Recessive distal renal tubular acidosis in Sarawak caused by AE1 mutations

Mutations and clinical characteristics of dRTA caused by SLC4A1 mutations: Analysis based on published patients

BACKGROUND AND AIMS

The genetic and clinical characteristics of patients with distal renal tubular acidosis (dRTA) caused by SLC4A1 mutations have not been systematically recorded before. Here, we summarized the SLC4A1 mutations and clinical characteristics associated with dRTA.

METHODS

Database was searched, and the mutations and clinical manifestations of patients were summarized from the relevant articles.

RESULTS

Fifty-three eligible articles involving 169 patients were included and 41 mutations were identified totally. Fifteen mutations involving 100 patients were autosomal dominant inheritance, 21 mutations involving 61 patients were autosomal recessive inheritance. Nephrocalcinosis or kidney stones were found in 72.27%, impairment in renal function in 14.29%, developmental disorders in 61.16%, hematological abnormalities in 33.88%, and muscle weakness in 13.45% of patients. The age of onset was younger (P < 0.01), urine pH was higher (P < 0.01), and serum potassium was lower (P < 0.001) in recessive patients than patients with dominant SLC4A1 mutations. Autosomal recessive inheritance was more often found in Asian patients (P < 0.05).

CONCLUSIONS

The children present with metabolic acidosis with high urinary pH, accompanying hypokalemia, hyperchloremia, nephrocalcinosis, growth retardation and hematological abnormalities should be suspected as dRTA and suggested a genetic testing. The patients with recessive dRTA are generally more severely affected than that with dominant SLC4A1 mutations. Autosomal recessive inheritance was more often found in Asian patients, and more attentions should be paid to the Asian patients.

Impaired trafficking and instability of mutant kidney anion exchanger 1 proteins associated with autosomal recessive distal renal tubular acidosis

Background

Mutations in solute carrier family 4 member 1 (SLC4A1) encoding anion exchanger 1 (AE1) are the most common cause of autosomal recessive distal renal tubular acidosis (AR dRTA) in Southeast Asians. To explain the molecular mechanism of this disease with hematological abnormalities in an affected family, we conducted a genetic analysis of SLC4A1 and studied wild-type and mutant AE1 proteins expressed in human embryonic kidney 293T (HEK293T) cells.

Methods

SLC4A1 mutations in the patient and family members were analyzed by molecular genetic techniques. Protein structure modeling was initially conducted to evaluate the effects of mutations on the three-dimensional structure of the AE1 protein. The mutant kidney anion exchanger 1 (kAE1) plasmid construct was created to study protein expression, localization, and stability in HEK293T cells.

Results

We discovered that the patient who had AR dRTA coexisting with mild hemolytic anemia carried a novel compound heterozygous SLC4A1 mutations containing c.1199_1225del (p.Ala400_Ala408del), resulting in Southeast Asian ovalocytosis (SAO), and c.1331C > A (p.Thr444Asn). Homologous modeling and in silico mutagenesis indicated that these two mutations affected the protein structure in the transmembrane regions of kAE1. We found the wild-type and mutant kAE1 T444N to be localized at the cell surface, whereas the mutants kAE1 SAO and SAO/T444N were intracellularly retained. The half-life of the kAE1 SAO, T444N, and SAO/T444N mutants was shorter than that of the wild-type protein.

Conclusion

These results suggest impaired trafficking and instability of kAE1 SAO/T444N as the likely underlying molecular mechanism explaining the pathogenesis of the novel SLC4A1 compound heterozygous mutation identified in this patient.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01381-y.

Genotypic analysis of SLC4A1 A858D mutation in Indian population associated with distal renal tubular Acidosis (dRTA) coupled with hemolytic anemia

INTRODUCTION

Although distinctive, distal renal tubular acidosis (dRTA) and Hereditary Spherocytosis (HS) shares a common protein, the anion exchanger1 (AE1) encoded by SLC4A1gene. In spite of this, the co-existence of dRTA and HS has rarely been observed. To date, 23 mutations have been identified in SLC4A1 gene causing both autosomal recessive (AR) and autosomal dominant (AD) forms of dRTA.

METHODS

We have assessed the applicability of the High Resolution Melting curve (HRM) method for the detection of SLC4A1 (A858D) mutation in 12 Indian families having AR dRTA coupled with HS. The reliability of the HRM analysis was verified by comparing the results of the HRM method with those of conventional methods such as Polymerase Chain Reaction-Restriction Fragment-Length Polymorphism (PCR-RFLP) and Sanger sequencing thereby confirming the diagnosis.

RESULTS

We here described the clinical, hematological and genetic data of 16 individuals from 12 families having AR dRTA coupled with HS. All patients carried homozygous SLC4A1 (A858D) mutation, whereas their family members had heterozygous A858D obtained by HRM analysis and confirmed by RFLP and Sanger sequencing.

CONCLUSION

Our data indicates that a missense mutation of A858D in SLC4A1 gene is the most common cause of dRTA coupled with HS in the Indian population. HRM analysis can be used as a rapid screening method for common SLC4A1 mutations that cause AR dRTA in the Indian population.

Improving outcomes for patients with distal renal tubular acidosis: recent advances and challenges ahead

Primary distal renal tubular acidosis (dRTA) is a rare genetic disorder caused by impaired distal acidification due to a failure of type A intercalated cells (A-ICs) in the collecting tubule. dRTA is characterized by persistent hyperchloremia, a normal plasma anion gap, and the inability to maximally lower urinary pH in the presence of systemic metabolic acidosis. Common clinical features of dRTA include vomiting, failure to thrive, polyuria, hypercalciuria, hypocitraturia, nephrocalcinosis, nephrolithiasis, growth delay, and rickets. Mutations in genes encoding three distinct transport proteins in A-ICs have been identified as causes of dRTA, including the B1/ATP6V1B1 and a4/ATP6V0A4 subunits of the vacuolar-type H+-ATPase (H+-ATPase) and the chloride-bicarbonate exchanger AE1/SLC4A1. Homozygous or compound heterozygous mutations in ATP6V1B1 and ATP6V0A4 lead to autosomal recessive (AR) dRTA. dRTA caused by SLC4A1 mutations can occur with either autosomal dominant or AR transmission. Red blood cell abnormalities have been associated with AR dRTA due to SLC4A1 mutations, including hereditary spherocytosis, Southeast Asia ovalocytosis, and others. Some patients with dRTA exhibit atypical clinical features, including transient and reversible proximal tubular dysfunction and hyperammonemia. Incomplete dRTA presents with inadequate urinary acidification, but without spontaneous metabolic acidosis and recurrent urinary stones. Heterozygous mutations in the AE1 or H+-ATPase genes have recently been reported in patients with incomplete dRTA. Early and sufficient doses of alkali treatment are needed for patients with dRTA. Normalized serum bicarbonate, urinary calcium excretion, urinary low-molecular-weight protein levels, and growth rate are good markers of adherence to and/or efficacy of treatment. The prognosis of dRTA is generally good in patients with appropriate treatment. However, recent studies showed an increased frequency of chronic kidney disease (CKD) in patients with dRTA during long-term follow-up. The precise pathogenic mechanisms of CKD in patients with dRTA are unknown.

Primary Autosomal Recessive Distal Renal Tubular Acidosis Caused by a Common Homozygous SLC4A1 Mutation in Two Lao Families

Primary distal renal tubular acidosis (dRTA) caused by mutations of the SLC4A1 gene, which encodes for erythroid and kidney isoforms of anion exchanger, shows marked difference in inheritance patterns and clinical features in different parts of the world. While the disease shows autosomal dominant inheritance without any red cell morphological abnormalities in the temperate countries, it is almost invariably recessive, and often accompanies red cell morphological abnormalities or hemolytic anemia in the tropics, especially in Southeast Asia. Here, we report three patients with autosomal recessive (AR) dRTA, presenting with typical findings of failure to thrive and rickets, from two unrelated Lao families. The mutational analyses revealed that all three patients harbored the same homozygous SLC4A1 mutation, p.Gly701Asp. Adequate supplementation of alkali and potassium resulted in remarkable improvement of growth retardation and skeletal deformities of the patients. This is the first case report of Lao patients with AR dRTA caused by SLC4A1 mutations.

Development and Diseases of the Collecting Duct System

The collecting duct of the mammalian kidney is important for the regulation of extracellular volume, osmolarity, and pH. There are two major structurally and functionally distinct cell types: principal cells and intercalated cells. The former regulates Na⁺ and water homeostasis, while the latter participates in acid-base homeostasis. In vivo lineage tracing using Cre recombinase or its derivatives such as CreGFP and CreER^T2 is a powerful new technique to identify stem/progenitor cells in their native environment and to decipher the origins of the tissue that they give rise to. Recent studies using this technique in mice have revealed multiple renal progenitor cell populations that differentiate into various nephron segments and collecting duct. In particular, emerging evidence suggests that like principal cells, most of intercalated cells originate from the progenitor cells expressing water channel Aquaporin 2. Mutations or malfunctions of the channels, pumps, and transporters expressed in the collecting duct system cause various human diseases. For example, gain-of-function mutations in ENaC cause Liddle's syndrome, while loss-of-function mutations in ENaC lead to Pseudohypoaldosteronism type 1. Mutations in either AE1 or V-ATPase B1 result in distal renal tubular acidosis. Patients with disrupted AQP2 or AVPR2 develop nephrogenic diabetes insipidus. A better understanding of the function and development of the collecting duct system may facilitate the discovery of new therapeutic strategies for treating kidney disease.

Genetic causes and mechanisms of distal renal tubular acidosis

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

dRTA and hemolytic anemia: first detailed description of SLC4A1 A858D mutation in homozygous state

Mutations in the anion exchanger 1 (AE1) gene encoding the erythroid and kidney anion (chloride-bicarbonate) exchanger 1 may result in familial distal renal tubular acidosis (dRTA) in association with membrane defect hemolytic anemia. Seven children presenting with hyperchloremic normal anion gap metabolic acidosis, failure to thrive, and compensated hemolytic anemia were studied. Analysis of red cell AE1/Band 3 surface expression by Eosin 5'-maleimide (E5M) was performed in patients and their family members using flow cytometry. Genetic studies showed that all patients carried a common SLC4A1 mutation, c.2573C>A; p.Ala858Asp in exon 19, found as homozygous (A858D/A858D) mutation in the patients and heterozygous (A858D/N) in the parents. Analysis by flowcytometry revealed a single uniform fluorescence peak, with the mean channel fluorescence (MCF) markedly reduced in cases with homozygous mutation, along with a left shift of fluorescence signal but was only mildly reduced in the heterozygous state. Red cell morphology showed striking acanthocytosis in the homozygous state [patients] and only a mild acanthocytosis in heterozygous state [parents]. In conclusion, this is the first description of a series of homozygous cases with the A858D mutation. The E5M flowcytometry test is specific for reduction in the Band 3 membrane protein and was useful in conjunction with a careful morphological examination of peripheral blood smears in our patient cohort.

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

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

The GPA-dependent, spherostomatocytosis mutant AE1 E758K induces GPA-independent, endogenous cation transport in amphibian oocytes

The previously undescribed heterozygous missense mutation E758K was discovered in the human AE1/SLC4A1/band 3 gene in two unrelated patients with well-compensated hereditary spherostomatocytic anemia (HSt). Oocyte surface expression of AE1 E758K, in contrast to that of wild-type AE1, required coexpressed glycophorin A (GPA). The mutant polypeptide exhibited, in parallel, strong GPA dependence of DIDS-sensitive (36)Cl(-) influx, trans-anion-dependent (36)Cl(-) efflux, and Cl(-)/HCO(3)(-) exchange activities at near wild-type levels. AE1 E758K expression was also associated with GPA-dependent increases of DIDS-sensitive pH-independent SO(4)(2-) uptake and oxalate uptake with altered pH dependence. In marked contrast, the bumetanide- and ouabain-insensitive (86)Rb(+) influx associated with AE1 E758K expression was largely GPA-independent in Xenopus oocytes and completely GPA-independent in Ambystoma oocytes. AE1 E758K-associated currents in Xenopus oocytes also exhibited little or no GPA dependence. (86)Rb(+) influx was higher but inward cation current was lower in oocytes expressing AE1 E758K than previously reported in oocytes expressing the AE1 HSt mutants S731P and H734R. The pharmacological inhibition profile of AE1 E758K-associated (36)Cl(-) influx differed from that of AE1 E758K-associated (86)Rb(+) influx, as well as from that of wild-type AE1-mediated Cl(-) transport. Thus AE1 E758K-expressing oocytes displayed GPA-dependent surface polypeptide expression and anion transport, accompanied by substantially GPA-independent, pharmacologically distinct Rb(+) flux and by small, GPA-independent currents. The data strongly suggest that most of the increased cation transport associated with the novel HSt mutant AE1 E758K reflects activation of endogenous oocyte cation permeability pathways, rather than cation translocation through the mutant polypeptide.

Hematological abnormalities in patients with distal renal tubular acidosis and hemoglobinopathies

Mutations of the human SLC4A1 gene encoding erythroid and kidney isoforms of anion exchanger 1 (AE1, band 3) result in erythrocyte abnormalities or distal renal tubular acidosis (dRTA) and such mutations are observed in Southeast Asia, where hemoglobinopathies are prevalent. Genetic and hematological studies in 18 Thai patients with dRTA have shown that 12 of them (67%) carried SLC4A1 mutations (7 G701D/G701D, 3 SAO/G701D, and 2 G701D/A858D). Of these 12 patients, three had homozygous G701D/G701D and heterozygous Hb E; one compound heterozygous SAO/G701D and heterozygous alpha(+)-thalassemia; and one compound heterozygous G701D/A858D and heterozygous Hb E. Of 6 patients without SLC4A1 mutation, two each carried heterozygous or homozygous Hb E and one of the latter also had Hb H disease (--(SEA)/-alpha(4.2)). The blood smears of patients with homozygous G701D/G701D showed approximately 25% ovalocytes. Strikingly, the patients with coexistence of homozygous G701D/G701D and heterozygous Hb E had 58% ovalocytes. Similarly, the patients who had compound heterozygous SAO/G701D showed 49% ovalocytes, but the patient with coexistence of compound heterozygous SAO/G701D and heterozygous alpha(+)-thalassemia had 70% ovalocytes. Our previous study has shown that under metabolic acidosis, the patients with homozygous G701D/G701D or compound heterozygous SAO/G701D had reticulocytosis, indicating compensated hemolysis. A patient with compound heterozygous SAO/G701D and heterozygous alpha(+)-thalassemia presented with hemolytic anemia and hepatosplenomegaly which was alleviated by alkaline therapy. Taken together, the coexistence of both homozygous or compound heterozygous SLC4A1 mutations and hemoglobinopathy has a combined effect on red cell morphology and degree of hemolytic anemia, which is aggravated by acidosis.

SLC4A11 mutations in Fuchs endothelial corneal dystrophy

The endothelial (posterior) corneal dystrophies, which result from primary endothelial dysfunction, include Fuchs endothelial corneal dystrophy (FECD), posterior polymorphous corneal dystrophy (PPCD) and congenital hereditary endothelial dystrophy (CHED). Mutations in SLC4A11 gene have been recently identified in patients with recessive CHED (CHED2). In this study, we show that heterozygous mutations in the SLC4A11 gene also cause late-onset FECD. Four heterozygous mutations [three missense mutations (E399K, G709E and T754M) and one deletion mutation (c.99-100delTC)] absent in ethnically matched controls were identified in a screen of 89 FECD patients. Missense mutations involved amino acid residues showing high interspecies conservation, indicating that mutations at these sites would be deleterious. Accordingly, immunoblot analysis, biochemical assay of cell surface localization and confocal immunolocalization showed that missense proteins encoded by the mutants were defective in localization to the cell surface. Our data suggests that SLC4A11 haploinsufficiency and gradual accumulation of the aberrant misfolded protein may play a role in FECD pathology and that reduced levels of SLC4A11 influence the long-term viability of the neural crest derived corneal endothelial cells.

Distal renal tubular acidosis associated with anion exchanger 1 mutations in children in Thailand

BACKGROUND

Mutations in the anion exchanger 1 (AE1) gene encoding the erythroid and kidney anion (chloride-bicarbonate) exchanger 1 may result in hereditary distal renal tubular acidosis (dRTA). Hemoglobinopathies are common in Thailand. We analyzed AE1 and hemoglobin mutations in children in Thailand with dRTA to evaluate their association with clinical manifestations.

STUDY DESIGN

Case series.

SETTING & PARTICIPANTS

17 patients were recruited from 6 referral hospitals in 4 regions of Thailand.

PREDICTORS

AE1 mutations were detected by means of nucleotide sequence alterations. Hemoglobin E (HbE) was detected by means of hemoglobin typing, and thalassemia, by means of analysis of globin genes. Hemolytic anemia was indicated by decreased hemoglobin and hematocrit values in the presence of reticulocytosis.

OUTCOMES & MEASUREMENTS

Leading clinical manifestations in patients were failure to thrive and muscle weakness. Compensated or overt anemia was identified in some cases. Coexistence of AE1 mutations with HbE or alpha(+)-thalassemia was present in a number of patients.

RESULTS

12 of 17 patients (70%) carried AE1 mutations, 7 patients (41%) had HbE, and 1 patient (6%) had alpha(+)-thalassemia. Patients with AE1 mutations presented with compensated hemolysis when they had metabolic acidosis. A patient with compound heterozygous Southeast Asian ovalocytosis/G701D and heterozygous alpha(+)-thalassemia showed severe hemolytic anemia.

LIMITATIONS

5 patients (30%) without detectable AE1 mutation also were unknown for other genetic abnormalities.

CONCLUSIONS

Most of the patients with dRTA studied carried autosomal recessive AE1 mutations. Metabolic acidosis, which could be alleviated by adequate alkaline therapy, induced variable degrees of hemolysis in patients with dRTA associated with autosomal recessive AE1 mutations, especially in the presence of thalassemia.

Trafficking defect of mutant kidney anion exchanger 1 (kAE1) proteins associated with distal renal tubular acidosis and Southeast Asian ovalocytosis

Compound heterozygous anion exchanger 1 (AE1) SAO/G701D mutations result in distal renal tubular acidosis with Southeast Asian ovalocytosis. Interaction, trafficking and localization of wild-type and mutant (SAO and G701D) kAE1 proteins fused with hemagglutinin, six-histidine, Myc, or green fluorescence protein (GFP) were examined in human embryonic kidney (HEK) 293 cells. When individually expressed, wild-type kAE1 was localized at cell surface while mutant kAE1 SAO and G701D were intracellularly retained. When co-expressed, wild-type kAE1 could form heterodimer with kAE1 SAO or kAE1 G701D and could rescue mutant kAE1 proteins to express on the cell surface. Co-expression of kAE1 SAO and kAE1 G701D also resulted in heterodimer formation but intracellular retention without cell surface expression, suggesting their trafficking defect and failure to rescue each other to the plasma membrane, most likely the molecular mechanism of the disease in the compound heterozygous condition.