Two novel mutations in the CLCNKB gene leading to classic Bartter syndrome presenting as syncope and hypertension in a 13-year-old boy

Abstract

Classic Bartter syndrome is a rare condition caused by mutations in the CLCNKB gene and characterised by metabolic alkalosis, hypokalaemia, hyper-reninaemia and hyperaldosteronism. Early signs and symptoms usually occur before a child’s sixth birthday and include polyuria and developmental delay. We treated a 13-year-old Vietnamese boy with this syndrome presenting with atypical presentations including syncope and hypertension, but normal growth and development. All common causes of hypertension were ruled out. Genetic testing found two novel mutations in the CLCNKB gene, that is, Ser12Ala (exon 2) and Glu192Ter (exon 6). His estimated glomerular filtration rate was 61 mL/min/1.73 m² and a kidney biopsy showed focal segmental glomerulosclerosis. He was well managed with long-term enalapril therapy instead of non-steroidalanti-inflammatory drugs which are recommended in managing the increased prostaglandin E2 production in Bartter syndrome. Paediatricians should be alerted with the variability in its presentation. To preserve the kidney function, treatment must include preventing factors damaging the kidneys.

Background

Bartter syndrome together with Gitelman syndrome is a group of autosomal recessive disorders featured by metabolic alkalosis, hypokalaemia, hyper-reninaemia and hyperaldosteronism.¹ Some patients also exhibit hypomagnesaemia and high levels of prostaglandin E2.^{1 2} These manifestations are due to the primary defect in sodium reabsorption in the thick ascending limb of the loop of Henle.³ Signs and symptoms may appear early in utero with polyhydramnios during pregnancy and premature delivery or later during childhood with polyuria and mental and growth retardation.⁴ Gitelman syndrome is caused by mutations of the SLC12A3 gene.⁵ Bartter syndrome is classified into four types including type I due to the SLC 12A1 gene mutation, type II (KCNJ1 mutation), type III (CLCNKB mutation (OMIM #607364)) and type IV (IVa due to the BSND mutation and IVb due to CLCNKA and CLCNKB mutations).^1–3 Of these types, types I, II and IV are called antenatal types, whereas type III is called classic type.¹ A wide severity spectrum has been documented in all forms of Bartter syndrome.¹ However, compared with other types, classic Bartter syndrome usually has less severe manifestations and often appears in childhood. In the late stages of classic Bartter syndrome, patients may develop proteinuria and declined glomerular filtration rate (GFR).⁶ Several mutations in the CLCNKB gene have been identified as the cause of classic Bartter syndrome.^{2 7} We present this case to add two novel mutations to those already described as well as demonstrate the variability in the clinical presentation of classic Bartter syndrome. This case also provides further robust evidence for the possible link between classic Bartter syndrome and focal segmental glomerulosclerosis (FSGS).

Case presentation

We present a case of a 13-year-old boy who was the only child of a non-consanguineous marriage. He fainted with a blood pressure of 190/120 mm Hg while he was attending a physical education class. He was referred to a local hospital where he was given an unknown antihypertensive medication and subsequently to our hospital. He was a full-term baby without any abnormalities at birth. Maternal polyhydramnios was uncertain. His parents divorced when he was 1 year old. He has been raised by his mother since then. He did not use any specific medication and had a normal family medical history, and especially no family history of HIV, kidney-related diseases, cardiovascular diseases or arterial hypertension.

Investigations

On admission, he was conscious and alerted without any focal neurological signs. His blood pressure was 150/110 mm Hg and he had no oedema. His weight (47.7 kg) and height (157 cm) were within the normal range. He had no retinopathy on eye examination. He had normal physical and cognitive development and his 24-hour urine volume was 2000 mL. No other signs and symptoms were recorded.

He was asked to undertake blood and imaging tests, urine electrolytes and urinalysis (urine dipstick) to identify the cause of hypertension and examine the renal function. Ultrasound of the renal and urinary system showed renal calcification in both kidneys. Doppler echocardiography result was normal except mild left ventricular hypertrophy. CT angiography of the renal arteries and veins showed no stenosis or obstruction. Plasma aldosterone concentration was normal, whereas there was a slight increase in plasma renin activity. Fasting plasma glucose levels, morning cortisol and adrenocorticotropic hormone levels were in normal ranges. Serum C3 and C4 levels were not decreased. His plasma creatinine concentration slightly increased at 124 µmol/L with an estimated glomerular filtration rate (eGFR) of 61 mL/min/ 1.73 m². Urine electrolytes showed high urine chloride concentration of 87.9 mmol/L (renal salt wasting is defined as >40 mEq/L),⁸ whereas sodium and potassium concentrations were within the normal ranges. His urinalysis showed minor proteinuria and no blood cell in the urine. Other blood test results indicated metabolic alkalosis, hypokalaemia, hyponatraemia, normal calcinemia and magnesaemia (table 1).

Table 1.

Blood and urine test results

Blood and urine tests	Results	Normal ranges
pH	7.54	7.33–7.42
HCO−3 (mEq/L)	29.6	21–25
Serum sodium (mEq/L)	128.7	135–145
Serum chloride (mEq/L)	95.4	98–106
Serum potassium (mEq/L)	3.21	3.3–4.6
Serum ionised calcium (mEq/L)	1.11	1.12–1.23
Serum magnesium (mmol/L)	0.96	0.6–0.95
Serum creatinine (µmol/L)	124	24.4–77.8
Plasma renin activity (ng/L)	129.85	7.54–63.3
Aldosterone/blood (ng/L)	17.7	2.5–31.5
Cortisol/blood/morning (μg/dL)	10.33	6.20–19.40
ACTH/blood/morning (pg/mL)	27.6	7.2–63.3
Proteinuria	(+)	Negative
Haematuria	(-)	Negative
24-Hour calcinuria (mmol/24 hours)	1.04	2.50–7.50

ACTH, adrenocorticotropic hormone.

Differential diagnosis

Differential diagnoses made after the clinical examination included renovascular diseases, parenchymal renal diseases and endocrine disorders (ie, primary aldosteronism, renin-secreting tumours and Cushing’s syndrome). The medical and family history and clinical examination together with the laboratory and imaging findings helped rule out renovascular diseases and endocrine disorders and support a diagnosis of parenchymal renal disease due to classic Bartter syndrome which led to genetic testing to detect the CLCNKB gene mutation. A kidney biopsy was also performed to further study the reason for the declined eGFR. Histopathology and immunofluorescence showed FSGS with a mild grade of IgM deposit at the mesangial regions (figure 1).

Figure 1.

Kidney biopsy with histopathology and immunofluorescence. Arrows indicate disruption of the glomerular tuft and adhesion to Bowman’s capsule. PAS, Periodic acid–Schiff.

Based on VarSome which is a human genomic variant search engine, CLCNKB has a total of 20 exons.⁹ After examining all exons, we found two mutations including a replacement of serine at position 12 (Ser-12) of exon 2 by alanine (Ser12Ala) and a replacement of glutamine at position 192 (Ser-12) of exon 6 by a stop codon (Glu192Ter—nonsense mutation). Molecular genetic testing for his mother also found mutant Ser12Ala. Unfortunately, his father could not be reached for testing.

Treatment

The patient was treated with enalapril. The dose was 0.5 mg/kg/day in the beginning and reduced gradually in response to the clinical improvement.

Outcome and follow-up

He was discharged after all disorders including hypertension, hypokalaemia and hyponatraemia were well corrected. After 6 months of treatment with enalapril (0.17 mg/kg/day) as an outpatient, he maintained normal plasma sodium and potassium concentrations. His blood pressure and plasma creatinine concentration fluctuated around the value of 110/70 mm Hg and 117.55 µmol/L, respectively. He was continued to follow-up as an outpatient every 3 months. His mother was advised about her child’s health condition including hypertension, chronic kidney disease and its progress, and the importance of compliance with follow-up and adherence to medication. Genetic counselling regarding the patient’s Bartter syndrome, how he develops this disease and the risk for him to pass the altered gene to the next generation was also conducted. The patient will be referred to a nephrologist when he is 18 years old according to the local policy.

Discussion

It is well documented that classic Bartter syndrome (OMIM #607364) is caused by mutations in the basolateral chloride channel CLC-Kb gene (CLCNKB).^{2 5} The CLCNKB gene belongs to the CLC family of chloride transporters and channels, which play a role in constructing chloride channels to generate and transmit cellular signals.⁷ In kidneys, these channels regulate the movement of ions into and out of kidney cells and are part of the salt reabsorption process from the urine back into the bloodstream.¹⁰ More than 30 mutations in the CLCNKB gene have been described.¹¹ The two mutations in our patient, including Ser12Ala (exon 2) and Glu192Ter (exon 6), are rare variants in accordance with the ACMG Standards and Guideline.¹² In addition, as these mutants have not been reported elsewhere, they are considered novel mutations. The patient’s mother also has Ser12Ala (exon 2). Therefore, although we could not perform molecular genetic testing for his father, we highly assumed that his father would have the same Ser12Ala (exon 2). In addition, as his mother does not have mutant Glu192Ter (exon 6), it is believed that this mutant detected from our patient may be either de novo mutant or inherited from his father.

Classic Bartter syndrome usually occurs during infancy or early childhood, with a broad spectrum of clinical features.⁵ The clinical symptoms may include hypokalaemic metabolic alkalosis, hyper-reninaemia and hyperaldosteronism which are usually found in the antenatal types or hypocalciuria and hypomagnesaemia which are found in Gitelman syndrome.^{5 13} Therefore, to make a definitive diagnosis of these syndromes including classic Bartter syndrome, performing genetic testing to detect the corresponding gene mutation is compulsory.⁵ Our patient had two mutations of the CLCNKB gene. Classic Bartter syndrome also results in growth retardation and developmental delay with different severity levels.⁵ Our patient exhibited the first recognisable clinical signs and symptoms at the age of 13, whereas he had normal physical and cognitive development. The primary disorder of classic Bartter syndrome is renal salt wasting.⁵ This leads to volume depletion and, therefore, activates the renin–angiotensin–aldosterone system together with increased distal flow and sodium delivery, which subsequently enhances potassium and hydrogen secretion at the secretory sites in the connecting tubules and collecting tubules.^{5 14} These physiological responses result in metabolic alkalosis, hypokalaemia, hyper-reninaemia and high or inappropriately normal aldosterone level.^{5 14} Our patient had a renal salt wasting condition demonstrated by high urine chloride concentration and expressed symptoms matching all these abnormalities suggesting Bartter syndrome.¹⁵ In addition, our patient had normal urinary calcium excretion, but nephrocalcinosis. Indeed, although patients with classic Bartter syndrome usually have normal urinary calcium excretion, some patients may have transient hypercalciuria and occasionally calcification.^{1 2 5} Bartter syndrome is not classically associated with proteinuria.⁵ This syndrome also consistently shows hyperplasia of the juxtaglomerular apparatus, with minimal or no glomerular or tubular abnormalities on renal biopsies.⁵ However, few cases of classic Bartter syndrome with proteinuria or FSGS and renal failure have been reported.⁵ Our patient had minor proteinuria and declined GFR which is considered as grade II chronic kidney disease in accordance with the Kidney Disease Improving Global Outcomes (KDIGO) 2012 classification.¹⁶ A renal biopsy with histopathology and immunofluorescence demonstrated FSGS with only minor mesangial IgM deposits. FSGS can be a primary FSGS or secondary to complement and immune-mediated nephropathy, diabetes, hypertension and infections such as HIV infection. Primary FSGS was excluded because this condition is usually associated with nephrotic syndrome.¹⁷ However, our patient did not have any risk factor, clinical and laboratory findings suggesting nephrotic syndrome. Besides, our patient’s FSGS did not resulted from complement and immune-mediated nephropathy based on the results of renal biopsy with histopathology and immunofluorescence demonstrating no mesangial complement and immune deposit, but only IgM deposits.¹⁷ Our patient also had a normal blood glucose level and no risk factors for and signs of HIV infection. In addition, secondary FSGS due to hypertension usually occurs in adults and thus, was ruled out.¹⁷ By applying the rule out method, it is confirmed that FSGS is secondary to classic Bartter syndrome in our patient. Indeed, there have been several case reports of patients with classic Bartter syndrome who developed FSGS which support a possible link between these two diseases.^{6 18–20} In light of this, kidney impairment in our patient can probably due to FSGS secondary to Bartter syndrome. Although hyper-reninaemic hyperaldosteronism is one of the main manifestations of Bartter syndrome, hypertension is not typically found in patients with this syndrome.⁵ Our patient’s FSGS together with chronic kidney disease helped explain his hypertensive symptoms.^{21 22}

Treatment of Bartter syndrome focuses on the maintenance of normal serum electrolytes and management of increased prostaglandin E2 production which may occur in severe Bartter syndrome.⁶ At this stage, non-steroidal anti-inflammatory drugs (NSAIDs) are the first-line medication to manage the increased prostaglandin E2 production even though long-term use of NSAIDs may lead to kidney and gastrointestinal injuries.²³ Our patient had chronic kidney disease, electrolyte disorder and hypertension with FSGS. Therefore, given NSAIDs were contraindicated in this case, in addition to electrolyte replacement, enalapril which is an ACE inhibitor was used to manage hypertension and increased prostaglandin E2 production. Enalapril also helps normalise serum potassium concentration as well as decrease angiotensin II and aldosterone secretion and therefore delays chronic kidney disease progression.²³

In conclusion, this case report adds to the body of knowledge of molecular pathogenesis of classic Bartter syndrome by describing two novel mutations. The case also demonstrates the variability in the clinical presentation of classic Bartter syndrome and this syndrome can possibly lead to FSGS which results in chronic kidney disease. Treatment must include preventing factors damaging the kidneys to preserve kidney function.

Patient’s perspective.

I did not feel anything different in my body until the day I fainted at the school when I was doing some exercise with my classmates. I did not know what was happening at that time, but when I woke up in a clinic, I was told to have a disease. Although I did not know what it was, I felt tired and was a little bit scared. I was transferred to a bigger, beautiful hospital where I was asked to undertake some blood and urine examinations. Mom said it helped me better. I felt good when staying in this hospital where I could eat good foods and find new friends. One day, mom cried and talked to me that I have a kidney disease that must be followed up for a long time. She cried and asked me to eat and sleep well to recover. I did not really understand why she cried and asked me to do so provided that I was doing very well, but I said yes to her to make her stop crying. I also did not understand why they said I was having a ‘disease’ because I felt that I was very well. The doctor came to my bed every day to ask me the same questions such as how do you feel today? Did you sleep well last night? I liked talking with him because he always made me laugh a lot. He and the nurses also used a machine to measure the so-called blood pressure. This activity was interesting as I always played a game with them in which we guessed how it changed every day. I was told that my high blood pressure was not good for me. One day, the doctor told me that my disease had been under good management. I can go home now, but I need to come back to visit him sometimes. I am very happy with this because I can see my classmates and still see him.

Learning points.

• Two novel mutations of the CLCNKB gene, that is, Ser12Ala (exon 2) and Glu192Ter (exon 6), can lead to classic Bartter syndrome.

• Classic Bartter syndrome can occur in late childhood with an atypical presentation, including syncope and hypertension, but normal growth and development.

• Long-term enalapril therapy may be a good alternative in managing patients with classic Bartter syndrome who develop chronic kidney disease, and thus non-steroidal anti-inflammatory drugs are contraindicated.