Distal renal tubular acidosis and high urine carbon dioxide tension in a patient with southeast Asian ovalocytosis

Autosomal dominant distal renal tubular acidosis in two pediatric patients with mutations in the SLC4A1 gene. Can the maximum urinary pCO2 test be normal?

Primary distal renal tubular acidosis (dRTA) is a rare tubulopathy characterised by the presence of hyperchloremic metabolic acidosis. It is caused by the existence of a defect in the function of the H+ -ATPase located on the luminal side of the α-intercalated cells or the Cl - HCO3- (AE1) anion exchanger located on the basolateral side. Patients do not acidify the urine after acid overload (NH4Cl) or after stimulating H+ secretion by obtaining a high intratubular concentration of an anion such as chlorine (pH is measured) or HCO3- (urinary pCO2 is measured). We present a family with autosomal dominant dRTA produced by a heterozygous mutation in the SLC4A1 gene in which the two paediatric members showed a test of normal maximum urinary pCO2. Our hypothesis is that since the H + -ATPase is intact, at least initially, the stimulation induced by intratubular electronegativity to secrete H + could be effective, which would allow the maximum urinary pCO2 to be paradoxically normal, which could explain the onset, moderate presentation of symptoms and late diagnosis in patients with this mutation. This is the first documented case of a dominant dRTA in Mexico.

Use of Urine Electrolytes and Urine Osmolality in the Clinical Diagnosis of Fluid, Electrolytes, and Acid-Base Disorders

We discuss the use of urine electrolytes and urine osmolality in the clinical diagnosis of patients with fluid, electrolytes, and acid-base disorders, emphasizing their physiological basis, their utility, and the caveats and limitations in their use. While our focus is on information obtained from measurements in the urine, clinical diagnosis in these patients must integrate information obtained from the history, the physical examination, and other laboratory data.

PDLIM5 links kidney anion exchanger 1 (kAE1) to ILK and is required for membrane targeting of kAE1

Anion exchanger 1 (AE1) mediates Cl⁻/HCO₃⁻ exchange in erythrocytes and kidney intercalated cells where it functions to maintain normal bodily acid-base homeostasis. AE1’s C-terminal tail (AE1C) contains multiple potential membrane targeting/retention determinants, including a predicted PDZ binding motif, which are critical for its normal membrane residency. Here we identify PDLIM5 as a direct binding partner for AE1 in human kidney, via PDLIM5’s PDZ domain and the PDZ binding motif in AE1C. Kidney AE1 (kAE1), PDLIM5 and integrin-linked kinase (ILK) form a multiprotein complex in which PDLIM5 provides a bridge between ILK and AE1C. Depletion of PDLIM5 resulted in significant reduction in kAE1 at the cell membrane, whereas over-expression of kAE1 was accompanied by increased PDLIM5 levels, underscoring the functional importance of PDLIM5 for proper kAE1 membrane residency, as a crucial linker between kAE1 and actin cytoskeleton-associated proteins in polarized cells.

Molecular Approach for Distal Renal Tubular Acidosis Associated AE1 Mutations

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

Dominant-negative effect of Southeast Asian ovalocytosis anion exchanger 1 in compound heterozygous distal renal tubular acidosis

The human chloride/bicarbonate AE1 (anion exchanger) is a dimeric glycoprotein expressed in the red blood cell membrane,and expressed as an N-terminal (Delta1-65) truncated form, kAE1(kidney AE1), in the basolateral membrane of alpha-intercalated cells in the distal nephron. Mutations in AE1 can cause SAO (Southeast Asian ovalocytosis) or dRTA (distal renal tubular acidosis), an inherited kidney disease resulting in impaired acid secretion. The dominant SAO mutation (Delta400-408) that results in an inactive transporter and altered erythrocyte shape occurs in manydRTA families, but does not itself result in dRTA. Compound heterozygotes of four dRTA mutations (R602H, G701D, DeltaV850 and A858D) with SAO exhibit dRTA and abnormal red blood cell properties. Co-expression of kAE1 and kAE1 SAO with the dRTAmutantswas studied in polarized epithelial MDCK(Madin-Darbycanine kidney) cells. Like SAO, the G701D and DeltaV850 mutants were predominantly retained intracellularly, whereas the R602H and A858D mutants could traffic to the basolateral membrane. When co-expressed in transfected cells, kAE1 WT (wild-type)and kAE1 SAO could interact with the dRTA mutants. MDCK cells co-expressing kAE1 SAO with kAE1 WT, kAE1 R602Hor kAE1 A858D showed a decrease in cell-surface expression of the co-expressed proteins. When co-expressed, kAE1 WT colocalized with the kAE1 R602H, kAE1 G701D, kAE1 DeltaV850 and kAE1 A858D mutants at the basolateral membrane, whereaskAE1 SAO co-localized with kAE1 WT, kAE1 R602H, kAE1 G701D, kAE1 DeltaV850 and kAE1 A858D in MDCK cells. The decrease in cell-surface expression of the dRTAmutants as a result of the interaction with kAE1 SAO would account for the impaired expression of functional kAE1 at the basolateral membrane of alpha-intercalated cells, resulting in dRTA in compound heterozygous patients.

An approach to the patient with severe hypokalaemia: the potassium quiz

The objective of this teaching session with Professor McCance is to develop an approach to the management of patients with a very low plasma potassium (K(+)) concentration (P(K)). The session begins with a quiz based on six recent medical consultations for a P(K) < 2 mmol/l. Professor McCance outlined how he would proceed with his diagnosis and therapy, using the synopsis that described each patient. This approach was then applied to a new patient, a 69-year-old woman who had a large volume of dependent oedema and developed a severe degree of weakness and hypokalaemia during more aggressive diuretic therapy that included a K(+)-sparing diuretic. The initial challenge for Professor McCance was to deduce why the K(+)-sparing diuretic was not effective in this patient. He also needed to explain why the P(K) was so low on admission.

High prevalence of Southeast Asian ovalocytosis in Malays with distal renal tubular acidosis

Southeast Asian ovalocytosis (SAO) is a red blood cell abnormality common in malaria-endemic regions and caused by a 27 nt deletion of the band 3 protein gene. Since band 3 protein, also known as anion exchanger 1, is expressed in renal distal tubules, the incidence of SAO was examined in distal renal tubular acidosis (dRTA) in Malays in Kelantan, Malaysia. Twenty-two patients with dRTA and 50 healthy volunteers were examined for complication of SAO by both morphological and genetic analyses. SAO was identified in 18 of the 22 dRTA patients (81.8%), but only two of the 50 controls (4%). The incidence of SAO was significantly high in those with dRTA (p<0.001), indicating a dysfunctional role for band 3 protein/anion exchanger 1 in the development of dRTA.

Band 3 mutations, distal renal tubular acidosis, and Southeast Asian ovalocytosis

Familial distal renal tubular acidosis (dRTA) and Southeast Asian ovalocytosis (SAO) may coexist in the same patient. Both can originate in mutations of the anion-exchanger 1 gene (AE1), which codes for band 3, the bicarbonate/chloride exchanger in both the red cell membrane and the basolateral membrane of the collecting tubule alpha-intercalated cell. Dominant dRTA is usually due to a mutation of the AE1 gene, which does not alter red cell morphology. SAO is caused by an AE1 mutation that leads to a nine amino acid deletion of red cell band 3, but by itself does not cause dRTA. Recent gene studies have shown that AE1 mutations are responsible for autosomal recessive dRTA in several countries in Southeast Asia; these patients may be homozygous for the mutation or be compound heterozygotes of two different AE1 mutations, one of which is usually the SAO mutation.

Genetic diseases of acid-base transporters

Genetic disorders of acid-base transporters involve plasmalemmal and organellar transporters of H(+), HCO3(-), and Cl(-). Autosomal-dominant and -recessive forms of distal renal tubular acidosis (dRTA) are caused by mutations in ion transporters of the acid-secreting Type A intercalated cell of the renal collecting duct. These include the AE1 Cl(-)/HCO3(-) exchanger of the basolateral membrane and at least two subunits of the apical membrane vacuolar (v)H(+)-ATPase, the V1 subunit B1 (associated with deafness) and the V0 subunit a4. Recessive proximal RTA with ocular disease arises from mutations in the electrogenic Na(+)-bicarbonate cotransporter NBC1 of the proximal tubular cell basolateral membrane. Recessive mixed proximal-distal RTA accompanied by osteopetrosis and mental retardation is associated with mutations in cytoplasmic carbonic anhydrase II. The metabolic alkalosis of congenital chloride-losing diarrhea is caused by mutations in the DRA Cl(-)/HCO3(-) exchanger of the ileocolonic apical membrane. Recessive osteopetrosis is caused by deficient osteoclast acid secretion across the ruffled border lacunar membrane, the result of mutations in the vH(+)-ATPase V0 subunit or in the CLC-7 Cl(-) channel. X-linked nephrolithiasis and engineered deficiencies in some other CLC Cl(-) channels are thought to represent defects of organellar acidification. Study of acid-base transport disease-associated mutations should enhance our understanding of protein structure-function relationships and their impact on the physiology of cell, tissue, and organism.