ATP6V1B1 Research Articles

Clinical Features and Unusual Heterozygous Mutations in Patients with Renal Hypokalemia.

Renal hypokalemia is associated with mutation. This study aimed to investigate the clinical features and pathogenic mutations in patients with renal hypokalemia. The patients with hypokalemia were enrolled, and the renal function, thyroid function, renin-aldosterone system, urinary potassium excretion, and exome sequencing were performed. The correlation between the clinical phenotypes and causative genes was assessed. Five patients with hypokalemia were enrolled and diagnosed as tubular hypokalemia. The patients with common clinical manifestations were difficult to differentiate based on atypical laboratory findings. The results of the genetic analysis were as follows: both patient 1 and patient 2 were heterozygous for the c.C625T mutation of the KCNJ1 gene, which is responsible for Bartter syndrome. Patient 3 was heterozygous for the c.G298A mutation of the ATP6V1B1 gene, which is responsible for renal tubular acidosis. Patient 4 had a compound heterozygous mutation of c.G893A of the BSND gene, responsible for Bartter syndrome, and c.1029+5G>A, the ATP6V0A4 gene responsible for distal renal tubular acidosis. Patient 5 had Gitelman syndrome and carried the compound heterozygous mutations c.C1963T and c.G2029A of the SLC12A3 gene. All the above loci were known heterozygous mutations. The unusual heterozygous mutations were identified in five renal hypokalemia patients. Molecular diagnosis of tubular hypokalemia was conducive to accurate diagnosis and treatment.

Neonatal Onset Distal Renal Tubular Acidosis: Description of Two Novel Variants on the ATP6V0A4 Gene and Review of the Literature on Associated Extrarenal Manifestations.

Distal renal tubular acidosis (dRTA) is an extremely rare disease that affects the distal tubule's ability to excrete proton cations, acidify urine, and maintain the acid-base balance. The clinical presentation of dRTA typically includes normal anion gap metabolic acidosis with decreased serum bicarbonate levels, hypokalemia, hypercalcemia, nephrocalcinosis, and alkaline urine. Hereditary causes of dRTA include pathogenic variants in ATP6V1B1 , ATP6V0A4 , SLC4A1 , FOXI1 , and WDR72 genes, which encode different transmembrane proteins on the apical surface of type A intercalated cells in the distal tubule. Variants in these genes lead to various defects in the function of the encoded proteins and can also account for extrarenal manifestations of dRTA due to the expression of these proteins in other organs, such as the stria vascularis of the inner ear. However, the literature on extrarenal manifestations, associated renal complications of hereditary dRTA, and appropriate investigations, and follow-up for patients with dRTA is scarce. In this article, we present a challenging case of neonatal-onset dRTA and contribute two novel variants of the ATP6V0A4 gene and a novel phenotype associated with a pathogenic variant on ATP6V0A4 to the scientific community. We also review the existing literature on hereditary causes of dRTA, with emphasis on associated renal and extrarenal complications.

#4989 FIRST INTERIM ANALYSIS OF THE INTERNATIONAL DISTAL RENAL TUBULAR ACIDOSIS REGISTRY

Abstract Background and Aims Primary Distal Renal Tubular Acidosis (dRTA) is a rare genetic condition characterized by impaired capacity of A-type intercalated cells to excrete excess of H+ ions. The disease is caused primarily by pathogenic variants in the SLC4A1 gene encoding for the basolateral anion exchanger 1 (AE1) or the ATP6V1B1 and ATP6V0A4 genes encoding for two subunits of the apical proton ATPase. The latter also cause variable degrees of sensorineural hearing loss (SNHL). Additional genes include WDR72 that is associated with amelogenesis imperfecta and FOXI1 that also causes SNHL. To understand better the impact of alkali treatment on the natural history of dRTA, a European prospective registry has been created in collaboration with the ERKNet European Reference Network. Method We present a first interim descriptive analysis of first 182 patients that have been collected from 2019 to 2022. Treatment data are under analysis and are not included. Results To date, mostly paediatric patients (79%) have been included in the registry. A genetic diagnosis was available in 111/182 (61%) of patients and showed the following distribution of underlying genes: ATP6V0A4: 45%, ATP6V1B1: 41%, SLC4A1: 14%. Variants in WDR72 and FOXI1 were reported in one patient each. Overall, we observed no major differences in blood pressure and in urinary/blood parameters, when comparing children vs adults or proton ATPase vs AE1 defects (partially shown in tables 1 and 2). Adult patients were on average diagnosed later, had more frequently nephrolithiasis and higher BMI. In part, differences may reflect selections biases that will be assessed with the collection of longitudinal data. As expected, patients with proton pump defects were diagnosed earlier and had more frequently SNHL. The degree of nephrocalcinosis and the estimated glomerular filtration rates were similar when comparing all groups. None of the patients had developed end stage kidney disease. Conclusion This first report demonstrates successful enrolment of a cohort of patients with dRTA. Enrolment is still ongoing. Longitudinal data will allow assessing the impact of alkali therapy on the long-term outcome.

Genotype–Phenotype Characteristics of Children with Distal Renal Tubular Acidosis Caused by WDR72 Mutations

Background: Distal renal tubular acidosis (dRTA) can be acquired or inherited. Hereditary types of dRTA are mostly seen in the pediatric population, whereas acquired forms predominate in adults. The diagnosis of hereditary dRTA can be confirmed by genetic testing. Five genes are known to cause the disease: ATP6V1B1, ATP6V0A4, FOXI1, SLC4A1, and WDR72. Most of the children with autosomal dominant forms carry mutations in the SLC4A1 gene, whereas the most common mutation in autosomal recessive type is ATP6V0A4 and ATP6V1B1 genes. Objective: In this systematic review of case reports, we discuss the clinical features and mutation variants of hereditary dRTA due to homozygous pathogenic variations in the WDR72 gene, which has been recently identified. Methods: The PubMed and Google Scholar databases were searched with defined search terms and eligibility criteria independently by different authors. Results: Of five full-text articles retrieved, four were finally included which provided data of 14 individuals, all of which had specific mutations in WDR72 gene. The majority of cases were attributed to individuals of Asian descent (71%) with equal distribution of males and females. The mean age of onset was 4.42 years. Amelogenesis imperfecta (AI) was described in 90% of patients, nephrocalcinosis in 62.5%, polyuria in 55.5%, proximal muscle weakness in 55.5%, and rickets in two patients. Conclusion: In patients exhibiting features such as metabolic acidosis, hypokalemia, AI, polyuria, nephrocalcinosis, and growth retardation, genetic analysis for WDR72 mutation should be considered.

Genotype-phenotype analysis and prognosis in children with primary distal renal tubular acidosis

Objective: The purpose of this study was to investigate the relationship between genotypes and clinical phenotypes of primary distal renal tubular acidosis (dRTA) in children. Methods: Clinical information, genetic testing information and follow-up data (until March 2021) of children with dRTA from Children's Hospital of Chongqing Medical University (from January 2010 to December 2020) were analyzed retrospectively. According to different pathogenic genes, patients were divided into SLC4A1 gene and ATP6V0A4+ATP6V1B1 gene groups. Age at onset, clinical manifestations and laboratory findings were compared. Self-comparisons of height standard deviation score (HtSDS), weight standard deviation score (WtSDS), blood pH and serum potassium before and after treatment were tested. T-test, Fisher's exact test and rank sum test were used to analyze among groups. Results: Among 27 children with dRTA (16 boys and 11 girls), the age of onset was 33.4 (10.0, 36.0) months.There were 22 patients (81%) with SLC4A1 gene variation, 3 patients (11%) with ATP6V1B1 gene variation and 2 patients (8%) with ATP6V0A4 gene variation. Totally 22 patients (81%) with renal calcium deposition, 19 patients (70%) hypokalemia, 18 patients (67%) short stature, 16 patients (59%) malnutrition, 16 patients (59%) rickets, and 15 patients (56%) polydipsia and polyuria. Noteworthily, the genotyping results indicated that the age at onset in SLC4A1 gene group was older than that in ATP6V0A4+ATP6V1B1 gene group, with a statistically significant difference (27.3 (12.0, 36.0) vs. 8.2 (2.5, 15.0) months, H=6.33, P=0.012). However, there were no significant differences in clinical manifestations or laboratory test results (all P>0.05). Furthermore, the course of disease was 3.9 (1.3, 6.0) years and the follow-up period was 3.1 (1.0, 4.5) years in 27 patients. In addition, there were no significant differences in recovery rate of clinical manifestations and last laboratory findings between SLC4A1 gene group and ATP6V0A4+ATP6V1B1 gene group (all P>0.05). HtSDS and WtSDS of those patients significantly increased after treatment (-3.2±1.9 vs. -2.1±1.1, -2.5±1.5 vs. 0±1.9, t=-2.94, -5.44, both P<0.01). Serum K+ and blood pH were restored eventually ((3.2±0.5) vs. (4.0±0.5) mmol/L, 7.27±0.07 vs. 7.37±0.07, t=-4.92, -5.25, both P<0.01). Totally 14 patients had normalized serum potassium, 12 patients had normalized blood pH, but only 4 patients had normalized serum bicarbonate concentration and normal base excess. Conclusions: The age of onset of patients who had SLC4A1 gene mutation was older than that of patients with ATP6V0A4 gene and ATP6V1B1 gene mutations. However, there was no obvious correlation between the condition and prognosis of the dRTA patients and pathogenic genes. Early diagnosis, early treatment, regular follow-up and timely adjustment of the dosage of medication can significantly improve the prognosis of dRTA in children. Serum bicarbonate concentration and actual base excess might not be the necessory indicators to assess clinical recovery.

Genetic testing enables a precision medicine approach for nephrolithiasis and nephrocalcinosis in pediatrics: a single-center cohort

ObjectiveHereditary factors are the main cause of pediatric nephrolithiasis (NL)/nephrocalcinosis (NC). We summarized the genotype–phenotype correlation of hereditary NL/NC in our center, to evaluate the role of genetic testing in early diagnosis.MethodsThe clinical data of 32 NL/NC cases, which were suspected to have an inherited basis, were retrospectively analyzed from May 2017 to August 2020. The trio-whole exome sequencing was used as the main approach for genetic testing, variants were confirmed by Sanger sequencing, and pathogenicity analysis according to protein function was predicted with custom-developed software.ResultsCausative monogenic mutations were detected in 24 of 32 NL/NC patients, and copy number variation was detected in one patient. A summary of manifestations in patients with inherited diseases revealed a significant degree of growth retardation, increased urinary excretion of the low-molecular weight protein, hypercalciuria, electrolyte imbalances, and young age of onset to be common in heredity disease. In addition, some patients had abnormal renal function (3 ppm 25). The most frequent pathology identified was distal renal tubular acidosis (with inclusion of SLC4A1, ATP6V1B1, and ATP6VOA4 genes), followed by Dent disease (CLCN5 and OCRL1 genes), primary hyperoxaluria (PH) (AGXT and HOGA1 genes) and Kabuki syndrome (KMT2D gene), which was more likely to present as NC or recurrent stone and having a higher correlation with a specific biochemical phenotype and extrarenal phenotype.ConclusionThe etiology of NL/NC is heterogeneous. This study explored in depth the relationship between phenotype and genotype in 32 patients, and confirmed that genetic testing and clinical phenotype evaluation enable the precision medicine approach to treating patients.

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.

Identification of seven exonic variants in the SLC4A1, ATP6V1B1, and ATP6V0A4 genes that alter RNA splicing by minigene assay.

Primary distal renal tubular acidosis (dRTA) is a rare tubular disease associated with variants in SLC4A1, ATP6V0A4, ATP6V1B1, FOXⅠ1, or WDR72 genes. Currently, there is growing evidence that all types of exonic variants can alter splicing regulatory elements, affecting the precursor messenger RNA (pre-mRNA) splicing process. This study was to determine the consequences of variants associated with dRTA on pre-mRNA splicing combined with predictive bioinformatics tools and minigene assay. As a result, among the 15 candidate variants, 7 variants distributed in SLC4A1 (c.1765C>T, p.Arg589Cys), ATP6V1B1 (c.368G>T, p.Gly123Val; c.370C>T, p.Arg124Trp; c.484G>T, p.Glu162* and c.1102G>A, p.Glu368Lys) and ATP6V0A4 genes (c.322C>T, p.Gln108* and c.1572G>A, p.Pro524Pro) were identified to result in complete or incomplete exon skipping by either disruption of exonic splicing enhancers (ESEs) and generation of exonic splicing silencers, or interference with the recognition of the classic splicing site, or both. To our knowledge, this is the first study on pre-mRNA splicing of exonic variants in the dRTA-related genes. These results highlight the importance of assessing the effects of exonic variants at the mRNA level and suggest that minigene analysis is an effective tool for evaluating the effects of splicing on variants in vitro.

Molecular aspects and long-term outcome of patients with primary distal renal tubular acidosis.

Primary distal renal tubular acidosis (dRTA) is a rare genetic disorder caused by impaired distal mechanisms of urinary acidification. Most cases are secondary to pathogenic variants in ATP6V0A4, ATP6V1B1, and SLC4A1 genes, which encode transporters regulating acid-base balance in the collecting duct. Retrospective study of molecular and clinical data from diagnosis and long-term follow-up (10, 20, and 40±10 years) of 16 patients with primary dRTA diagnosed in childhood. Molecular analyses revealed nine patients had ATP6V0A4 pathogenic variants, five in ATP6V1B1, and two in SLC4A1. A novel intragenic deletion and a common ATP6V0A4 gene variant (c.1691 + 2dupT) in ATP6V0A4 occurred in two-thirds of these patients, suggesting a founder effect. Median age at diagnosis was 3.25 months (IQR 1, 13.5), which was higher in the SLC4A1 group. Median SDS height at diagnosis was -1.02 (IQR -1.79, 0.14). Delayed clinical diagnosis was significantly related to growth failure (P = 0.01). Median SDS height at 20 years follow-up was -1.23 (IQR -1.71, -0.48), and did not significantly improve from diagnosis (P=0.76). Kidney function declined over time: at last follow-up, 43% had moderate to severe chronic kidney disease (CKD). Adequate metabolic control was not related to CKD development. Incidence of sensorineural hearing loss (SNHL) was high in ATP6V1B1 patients, though not universal. Patients harboring ATP6V0A4 variants also developed SNHL at a high rate (80%) over time. Patients with dRTA can develop moderate to severe CKD over time with a high frequency despite adequate metabolic control. Early diagnosis ameliorates long-term height prognosis.

Phenotype and Genotype Profile of Children with Primary Distal Renal Tubular Acidosis: A 10-Year Experience from a North Indian Teaching Institute.

Primary distal renal tubular acidosis (dRTA) or Type 1 RTA in children is caused by a genetic defect (involved genes ATP6V0A4 , ATP6V1B1 , SLC4A1 , FOXI1 , or WDR72 ), which causes tubular transport defects characterized by an inability to appropriately acidify urine with resultant persistent hyperchloremic metabolic acidosis. Retrospective analysis of 28 children (14 males) under the age of 14 years with dRTA seen from 2010 to 2019 was reviewed, and detailed clinic records were analyzed. The clinical features, investigations, and response to treatment were recorded. The median age of the children at presentation was 30 months (range: 9.25-72 months), and the median age at onset of symptoms was 2 months. All the children had growth failure, polyuria, and polydipsia at presentation. Mean serum potassium, pH, bicarbonate, and anion gap at presentation was 2.3 ± 0.5 mmol/L, 7.22 ± 0.09, 13.28 ± 4.37 mmol/L, and 9.3 ± 2.18, respectively. Mean serum potassium, pH, bicarbonate at follow-up was 3.88 ± 0.6 mmol/L, 7.35 ± 0.06, and 20.13 ± 4.17 mmol/L, respectively. The median z-score for the weight for age and height for age at initial presentation was -4.77 (-7.68 to -3.74) and -4.21 (-5.42 to -2.37) and at follow-up was -3.35 (-5.29 to -1.55) and -3.84 (-5.36 to -1.63), respectively. Twenty-two (78.6%) children had medullary nephrocalcinosis. Four children had sensorineural hearing loss. Seven children had genetic testing done, and six had pathogenic or likely pathogenic variants in ATP6V1B1 and ATP6V0A4 gene. Children with dRTA have a guarded prognosis and ATP6V1B1 and ATP6V0A4 mutations are the most common implicated genetic defect in Indian children with distal RTA.

Evaluation of phenotypic and genotypic features of children with distal kidney tubular acidosis.

The present study aimed to assess genotype-phenotype correlations with long-term prognosis in children with distal kidney tubular acidosis (dKTA). The kidney function of children with dKTA could be impaired in the long-term. Thirty-one children with dKTA from 23 families were included in the present study. Demographic features, growth parameters, clinical manifestations, follow-up results, and genetic analysis results of the patients were recorded. Eighteen children (58.1%) were male. The median age at diagnosis was 3months. The median follow-up period was 77months and the longest was 23.5years. Eight (28.8%) patients had chronic kidney disease (CKD) stage 2 or 3. Three patients aged 24, 23, and 19years had CKD stage 3 with an estimated glomerular filtration rate of 54, 57, and 48mL/min/1.73m2, respectively. Thirteen patients had mutations in the ATP6V0A4 gene, eight had mutations in the ATP6V1B1 gene, and three had mutations in the SLC4A1 gene. There was no significant correlation between molecular diagnosis and CKD. Growth retardation with a height below a standard deviation (SD) score of - 2 was found in 14 patients (45.1%) at the time of diagnosis. The mean height SD score at the last visit was significantly higher in patients who had adequate metabolic control at > 75% of all visits as compared with that in patients who did not. Patients with dKTA usually have a good clinical prognosis in childhood with appropriate treatment; however, dRTA is characterized by deterioration of kidney function in adulthood, particularly after puberty.

Analysis of correlation between clinical phenotypes and genotypes in children with distal renal tubular acidosis

Objective To analyze the correlation between clinical phenotypes and genotypes in 6 children with primary distal renal tubular acidosis (dRTA). Methods The clinical data of 6 children confirmed as dRTA in Wuhan Children′s Hospital, Tongji Medical College, Huazhong University of Science & Technology from November 2017 to August 2019 were collected, and related auxiliary examination was performed to assess their growth and development.The venous whole blood was reserved for Trio whole exome sequencing, and full spectrum genetic disease accurate diagnosis cloud platform was applied to systematic data screening and analysis.The suspected mutations were checked by Sanger sequencing, and then the role of protein was predicted by software. Results Clinical manifestations, signs and auxiliary examination results of the 6 children accorded with the diagnostic criteria of dRTA, and the prominent characteristics was growth retardation.One case had knee valgus, one had osteoporosis, and the auxiliary examination results showed that both of them had alkaline urine, metabolic acidosis, and hypokalemia.Three children had nephrocalcinosis, and 2 children had nephrolithiasis.The parents of the 6 patients were all normal without phenotypes.Mutations in the SLC4A1 gene were identified in 4 patients, including 1 child with a reported homozygous autosomal recessive missense mutation(c.2102G>A, p.G701D), who had dRTA and hemolytic anemia, and 3 children with the reported de novo heterozygous autosomal dominant missense mutation(c.1766G>A, p.R589H, c.1765C>T, p.R589C), whose age at diagnosis was related to abnormal renal imaging.Compound heterozygous autosomal recessive mutations in the ATPV1B1 gene were identified in 1 patient, and they were novel heterozygous missense mutations (1153C>A, p.P385T and c. 806C>T, p.P269L). A novel homozygous autosomal recessive missense mutation was identified in 1 patient in the ATPV0A4 gene(c.1899C>A, p.Y633X, 208). Conclusions Mutations in SLC4A1, ATP6V1B1, ATP6V0A4 genes are identified as the main causes of the primary dRTA, and the phenotypes was related to the mutation features and genotypes.Genetic test should be conducted on patients suspected as dRTA for early molecular diagnosis, thereby improving clinical phenotypic screening and individualized treatment. Key words: Distal renal tubular acidosis; Genetic mutation; SLC4A1 gene; ATP6V1B1 gene; ATP6V0A4 gene

Screening and function discussion of a hereditary renal tubular acidosis family pathogenic gene

Hereditary distal renal tubular acidosis (dRTA) is a rare disease of H+ excretion defect of α-intercalated cells in renal collecting duct, caused by decreased V-ATPase function due to mutations in the ATP6V1B1 or ATP6V0A4 genes. In the present study, a genetic family with 5 members of the complete dRTA phenotype were found with distal tubule H+ secretion disorder, hypokalemia, osteoporosis, and kidney stones. A variant NM_020632.2:c.1631C > T (p.Ser544Leu) in exon 16 on an ATP6V0A4 gene associated with dRTA was detected by next generation sequencing target region capture technique and verified by Sanger sequencing, which suggested that except for one of the patients who did not receive the test, the other four patients all carried the p.S544L heterozygote. In transfected HEK293T cells, cells carrying p.S544L-mut showed early weaker ATPase activity and a slower Phi recovery rate after rapid acidification. By immunofluorescence localization, it was observed that the expression level of p.S544L-mut on the cell membrane increased and the distribution was uneven. Co-immunoprecipitation showed the a4 subunit of ATP6V0A4/p.S544L-mut could not bind to the B1 subunit, which might affect the correct assembly of V-ATPase. The present study of dRTA family suggests that the p.S544L variant may be inherited in a dominant manner.

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.

Novel compound heterozygous ATP6V1B1 mutations in a Chinese child patient with primary distal renal tubular acidosis: a case report

BackgroundDistal renal tubular acidosis (dRTA) is a heterogeneous disorder characterized by normal anion gap metabolic acidosis. Autosomal recessive dRTA is usually caused by mutations occurring in ATP6V1B1 and ATP6V0A4 genes,encoding subunits B1 and a4 of apical H+-ATPase, respectively. The heterogeneous clinical manifestations of dRTA have been described in different ethnic groups harboring distinct mutations. Most of the reported cases are from Europe and Africa. At present, the prevalence of primary dRTA is still poorly elucidated in Chinese population.Case presentationA 2-year and six-month-old female patient was hospitalized because of recurrent hypokalemia, hyperchloremic metabolic acidosis and growth retardation. Laboratory investigations presented a normal anion gap hyperchloremic metabolic acidosis, hypokalemia, and inappropriate alkaline urine. Renal ultrasound indicated bilateral nephrocalcinosis. Bilateral sensorineural hearing loss (SNHL) was confirmed with moderately severe (45 dB) on the left ear and severe (80 dB) on the right ear, which was accompanied with enlarged vestibular aqueduct (EVA) on both sides. According to these findings, a diagnosis of dRTA was made. To identify the pathogenic gene mutation, all coding regions of ATP6V1B1 and ATP6V0A4 gene, including intron-exon boundaries, were analyzed using PCR followed by direct sequence analysis. The splicing variants were verified in peripheral blood leucocytes of the patient by RT-PCR. As a result, two novel heterozygous mutations in ATP6V1B1 were identified in the child. One mutation was a successive 2-nucleotide deletion in exon 2(c.133-134delTG), which caused a marked nonsense mediated mRNA decay. The other was a guanine to adenine substitution of the first nucleotide of intron 8(c.785 + 1 G > A), which led to the exclusion of exon 8. After treatment with sodium citrate, potassium citrateand citric acid, metabolic acidosis and hypokalemia were corrected, but her hearing decreased gradually during the 2 years and had to accept the use of bilateral hearing aids.ConclusionsWe described two novel dRTA associated mutations in ATP6V1B1 identified in a Chinese child patient accompanying with SNHL and EVA. Our study will help to expand the understanding of this rare disease in Chinese population.

Etiology and pathogenesis of hereditary renal tubular acidosis

Renal tubular acidosis(RTA) is a hyperchloremic metabolic acidosis characterized by a normal anion gap due to reduced urinary acidification.Multiple mechanisms act in the renal tubule to maintain balance in hydrogen ion secretions and bicarbonate absorption.In recent years, more and more proteins have been found to be associated with RTA, so hereditary RTA has attracted people′s attention.Mutations in the gene SLC4A1, encoding Cl-/HCO3- exchanger (kAE1); in the gene ATP6V1B1, encoding B1 subunit of H+ -ATP ase; in the gene ATP6V0A4, encoding a4 subunit of H+ -ATP ase; in the gene SLC4A4, encoding Na+ /HCO3-cotransporter(kNBCe1); in the gene CA2, encoding carbonic anhydrase Ⅱ are identified in the pathogenesis of hereditary RTA.The search for pathogenetic genes for clinically suspected hereditary RTA patients can help us to make accurate genetic diagnosis and provide targeted treatment interventions. Key words: Renal tubular acidosis; Hereditary; Pathogenetic gene

Distal renal tubular acidosis. Clinical manifestations in patients with different underlying gene mutations.

To evaluate whether there are differences in the phenotype of primary distal renal tubular acidosis (dRTA) patients according to the causal defective gene. Twenty-seven non-oriental patients with genetically confirmed dRTA were grouped according to the identified underlying mutations in either ATP6V1B1 (n= 10), ATP6V0A4 (n= 12), or SLC4A1 (n= 5) gene. Demographic features, growth impairment, biochemical variables and presence of deafness, nephrocalcinosis, and urolithiasis at diagnosis were compared among the three groups. Patients with SLC4A1 mutations presented later than those with ATP6V1B1 or ATP6V0A4 defects (120 vs. 7 and 3months, respectively). Hearing loss at diagnosis was present in the majority of patients with ATP6V1B1 mutations, in two patients with ATP6V0A4 mutations, and in none of cases harboring SLC4A1 mutations. Serum potassium concentration (X ± SD) was higher in SLC4A1 group (3.66 ± 0.44mEq/L) than in ATP6V0A4 group (2.96 ± 0.63mEq/L) (p= 0.046). There were no differences in the other clinical or biochemical variables analyzed in the three groups. This study indicates that non-oriental patients with dRTA caused by mutations in the SLC4A1 gene present later and have normokalemia or milder hypokalemia. Hypoacusia at diagnosis is characteristically associated with ATP6V1B1 gene mutations although it may also be present in infants with ATP6V0A4 defects. Other phenotypical manifestations do not allow predicting the involved gene.

Haploinsufficiency of the Mouse Atp6v1b1 Gene Leads to a Mild Acid-Base Disturbance with Implications for Kidney Stone Disease

Background/Aims: Homozygous mutations or deletion of the ATP6V1B1 gene encoding for the B1 subunit of the vacuolar H⁺-ATPase leads to distal renal tubular acidosis in man and mice. In humans, heterozygous carriers of B1 mutations can develop incomplete dRTA with nephroclacinosis. Here, we investigated whether Atp6v1b1^+/- mice also develop acid-base disturbances during an HCl acid load. Methods: We subjected Atp6v1b1^+/+, Atp6v1b1^+/-, Atp6v1b1^-/- to an HCl-load for 7 days and investigated acid-base status, kidney function, and expression of renal acid-base transport proteins. Results: Atp6v1b1^-/- mice had more alkaline urine and low ammoniuria, whereas Atp6v1b1^+/- mice showed no difference in their urine parameters but higher blood chloride and lower blood pCO₂ compared to controls. Subcellular localization of a4 and B2 subunits of H⁺-ATPase were unchanged within the 3 genotypes and Atp6v1b1^+/+ and Atp6v1b1^+/- mice exhibited a similar luminal localization of B1 subunit in intercalated cells. However, B1, B2 and a4 expression were decreased in renal membrane fractions from Atp6v1b1^+/- mice compared to Atp6v1b1^+/+ while B2 and a4 were unchanged and B1 protein was reduced in Atp6v1b+^-/- kidneys. Compensatory mechanisms of B1 ablation were found only in the collecting duct with a down-regulation of pendrin in Atp6v1b1^-/- mice. Conclusions: In conclusion, 1) Atp6v1b1^+/- mice developed a mild incomplete dRTA. dRTA is partly compensated by respiration. 2) Compensatory mechanisms for the absence of B1 take place only in the collecting duct of Atp6v1b1^-/- kidneys.

An in vitro splicing assay reveals the pathogenicity of a novel intronic variant in ATP6V0A4 for autosomal recessive distal renal tubular acidosis

BackgroundAutosomal recessive distal renal tubular acidosis (dRTA) is a rare hereditary disease caused by pathogenic variants in the ATP6V0A4 gene or ATP6V1B1 gene, and characterized by hyperchloremic metabolic acidosis with normal anion gap, hypokalemia, hypercalciuria, hypocitraturia and nephrocalcinosis. Although several intronic nucleotide variants in these genes have been detected, all of them fell in the apparent splice consensus sequence. In general, transcriptional analysis is necessary to determine the effect on function of the novel intronic variants located out of splicing consensus sequences. In recent years, functional splicing analysis using minigene construction was used to assess the pathogenicity of novel intoronic variant in various field.MethodsWe investigated a sporadic case of dRTA with a compound heterozygous mutation in the ATP6V0A4 gene, revealed by next generation sequencing. One variant was already reported as pathogenic; however, the other was a novel variant in intron 11 (c.1029 + 5G > A) falling outside of the apparent splicing consensus sequence. Expression of ATP6V0A4 was not detected in peripheral leukocytes by RT-PCR analysis. Therefore, an in vitro functional splicing study using minigene construction was conducted to analyze the splicing pattern of the novel variant.ResultsA minigene assay revealed that the novel intronic variant leads to a 104 bp insertion immediately following exon 11. In addition, this result was confirmed using RNA extracted from the patient’s cultured leukocytes.ConclusionThese results proved the pathogenicity of a novel intronic variant in our patient. We concluded that the minigene assay is a useful, non-invasive method for functional splicing analysis of inherited kidney disease, even if standard transcriptional analysis could not detect abnormal mRNA.

Distal renal tubular acidosis in a Libyan patient: Evidence for digenic inheritance

Aim of the studyRecent advances in understanding the underlying molecular mechanism for distal renal tubular acidosis (dRTA), led to an increased attention towards the primary and the familial forms of the disease. Mutations in ATP6V1B1 and ATP6V0A4 are usually responsible for the recessive form of the disease. Mutations in gene AE1 encoding the Cl-/HCO3- exchanger, usually present as dominant dRTA, but a recessive pattern has been recently described. Our objective is to identify the mutational spectrum responsible of dRTA in a consanguineous Libyan family. Materials and methodsBoth ATP6V0A4 and ATP6V1B1 genes were preferentially screened in our patient. Additional whole exome sequencing (WES) in the same patient, offered a wider view on potential chromosomal rearrangements as well as the mutational spectrum of other genes involved in this disease. ResultsThe patient is a heterozygote for two different mutations, one in each of the genes ATP6V0A4 and ATP6V1B1, while no deleterious variation was detected in the remaining genes responsible for the recessive form of dRTA. Homozygosity mapping and WES confirmed our findings and supported the hypothesis of a digenic inheritance model existing as an explanation for dRTA. ConclusionsTo our knowledge, this is the first report describing a Libyan patient with dRTA who suffered from early-onset sensorineural hearing loss, with a digenic mode of inheritance, supported by the identification of two novel mutations. This study increases the understanding of how dRTA is genetically transmitted, while offers a good outline towards the molecular diagnostics and genetic counseling for dRTA in Lybians.