Mutation analysis of 5 children with primary distal renal tubular acidosis

Objective To analyze the mutations of causal genes in 5 children with primary distal renal tubular acidosis (dRTA), and explore their association of genotype and phenotype, so as to raise the awareness of the disease. Methods The whole exome sequencing was used to identify mutations in these 5 children from 5 families. Results A total of 4 different mutations of ATP6V0A4 gene were found in 2 dRTA children, including a novel heterozygous intron mutation (c.639+1G＞A), a reported heterozygous nonsense variant (c.580C＞T, p.Arg194*) and 2 novel heterozygous duplications (c.1504dupT, p.Tyr502Leufs*22; c.2351dupT, p.Phe785Ilefs*28). Two novel heterozygous missense mutations of ATP6V1B1 gene (c.409C＞T, p.Pro137Ser; c.904C＞T, p.Arg302Trp) were identified in the third child, and a heterozygous missense mutation of SLC4A1 gene (c.1765C＞A, p.Arg589Ser) previously reported was found in the fourth child. No mutation of the dRTA-related causal genes was found in the fifth child. Furthermore, the mutations of causal genes in each of the first three children were compound heterozygous, which were consistent with the autosomal recessive inheritance pattern, and the variant from the fourth child was de novo. Conclusions The present study has found 7 mutations, including 5 novel variants, which enriches the human gene mutation database (HGMD) and contributes to a better understanding of the disease mechanisms.     

Mutation analysis and audiologic assessment in six Chinese children with primary distal renal tubular acidosis

The objective of this study is to identify ATP6V1B1, ATP6V0A4 and SLC4A1 genes mutations and assess audiologic characteristics in six Chinese children with primary distal renal tubular acidosis from four unrelated families between the ages of 2 and 13 years. Both ATP6V0A4 and ATP6V1B1 genes were preferentially screened in all index cases by direct sequence analysis. If inconclusive then SLC4A1 gene should be analyzed for mutation. Their clinical features, hearing status and inner ear imaging structure were also investigated. Six loss-of-function mutations were identified in six patients. Two novel mutations were identified in either of ATP6V0A4 and ATP6V1B1 genes, respectively. Two probands from different kindreds with mutations in ATP6V1B1 presented early onset profound sensorineural hearing loss (SNHL) and enlarged vestibular aqueduct (EVA). Two from different families carrying ATP6V0A4 mutations manifested early onset moderate mixed HL and moderate SNHL, respectively, the former comorbid with EVA, while the latter not; however, both their elder sisters showed normal hearing and inner ear. These findings expand the spectrum of mutations in the ATP6V0A4 and ATP6V1B1 genes associated with primary dRTA. Our study confirms the association of EVA and mutations in either of these two genes. More studies are necessary to clarify the relationship between dRTA, SNHL, EVA, and gene mutations.     

常染色体隐性遗传性远端肾小管酸中毒患儿ATP6V0A4和ATP6V1B1基因突变分析

目的 分析常染色体隐性遗传性远端肾小管酸中毒(rdRTA)患儿ATP6V0A4和ATP6V1B1基因的突变,进行基因型和表型的相关性研究.方法 PCR扩增基因组DNA,直接测序分析来自3个家系3例患儿的ATP6V0A4和ATP6V1B1基因的突变位点,选取不相关的100例健康人作为对照.结果 1例患儿携带ATP6V0A4基因的1个新的纯合无义突变(p.R194X);1例患儿携带ATP6V1B1基因1个新的杂合无义突变(p.R114X)和1个已经报道过的杂合突变p.I386fsX441;第3例患儿未发现以上2个基因的突变.结论 对中国rdRTA患者基因突变分析有利于了解该类疾病的基因型和表型的相关性,增强临床医生对该类疾病的认识和治疗.     

ATP6V1B1 mutations in distal renal tubular acidosis and sensorineural hearing loss: clinical and genetic spectrum of five families

Abstract

Distal renal tubular acidosis (DRTA) is characterized by tubular defects in urinary acidification and hyperchloremic metabolic acidosis, hypokalemia, hypercalciuria, hypocitraturia, nephrocalcinosis and nephrolithiasis. Mutations in ATP6V1B1 cause DRTA associated with sensorineural hearing loss. The objective of this multicenter study is to screen DRTA patients with sensorineural hearing loss for ATP6V1B1 gene mutations and present genotype/phenotype correlation. Clinical data in five unrelated consanguineous families with DRTA and hearing loss were obtained in Turkey. For mutation screening, all coding exons of ATP6V1B1 were PCR-amplified and sequenced from genomic DNA. In our cohort of five families, there were four different homozygous ATP6V1B1 mutations in affected individuals: c.91C>T (p.R31X), c.232G>A (p.G78R), c.497delC (p.T166RfsX9) and c.1155dupC (p.I386HfsX56). Our study shows that rare and family-specific variants in ATP6V1B1 are responsible for DRTA and sensorineural hearing loss syndrome in Turkey. While firm genotype–phenotype correlations are not available, detailed clinical and molecular analyses provide data to be used in genetic counseling.     

Val2Ala mutation in the Atp6v0a4 gene causes early-onset sensorineural hearing loss in children with recessive distal renal tubular acidosis: a case report

A young female patient born to consanguineous parents was admitted to our clinic at the age of 3 years with a 5-month history of weight loss and recurrent urinary tract infections. Based on clinical findings (delayed growth and O-bein deformity) and laboratory tests (hypokalemia, hyperchloremia, partially compensated metabolic acidosis, alkaline urine and nephrocalsinosis), a diagnosis of distal renal tubular acidosis (dRTA) was made. Then, the audiogram revealed a bilateral sensorineural hearing loss (SNHL). On follow-up, bilateral SNHL progressively worsened requiring the need for hearing aid. The ATP6V0A4 gene mutation analysis showed homozygote Val2Ala mutation. To the best of our knowledge, this is the first report describing a Turkish girl with dRTA who suffered from early-onset SNHL caused by Val2Ala mutation in the ATP6V0A4 gene.     

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.     

Hyperkalemic distal renal tubular acidosis associated with Rett syndrome

Renal function was studied in a 7-year-old girl with Rett syndrome (RS) complicated by persistent hyperchloremic hyperkalemic metabolic acidosis. The acidosis was associated with a urine pH above 5.5, positive urinary anion gap and decreased potassium excretion. Plasma renin activity, aldosterone and cortisol levels were normal. Therapy with sodium bicarbonate failed to lower urine pH below 5.5 or increase potassium excretion. Hydrochlorothiazide administration resulted in a fall in urine pH below 5.5 and an increase in potassium excretion as a result of increased distal sodium delivery and increased sodium reabsorption in the distal nephron. We conclude that a voltage-dependent type of derangement in the distal nephron, rather than aldosterone deficiency, is responsible for the impairment in urinary acidification observed in this patient. Early detection of impaired renal acidification in RS may prevent or slow the progression of growth failure.     

Incomplete distal renal tubular acidosis affects growth in children.

BACKGROUND: Incomplete distal renal tubular acidosis (idRTA) is recognized as an underlying aetiology in recurrent nephrolithiasis. Until the recently reported high prevalence of idRTA in adults with osteoporosis, the effect of idRTA on skeletal parameters was not known. We hypothesize that idRTA has a potential to affect height in the paediatric population. METHODS: In a cross-sectional study, the children with posterior urethral valves (PUV), with normal estimated glomerular filtration rates, were evaluated for idRTA and complete dRTA. The idRTA evaluation was done by short ammonium chloride acidification test. The height standard deviation scores (SDS) in the idRTA group were compared with PUV children without dRTA, with complete dRTA, and to age and gender matched controls with no renal issue (n = 50). RESULTS: The idRTA group (n = 17) manifested a significantly lower mean height SDS (-1.94 +/- 0.41 vs -0.46 +/- 0.28; P < 0.001) and a higher short stature prevalence (height SDS below 2) (18% vs 0; P = 0.06) as compared with those without dRTA (n = 23). The matched controls showed a significantly higher height SDS as compared with the idRTA group (-0.39 +/- 0.25 vs -1.94 +/- 0.41; P < 0.001). As compared with the complete dRTA group (n = 9), the children with idRTA did have significantly higher height SDS (-1.94 +/- 0.41 vs -5.31 +/- 1.95; P = 0.002), and a lower short stature prevalence (18% vs 78%; P = 0.001). On multivariate analysis, dRTA was significantly associated with the height SDS (= -0.88; P < 0.001). CONCLUSIONS: Incomplete dRTA affects height in children. This observation needs validation in longitudinal studies.     

Transient neonatal distal renal tubular acidosis with secondary hyperparathyroidism

We describe a neonate with distal renal tubular acidosis with secondary hyperparathyroidism manifesting as hyperchloraemia, hypercalcaemia, elevated serum parathyroid hormone (PTH) and life-threatening metabolic acidosis. He exhibited general weakness, tachypnoea, dry skin and weight loss. Urinary excretion of titratable acid and ammonium was decreased. Daily alkali (2.5 mEq/kg body weight) was required to maintain a normal plasma bicarbonate (HCO₃–). With alkali therapy, the fractional excretion of HCO₃– was below 5%. Serum calcium and PTH were restored to normal promptly on initiation of alkali therapy. After 5 months of alkali therapy, normal growth and urine acidifying ability were restored and alkali therapy was discontinued. The acidification defect in this patient was transient. We consider this patient to be consistent with Lightwood's syndrome of transient infantile renal tubular acidosis.     

Case of hypoparathyroidism, deafness and renal dysplasia (HDR) syndrome associated with nephrocalcinosis and distal renal tubular acidosis

Hypoparathyroidism, deafness and renal dysplasia (HDR) syndrome is an autosomal dominant disorder characterized by hypoparathyroidism, sensorineural deafness and renal dysplasia. Herein, we report a case of HDR syndrome associated with nephrocalcinosis and distal renal tubular acidosis. A 34-year-old woman was admitted to investigate recurrent stone formation and bilateral nephrocalcinosis. As a 3-year-old child, she had been diagnosed with HDR syndrome without chromosome evaluation. She had spontaneous stone passages on several occasions. On laboratory examination, serum calcium and intact parathyroid hormone at lower levels. Urinary citrate excretion was extremely low at 51.6 mg/day. On an ammonium chloride loading test, complete distal renal tubular acidosis was proved. To prevent the nephrocalcinosis from deteriorating, she was given potassium-sodium citrate. Since administration, she has not experienced spontaneous stone passage or renal colic. Nephrocalcinosis and recurrent urolithiasis will strongly affect renal prognosis in this case and we consider that citrate medication is an effective therapy in avoiding progress of her nephrocalcinosis.     

Benign methylmalonic acidemia in a sibship with distal renal tubular acidosis

Two male infants born to consanguineous parents were investigated for feeding difficulties in the 1st month of life. Both were found to have distal renal tubular acidosis (dRTA) with hypercalciuria. Nephrocalcinosis was present in the first child but not in the second. Urinary organic acid profile demonstrated an excess of methylmalonic acid (MMA) in both children in the absence of any other organic acid. MMA mutase activity and propionate incorporation were normal. There have been no neurological symptoms in either child. The first child has normal growth and psychomotor development at 4 years. His brother, who also has significant gastro-oesophageal reflux, has failed to thrive and currently requires nasogastric feeding and caloric supplements to maintain weight along the 3rd percentile. Urinary and plasma MMA continue to be raised in both cases. The association of increased urinary and plasma MMA and dRTA presenting in the 1st month of life has not previously been reported and may represent a new syndrome of autosomal recessive inheritance. Received January 12, 1998; received in revised form and accepted March 26, 1998     

Acidosis increases magnesiuria in children with distal renal tubular acidosis

In experimental animals, metabolic acidosis increases renal magnesium (Mg) excretion, whereas metabolic alkalosis reduces it. The objective of this study was to examine renal magnesium handling (U_Mg) in children with primary distal renal tubular acidosis (DRTA). We measured U_Mg in 11 children (3 females, 8 males, aged 6.9±4.9 years) with primary DRTA. They were studied either during spontaneous acidosis post treatment removal (3 patients) or after ammonium chloride (100 mmol/m²) induced acidosis (8 patients), and then following oral sodium bicarbonate load (4 g/1.73 m²). During acidosis (plasma pH 7.28±0.09, bicarbonate 13.2±4.3 mEq/l), U_Mg was elevated (U_Mg/Cr 0.18±0.06 mg/mg, normal values 0.1±0.06, P =0.003) although plasma Mg (P_Mg) was in the normal range (1.93±0.31 mg/dl, controls 1.77±0.19, P =NS). After acute correction of metabolic acidosis (plasma pH 7.44±0.05, bicarbonate 25.6±1.6 mEq/l, P <0.001; urine pH 7.52±0.28, bicarbonate 86.9±39.1 mEq/l), U_Mg decreased significantly ( P =0.003), returning to control values after about 2 h (U_Mg/Cr 0.09±0.06 mg/mg). Bicarbonate load resulted not only in reduction in U_Mg but also in a decrease in urinary calcium excretion (U_Ca/Cr) from 0.46±0.17 mg/mg to 0.14±0.12 mg/mg ( P <0.001). We conclude that in children with primary DRTA, urinary Mg excretion is markedly increased and that this defect, like the hypercalciuric defect, is correctable by sodium bicarbonate administration.     

常染色体显性遗传性远端肾小管酸中毒一家系SLC4A1基因突变分析

目的 研究肾小管酸中毒（RTA）一家系的遗传方式和临床特征及分析致病基因SLC4A1突变情况。 方法 通过家系调查和检测肾小管酸化功能,研究分析遗传特点和肾小管缺陷的定位。通过直接测序法分析SLC4A1基因。100例健康正常人作为对照。 结果 该家系共有6例患者确诊为远端肾小管酸中毒（dRTA）,6例均有严重的临床表型（发育障碍、肾脏钙质沉着和肾功能不全）;遗传特点符合常染色体显性遗传。SLC4A1基因分析发现位于20号外显子的一个新突变位点（c.2713dupG,band 3 Qingdao）,该突变导致第905位密码子起始的移码突变（p.Asp905Glyfs15）。 结论 常染色体显性遗传性dRTA可表现严重的临床表型。1个新的SLC4A1突变位点被确定与显性遗传性dRTA有关。这是国内首次对遗传性RTA的报道。     

Atypical distal renal tubular acidosis confirmed by mutation analysis

In autosomal dominant distal renal tubular acidosis type I (dRTA) impaired hydrogen ion secretion is associated with metabolic acidosis, hyperchloremic hypokalemia, hypercalciuria, nephrocalcinosis, and/or nephrolithiasis. A retardation of growth is commonly observed. In this report we present a family with autosomal dominant dRTA with an atypical and discordant clinical picture. The father presented with severe nephrocalcinosis, nephrolithiasis, and isosthenuria but metabolic acidosis was absent. His 6-year-old daughter, however, suffered from metabolic acidosis, hypokalemia, and hypercalciuria. In addition, sonography revealed multiple bilateral renal cysts but no nephrocalcinosis. Mutation analysis of the AE1 gene coding for the renal Cl^–/HCO₃ ^–- exchanger AE1 displayed a heterozygous Arg589Cys exchange in both patients but not in the healthy family members. This point mutation is frequently associated with autosomal dominant dRTA. Diagnosis of autosomal dominant dRTA is supported in this family by results of AE1 mutation analysis. Received: 13 April 2000 / Revised: 23 June 2000 / Accepted: 26 June 2000     

远端肾小管酸中毒的诊断和治疗

报告远端肾小管酸中毒１６例，其中完全型２例，不完全型１４例。完全型有高氯低钾性酸中毒，不完全型无酸中毒，但氯化铵负荷试验阳性。在口服枸橼酸钾期间，两型均观察到尿钙明显降低，尿ｐＨ和枸橼酸显著升高，完全型代谢性酸中毒得到纠正。对远端肾小管酸中毒的诊断和治疗进行了讨论。     

Recessive distal renal tubular acidosis in Sarawak caused by AE1 mutations

Mutations of the AE1 (SLC4A1, Anion-Exchanger 1) gene that codes for band 3, the renal and red cell anion exchanger, are responsible for many cases of familial distal renal tubular acidosis (dRTA). In Southeast Asia this disease is usually recessive, caused either by homozygosity of a single AE1 mutation or by compound heterozygosity of two different AE1 mutations. We describe two unrelated boys in Sarawak with dRTA associated with compound heterozygosity of AE1 mutations. Both had Southeast Asian ovalocytosis (SAO), a morphological abnormality of red cells caused by a deletion of band 3 residues 400–408. In addition, one boy had a DNA sequence abnormality of band 3 residue (G701D), which has been reported from elsewhere in Southeast Asia. The other boy had the novel sequence abnormality of band 3 (Q759H) and profound hemolytic anemia.     

Bicarbonate therapy improves growth in children with incomplete distal renal tubular acidosis

Incomplete distal renal tubular acidosis (idRTA) has recently been associated with osteoporosis and growth retardation, attributed to the mild persistent metabolic acidosis. We hypothesized a therapeutic benefit from bicarbonate therapy on growth parameters in children with idRTA. In a study group of 40 surgically treated patients with posterior urethral valve (PUV) and normal estimated glomerular filtration rate, we evaluated the change in height standard deviation scores (SDSs) while they were on bicarbonate therapy in the presence of idRTA and complete distal renal tubular acidosis (dRTA). Age- and gender-matched healthy subjects constituted the control group (n = 55). Incomplete dRTA was evaluated by ammonium chloride acidification. The baseline height SDS of −1.94 ± 0.41 and −5.31 ± 1.95 in the groups with idRTA and complete dRTA, respectively, were significantly lower than that of the controls. After a follow-up period of 24.7 ± 8.3 months on sodium bicarbonate therapy, the idRTA patients had a 66% increase in height SDS compared with 26% and 3% increases in the patients with PUV with complete dRTA and without dRTA, respectively. At the end of follow-up, mean height SDS in the group with idRTA no longer remained significantly lower than that of the controls (P = 0.42). We concluded that bicarbonate therapy improves height SDS in idRTA. This issue needs further validation in larger studies.