The SLC4A1 gene is one of the bicarbonate anion transporters of the solute carrier family 4 (SLC4) gene family (1). This gene encodes anion exchanger member 1 (AE1), which is expressed in both erythrocytes and the acid-secreting α- intercalated cells of the kidney (2). The erythroid isoform of AE1(eAE1) is expressed in the red blood cells. The kidney anion exchanger 1 (kAE1) is highly expressed in the kidney and located in the basolateral membrane of the α-intercalated cell (3). The kAE1 protein plays an essential role in bicarbonate ion (HCO3⁻) in exchange with chloride ion (Cl⁻) during the apical secretion of hydrogen ion (H⁺) into the tubular lumen (4–6).

Mutations in SCL4A1 may cause distal renal tubular acidosis (dRTA). Distal renal tubular acidosis is a disease of defective urinary acidification characterized by impaired H+ secretion into the urine leading to metabolic acidosis, hypokalemia, nephrocalcinosis, and failure to thrive (7, 8). The mutations in SLC4A1 have been reported in both autosomal dominant (AD) and recessive (AR) types of dRTA. Bruce et al. first reported that dominant dRTA was attributed to SLC4A1 gene defect in 1997 (9). Mutations associated with AR types of dRTA have usually been found in homozygous and compound heterozygous conditions (10–14).

Our understanding of dRTA caused by SLC4A1 mutations is limited by low incidence and phenotypic variability, and mutations in SLC4A1 have not been systematicly documented. However, diagnosis and treatment are often delayed due to clinical variability of the disease, and hereditary kidney disease compromises the quality of life of patients. Herein we analyzed the genetic defects in SLC4A1 and clinical phenotypes of the patients to facilitate the diagnosis and treatment of dRTA with SLC4A1 mutations.

To our knowledge, this is the first study to systematically and comprehensively analyze reported dRTA patients with SLC4A1 mutations. In this work, we summarized the mutation spectrum and clinical characteristics in patients with SLC4A1 mutations. SLC4A1 is located on chromosome 17 and encodes anion exchanger 1 protein in erythrocyte cell membranes and α-intercalated cells of the kidney (15).

There are several possible explanations for the pathogenesis of the acid secreting defect secondary to SLC4A1. A mouse model lacking AE1 exhibits spontaneous hyperchloremic metabolic acidosis with reduced acid excretion, and inappropriately alkaline urine (16). Basolateral Cl-/HCO3- exchange activity was reduced, while alternate bicarbonate transport pathways were upregulated. Dysregulated expression and localization of the aquaporin-2 water channel were observed in mice lacking AE1, accompanied by severe urinary concentration defect (16). KAE1 forms functional homodimers or etero-oligomers, and oligomerization of a dominant negative mutant AE1 with a wild-type polypeptide in the heterozygote A-IC likely explains some dominant disease. The mutant/wild-type heterodimer fails to traffic to the cell surface or traffics normally but may not function normally at the cell surface (16–18). Dominant dRTA might be caused by inappropriate targeting to A-IC apical membrane, with resultant apical bicarbonate secretion likely short-circuiting luminal acid secretion (19, 20). Whereas, the study by Mumtaz R et al. generated a mouse corresponding to the dominant dRTA mutation in human AE1 R589H, suggesting normal targeting of the pathologic R607H variant, but the mutant mice exhibited reduced expression of V-type ATPase and compromised targeting of this proton pump to the plasma membrane upon acid challenge (21). Homozygous patients may not have any kAE1 isoform at the plasma membrane of a-intercalated cells in the distal nephron since these cells lack glycophorin A and develop dRTA consequently (17). Previous study has also reported that the integrity of the cytosolic COOH terminus played a crucial role in normal kAE1 targeting to the cell surface. Truncation of the last 11 amino acids increased the kAE1 endocytosis rate and reduced recycle to the plasma membrane. The decreased abundance at the plasma membrane and altered recycling provide a possible physiological mechanism of distal renal tubular acidosis in patients carrying SLC4A1 mutations (22).

The defect of kAE1 is associated with inherited distal renal tubular acidosis (dRTA) (9, 23–26). In addition, eAE1 is an important structural component of the red cell membrane, which ensures the normal skeleton structure and ion transport of erythrocytes (27). Consequently, SLC4A1 mutations that alter AE1 composition are often associated with disorders of red cell membrane integrity including SAO and HS, which are usually autosomal recessive inheritance (28, 29). SAO is a morphological erythrocyte abnormality caused by a mutational deletion of 27 bp in exon 11 of SLC4A1, resulting in a frame 9 amino acid deletion at the junction between the N-terminal domain of eAE1 and the first transmembrane segment, involving codons 400–408 (30). These patients are usually homozygous for a single SLC4A1 mutation or compound heterozygotes of two different SLC4A1 mutations found throughout Southeast Asia most commonly, one of which is usually the SAO mutation.

SLC4A1-dRTA exhibits different behaviors with regard to different ethnicities. The G701D mutation was first identified in a Thai family (11), which was recessive and frequently observed in Southeast Asia in both homozygote and compound heterozygote with SAO mutation (31). These patients usually accompanied with anemia, jaundice, and splenomegaly; spherocytosis and ovalocytosis could be found in in blood smear (14, 32). Our study showed that almost half of the patients with AR dRTA had hematological abnormalities, while it was uncommon in patients with AD dRTA. Dominant mutant proteins tend to maintain normal Cl-/HCO3- exchange function in erythrocytes, hemolytic anemia was rarely observed consequently (9). The Arg589 is a hotspot mutation site, which is located in the intracellular domain between the sixth and the seventh transmembrane regions of the AE1 protein, more commonly seen in Caucasian (33–35). Even though SAO and G701D are more common in Southeast Asians (10, 14, 31). Since Southeast Asia historically have had a high incidence of Plasmodium falciparum malaria, a presumable supposition is that SAO is considered to have evolved, which might provide a protection against the clinical effects of Plasmodium falciparum malaria (36, 37).

Our study showed that the age of onset varies from birth to adulthood, mostly in childhood. Common clinical manifestations include vomit, failure to thrive, nephrocalcinosis, nephrolithiasis, rickets and muscle weakness. The spectrum of phenotypic severity in dRTA is wide, ranging from growth impairment to renal function impairment. In dRTA, ɑ-intercalated cells in the collecting duct are unable to secrete H+  ions and acidify urine, and result in metabolic acidosis and hypokalemia. Severe hypokalemia could result in vomit and muscle weakness (12, 26, 38). The metabolic acidosis is mainly buffered by the bone, and bone demineralization leads to failure to thrive and rickets. The excess of calcium in the blood and decreased expression of kidney calcium transporter proteins cause hypercalciuria, which results in nephrocalcinosis and/or nephrolithiasis, eventually leads to renal insufficiency (25, 36). The patients often visit different departments due to various clinical manifestations, therefore clinical workers should raise general awareness of the disease.

Our review of the clinical characteristics of these patients revealed the differences between the dominant and recessive forms of patients with SLC4A1 mutations. The clinical phenotypes of dRTA caused by SLC4A1 mutations are of great heterogenity. The percentage of homozygous subjects is greater in the Asian population and for this reason they present a more severe clinical picture. Consistent with already published data, the clinical manifestations and biochemical phenotypes of AD are usually milder compared with that of patients with AR mutations (39). Compared with the patients with autosomal dominant mutations, the patients with autosomal recessive mutations had more severe alkaline urine and hypokalemia, and the age of onset was much younger. Probably because the dominant mutants retain wild-type kAE1 protein intracellularly while the recessive kAE1 mutants do not. The patients with the recessive mutants show more severe trafficking defects (37). Based on the above genotype-phenotype correlations, a molecular diagnosis is necessary due to the implications for treatment, prognosis and family risk. For the patients with AR mutations, more attention needs to be paid.

For patients with dRTA, the clinical management is a critical issue that needs to be taken seriously. Since acid-base homeostasis is essential for normal growth and development, the treatments of dRTA should not be limited to correcting the biochemical abnormalities, but also to preventing disease-related abnormalities such as failure to thrive, growth retardation, rickets, osteoporosis, nephrolithiasis, and nephrocalcinosis (40, 41). Since the progression of nephrocalcinosis may lead to chronic kidney disease and end-stage kidney disease in dRTA patients, prevention of nephrocalcinosis is particularly important (40). As citrate salts can prevent nephrolithiasis, potassium citrate is usually recommended (23, 40).

Our results also suggested over half children presented with growth retardation, which has been a major problem in children with renal tubular acidosis, deserving our concerns. At an early stage, alkali and potassium supplementation could correct metabolic acidosis, regulating acid-base balance and significantly improve growth and skeletal deformities (13, 42). And lifelong treatment is recommended (23). Although calcium and active vitamin D are beneficial in the treatment of osteoporosis and osteomalacia, they promote the formation of kidney stones and should be avoided (43). It was reported that combined conventional alkali supplementation with recombinant human growth hormone (rhGH) therapies might have a beneficial effect on growth in dRTA patients, however, further high-quality clinical studies involving more patients are needed to confirm this observation (44). For patients with erythrocyte membrane disorders, be of benefit in patients with severe and moderate hemolytic anemia, but is not necessary in mild cases generally (45).

Our study has several limitations. First, the number of reported cases is not large enough to permit definite genotype-phenotype correlations. Second, this study might have selective bias, since typical or more severe patients tend to be diagnosed and reported. Additionally, the biochemical indexes were from different laboratory, which might cause a slight deviation. And further high-quality studies are needed for the purpose of explaining the more precise molecular mechanism.

In summary, our study firstly summarized mutations and clinical characteristics of dRTA caused by SLC4A1 mutations. The patients with the presence of metabolic acidosis, hypokalemia, hyperchloremia, nephrocalcinosis and growth retardation should be prompt a genetic test as soon as possible. The patients with recessive dRTA are generally more severely affected and the age of onset is earlier than that with dominant SLC4A1 mutations, and autosomal recessive inheritance was more often found in Asian patients. Early identification and early treatment are very essential for the prognosis of patients, especially the Asian patients.