3 subunit expression is limited to the kidney. Here we have shown wide-spread transcription of murine G3 in specific segments of microdissected nephron, and demonstrated additional G3 expression in epithelial fragments from human inner ear. We raised a polyclonal G3-specific antibody, which specifically detects G3 from human, mouse, and rat kidney lysates, and displays no cross-reactivity with G1 or G2. However, immunolocalization using this antibody on human and mouse kidney sections was unachievable, suggesting epitope masking. Phage display analysis and subsequent enzyme-linked immunosorbent assay, using the G3 antibody epitope peptide as bait, identified a possible interaction between the G3 subunit and the a4 subunit of the H(+)-ATPase. This interaction was verified by successfully using purified, immobilized full-length G3 to pull down the a4 subunit from human kidney membrane preparations. This confirms that a4 and G3 are component subunits of the same proton pump and explains the observed epitope masking. This interaction was also found to be a more general feature of human H(+)-ATPases, as similar G1/a1, G3/a1, and G1/a4 interactions were also demonstrated. These interactions represent a novel link between the V(1) and V(0) domains in man, which is known to be required for H(+)-ATPase assembly and regulation.

Kidney Vacuolar H+-ATPase: Physiology and Regulation

The vacuolar H(+)-ATPase is a multisubunit protein consisting of a peripheral catalytic domain (V(1)) that binds and hydrolyzes adenosine triphosphate (ATP) and provides energy to pump H(+) through the transmembrane domain (V(0)) against a large gradient. This proton-translocating vacuolar H(+)-ATPase is present in both intracellular compartments and the plasma membrane of eukaryotic cells. Mutations in genes encoding kidney intercalated cell-specific V(0) a4 and V(1) B1 subunits of the vacuolar H(+)-ATPase cause the syndrome of distal tubular renal acidosis. This review focuses on the function, regulation, and the role of vacuolar H(+)-ATPases in renal physiology. The localization of vacuolar H(+)-ATPases in the kidney, and their role in intracellular pH (pHi) regulation, transepithelial proton transport, and acid-base homeostasis are discussed.

Vacuolar H+-ATPase B1 Subunit Mutations that Cause Inherited Distal Renal Tubular Acidosis Affect Proton Pump Assembly and Trafficking in Inner Medullary Collecting Duct Cells

Point mutations in the B1 subunit of vacuolar H+ -ATPase are associated with impaired ability of the distal nephron to secrete acid (distal renal tubular acidosis). For testing of the hypothesis that these mutations interfere with assembly and trafficking of the H+ -ATPase, constructs that mimic seven known point mutations in inherited distal renal tubular acidosis (M) or wild-type (WT) B1 were transfected into a rat inner medullary collecting duct cell line to express green fluorescence protein (GFP)-B1WT or GFP-B1M fusion proteins. In co-immunoprecipitation studies, GFP-B1WT formed complexes with other H+ -ATPase subunits (c, H, and E), whereas GFP-B1M did not. Proteins that were immunoprecipitated with anti-GFP antibody from GFP-B1WT cells had ATPase activity, whereas proteins from GFP-B1M cells did not. Proton pump-mediated intracellular pH transport was inhibited in GFP-B1M-transfected cells but not in GFP-B1WT cells. GFP-B1WT and GFP-B1M are present in the apical membrane and increased with cellular acidification. In GFP-B1WT cells, the apical membrane fraction of GFP-B1, endogenous B1, and the 31-kD subunits of the H+ -ATPase increased with cell acidification. In GFP-B1M cells, the endogenous B1 and 31-kD subunits did not increase with acidification. B1 point mutations prevent normal assembly of the H+ -ATPase and also may act as an inhibitor of H+ -ATPase function by competing with endogenous intact H+ -ATPase for trafficking in inner medullary collecting duct cells.

Molecular investigation and long-term clinical progress in Greek Cypriot families with recessive distal renal tubular acidosis and sensorineural deafness due to mutations in the ATP6V1B1 gene

The spectrum of distal renal tubular acidosis (dRTA) includes a genetically heterogeneous group of inherited conditions of both autosomal-dominant and recessive mode of inheritance. The basic defect is linked to the renal part of acid-base homeostasis, which is partly achieved by the regulated luminal secretion of H+ at the apical surface of the alpha-intercalated cells of renal collecting ducts. This is coupled to bicarbonate reabsorption with chloride counter transport across the basolateral membranes. Here, we describe the molecular findings of the first two Greek Cypriot families with recessive dRTA and the long-term clinical findings in four of five affected members. DNA linkage analysis with four polymorphic markers flanking the ATP6V1B1 gene on chromosome 2 gave evidence for positive linkage; direct DNA analysis by automated DNA sequencing revealed that patients in one family were homozygous for mutation 229+1G>T (IVS7+1G>T) and that patients in the second family were compound heterozygous for 229+1G>T and R157C. The mutations were found on four different haplotypes. Both the mutations were previously reported in patients of Turkish origin. Three known polymorphic variants were also identified. The five patients demonstrated the whole clinical spectrum of the disease including death in infancy, failure to thrive, rickets, nephrocalcinosis, nephrolithiasis, and episodes of hypokalemic paralysis. Some of the family members are now in their mid 30s and late 20s, and nephrolithiasis with recurrent renal colics is their main problem. Renal function has remained normal. In conclusion, early diagnosis in infancy and prompt treatment with alkali and potassium supplements is of great benefit to the patient with dRTA and ensures normal growth. The identification of responsible mutations facilitates antenatal or postnatal diagnosis in concerned families and improves the prognosis.

Renal tubular acidosis: developments in our understanding of the molecular basis

Renal tubular acidosis is a metabolic acidosis due to impaired acid excretion by the kidney. Hyperchloraemic acidosis with a normal anion gap and normal (or near normal) glomerular filtration rate, and in the absence of diarrhoea, defines this disorder. However, systemic acidosis is not always evident and renal tubular acidosis can present with hypokalaemia, medullary nephrocalcinosis and recurrent calcium phosphate stone disease, as well as growth retardation and rickets in children, or short stature and osteomalacia in adults. Renal dysfunction in renal tubular acidosis is not always confined to acid excretion and can be part of a more generalised renal tubule defect, as in the renal Fanconi syndrome. Isolated renal tubular acidosis is more usually acquired, due to drugs, autoimmune disease, post-obstructive uropathy or any cause of medullary nephrocalcinosis. Less commonly, it is inherited and may be associated with deafness, osteopetrosis or ocular abnormalities. The clinical classification of renal tubular acidosis has been correlated with our current physiological model of how the nephron excretes acid, and this has facilitated genetic studies that have identified mutations in several genes encoding acid and base ion transporters. In vitro functional studies of these mutant proteins in cell expression systems have helped to elucidate the molecular mechanisms underlying renal tubular acidosis, which ultimately may lead to new therapeutic options in what is still treatment only by giving an oral alkali.

Band 3Tambau: a de novo mutation in the AE1 gene associated with hereditary spherocytosis. Implications for anion exchange and insertion into the red blood cell membrane

Hereditary spherocytosis (HS) is attributed to red blood cell membrane protein defects, caused by mutations in ankyrin, spectrin, band 3 and protein 4.2. In this study, the presence of band 3 mutations was investigated in a patient presenting mild HS and band 3 deficiency. Using single strand conformation polymorphism analysis, a shift in exon 16 of the band 3 gene was found. DNA sequencing revealed a point mutation 2102 T>C, changing methionine at position 663 to lysine. The M663K substitution was not found in either the parents or in the siblings, and the restriction fragment length polymorphism analysis of 100 alleles from a random Brazilian population did not reveal this mutation, suggesting that this gene defect is more likely to be a de novo mutation, causing HS. Flow cytometry of eosin-5-isothiocyanate (EITC)-labelled erythrocytes showed, in the patient, 54% of band 3 protein content vs. 78% based on the sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, suggesting that flow cytometry is a more sensitive method and may be used as a diagnostic tool in membrane disorders related to band 3 deficiency. The characterisation of novel AE1 mutations is helpful to improve the understanding of the role of band 3 protein in cell physiology.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Distal renal tubular acidosis

PURPOSE OF REVIEW

Research in the past several years has led to the understanding of numerous genetic mutations that lead to inheritable forms of distal renal tubular acidosis (dRTA). Most of these mutations affect the physiology of the A-intercalated cells of the renal cortical collecting duct. These include mutations of genes encoding carbonic anhydrase II, kidney anion exchanger 1, and different subunits of the H+-ATPase proton pump. Genetic defects in any one of these components may impair renal acidification and thereby result in persistent acidosis, failure to thrive, and nephrocalcinosis.

RECENT FINDINGS

The present review provides a summary of the most recently identified genetic mutations resulting in a dRTA phenotype and, when possible, describes a mechanism. Most causes of dRTA are due to loss of function or inappropriate targeting of transporters.

SUMMARY

The collaboration of clinicians, geneticists, and renal physiologists has enabled us to better understand at the cellular level the different mechanisms leading to dRTA. Such information should lead to earlier diagnosis and treatment, thereby minimizing the irreversible complications affecting patients with this or similar diseases.

A novel mutation in the anion exchanger 1 gene is associated with familial distal renal tubular acidosis and nephrocalcinosis

OBJECTIVE

The anion exchanger gene (AE1) or band 3 encodes a chloride-bicarbonate (Cl(-)/HCO(3)(-)) exchanger expressed in the erythrocyte and in the renal alpha-intercalated cells involved in urine acidification. The purpose of the present study was to screen for mutations in the AE1 gene in 2 brothers (10 and 15 years of age) with familial distal renal tubular acidosis (dRTA), nephrocalcinosis, and failure to thrive.

METHODS

AE1 mutations were screened by single-strand conformation polymorphism, cloning, and sequencing.

RESULTS

A complete form of dRTA was confirmed in the 2 affected brothers and an incomplete form in their father. All 3 were heterozygous for a novel 20-bp deletion in exon 20 of the AE1 gene. This deletion resulted in 1 mutation in codon 888 (Ala-888-->Leu) followed by a premature termination codon at position 889, truncating the protein by 23 amino acids. As band 3 deficiency might lead to spherocytic hemolytic anemia or ovalocytosis, erythrocyte abnormalities were also investigated, but no morphologic changes in erythrocyte membrane were found and the osmotic fragility test was normal.

CONCLUSIONS

A novel mutation in the AE1 gene was identified in association with autosomal dominant dRTA. We suggest that RTA be considered a diagnostic possibility in all children with failure to thrive and nephrocalcinosis.

ATP6B1 gene mutations associated with distal renal tubular acidosis and deafness in a child

A large proportion of autosomal recessive distal renal tubular acidosis (RTA) is associated with mutations in the ATP6B1 gene encoding the B1 subunit of H+-ATPase. H+-ATPase is one of the key membrane transporters for net acid excretion in the alpha-intercalated cells of the medullary collecting duct. Sensorineural hearing loss frequently accompanies this type of distal RTA. Mutational analysis of the ATP6B1 gene in a 9-year-old Korean boy with distal RTA and sensorineural hearing loss found 2 heterozygous missense point mutations. Although a single case report, this is the second report documenting ATP6B1 mutations in recessive distal RTA with sensorineural hearing loss after the original report by Karet et al and confirms the novelty of these mutations.

Novel ATP6V1B1 and ATP6V0A4 mutations in autosomal recessive distal renal tubular acidosis with new evidence for hearing loss

Autosomal recessive distal renal tubular acidosis (rdRTA) is characterised by severe hyperchloraemic metabolic acidosis in childhood, hypokalaemia, decreased urinary calcium solubility, and impaired bone physiology and growth. Two types of rdRTA have been differentiated by the presence or absence of sensorineural hearing loss, but appear otherwise clinically similar. Recently, we identified mutations in genes encoding two different subunits of the renal alpha-intercalated cell's apical H(+)-ATPase that cause rdRTA. Defects in the B1 subunit gene ATP6V1B1, and the a4 subunit gene ATP6V0A4, cause rdRTA with deafness and with preserved hearing, respectively. We have investigated 26 new rdRTA kindreds, of which 23 are consanguineous. Linkage analysis of seven novel SNPs and five polymorphic markers in, and tightly linked to, ATP6V1B1 and ATP6V0A4 suggested that four families do not link to either locus, providing strong evidence for additional genetic heterogeneity. In ATP6V1B1, one novel and five previously reported mutations were found in 10 kindreds. In 12 ATP6V0A4 kindreds, seven of 10 mutations were novel. A further nine novel ATP6V0A4 mutations were found in "sporadic" cases. The previously reported association between ATP6V1B1 defects and severe hearing loss in childhood was maintained. However, several patients with ATP6V0A4 mutations have developed hearing loss, usually in young adulthood. We show here that ATP6V0A4 is expressed within the human inner ear. These findings provide further evidence for genetic heterogeneity in rdRTA, extend the spectrum of disease causing mutations in ATP6V1B1 and ATP6V0A4, and show ATP6V0A4 expression within the cochlea for the first time.

Inherited disorders of the H+-ATPase

PURPOSE OF REVIEW

The alpha-intercalated cell in the distal nephron shares a number of molecular features with the osteoclast, including site-limited proton pumps that are present at high density. These are multisubunit H -ATPases, which are essential for acid-base homeostasis and for the maintenance of normal bone turnover. In recent years it has become evident that some rare inherited human disorders are due to pump dysfunction in kidney or in bone; these are reviewed here.

RECENT FINDINGS

The present review provides an overview of acid secretion in both kidney and bone, and describes the recently identified diseases that are associated with mutations in tissue-specific subunits of these pumps.

SUMMARY

Elucidation of the molecular bases of a number of inherited renal acidopathies and bone disorders raises the possibility that additional tissue-specific subunits of these important pumps will be identified, gives hope for a better understanding of normal function at the molecular level, and may have implications for future therapeutic development.