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

Primary Distal Renal Tubular Acidosis: Toward an Optimal Correction of Metabolic Acidosis

The term classic, type 1 renal tubular acidosis or primary distal renal tubular acidosis is used to designate patients with impaired ability to excrete acid normally in the urine as a result of tubular transport defects involving type A intercalated cells in the collecting duct. The clinical phenotype is largely characterized by the complications of chronic metabolic acidosis (MA): stunted growth, bone abnormalities, and nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria and hypocitraturia. All these manifestations are preventable with early and sustained correction of MA with alkali therapy. The optimal target for plasma bicarbonate should be as close as possible to the range considered normal by current standards (between 23 and 28 mEq/L.). Most of the benefits of alkali therapy are tangible early in the course of the disease in childhood, but life-long treatment is required to prevent the vast array of complications attributable to chronic MA.

Renal calcification in children with renal tubular acidosis: What a paediatrician ‎should ‎know‎

Renal tubular acidosis (RTA) can lead to renal calcification in children, which can cause various complications and impair renal function. This review provides pediatricians with a comprehensive understanding of the relationship between RTA and renal calcification, highlighting essential aspects for clinical management. The article analyzed relevant studies to explore the prevalence, risk factors, underlying mechanisms, and clinical implications of renal calcification in children with RTA. Results show that distal RTA (type 1) is particularly associated with nephrocalcinosis, which presents a higher risk of renal calcification. However, there are limitations to the existing literature, including a small number of studies, heterogeneity in methodologies, and potential publication bias. Longitudinal data and control groups are also lacking, which limits our understanding of long-term outcomes and optimal management strategies for children with RTA and renal calcification. Pediatricians play a crucial role in the early diagnosis and management of RTA to mitigate the risk of renal calcification and associated complications. In addition, alkaline therapy remains a cornerstone in the treatment of RTA, aimed at correcting the acid-base imbalance and reducing the formation of kidney stones. Therefore, early diagnosis and appropriate therapeutic interventions are paramount in preventing and managing renal calcification to preserve renal function and improve long-term outcomes for affected children. Further research with larger sample sizes and rigorous methodologies is needed to optimize the clinical approach to renal calcification in the context of RTA in the pediatric population.

Anterior cruciate ligament rupture in a patient with Albers-Schonberg disease

Background

Osteopetrosis is an uncommon inherited disease marked with elevated bone density and frequent bone fractures owing to flawed osteoclast activity. Autosomal dominant osteopetrosis type 2 (ADO-2), a benign form of osteopetrosis, is also known as Albers-Schonberg disease.

Case presentation

We report the first successful anterior cruciate ligament (ACL) reconstruction surgery for ACL rupture treatment in a 30-year-old female with ADO-2, who carried a heterozygous missense mutation c.2227C > T (p.Arg743Trp) in exon 23 of the chloride channel 7 (CLCN7) gene. Histopathological analysis of the ruptured ACL sample revealed massive calcium salt deposition in the ligament tissue. A ligament advanced reinforcement system (LARS) artificial ligament was employed in her ACL reconstruction surgery. At her final 16 month’s follow-up, she reported no knee instability symptoms and other complications. The range of motion of the affected knee was good. The side-to-side difference in knee laxity, as evidenced by a KT-1000 arthrometer was 0.9 mm. The Lysholm score improved from 45 before operation to 83 after operation. The Tegner activity score improved from 1 before operation to 4 after operation.

Conclusions

Our findings further confirmed that the newly identified mutated locus (p.Arg743Trp) may lead to acid secretion disorders at different sites (including calcified ACL in our case). In terms of clinical treatment, ligament reconstruction surgery in patients with Albers-Schonberg disease presents a unique challenge to orthopedic surgeons and requires further preparation and time.

Genetic heterogeneity in GJB2, COL4A3, ATP6V1B1 and EDNRB variants detected among hearing impaired families in Morocco

BACKGROUND

Deafness is the most prevalent human sensorineural defect. It may occur as a result of an external auditory canal involvement, or a deficiency in the sound conduction mechanism, or an impairment of the cochlea, the cochlear nerve or central auditory perception. The genetic causes are the most common, as approximately 70% of hearing disorders are of hereditary origin, divided into two groups, syndromic (associated with other symptoms) and no syndromic (isolated deafness).

METHODS

A whole exome sequencing was performed to identify the genetic cause of hearing loss in six Moroccan families and Sanger sequencing was used to validate mutations in these genes.

THE RESULTS

The results of four out of the six families revealed four genetic variants in the genes GJB2, COL4A3, ATP6V1B1 and EDNRB responsible for non-syndromic and syndromic hearing loss. Multiple Bioinformatics programs and molecular modelling predicted the pathogenic effect of these mutations.

CONCLUSIONS

We identified in Moroccan deaf patients four homozygous mutations. These results show the importance of whole exome sequencing to identify pathogenic mutations in heterogeneous disorders with multiple genes responsible.

Custom Next-Generation Sequencing Identifies Novel Mutations Expanding the Molecular and clinical spectrum of isolated Hearing Impairment or along with defects of the retina, the thyroid, and the kidneys

Background

In the Tunisian population, the molecular analysis of hearing impairment remains based on conventional approaches, which makes the task laborious and enormously expensive. Exploration of the etiology of Hearing Impairment and the early diagnosis of causal mutations by next‐generation sequencing help significantly alleviate social and economic problems.

Methods

We elaborated a custom SureSelect^QXT panel for next‐generation sequencing of the coding sequences of 42 genes involved in isolated hearing impairment or along with defects of the retina, the thyroid, and the kidneys.

Results

We report eight pathogenic variants, four of which are novel in patients with isolated hearing impairment, hearing impairment, and renal tubular acidosis, Usher syndrome and Pendred syndrome. Functional studies using molecular modeling showed the severe impact of the novel missense mutations on the concerned proteins. Basically, we identified mutations in nuclear as well as mitochondrial genes in a Tunisian family with isolated hearing impairment, which explains definitely the phenotype detected since 2006.

Conclusion

Our results expanded the mutation spectrum and genotype‒phenotype correlation of isolated and syndromic hearing loss and also emphasized the importance of combining both targeted next‐generation sequencing and detailed clinical evaluation to elaborate a more accurate diagnosis for hearing impairment and related phenotypes especially in North African populations.

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

BACKGROUND

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.

METHODS

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.

RESULTS

Eighteen children (58.1%) were male. The median age at diagnosis was 3 months. The median follow-up period was 77 months and the longest was 23.5 years. Eight (28.8%) patients had chronic kidney disease (CKD) stage 2 or 3. Three patients aged 24, 23, and 19 years had CKD stage 3 with an estimated glomerular filtration rate of 54, 57, and 48 mL/min/1.73 m², 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.

CONCLUSION

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.

Molecular Mechanisms and Biological Functions of Autophagy for Genetics of Hearing Impairment

The etiology of hearing impairment following cochlear damage can be caused by many factors, including congenital or acquired onset, ototoxic drugs, noise exposure, and aging. Regardless of the many different etiologies, a common pathologic change is auditory cell death. It may be difficult to explain hearing impairment only from the aspect of cell death including apoptosis, necrosis, or necroptosis because the level of hearing loss varies widely. Therefore, we focused on autophagy as an intracellular phenomenon functionally competing with cell death. Autophagy is a dynamic lysosomal degradation and recycling system in the eukaryotic cell, mandatory for controlling the balance between cell survival and cell death induced by cellular stress, and maintaining homeostasis of postmitotic cells, including hair cells (HCs) and spiral ganglion neurons (SGNs) in the inner ear. Autophagy is considered a candidate for the auditory cell fate decision factor, whereas autophagy deficiency could be one of major causes of hearing impairment. In this paper, we review the molecular mechanisms and biologic functions of autophagy in the auditory system and discuss the latest research concerning autophagy-related genes and sensorineural hearing loss to gain insight into the role of autophagic mechanisms in inner-ear disorders.

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.

A novel homozygous deletion in ATP6V0A4 causes distal renal tubular acidosis: A case report

RATIONALE

Autosomal recessive distal renal tubular acidosis (dRTA) is a rare condition, most cases of which are caused by genetic mutations. Several loss-of-function mutations in the ATP6V0A4 gene have been recently reported.

PATIENT CONCERNS

A 2-month, 24-day-old Chinese girl presenting with vomiting and diarrhea.

DIAGNOSIS

dRTA was established by metabolic acidosis and hypokalemia. Mutational analysis of the ATP6V0A4 gene revealed a homozygous deletion of exons 13 and 14. The father was found to have a heterozygous loss of both exons, whereas the mother was normal.

INTERVENTIONS

Patient was treated with potassium citrate.

OUTCOMES

The patient has shown normal pH and potassium levels.

LESSONS

This is the first case of a homozygous deletion in ATP6V0A4 reported in the literature. Although the initial auditory screening was normal in this case, this patient will nevertheless undergo long-term auditory testing.

A subunit of V-ATPases, ATP6V1B2, underlies the pathology of intellectual disability

Background

Dominant deafness-onychodystrophy (DDOD) syndrome is a rare disorder mainly characterized by severe deafness, onychodystrophy and brachydactyly. We previously identified c.1516C > T (p.Arg506X) in ATP6V1B2 as cause of DDOD syndrome, accounting for all cases of this genetic disorder. Clinical follow-up of DDOD syndrome patients with cochlear implantation revealed the language rehabilitation was unsatisfactory although the implanted cochlea worked well, which indicates there might be learning and memory problems in DDOD syndrome patients. However, the underlying mechanisms were unknown.

Methods

atp6v1b2 knockdown zebrafish and Atp6v1b2 c.1516C > T knockin mice were constructed to explore the phenotypes and related mechanism. In mutant mice, auditory brainstem response test and cochlear morphology analysis were performed to evaluate the auditory function. Behavioral tests were used to investigate various behavioral and cognitive domains. Resting-state functional magnetic resonance imaging was used to evaluate functional connectivity in the mouse brain. Immunofluorescence, Western blot, and co-immunoprecipitation were performed to examine the expression and interactions between the subunits of V-ATPases.

Findings

atp6v1b2 knockdown zebrafish showed developmental defects in multiple organs and systems. However, Atp6v1b2 c.1516C > T knockin mice displayed obvious cognitive defects but normal hearing and cochlear morphology. Impaired hippocampal CA1 region and weaker interaction between the V1E and B2 subunits in Atp6v1b2^{Arg506X//Arg506X} mice were observed.

Interpretation

Our study extends the phenotypic range of DDOD syndrome. The impaired hippocampal CA1 region may be the pathological basis of the behavioral defects in mutant mice. The molecular mechanism underlying V-ATPases dysfunction involves a weak interaction between subunits, although the assembly of V-ATPases can still take place.

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

BACKGROUND

Distal 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 PRESENTATION

A 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.

CONCLUSIONS

We 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.

Nephrolithiasis secondary to inherited defects in the thick ascending loop of henle and connecting tubules

Twin and genealogy studies suggest a strong genetic component of nephrolithiasis. Likewise, urinary traits associated with renal stone formation were found to be highly heritable, even after adjustment for demographic, anthropometric and dietary covariates. Recent high-throughput sequencing projects of phenotypically well-defined cohorts of stone formers and large genome-wide association studies led to the discovery of many new genes associated with kidney stones. The spectrum ranges from infrequent but highly penetrant variants (mutations) causing mendelian forms of nephrolithiasis (monogenic traits) to common but phenotypically mild variants associated with nephrolithiasis (polygenic traits). About two-thirds of the genes currently known to be associated with nephrolithiasis code for membrane proteins or enzymes involved in renal tubular transport. The thick ascending limb of Henle and connecting tubules are of paramount importance for renal water and electrolyte handling, urinary concentration and maintenance of acid-base homeostasis. In most instances, pathogenic variants in genes involved in thick ascending limb of Henle and connecting tubule function result in phenotypically severe disease, frequently accompanied by nephrocalcinosis with progressive CKD and to a variable degree by nephrolithiasis. The aim of this article is to review the current knowledge on kidney stone disease associated with inherited defects in the thick ascending loop of Henle and the connecting tubules. We also highlight recent advances in the field of kidney stone genetics that have implications beyond rare disease, offering new insights into the most common type of kidney stone disease, i.e., idiopathic calcium stone disease.

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

BACKGROUND

To evaluate whether there are differences in the phenotype of primary distal renal tubular acidosis (dRTA) patients according to the causal defective gene.

METHODS

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.

RESULTS

Patients with SLC4A1 mutations presented later than those with ATP6V1B1 or ATP6V0A4 defects (120 vs. 7 and 3 months, 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 ± 0.44 mEq/L) than in ATP6V0A4 group (2.96 ± 0.63 mEq/L) (p = 0.046). There were no differences in the other clinical or biochemical variables analyzed in the three groups.

CONCLUSIONS

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.

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.