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

Abstract

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.

• ATP6V0A4 gene
• childhood
• distal renal tubular acidosis
• early-onset sensorineural hearing loss
• val2ala mutation

Introduction

Primary distal renal tubular acidosis (dRTA) is a rare genetic disease due to inability to secrete the hydrogen ion in the distal nephron. Clinical and biologic features include hyperchloremic metabolic acidosis, hypokalemia, hypocalcemia, hypercalciuria, hypocitraturia, medullar nephrocalcinosis, growth retardation and rickets or osteomalacia.1,2 Autosomal dominant (Cl⁻/ exchanger) and autosomal recessive (H⁺ ATPase channel) forms were described. ATP6V1B1 and ATP6V0A4 are the main gene mutations in autosomal recessive form.

Sensorineural hearing loss (SNHL) is a rare entity of this disease, especially in patients with ATP6V1B1 (subunit β1) and ATP6V0A4 (subunit α4) mutations in H⁺ ATPase gene. ATP6V0A4 gene mutation has been particularly known to be associated with late-onset SNHL.3,4

Although, some mutations in ATP6V0A4 gene (Gly175Asp, Arg449His, Pro524Leu, Met580Thr, Arg807Gln, Gly820Arg) associated with SNHL has been described, currently Val2Ala mutation has been defined as a “variant of unknown significance”.5

To the best of our knowledge, this is the first report describing a Turkish girl with dRTA who suffered from early-onset (before the age of 10 years) SNHL caused by Val2Ala amino acid mutation in ATP6V0A4 gene.

Case

An 18-year-old Turkish girl was admitted to our hospital at the age of 3 years with a 5-month history of weight loss and recurrent urinary tract infections. The child weighing 3100 g was born to consanguineous parents after a full-term uneventful pregnancy. Her history during the postnatal and early infancy period was unremarkable. There was no history of renal failure, hearing loss and genetic disease or congenital abnormalities in her family. On admission, growth retardation (weight: 3–10th percentile, height: 10–25th percentile) and O-bein deformity was detected. Respiratory rate, heart rate and blood pressure were 23 per minute, 96 per minute and 130/80 mm Hg (>95th percentile for age, sex and height), respectively. No dysmorphic appearance was noted. Other system examinations were normal.

Laboratory investigations showed hypokalemia (K: 2.9 mEq/L), hyperchloremia (Cl: 117 mEq/L), partially compensated metabolic acidosis (pH: 7.33, pCO₂: 29 mmHg, HCO₃: 17.2 mmol/L, base excess: −11.8 mmol/L) and alkaline urine (pH: 7.5). Serum sodium, urea, creatinine levels and other biochemical parameters were normal. Renal ultrasound and dimercaptosuccinic acid (DMSA) scan showed nephrocalcinosis and bilateral renal cortical dysfunction, respectively. Metabolic acidosis was substantiated by ammonium chloride acid titration test. Based on these clinical findings and laboratory tests as well as consanguineous parents’ history, a diagnosis of autosomal recessive dRTA was made. At that time, the audiogram revealed a bilateral SNHL. The patient was started with a combination treatment consisting of potassium citrate, enalapril and sodium bicarbonate.

On follow-up, at the age of 10 years, bilateral SNHL progressively worsened requiring the need for hearing aid.

On her last physical examination, at the age of 18 years old, weight and height were 44 kg (<3th percentile) and 150 cm (3–10th percentile), respectively. Her laboratory findings were as follows: sodium: 140 mEq/L, potassium: 4.0 mEq/L, chloride: 117 mEq/L, urea: 15 mg/dL, creatinine: 0.8 mg/dL, blood pH: 7.37, pCO₂: 35 mmHg, HCO₃: 24.3 mmol/L, base excess: +1.5 mmol/L, ClCr: 67 mL/min/1.73 m². At that time, while investigating to see if the patient might have enlarged vestibular ducts, the cranial magnetic resonance imaging (MRI) (Philips Achieva 1.5 T) performed has showed no enlargement of the vestibular aqueducts. Also, otoacoustic emission and audiograms confirmed complete hearing loss in both ears (right: <95 dB, left: <95 dB) (Figure 1). Additionally, the patient was also analyzed for mutations in ATP6V0A4 gene by DNA resequencing.

Figure 1. Results of free field measurement and inserted right and left ear audiogram.

Mutation analysis

Polymerase chain reaction was performed to genomic DNA obtained from the peripheral blood of the patient ATP6V0A4 gene encoding a protein consisting of 840 amino acids of all 21 exons by using the exon–intron regions synthetic specific primers. Mutation analysis was performed after cycle-sequencing PCR that carry out with the Big Dye v.3.1 Terminator Cycle Sequencing Ready Reaction Kit (Applied Biosystems, Foster City, CA). Nucleotide sequence was compared with the reference sequence (NM_020632) and the possible reflection of changes in nucleotide position to the protein structure was compared with the reference sequence of the NP_065683 protein by SeqScape 2.0 programme.

As a result of the gene mutation analysis, Val2Ala homozygote mutation was detected in fifth nucleotide position of ATP6V0A4 gene (Figure 2). No other nucleotide displacement was detected in the cDNA of the ATP6V0A4 gene consisting of 3130 bp.

Figure 2. Val2Ala homozygote mutation in the fifth nucleotide position of ATP6V0A4 gene.

Discussion

The most important aspect of this paper is that it is, to the best of our knowledge, the first report describing an association between early-onset SNHL and Val2Ala mutation in the ATP6V0A4 gene in a Turkish girl with dRTA. In considering the database of human missense variants mapped to 3D protein structures and annotation pipeline, Val2Ala mutation in the ATP6V0A4 gene has been known as a variant of unknown significance, to date.5 In our opinion, this association presented in this paper will aid to clinicians to understand the clinical implication of this genetic variation.

Primary dRTA with SNHL is a rare entity, especially in patients with ATP6V1B1 (subunit β1) and ATP6V0A4 (subunit α4) mutations in H⁺ ATPase gene. Mutations in the ATP6V1B1 gene were more frequent than mutations in the ATP6V0A4 gene.6,7 It has been reported that mutations in ATP6V1B1 gene cause dRTA with early-onset SNHL, whereas dRTA with preserved hearing or late-onset (between the ages of 10 and 40 years) hearing loss is caused by mutations in ATP6V0A4 gene.4 However, it has become clear from recent genetic investigations that SNHL arises in both. In a recent study of 31 families with V-ATPase mutations, early-onset SNHL was observed in 70% of cases with ATP6V1B1 gene mutations and in 39% of cases with ATP6V0A4 gene mutations.8 So far, no mechanism has yet been proposed to account for the variability of SNHL in terms of both severity and age at onset, depending on the gene affected. Also the spectrum of severity of SNHL and the range of ages over which hearing loss occurs in patients with ATP6V0A4 mutations are unclear.8

To date, the known mutations in the ATP6V0A4 gene associated with SNHL in dRTA patients are Gly175Asp, Arg449His, Pro524Leu, Met580Thr, Arg807Gln, Gly820Arg. In our patient with dRTA, we described a novel association between the early-onset SNHL and Val2Ala mutation in the ATP6V0A4 gene.

It has been reported that inner ear morphological abnormalities such as the enlarged vestibular aqueduct syndrome can be found in a variety of different etiologies for hearing loss in children.9 In dRTA with SNHL associations between an enlarged vestibular aqueduct and both autosomal recessive genes (ATP6V0A4 and ATP6V1B1) have been made, however a clear link between the degree of hearing loss and degree of enlarged vestibular aqueduct could not be established yet.10,11 We, therefore, wanted to see if our patient might have enlarged vestibular aqueducts and performed cranial MRI. A cranial MRI revealed no enlargement of the vestibular aqueducts.

In conclusion, in this report, we describe for the first time a novel association between Val2Ala mutation in the ATP6V0A4 gene, formerly known as a variant of unknown significance, and early-onset SNHL in a Turkish girl with dRTA. However, further studies on larger series will be important to confirm a causal relationship between mutations and phenotypes.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.