Molecular pathophysiology of renal tubular acidosis

A review of renal tubular acidosis

OBJECTIVE

To review the current scientific literature on renal tubular acidosis (RTA) in people and small animals, focusing on diseases in veterinary medicine that result in secondary RTA.

DATA SOURCES

Scientific reviews and original research publications on people and small animals focusing on RTA.

SUMMARY

RTA is characterized by defective renal acid-base regulation that results in normal anion gap hyperchloremic metabolic acidosis. Renal acid-base regulation includes the reabsorption and regeneration of bicarbonate in the renal proximal tubule and collecting ducts and the process of ammoniagenesis. RTA occurs as a primary genetic disorder or secondary to disease conditions. Based on pathophysiology, RTA is classified as distal or type 1 RTA, proximal or type 2 RTA, type 3 RTA or carbonic anhydrase II mutation, and type 4 or hyperkalemic RTA. Fanconi syndrome comprises proximal RTA with additional defects in proximal tubular function. Extensive research elucidating the genetic basis of RTA in people exists. RTA is a genetic disorder in the Basenji breed of dogs, where the mutation is known. Secondary RTA in human and veterinary medicine is the sequela of diseases that include immune-mediated, toxic, and infectious causes. Diagnosis and characterization of RTA include the measurement of urine pH and the evaluation of renal handling of substances that should affect acid or bicarbonate excretion.

CONCLUSIONS

Commonality exists between human and veterinary medicine among the types of RTA. Many genetic defects causing primary RTA are identified in people, but those in companion animals other than in the Basenji are unknown. Critically ill veterinary patients are often admitted to the ICU for diseases associated with secondary RTA, or they may develop RTA while hospitalized. Recognition and treatment of RTA may reverse tubular dysfunction and promote recovery by correcting metabolic acidosis.

Pembrolizumab-induced Acute Tubulointerstitial Nephritis Accompanying Fanconi Syndrome and Type 1 Renal Tubular Acidosis

Pembrolizumab, an immune checkpoint inhibitor, is used to treat a variety of refractory malignancies. However, these agents are sometimes associated with immune-related adverse events. A 71-year-old woman received pembrolizumab-integrated chemotherapy to treat her recurrent mandibular gingival cancer. Five months after stopping pembrolizumab, she developed acute tubulointerstitial nephritis associated with Fanconi syndrome and type 1 renal tubular acidosis, which resolved with steroid therapy. We experienced a case of pembrolizumab-induced Fanconi syndrome and type 1 renal acidosis. We recommend follow-up of the tubular function in addition to the renal function even after discontinuation of pembrolizumab.

Renal Tubular Acidosis in the Neonate

See Bonus NeoBriefs videos and downloadable teaching slides Metabolic acidosis can manifest in the neonatal period and cause significant morbidity and mortality in neonates. Preterm infants are at an even higher risk of developing metabolic acidosis. If the acidosis results from a dysfunction of acid-base homeostasis by the renal system, the disorder is known as renal tubular acidosis (RTA). In this review, we will describe renal development and normal acid-base homeostasis by the renal system. We will also discuss the pathophysiology of the different types of RTA, laboratory findings to aid in diagnosis, and treatment considerations. Understanding RTA will help neonatal clinicians recognize and diagnose an infant affected by RTA and initiate treatment in a timely manner.

Rare Case of Hemolytic Anemia and Distal Renal Tubular Acidosis in an adult due to Homozygous SLC4A1 Mutation

In this case study, we report an adult patient presenting with generalized weakness, marked anemia, spherocytosis, and no features of thalassemia. The patient was treated for suspicion of autoimmune hemolytic anemia but was recalcitrant to treatment. Genetic analysis revealed the patient to be homozygous for SLC4A1 c.2573C>A (p.Ala858Asp). Distal renal tubular acidosis (dRTA) can be caused by mutations in SLC4A1, which encodes the Cl-/HCO3- exchanger of the renal type A intercalated cell, kidney AE1. SLC4A1 variants have been reported in dRTA patients from North America, Europe, and Southeast Asia. In some rare instances, SLC4A1 dRTA can present with hemolytic anemia resulting in marked anemia that is not responsive to standard interventions. This report identifies an autosomal recessive inheritance pattern for SLC4A1 variants in a patient presenting with dRTA and hemolytic anemia.

Dynamic transcriptome and LC-MS/MS analysis revealed the important roles of taurine and glutamine metabolism in response to environmental salinity changes in gills of rainbow trout (Oncorhynchus mykiss)

Recently, the frequent salinity fluctuation has become a growing threat to fishes. However, the dynamic patterns of gene expression in response to salinity changes remain largely unexplored. In the present study, 18 RNA-Seq datasets were generated from gills of rainbow trout at different salinities, including 0 ‰, 6 ‰, 12 ‰, 18 ‰, 24 ‰ and 30 ‰. Based on the strict thresholds, we have identified 63, 1411, 2096, 1031 and 1041 differentially expressed genes in gills of rainbow trout through pairwise comparisons. Additionally, weighted gene co-expression network analysis was performed to construct 18 independent modules with distinct expression patterns. Of them, green and tan modules were found to be tightly related to salinity changes, several hub genes of which are known as the important regulators in taurine and glutamine metabolism. To further investigate their potential roles in response to salinity changes, taurine, glutamine, and their metabolism-related glutamic acid and α-ketoglutaric acid were accurately quantitated using liquid chromatography-tandem mass spectrometry analysis. Results clearly showed that their concentrations were closely associated with salinity changes. These findings suggested that taurine and glutamine play important roles in response to salinity changes in gills of rainbow trout, providing new insights into the molecular mechanism of fishes in salinity adaptation.

Hypokalemic Periodic Paralysis: An Atypical Presentation of Non-autoimmune Hypothyroidism With Distal Renal Tubular Acidosis

Hypokalemic periodic paralysis (hypo KPP) is a rare form of autosomal dominant channelopathy characterized by muscular weakness and paralysis caused by decreased potassium levels. Precipitating factors are a diet rich in starches and sweets, and rest after an unusual degree of exercise. Paralytic attacks are more common between the ages of 15 and 40 years. The presentation can be a total paralysis or severe quadriplegia or mild weakness in certain group of muscles. During the acute episode of weakness proximal muscles are involved initially with gradual spread to the distal muscles. Deep reflexes are decreased or absent but the cognitive functions and sensory systems are intact. The paralysis may last for few hours to several days, but recovery is usually sudden in most patients. Hypo KPP is usually associated with thyroid disorders and distal renal tubular acidosis (DRTA). Here we report a case of young female patient who presented in emergency with two days history of weakness of all four limbs. The patient also had two episodes of similar illness in the last two and half years. On examination she had decreased tone and power in all four limbs with absent deep tendon reflexes, and plantar reflexes were down going bilaterally. On initial laboratory workup, patient was diagnosed to have hypokalemic, hyperchloremic metabolic acidosis with alkaline urine secondary to hypothyroidism. Features of hypokalemia with metabolic acidosis and failure to acidify urine was consistent with DRTA. Intravenous potassium chloride and bicarbonate replacement resulted in biochemical and clinical improvement.

Molecular Diagnosis of Primary Hyperoxaluria Type 1 and Distal Renal Tubular Acidosis in Moroccan Patients With Nephrolithiasis and/or Nephrocalcinosis

Nephrolithiasis (NL) and urolithiasis (UL) are usual reasons for hospitalization and presentation in pediatric outpatient departments and their incidence continues to rise worldwide. In Morocco, a previous epidemiological study done in the Fez region between January 2003 and November 2013 reported a prevalence of 0.83% of childhood UL. In two studies, heritability accounted for almost half of all NL or nephrocalcinosis (NC) prevalence. Genetic factors must be considered in the etiological diagnosis of urinary lithiasis in Morocco since the frequency of consanguineous marriages is high. Hereditary tubular disorders, especially distal renal tubular acidosis (dRTA) and Dent disease, and metabolic disorders like idiopathic hypercalciuria and hyperoxaluria are the most common causes of medullary NC. Primary hyperoxaluria type 1 (PH1), which can generate an early onset of NC, and often chronic kidney disease (CKD) should always be considered and thoroughly diagnosed. The aim of this work was to establish a molecular diagnosis of PH1 and dRTA and, thus, to predict and explain the disease phenotype in a cohort of 44 Moroccan patients with NL and/or NC by analyzing the AGXT and ATP6V1B1 genes that cause NL and/or NC when mutated. Disease phenotype was molecularly explained and solved in six of 44 individuals with NL and/or NC (13.6%). In the pediatric subgroup of individuals, a causative mutation in 16.2% was identified, whereas in the adult cohort no pathogenic mutation was detected. In our patients, PH1 was objectified in 67% of cases followed by dRTA in 33% of cases. We suggest that prompt detection and prophylactic treatment of UL are necessary to limit the risk of everlasting renal damage and thus prevent or delay the progression to CKD.

Renal Tubular Acidosis Manifesting as Severe Metabolic Bone Disease

Renal tubular acidosis (RTA) is a condition characterized by normal anion gap metabolic acidosis. Type 1 and type 2 RTA are the most common, and are caused by defective secretion of hydrogen ions and impaired absorption of bicarbonate, respectively. Long-standing uncorrected acidosis can lead to metabolic bone disease (MBD). Rickets and osteomalacia remain the commonest manifestations of uncorrected RTA. In addition, there can be a myriad of other skeletal manifestations like fractures, pseudofractures, secondary osteoporosis and even sclerotic bone disease. The postulated mechanism for bone involvement includes acidosis-mediated exaggerated osteoclastic bone resorption. Other contributory factors include abnormal renal handling of phosphate leading to hypophosphataemia in proximal RTA, and impaired vitamin D metabolism and action. In distal RTA, hypercalciuria and secondary hyperparathyroidism may play a key role for bone involvement. Recognizing the disease in its early course is important to prevent permanent sequelae of skeletal involvement. Most of these patients may, in fact, undergo orthopaedic interventions without primary correction of acidosis. We describe five cases who presented with MBD in varied forms. While evaluating the aetiology of MBD, they were diagnosed with RTA. Subsequently, we attempted to analyse the causes of RTA. Although the common causes were ruled out, genetic aetiology could not be ascertained due to resource constraints. RTA remains an important differential diagnosis of MBD. More awareness is required to diagnose the disease early and to treat it adequately. Our case series is an attempt to provide the clinical, biochemical and skeletal spectrum of RTA. In addition, we have attempted to provide algorithms for the approach and evaluation of RTA along with their varied causes.

Enamel Renal Syndrome: A Systematic Review

The enamel renal syndrome (ERS) is a rare autosomal recessive disease that is associated with mutations in the FAM20A gene. The syndrome is characterized by impaired amelogenesis of the hypoplastic type and nephrocalcinosis, presenting with presence of thin or absence of enamel, late dental eruption, intrapulpal calcifications, bilateral nephrocalcinosis, and normal plasma calcium level. The objective is to characterize ERS by systematically literature reviewing, highlighting the main findings of the syndrome to increase knowledge about this condition in the health professionals. The study is a systematic review of the scientific literature, whose research was developed in the PubMed database in March 2018. A total of 69 articles were found. Two authors analyzed their abstracts and selected, according to the language and main subject, 30 articles to write this study. A total of 69 patients were cited in the studies and their data were analysed. There was gender equivalence and the ages ranged from 1 to 64 years old. There is a clear hereditary relation of the syndrome, since there was consanguinity in 18 cases, indicating a percentage of 26.08% and family history in 30 cases (43.47%). Laboratory changes vary greatly from patient to patient and may even remain unchanged. The relationship between the syndrome and the mutation in the FAM20A gene can be proven from the data, since all patients with ERS screened by the mutation were positive. With the advancement of the ERS studies, some associations with the syndrome are suspected, such as the presence of gingival fibromatosis, hearing loss, and hypertrichosis. Thus, it is noticed that the syndrome does not show a predilection for gender or age and there is a strong hereditary character, marked by the consanguinity and family history of the patients. The association with the FAM20A gene is reinforced, since the mutation was identified in all patients analyzed.

Functional Imaging Using Fluorine (19F) MR Methods: Basic Concepts

Kidney-associated pathologies would greatly benefit from noninvasive and robust methods that can objectively quantify changes in renal function. In the past years there has been a growing incentive to develop new applications for fluorine (¹⁹F) MRI in biomedical research to study functional changes during disease states. ¹⁹F MRI represents an instrumental tool for the quantification of exogenous ¹⁹F substances in vivo. One of the major benefits of ¹⁹F MRI is that fluorine in its organic form is absent in eukaryotic cells. Therefore, the introduction of exogenous ¹⁹F signals in vivo will yield background-free images, thus providing highly selective detection with absolute specificity in vivo. Here we introduce the concept of ¹⁹F MRI, describe existing challenges, especially those pertaining to signal sensitivity, and give an overview of preclinical applications to illustrate the utility and applicability of this technique for measuring renal function in animal models.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Renal Tubular Acidosis Manifesting as Severe Metabolic Bone Disease

Collapse

Renal Tubular Acidosis and Immune Checkpoint Inhibitor Therapy: An Immune-Related Adverse Event of PD-1 Inhibitor-A Report of 3 Cases

The main cause of acute kidney injury in patients receiving immune checkpoint inhibitors (ICIs) is acute interstitial nephritis. However, as their use continues to increase, other kidney manifestations are being described. We report 3 cases of patients treated with ICIs who developed predominantly electrolyte disorders secondary to renal tubular acidosis as an immune-related adverse event and discuss the potential mechanism. Nongap acidosis in combination with hypokalemia should raise suspicion for distal renal tubular acidosis in patients treated with ICIs.

Body Fluid Compartments, Cell Membrane Ion Transport, Electrolyte Concentrations, and Acid-Base Balance

Measurements made in the blood, part of the extracellular compartment, are used in the clinical assessment of acid-base disorders; however, intracellular events determine much of the metabolic importance of these disorders. Intracellular and interstitial compartment acid-base balance is complex and varies in different tissues. This review considers the determination of extracellular pH in the context of ion transport processes at the interface of cells and the interstitial fluid, and between epithelial cells lining the transcellular contents of the gastrointestinal and urinary tracts that open to the external environment. A further consideration is the role of these membrane transporters in the generation of acid-base disorders and the associated disruption of electrolyte balance. This review suggests a process of internal and external balance for pH regulation similar to that of potassium, and considers the role of secretory gastrointestinal epithelia and renal epithelia with respect to normal pH homeostasis and clinical disorders. Electroneutrality of electrolytes in the extracellular fluid is a fundamental feature of reciprocal changes in Cl^- or non-Cl^- anions and HCO₃^-. Normal mechanisms for protecting cell pH and producing normal gastrointestinal and renal secretions in healthy states also may result in disease when abnormal. In a similar manner, organic anions such as ketoacid anions and lactate, normally transported as fuels between organs, result in acid-base disturbances in disease. Understanding the genomic basis of these transporters may contribute to specific treatments.

ATP6V1B1 recurrent mutations in Algerian deaf patients associated with renal tubular acidosis

Hereditary distal renal tubular acidosis (dRTA) is a rare disorder characterized by metabolic acidosis due to impaired renal acid excretion. To date, three genes (ATP6V1B1, ATP6V0A4 and SLC4A1) have been reported to be responsible for this genetic disorder. Notably, mutations of ATP6V1B1 gene, which encode B1-subunit of H + -ATPase pump cause distal renal tubular acidosis often, associated with sensorineural hearing loss (SNHL). Furthermore, enlarged vestibular aqueduct (EVA) was also described in some patients with ATP6V1B1 mutations. Four Algerian unrelated patients presented with dRTA and SNHL were recruited. The ATP6V1B1 gene was preferentially analyzed in all these patients by Sanger sequencing. We identified two previously reported variants in ATP6V1B1 gene: a frameshift mutation (c.1155dupC: p.(Ile386Hisfs*56) in exon 12 and a splicing mutation in intron 2 (c.175-1G > C: p?). Both mutations were homozygous in affected members. Interestingly, one patient with p.(Ile386Hisfs*56) mutation presented profound SNHL and bilateral enlarged vestibular aqueduct (EVA). Our study indicates the importance contribution of ATP6V1B1 gene mutations to the pathogenesis of the dRTA in the Algerian population and will contribute to introducing principles to predict the characteristics of the dRTA in patients. Thus, screening for this gene could allow rapid patient management and provide adequate genetic counseling.

Saccharomyces cerevisiae: First Steps to a Suitable Model System To Study the Function and Intracellular Transport of Human Kidney Anion Exchanger 1

Distal renal tubular acidosis (dRTA) is a common kidney dysfunction characterized by impaired acid secretion via urine. Previous studies revealed that α-intercalated cells of dRTA patients express mutated forms of human kidney anion exchanger 1 (kAE1) which result in inefficient plasma membrane targeting or diminished expression levels of kAE1. However, the precise dRTA-causing processes are inadequately understood, and alternative model systems are helpful tools to address kAE1-related questions in a fast and inexpensive way. In contrast to a previous study, we successfully expressed full-length kAE1 in Saccharomyces cerevisiae. Using advanced microscopy techniques as well as different biochemical and functionality assays, plasma membrane localization and biological activity were confirmed for the heterologously expressed anion transporter. These findings represent first important steps to use the potential of yeast as a model organism for studying trafficking, activity, and degradation of kAE1 and its mutant variants in the future.

Saccharomyces cerevisiae has been frequently used to study biogenesis, functionality, and intracellular transport of various renal proteins, including ion channels, solute transporters, and aquaporins. Specific mutations in genes encoding most of these renal proteins affect kidney function in such a way that various disease phenotypes ultimately occur. In this context, human kidney anion exchanger 1 (kAE1) represents an important bicarbonate/chloride exchanger which maintains the acid-base homeostasis in the human body. Malfunctions in kAE1 lead to a pathological phenotype known as distal renal tubular acidosis (dRTA). Here, we evaluated the potential of baker's yeast as a model system to investigate different cellular aspects of kAE1 physiology. For the first time, we successfully expressed yeast codon-optimized full-length versions of tagged and untagged wild-type kAE1 and demonstrated their partial localization at the yeast plasma membrane (PM). Finally, pH and chloride measurements further suggest biological activity of full-length kAE1, emphasizing the potential of S. cerevisiae as a model system for studying trafficking, activity, and/or degradation of mammalian ion channels and transporters such as kAE1 in the future.

IMPORTANCE Distal renal tubular acidosis (dRTA) is a common kidney dysfunction characterized by impaired acid secretion via urine. Previous studies revealed that α-intercalated cells of dRTA patients express mutated forms of human kidney anion exchanger 1 (kAE1) which result in inefficient plasma membrane targeting or diminished expression levels of kAE1. However, the precise dRTA-causing processes are inadequately understood, and alternative model systems are helpful tools to address kAE1-related questions in a fast and inexpensive way. In contrast to a previous study, we successfully expressed full-length kAE1 in Saccharomyces cerevisiae. Using advanced microscopy techniques as well as different biochemical and functionality assays, plasma membrane localization and biological activity were confirmed for the heterologously expressed anion transporter. These findings represent first important steps to use the potential of yeast as a model organism for studying trafficking, activity, and degradation of kAE1 and its mutant variants in the future.

Tenofovir disoproxil fumarate-induced distal renal tubular acidosis: A case report

Tenofovir disoproxil fumarate (TDF) is an anti-retroviral drug that is known to cause nephrotoxicity including renal tubular acidosis (RTA). With increasing literature on proximal RTA caused by TDF, reports on distal RTA are scarce, with only one case reported so far. We report a case of distal RTA in patient living with human immunodeficiency virus, who presented with nausea and fatigue giving a history of TDF-based therapy for two years. Laboratory investigations revealed non-anion gap metabolic acidosis, positive urine anion gap, hyperchloremia, and hypokalemia. The patient improved after discontinuing TDF and supportive management.

Association of urinary acidification function with the progression of diabetic kidney disease in patients with type 2 diabetes

OBJECTIVE

Although diabetic kidney disease (DKD) has been considered as a glomerulocentric disease in the past few decades, growing evidence demonstrated that tubular damage was indispensable in its pathogenesis and progression. This study was designed to investigate the association of urinary acidification dysfunction with the progression of DKD in type 2 diabetic patients.

METHODS

Here the urinary acidification functions were measured from 80 participants with renal biopsy-proven DKD. The different kinds of renal tubular transportation dysfunction were analyzed, including the dysfunction of bicarbonate reabsorption, titratable acid secretion, and ammonium secretion. In addition, patients were followed up for 17 (interquartile range, 11-32) months to evaluate the effect of urinary acidification dysfunction in the progression of DKD.

RESULTS

The most common urinary acidification dysfunction was the disorder of ammonium secretion, accounting for 53.75%. The more proteinuria excretion and the lower glomerular filtration rate (GFR) were observed in the urinary titratable acid disorder group than the normal group, and the same results were obtained for ammonium secretion disorder. Urine titratable acid was positively correlated with eGFR whereas it was inversely correlated with proteinuria, serum creatinine, and BUN. Moreover, 24 h urine protein, serum creatinine, BUN and cystatin C increased from DKD stage II to stage IV, whereas the eGFR and urine titratable acid decreased in the same way. Furthermore, Kaplan-Meier analysis and Cox regression showed that the disorder of titratable acid was an independent risk factor for DKD progression.

CONCLUSIONS

The dysfunction of urinary titratable acid is a potential biomarker for the severity of proteinuria, eGFR and glomerular lesions in patients with DKD. Moreover, the titratable acid disorder is an independent risk factor of the DKD progression.

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.

The kidney anion exchanger 1 affects tight junction properties via claudin-4

In the renal collecting duct, intercalated cells regulate acid-base balance by effluxing protons through the v-H⁺-ATPase, and bicarbonate via apical pendrin or the basolateral kidney anion exchanger 1 (kAE1). Additionally, collecting duct cells play an essential role in transepithelial absorption of sodium and chloride. Expression of kAE1 in polarized MDCK I cells was previously shown to decrease trans-epithelial electrical resistance (TEER), suggesting a novel role for kAE1 in paracellular permeability. In our study, we not only confirmed that inducible expression of kAE1 in mIMCD3 cells decreased TEER but we also observed (i) increased epithelial absolute permeability to both sodium and chloride, and (ii) that this effect was dependent on kAE1 activity. Further, kAE1 regulated tight junction properties through the tight junction protein claudin-4, a protein with which it physically interacts and colocalizes. These findings unveil a novel interaction between the junctional protein claudin-4 and the kidney anion exchanger, which may be relevant to ion and/or pH homeostasis.

Phosphaturia in kidney stone formers: Still an enigma

Calcium kidney stones are common worldwide. Most are idiopathic and composed of calcium oxalate. Calcium phosphate is present in around 80% and may initiate stone formation. Stone production is multifactorial with a polygenic genetic contribution. Phosphaturia is found frequently among stone formers but until recently received scant attention. This review examines possible mechanisms for the phosphaturia and its relevance to stone formation from a wide angle. There is a striking lack of clinical data. Phosphaturia is associated, but not correlated, with hypercalciuria, increased 1,25 dihydroxy-vitamin D [1,25 (OH)₂D], and sometimes evidence of disturbances in proximal renal tubular function. Phosphate reabsorption in the proximal renal tubules requires tightly regulated interaction of many proteins. Paracellular flow through intercellular tight junctions is the major route of phosphate absorption from the intestine and can be reduced therapeutically in hyperphosphatemic patients. In monogenic defects stones develop when phosphaturia is associated with hypercalciuria, generally explained by increased 1,25 (OH)₂D production in response to hypophosphatemia. Calcification does not occur in disorders with increased FGF23 when phosphaturia occurs in isolation and 1,25 (OH)₂D is suppressed. Candidate gene studies have identified mutations in the phosphate transporters, but in few individuals. One genome-wide study identified a polymorphism of the phosphate transporter gene SLC34A4 associated with stones. Others did not find mutations obviously linked to phosphate reabsorption. Future genetic studies should have a wide trawl and should focus initially on groups of patients with clearly defined phenotypes. The global data should be pooled.

Primary biliary cirrhosis with refractory hypokalemia: A case report

RATIONALE

Renal tubular acidosis (RTA) represents a class of metabolic disorders characterized by metabolic acidosis with a normal plasma anion gap. As a rare complication of primary biliary cirrhosis (PBC), RTA is easily overlooked, likely leading to misdiagnosis.

PATIENT CONCERNS

A 32-year-old woman who had been diagnosed with PBC at our hospital was found to have hypokalemia due to repeated fatigue for 2 years, and the etiology was unknown.

DIAGNOSES

Due to the laboratory test results, radiographic findings, and pathologic results, she was diagnosed with PBC associated with RTA.

INTERVENTIONS

She was then treated with ursodeoxycholic acid, potassium citrate, and calcium supplements together with activated vitamin D.

OUTCOMES

Thus far, the patient showed a good response to ursodeoxycholic acid, and the clinical symptoms and liver function were significantly improved.

LESSONS

Physicians that encounter refractory hypokalemia in a patient with PBC should be aware of the presence of RTA. The early diagnosis and treatment of such patients are of paramount importance to alleviate clinical symptoms and delay disease progression.

Prospective relation of adolescent citrate excretion and net acid excretion capacity with blood pressure in young adulthood

Experimental data and observational studies in adults suggest that even subtle changes in acid-base balance, indicative of a higher systemic proton load, are related to higher blood pressure (BP) levels and an increased hypertension risk. However, these associations have not been investigated during growth. The kidney is the central organ in regulating excretion of nonvolatile acids, and renal citrate excretion has been shown to be a sensitive, noninvasive marker of changes in systemic acid balance. We thus analyzed the prospective relation of 24-h citrate excretion, as well as net acid excretion capacity (NAEC; a noninvasive indicator of the renal ability to excrete protons), during adolescence (boys: 10-15 yr; girls: 9-14 yr) with BP levels in young adulthood (18-30 yr) in 374 healthy participants of the Dortmund Nutritional and Anthropometric Longitudinally Designed (DONALD) Study. In linear-regression analyses adjusted for age, sex, 24-h urinary excretions of sodium and potassium, as well as further relevant confounders, a 1-mmol/1.73 m²/day higher adolescent citrate excretion was related to 1.2 mmHg lower systolic BP ( P = 0.02) but not to diastolic BP ( P = 0.6). A 10-mEq higher NAEC during adolescence was related to 1.7 mmHg lower systolic BP in young men, but this association was statistically nonsignificant ( P = 0.07) after multivariable adjustment. Additional adjustment for adult body mass index did not alter these findings. To conclude, subtle changes in systemic acid-base balance during adolescence are already indicative for later BP. Potential sex differences in these associations should be investigated in further studies.

Extracellular Acid-Base Balance and Ion Transport Between Body Fluid Compartments

Clinical assessment of acid-base disorders depends on measurements made in the blood, part of the extracellular compartment. Yet much of the metabolic importance of these disorders concerns intracellular events. Intracellular and interstitial compartment acid-base balance is complex and heterogeneous. This review considers the determinants of the extracellular fluid pH related to the ion transport processes at the interface of cells and the interstitial fluid, and between epithelial cells lining the transcellular contents of the gastrointestinal and urinary tracts that open to the external environment. The generation of acid-base disorders and the associated disruption of electrolyte balance are considered in the context of these membrane transporters. This review suggests a process of internal and external balance for pH regulation, similar to that of potassium. The role of secretory gastrointestinal epithelia and renal epithelia with respect to normal pH homeostasis and clinical disorders are considered. Electroneutrality of electrolytes in the ECF is discussed in the context of reciprocal changes in Cl^- or non Cl^- anions and [Formula: see text].

The genetic and clinical spectrum of a large cohort of patients with distal renal tubular acidosis

Primary distal renal tubular acidosis is a rare genetic disease. Mutations in SLC4A1, ATP6V0A4, and ATP6V1B1 genes have been described as the cause of the disease, transmitted as either an autosomal dominant or recessive trait. Particular clinical features, such as sensorineural hearing loss, have been mainly described in association with mutations in one gene instead of the others. Nevertheless, the diagnosis of distal renal tubular acidosis is essentially based on clinical and laboratory findings, and the series of patients described so far are usually represented by small cohorts. Therefore, a strict genotype-phenotype correlation is still lacking, and questions about whether clinical and laboratory data should direct the genetic analysis remain open. Here, we applied next-generation sequencing in 89 patients with a clinical diagnosis of distal renal tubular acidosis, analyzing the prevalence of genetic defects in SLC4A1, ATP6V0A4, and ATP6V1B1 genes and the clinical phenotype. A genetic cause was determined in 71.9% of cases. In our group of sporadic cases, clinical features, including sensorineural hearing loss, are not specific indicators of the causal underlying gene. Mutations in the ATP6V0A4 gene are quite as frequent as mutations in ATP6V1B1 in patients with recessive disease. Chronic kidney disease was frequent in patients with a long history of the disease. Thus, our results suggest that when distal renal tubular acidosis is suspected, complete genetic testing could be considered, irrespective of the clinical phenotype of the patient.

Amelogenesis Imperfecta with Distal Renal Tubular Acidosis: A Novel Syndrome?

Amelogenesis imperfecta (AI) is a heterogeneous group of inherited dental enamel defects. It has rarely been reported in association with multiorgan syndromes and metabolic disorders. The metabolic disorders that have been reported in association with AI include hypocalciuria, impaired urinary concentrating ability, and Bartter-like syndrome. In literature, only three cases of AI and distal renal tubular acidosis (dRTA) have been described: two cases in adults and a solitary case in the pediatric age group. Here, we report a child with AI presenting with dRTA; to the best of our knowledge, our reported case is the only second such case in pediatric age group. Our case highlights the importance of recognizing the possibility of renal abnormalities in patients with AI as it will affect the long-term prognosis.

Chronic Metabolic Acidosis Activates Renal Tubular Sodium Chloride Cotransporter through Angiotension II-dependent WNK4-SPAK Phosphorylation Pathway

The mechanism by which chronic metabolic acidosis (CMA) regulates sodium (Na(+))-chloride (Cl(-)) cotransporter (NCC) in the renal distal convoluted tubules remains unexplored. We examined the role of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and with-no-lysine kinase 4 (WNK4) on expression of NCC in mouse models of CMA. CMA was induced by NH4Cl in wild type mice (WTA mice), SPAK, and WNK4 knockout mice. The quantities of Ncc mRNA, expression of total NCC, phosphorylated (p)-NCC, SPAK and WNK4 in the kidneys as well as NCC inhibition with hydrochlorothiazide and Na(+) balance were evaluated. Relative to WT mice, WTA mice had similar levels of Ncc mRNA, but increased expression of total and p-NCC, SPAK, and WNK4 and an exaggerated response to hydrochlorothiazide which could not be observed in SPAK or WNK4 knockout mice with CMA. In WTA mice, increased plasma renin activity, aldosterone and angiotensin II concentrations accompanied by a significantly negative Na(+) balance. High Na(+) diet abolished the enhanced NCC expression in WTA mice. Furthermore, an angiotensin II type 1 receptor blocker rather than a mineralocorticoid receptor antagonist exerted a marked inhibition on Na(+) reabsorption and NCC phosphorylation in WTA mice. CMA increases WNK4-SPAK-dependent NCC phosphorylation and appears to be secondary to previous natriuresis with volume-dependent angiotensin II activation.

Dental findings in Brazilian patients with Fanconi syndrome

BACKGROUND

Fanconi syndrome (FS) is a generalized dysfunction of renal proximal tubules resulting in impaired reabsorption of amino acids, phosphate, urate, bicarbonate, glucose, and electrolytes. FS can be a primary renal impairment due to genetic origin or secondary to inborn errors of metabolism or systemic diseases. Main clinical findings are polyuria, failure to thrive, rickets/osteomalacia, metabolic acidosis, bone deformities, and, occasionally, nephrocalcinosis. Although dental abnormalities have been reported in patients with tubulopathies, oral manifestations in FS were very rarely described.

CASE REPORT

Two patients with FS exhibiting important dental alterations were reported. Oral examination of the first patient revealed teeth with rough surfaces and white opaque areas. Early loss of primary teeth and delayed eruption of permanent teeth were also observed. Oral examination of the second patient detected delayed eruption of permanent teeth and X-ray images revealed loss of contrast between enamel and dentin. In both patients with FS, loss of bone density in maxilla and mandible was observed in X-ray images.

CONCLUSIONS

This study is the first report of dental abnormalities in Brazilian patients with FS. Furthermore, our findings support the general idea that renal tubular disorders might impact on mineralized tissues of the oral cavity.

Whole-exome sequencing as a diagnostic tool for distal renal tubular acidosis

OBJECTIVE

Distal renal tubular acidosis (dRTA) is characterized by metabolic acidosis due to impaired renal acid excretion. The aim of this study was to demonstrate the genetic diagnosis of four children with dRTA through use of whole-exome sequencing.

METHODS

Two unrelated families were selected; a total of four children with dRTA and their parents, in order to perform whole-exome sequencing. Hearing was preserved in both children from the first family, but not in the second, wherein a twin pair had severe deafness. Whole-exome sequencing was performed in two pooled samples and findings were confirmed with Sanger sequencing method.

RESULTS

Two mutations were identified in the ATP6V0A4 and ATP6V1B1 genes. In the first family, a novel mutation in the exon 13 of the ATP6V0A4 gene with a single nucleotide change GAC → TAC (c.1232G>T) was found, which caused a substitution of aspartic acid to tyrosine in position 411. In the second family, a homozygous recurrent mutation with one base-pair insertion (c.1149_1155insC) in exon 12 of the ATP6V1B1 gene was detected.

CONCLUSION

These results confirm the value of whole-exome sequencing for the study of rare and complex genetic nephropathies, allowing the identification of novel and recurrent mutations. Furthermore, for the first time the application of this molecular method in renal tubular diseases has been clearly demonstrated.

Bilateral nephrocalcinosis and amelogenesis imperfecta: A case report

Amelogenesis imperfecta (AI) is a group of hereditary disorders that affect the quality and/or quantity of dental enamel. This paper describes the clinicopathological features of a patient who was born of nonconsanguineous parents and who presented with oral alterations, including yellow and misshapen teeth, intrapulpal calcifications, delayed tooth eruption, and gum enlargement. Scanning electron microscopy of the teeth revealed hypoplastic enamel, and a renal ultrasound detected bilateral nephrocalcinosis, leading to a diagnosis of AI and nephrocalcinosis syndrome. Since nephrocalcinosis is often asymptomatic and can be associated with impaired renal function, dentists who see children with a generalized and thin hypoplastic AI should consider a renal ultrasound scan and referral to a Nephrologist. Children with nephrocalcinosis should also be considered for a dental check.

Clinical evaluation of Chinese patients with primary distal renal tubular acidosis

OBJECTIVE

Distal renal tubular acidosis (dRTA) is a hyperchloremic metabolic acidosis disorder characterized by a normal anion gap with abnormal urinary hydrogen (H(+)) excretion. At present, there are few available reports regarding the clinical status of primary dRTA. The primary objective of this study was to analyze the clinical features and outcomes of primary dRTA.

METHODS

This was a retrospective study performed in patients with primary dRTA who were hospitalized at Ruijin Hospital between March 1996 and July 2009; the clinical features of these patients were analyzed.

RESULTS

This study included 95 consecutive inpatients: 40 men (42.11%) and 55 women (57.89%). Among them, 60 had hypokalemia (63.12%), 29 had complete dRTA and 66 had incomplete dRTA. The mean urine calcium levels of the patients with and without urinary lithiasis were 0.10±0.04 and 0.07±0.05 mmol/24 h・kg, respectively (p=0.04). The blood pH values of the patients with and those without bone disease were 7.37±0.06 and 7.32±0.06, respectively (p=0.01). A total of 8.33% (8/27) of the patients had tubular proteinuria.

CONCLUSION

Hypokalemia is the most common clinical manifestation of primary dRTA. Primary dRTA can also be accompanied by proximal tubular dysfunction. Controlling the urine calcium and citrate levels is crucial for the treatment of nephrocalcinosis and/or nephrolithiasis, while restoring the blood pH to the normal level is essential for controlling bone disease.

Cystinosis as a lysosomal storage disease with multiple mutant alleles: Phenotypic-genotypic correlations

Cystinosis is an autosomal recessive lysosomal storage disease with an unclear enzymatic defect causing lysosomal cystine accumulation with no corresponding elevation of plasma cystine levels leading to multisystemic dysfunction. The systemic manifestations include a proximal renal tubular defect (Fanconi-like), endocrinal disturbances, eye involvements, with corneal, conjunctival and retinal depositions, and neurological manifestations in the form of brain and muscle dysfunction. Most of the long-term ill effects of cystinosis are observed particularly in patients with long survival as a result of a renal transplant. Its responsible CTNS gene that encodes the lysosomal cystine carrier protein (cystinosin) has been mapped on the short arm of chromosome 17 (Ch17 p13). There are three clinical forms based on the onset of main symptoms: nephropathic infantile form, nephropathic juvenile form and non-nephropathic adult form with predominant ocular manifestations. Avoidance of eye damage from sun exposure, use of cystine chelators (cysteamine) and finally renal transplantation are the main treatment lines. Pre-implantation genetic diagnosis for carrier parents is pivotal in the prevention of recurrence.

Sulfatides are required for renal adaptation to chronic metabolic acidosis

Urinary ammonium excretion by the kidney is essential for renal excretion of sufficient amounts of protons and to maintain stable blood pH. Ammonium secretion by the collecting duct epithelia accounts for the majority of urinary ammonium; it is driven by an interstitium-to-lumen NH3 gradient due to the accumulation of ammonium in the medullary and papillary interstitium. Here, we demonstrate that sulfatides, highly charged anionic glycosphingolipids, are important for maintaining high papillary ammonium concentration and increased urinary acid elimination during metabolic acidosis. We disrupted sulfatide synthesis by a genetic approach along the entire renal tubule. Renal sulfatide-deficient mice had lower urinary pH accompanied by lower ammonium excretion. Upon acid diet, they showed impaired ammonuria, decreased ammonium accumulation in the papilla, and chronic hyperchloremic metabolic acidosis. Expression levels of ammoniagenic enzymes and Na(+)-K(+)/NH4(+)-2Cl(-) cotransporter 2 were higher, and transepithelial NH3 transport, examined by in vitro microperfusion of cortical and outer medullary collecting ducts, was unaffected in mutant mice. We therefore suggest that sulfatides act as counterions for interstitial ammonium facilitating its retention in the papilla. This study points to a seminal role of sulfatides in renal ammonium handling, urinary acidification, and acid-base homeostasis.

Typical Features of Amelogenesis Imperfecta in Two Patients with Bartter's Syndrome

BACKGROUND/AIMS

Amelogenesis imperfecta (AI) is due to many inherited defects of enamel formation that affect the quantity and quality of enamel, leading to delay in tooth eruption and cosmetic consequences. AI has been described in association with nephrocalcinosis, which is called the enamel-renal syndrome. The aim of this case report is to describe typical features of AI in 2 patients with Bartter's syndrome (BS) for the first time.

METHODS

-Eight patients with confirmed BS were systematically screened for dental abnormalities as part of protocol. Those with suggestive clinical features of AI were submitted to panoramic X-ray and decayed teeth were analyzed by scanning electron microscopy.

RESULTS

Typical features of AI were detected in 2 girls with BS. These 2 patients showed nephrocalcinosis, and diagnosis and adequate clinical control were delayed. Genetic analysis detected the mutation responsible for BS in 1 of these patients. In this case, BS was due to a homozygous mutation of exon 5 of the KCNJ1 gene resulting in a substitution of valine for alanine at the codon 214 (A214V).

CONCLUSIONS

The finding of typical features of AI in BS might constitute preliminary evidence that abnormalities of the biomineralization process found in patients with renal tubular disorders might also affect calcium deposition in dental tissues.

Fanconi-Bickel syndrome as an example of marked allelic heterogeneity

Renal tubular acidosis (RTA) encompasses many renal tubular disorders characterized by hyperchloremic metabolic acidosis with a normal anion gap. Untreated patients usually complain of growth failure, osteoporosis, rickets, nephrolithiasis and eventually renal insufficiency. Fanconi-Bickel syndrome (FBS) is an example of proximal RTA due to a single gene disorder; it is caused by defects in the facilitative glucose transporter 2 gene that codes for the glucose transporter protein 2 expressed in hepatocytes, pancreatic β-cells, enterocytes and renal tubular cells. It is a rare inherited disorder of carbohydrate metabolism manifested by huge hepatomegaly [hence it is classified as glycogen storage disease (GSD) type XI; GSD XI], severe hypophosphatemic rickets and failure to thrive due to proximal renal tubular dysfunction leading to glucosuria, phosphaturia, generalized aminoaciduria, bicarbonate wasting and hypophosphatemia. The disorder has been reported from all parts of Europe, Turkey, Israel, Arabian countries, Japan and North America. Many mutant alleles have been described, its exact frequency is unknown and there is no single mutation found more frequently than the others. The presence of consanguinity in affected families suggests an autosomal recessive pattern of inheritance. New cases of FBS have been recently reported in the Middle and Far East in collaboration with specialized centers. Two novel mutations have been discovered in two unrelated Egyptian families. The first was two bases deletion, guanine and adenine, (c.253_254delGA) causing a frameshift mutation (p. Glu85fs) and the second is mutation in exon6 in splicing acceptor site with intron5 (c.776-1G>C or IVS5-1G>A). Moreover, a new different mutation was described in a 3 year old Indian boy.

Iopamidol as a responsive MRI-chemical exchange saturation transfer contrast agent for pH mapping of kidneys: In vivo studies in mice at 7 T

Iopamidol (Isovue®-Bracco Diagnostic Inc.) is a clinically approved X-Ray contrast agent used in the last 30 years for a wide variety of diagnostic applications with a very good clinical acceptance. Iopamidol contains two types of amide functionalities that can be exploited for the generation of chemical exchange saturation transfer effect. The exchange rate of the two amide proton pools is markedly pH-dependent. Thus, a ratiometric method for pH assessment has been set-up based on the comparison of the saturation transfer effects induced by selective irradiation of the two resonances. This ratiometric approach allows to rule out the concentration effect of the contrast agent and provides accurate pH measurements in the 5.5-7.4 range. Upon injection of Iopamidol into healthy mice, it has been possible to acquire pH maps of kidney regions. Furthermore, it has been also shown that the proposed method is able to report about pH-changes induced in control mice fed with acidified or basified water for a period of a week before image acquisition.

Severe metabolic acidosis causes early lethality in NBC1 W516X knock-in mice as a model of human isolated proximal renal tubular acidosis

We have identified a novel homozygous nonsense mutation (W516X) in the kidney-type electrogenic sodium bicarbonate cotransporter 1 (NBC1) in a patient with isolated proximal renal tubular acidosis (pRTA). To specifically address the pathogenesis of this mutation, we created NBC1 W516X knock-in mice to match the patient's abnormalities. The expression of NBC1 mRNA and protein in the kidneys of NBC1(W516X/W516X) mice were virtually absent, indicating that nonsense-mediated mRNA decay (NMD) is involved in the defective transcription and translation of this mutation. These mice not only recapitulated the phenotypes of this patient with growth retardation, pRTA, and ocular abnormalities, but also showed anemia, volume depletion, prerenal azotemia, and several organ abnormalities, culminating in dehydration and renal failure with early lethality before weaning. In isolated renal proximal tubules, both NBC1 activity and the rate of bicarbonate absorption were markedly reduced. Unexpectedly, there was no compensatory increase in mRNA of distal acid/base transporters. Sodium bicarbonate but not saline administration to these mutant mice markedly prolonged their survival, decreased their protein catabolism and attenuated organ abnormalities. The prolonged survival time uncovered the development of corneal opacities due to corneal edema. Thus, NBC1(W516X/W516X) mice with pRTA represent an animal model for metabolic acidosis and may be useful for testing therapeutic inhibition of NMD in vivo.

Molecular Approach for Distal Renal Tubular Acidosis Associated AE1 Mutations

The molecular approaches to distal renal tubular acidosis (dRTA) associated AE1 mutations lead us to understand the genetic and pathophysiological aspects of the acidification defects. An unanticipated high value of the urine-blood (U-B) PCO(2) after NaHCO(3) loading observed in a case of dRTA and southeast Asian ovalocytosis (SAO) might be from a mistarget of the AE1 to the luminal membrane of type A intercalated cells. The mutations of the AE1 gene resulted in SAO and also affected renal acidification function. Notwithstanding, after the NH4Cl loading in 20 individuals with SAO, the acidification in the distal nephron was normal. The presence of both SAO and G701D mutations of AE1 gene would explain the abnormal urinary acidification in the patients with the compound heterozogosity. In terms of the effect of the mutations on trafficking of AE1, truncated kidney isoform (kAE1) of wild-type showed a 'dominant-positive effect' in rescuing the recessive mutant kAE1 (S773P or G701D) trafficking to the plasma membrane, in contrast with the dominant mutant kAE1 (R589H) resulting in a 'dominant-negative effect' when heterodimerized with the wild-type kAE1. It is notable that the dominant mutants kAE1 (R901X or G609R) expression in MDCK cells clearly results in aberrant surface expression with some mutant protein appearing at the apical membrane. These might result in net bicarbonate secretion and increasing U-B PCO(2) in the distal nephron. The molecular physiological and genetic approaches have permitted identification of the molecular defects, predominantly in transporter proteins, and should in turn prompt development of novel therapeutic strategies.