Reversible low-molecular-weight proteinuria in patients with distal renal tubular acidosis

Coexistence of Sjögren's Syndrome-associated Interstitial Nephritis and Hypokalemic Nephropathy in a Patient with Distal Renal Tubular Acidosis: A Case Report

A 42-year-old woman presented with muscle weakness and hypokalemic distal renal tubular acidosis (dRTA). Investigations revealed concurrent Sjögren's syndrome (SS) and Hashimoto's thyroiditis contributing to hypokalemic dRTA. A renal biopsy revealed focal tubulointerstitial nephritis (TIN) suggestive of SS-related renal involvement, along with distinctive ischemic glomerular changes and tubular alterations consistent with hypokalemic nephropathy. Rapid improvement in tubular injury markers and hypobicarbonemia followed potassium supplementation, suggesting that hypokalemia contributed to proximal tubular injury. This case underscores the diagnostic challenge posed by the simultaneous presence of TIN and hypokalemic nephropathy, potentially masking hypokalemic nephropathy in patients with hypokalemic dRTA secondary to SS-TIN.

An Infant Case of Transient Distal Renal Tubular Acidosis and Fanconi Syndrome Caused by Rotavirus Gastroenteritis

We report an infant case of transient distal renal tubular acidosis and Fanconi syndrome caused by rotavirus gastroenteritis. A 10-month-old boy was admitted to the hospital because of frequent vomiting, lack of vitality, and dehydration. He was diagnosed with rotavirus gastroenteritis on account of his positive stool rotavirus antigen test. Although he presented with acidemia and severe mixed metabolic acidosis, he also had a urine pH of 6.0, indicating impaired urinary acidification. Therefore, he was diagnosed with distal renal tubular acidosis. On the third day of hospitalization, a relatively low %tubular reabsorption of phosphate level with hypophosphatemia, increased fractional excretion of uric acid with hypouricemia, and high urinary β2-microglobulin levels were observed. Moreover, he was diagnosed with Fanconi syndrome on account of multiple proximal tubular dysfunctions. After remission of rotavirus gastroenteritis, the signs of renal tubular dysfunction improved. This was a case of rotavirus gastroenteritis-caused transient distal renal tubular acidosis and Fanconi syndrome. Severe metabolic acidosis resulted from anion-gap metabolic acidosis due to acute kidney injury by rotavirus gastroenteritis and normal anion-gap acidosis due to renal tubular acidosis. When renal tubular acidosis is associated with a disease that causes anion-gap metabolic acidosis, mixed metabolic acidosis occurs and becomes exacerbated. Furthermore, it is important to consider the complications of renal tubular acidosis in the case of severe metabolic acidosis.

Renal tubular acidosis without interstitial nephritis in Sjögren's syndrome: a case report and review of the literature

BACKGROUND

Renal tubular acidosis is the principal clinical feature associated with tubulointerstitial nephritis in patients with primary Sjögren's syndrome. Renal tubular dysfunction due to interstitial nephritis has been considered the underlying pathophysiology connecting renal tubular acidosis and primary Sjögren's syndrome. However, the detailed mechanisms underlying the pathophysiology of renal tubular acidosis in primary Sjögren's syndrome is not fully understood.

CASE PRESENTATION

A 30-year-old woman was admitted with complaints of weakness in the extremities. The patient was hospitalized thirteen years earlier for similar issues and was diagnosed with hypokalemic paralysis due to distal renal tubular acidosis with primary Sjögren's syndrome. This diagnosis was based on a positive Schirmer's test. Besides, anti-Sjögren's syndrome-related antigen A was also detected. Laboratory tests indicated distal RTA; however, a renal biopsy showed no obvious interstitial nephritis. Laboratory tests conducted during the second admission indicated distal renal tubular acidosis. Therefore, a renal biopsy was performed again, which revealed interstitial nephritis. Histological analysis of acid-base transporters revealed the absence of vacuolar type H+-ATPases in the collecting duct. The vacuolar type H+-ATPase was also absent in the past renal biopsy, suggesting that the alteration in acid-base transporters is independent of interstitial nephritis.

CONCLUSIONS

This case study demonstrates that vacuolar-type H+-ATPases are associated with distal renal tubular acidosis, and distal renal tubular acidosis precedes interstitial nephritis in patients with primary Sjögren's syndrome.

Hereditary distal renal tubular acidosis: Genotypic correlation, evolution to long term, and new therapeutic perspectives

Distal renal tubular acidosis (DRTA) is a rare disease resulting from a failure in the normal urine acidification process at the distal tubule and collecting duct level. It is characterised by persistent hyperchloremic metabolic acidosis, with a normal anion gap in plasma, in the presence of high urinary pH and low urinary excretion of ammonium. To date, 5 genes whose mutations give rise to primary DRTA have been described. Alterations in the ATP6V1B1 and ATP6V0A4 genes are inherited recessively and are associated with forms of early onset and, in many cases, with neurosensorial deafness. Pathogenic variants in the SLC4A1 gene are habitually inherited dominantly and give rise to milder symptoms, with a later diagnosis and milder electrolytic alterations. Nonetheless, evolution to nephrocalcinosis and lithiasis, and the development of chronic kidney disease in the medium to long term has been described in a similar manner in all 3 groups. Lastly, recessive forms of DTRA associated to mutations in the FOXI1 and WDR72 genes have also been described. The clinical management of DTRA is based on bicarbonate or citrate salts, which do not succeed in correcting all cases of the metabolic alterations described and, thus, the consequences associated with them. Recently, a new treatment based on slow-release bicarbonate and citrate salts has received the designation of orphan drug in Europe for the treatment of DTRA.

Distal Renal Tubular Acidosis: ERKNet/ESPN Clinical Practice Points

Distal renal tubular acidosis (dRTA) is characterised by an impaired ability of the distal tubule to excrete acid, leading to metabolic acidosis. Associated complications include bone disease, growth failure, urolithiasis and hypokalaemia. Due to its rarity, there is a limited evidence to guide diagnosis and management, however, available data strongly suggest that metabolic control of the acidosis by alkali supplementation can halt or revert almost all complications. Despite this, cohort studies show that adequate metabolic control is present in only about half of patients, highlighting problems with treatment provision or adherence. With these clinical practice points the authors, part of the working groups tubulopathies in the European Rare Kidney Disease Reference network (ERKnet) and inherited kidney diseases of the European Society for Paediatric Nephrology (ESPN) aim to provide guidance for the management of patients with dRTA to facilitate adequate treatment and establish an initial best practice standard against which treatment of patients can be audited.

Hereditary distal renal tubular acidosis: Genotypic correlation, evolution to long term, and new therapeutic perspectives

Distal renal tubular acidosis (DRTA) is a rare disease resulting from a failure in the normal urine acidification process at the distal tubule and collecting duct level. It is characterised by persistent hyperchloremic metabolic acidosis, with a normal anion gap in plasma, in the presence of high urinary pH and low urinary excretion of ammonium. To date, 5 genes whose mutations give rise to primary DRTA have been described. Alterations in the ATP6V1B1 and ATP6V0A4 genes are inherited recessively and are associated with forms of early onset and, in many cases, with neurosensorial deafness. Pathogenic variants in the SLC4A1 gene are habitually inherited dominantly and give rise to milder symptoms, with a later diagnosis and milder electrolytic alterations. Nonetheless, evolution to nephrocalcinosis and lithiasis, and the development of chronic kidney disease in the medium to long term has been described in a similar manner in all 3groups. Lastly, recessive forms of DTRA associated to mutations in the FOXI1 and WDR72 genes have also been described. The clinical management of DTRA is based on bicarbonate or citrate salts, which do not succeed in correcting all cases of the metabolic alterations described and, thus, the consequences associated with them. Recently, a new treatment based on slow-release bicarbonate and citrate salts has received the designation of orphan drug in Europe for the treatment of DTRA.

Rickets with hypophosphatemia, hypokalemia and normal anion gap metabolic acidosis: not always an easy diagnosis

Rickets other than those associated with advanced kidney disease, isolated distal renal tubular acidosis (dRTA) and hypophosphatasia (defective tissue non-specific alkaline phosphatase) are associated with hypophosphatemia due to abnormal proximal tubular reabsorption of phosphate. dRTA, however, at times is associated with completely reversible proximal tubular dysfunction. On the other hand, severe hypophosphatemia of different aetiologies may also interfere with both distal tubular acid excretion and proximal tubular functions giving rise to transient secondary renal tubular acidosis (distal and/or proximal). Hypophosphatemia and non-anion gap metabolic acidosis thus pose a diagnostic challenge occasionally. A definitive diagnosis and an appropriate management of the primary defect results in complete reversal of the secondary abnormality. A child with vitamin D resistant rickets was thoroughly evaluated and found to have primary dRTA with secondary proximal tubular dysfunction in the form of phosphaturia and low molecular weight proteinuria. The child was treated only with oral potassium citrate. A complete clinical, biochemical and radiological improvement was noticed in follow-up.

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.

Low Molecular Weight Proteinuria in Children with Distal Renal Tubular Acidosis

Distal renal tubular acidosis (dRTA) (MIM #267300, #602722 and #179800) is a rare inherited tubulopathy characterized by the inability of the distal tubule to acidify the urine with consecutive systemic acidosis. The clinical features include polyuria, polydipsia, poor appetite, failure to thrive, short stature and rickets. Prominent biochemical features are hypokalemia, hypercalciuria and hypocitraturia. There are reports on patients who presented with unusual biochemical features such as low molecular proteinuria, hypophosphatemia, hypouricemia, generalized hyperaminioaciduria, hyperoxaluria and other making diagnostic confusion to the clinicians. In this work, we report on a series of 8 children with clinically, biochemically and genetically proven dRTA who present with low molecular proteinuria at the disease onset. With metabolic compensation of the disease, there was complete resolution of the low molecular weight protenuria and other proximal tubular abnormalities in all children. Late recognition of the disease with long standing hypokalemia and acidosis may result in abnormal expression and function of the transporters in the proximal tubules. Sodium dodecyl sulphate polyacrylamide gel electrophoeresis is an accurate method for detection and follow up of patients with low molecular weight proteinuria.

Clinical and molecular aspects of distal renal tubular acidosis in children

BACKGROUND

Distal renal tubular acidosis (dRTA) is characterized by hyperchloraemic metabolic acidosis, hypokalaemia, hypercalciuria and nephrocalcinosis. It is due to reduced urinary acidification by the α-intercalated cells in the collecting duct and can be caused by mutations in genes that encode subunits of the vacuolar H⁺-ATPase (ATP6V1B1, ATP6V0A4) or the anion exchanger 1 (SLC4A1). Treatment with alkali is the mainstay of therapy.

METHODS

This study is an analysis of clinical data from a long-term follow-up of 24 children with dRTA in a single centre, including a genetic analysis.

RESULTS

Of the 24 children included in the study, genetic diagnosis was confirmed in 19 patients, with six children having mutations in ATP6V1B1, ten in ATP6V0A4 and three in SLC4A1; molecular diagnosis was not available for five children. Five novel mutations were detected (2 in ATP6V1B1 and 3 in ATP6V0A4). Two-thirds of patients presented with features of proximal tubular dysfunction leading to an erroneous diagnosis of renal Fanconi syndrome. The proximal tubulopathy disappeared after resolution of acidosis, indicating the importance of following proximal tubular function to establish the correct diagnosis. Growth retardation with a height below -2 standard deviation score was found in ten patients at presentation, but persisted in only three of these children once established on alkali treatment. Sensorineural hearing loss was found in five of the six patients with an ATP6V1B1 mutation. Only one patient with an ATP6V0A4 mutation had sensorineural hearing loss during childhood. Nine children developed medullary cysts, but without apparent clinical consequences. Cyst development in this cohort was not correlated with age at therapy onset, molecular diagnosis, growth parameters or renal function.

CONCLUSION

In general, the prognosis of dRTA is good in children treated with alkali.

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.

An expanded syndrome of dRTA with hearing loss, hyperoxaluria and beta2-microglobulinuria

We describe a 7-month-old male with atypical features of autosomal recessive distal renal tubular acidosis (dRTA) with sensorineural hearing loss. Uncharacteristically, he presented with mild acidosis, hypokalaemia and hypocalciuria as well as unilateral sensorineural hearing loss. Subsequent investigations led to the discovery of both hyperoxaluria and beta2-microglobulinuria, thereby expanding the differential diagnosis to include both primary hyperoxaluria and Dent disease. Two mutations in the ATPV1B1 gene, one of which was novel, confirmed the diagnosis of dRTA. We consider the overlapping features of and diagnostic dilemmas involved in making a diagnosis of dRTA, primary hyperoxaluria and Dent disease in patients with infantile nephrocalcinosis. We highlight the occurrence of hyperoxaluria and low-molecular-weight proteinuria in patients with dRTA and propose that the phenotype of autosomal recessive dRTA with sensorineural hearing loss be broadened to include both hyperoxaluria and increased urinary excretion of beta2-microglobulin.

Atypical presentation of distal renal tubular acidosis in two siblings

Primary distal renal tubular acidosis (dRTA) is an inherited disease characterized by the inability of the distal tubule to lower urine pH <5.50 during systemic acidosis. We report two male siblings who presented with severe hyperchloremic metabolic acidosis, high urinary pH, nephrocalcinosis, growth retardation, sensorineural hearing loss, and hypokalemic paralysis. Laboratory investigations revealed proximal tubular dysfunction (low molecular weight proteinuria, generalized hyperaminoaciduria, hypophosphatemia with hyperphosphaturia, and hypouricemia with hyperuricosuria). There was significant hyperoxaluria and laboratory evidence for mild rhabdomyolysis. Under potassium and alkali therapy, proximal tubular abnormalities, muscular enzymes, and oxaluria normalized. A homozygous mutation in the ATP6V1B1 gene, which is responsible for dRTA with early hearing loss, was detected in both siblings. In conclusion, proximal tubular dysfunction and hyperoxaluria may be found in children with dRTA and are reversible under appropriate therapy.

Medullary sponge kidney associated with distal renal tubular acidosis in a 5-year-old girl

INTRODUCTION

Medullary sponge kidney (MSK) is characterized by cystic dilatation of the inner medullary collecting ducts, which causes the kidneys to resemble a sponge.

CASE REPORT

Although distal renal tubular acidosis (dRTA) is commonly observed in patients with MSK, we report a 5-year-old girl with MSK who had features of both dRTA (nephrocalcinosis, hypercalciuria, hypocitraturia) and proximal tubular dysfunction (hyperuricosuria, impaired tubular phosphate reabsorption and proteinuria).

DISCUSSION

Metabolic acidosis, hypercalciuria, hypocitraturia, tubular phosphate reabsorption and growth retardation in the patient improved with alkali therapy.

Proximal renal tubular dysfunction in primary distal renal tubular acidosis

Low-molecular-weight (LMW) proteinuria has been described in patients with primary distal renal tubular acidosis (dRTA). However, other proximal renal tubular dysfunctions have rarely been reported. In this report we describe reversible and multiple proximal renal tubular cell dysfunctions in a patient with dRTA. A 4-year-old girl was admitted to our hospital for investigation of short stature and proteinuria. Laboratory studies revealed a hyperchloremic metabolic acidosis without aciduria, hypokalemia, hypouricemia with uricosuria, hypercalciuria, LMW proteinuria, phosphaturia, and generalized aminoaciduria. The patient was diagnosed as having dRTA with multiple proximal renal tubular dysfunctions. All proximal renal tubular dysfunction subsided 1.5 years after starting alkali therapy. The precise pathogenic mechanisms underlying the development of multiple proximal renal tubular dysfunctions in dRTA remained unclear. However, proximal renal tubular endosomal dysfunction resulting from a profound intracellular acidosis caused by vacuolar H+-ATPase dysfunction or hypokalemic nephropathy might contribute to the development of proximal renal tubular dysfunctions in patients with dRTA.