Association of the CLC-Kb T481S polymorphism with childhood hypertension

doi:10.4236/health.2010.27103

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Vol.2, No.7, 682-684 (2010)

doi:10.4236/health.2010.27103

Health

Association of the CLC-Kb T481S polymorphism with

childhood hypertension

Rainer Büscher

University of Duisburg-Essen, Children’s Hospital, Pediatrics II, Essen, Germany;

rainer.buescher@uk-essen.de

Received 17 March 2010; revised 29 March 2010; accepted 31 March 2010.

ABSTRACT

Essential hypertension is a difficul t diagnosis in

children and the gene of the renal-epithelial

chloride channel ClC-Kb is potentially predis-

posing. In vitro studies have shown that a co-

mmon ClC-Kb threonine481serine (T481S) poly-

morphism leads to enhanced chloride channel

activity and may predispose for hypertension

(HT). We therefore analysed children at risk for

HT for the T481S polymorphism and associ-

ated genotype with blood pressure (BP) status.

A total of 48 children with essential hyperten-

sion (mean age 14.4 ± 2.7 years, 26 male; 22

female; mean BP 143.4 ± 7.5/88 ± 5.8 mmHg)

were compared with 78 children with white-coat

HT (WCHT), who showed occasionally hyper-

tensive BP values, which were not confirmed by

ambulatory blood pressure monitoring (mean

age 13.7 ± 2.5 years, 49 male, 29 female; mean

BP 122.4 ± 4.3/68.2 ± 3.5 mmHg). Other causes

of HT were excluded. Allelic frequencies of hy-

pertensive patients were not significantly dif-

ferent from those w ith WCHT (HT: A 0.84; T 0.16

vs. WCHT: A 0.85; T 0.15). However, the T-allele

was observed more frequently in WCHT sub-

jects with systolic and diastolic BP exceeding

the 90th percentile (A 0.71 ; T 0.29, n = 34, p < 0.05,

considered as borderline hypertensive). The

preliminary data suggest that children with

WCHT carry the ClC-Kb T481S polymorphism

more often and that this variant may predispose

for development of arterial HT.

Keywords: Essential Hypertension;

White-Coat Hypertension;

Ambulatory Blood Pressure Monitoring (Abpm);

Clc-Kb T481s Polymorphism;

Renal-Epithelial Chloride Channel

1. INTRODUCTION

Essential hypertension in childhood is rare and in most

cases, diagnosis can only be confirmed by exclusion of

other causes of hypertension (HT) such as renovascular

disease, endocrine or cardiovascular disorders 1,2. In

childhood, HT is defined as blood pressure (BP) ex-

ceeding the 95th percentile for healthy children and Am-

bulatory Blood Pressure Monitoring (ABPM) is an im-

portant method for the detection and therapeutic moni-

toring of children and adolescents with HT 3-5. White

Coat Hypertension (WCHT) is the most common diag-

nosis in children referred for evaluation of HT (> 50%)

but there is doubt whether occasionally occurring hyper-

tensive episodes predispose for end-organ damage as

adult patient 6.

There is increasing evidence that disturbances in the

renal tubular sodium chloride (NaCl) reabsorption are

predisposing for the development of HT. The renal-

epithelial chloride channel ClC-Kb is expressed in the

baso-lateral cell membrane of the distal nephron and is

involved in NaCl reabsorption, therefore, the ClC-Kb

gene might influence development of HT. Recently, a

frequent polymorphism has been detected in the ClC-Kb

gene (threonine to serine amino acid change, T481S) and

in vitro data confirmed that the ClC-Kb T481S poly-

morphism leads to enhanced activation of the chloride

channel 7,8. Carriers of the ClC-Kb T481S polymor-

phism might therefore be at risk for elevated BP. How-

ever, this hypothesis has not been verified in a cohort of

hypertensive adult or paediatric patients so far.

The aim of our study is to geno type children and ado-

lescents at risk for development of essential hyperten-

sion for the ClC-Kb T481S polymorphism and associate

the presence of the polymorphism with elevated BP.

2. MATERIALS AND METHODS

Over the past 15 years ABPM has been performed in

more than 3000 children in our department. Among th-

R. Büscher / HEALTH 2 (2010) 682-684

683

ose, 126 Caucasian patients, who were seen at least three

times in our clinic, were diagnosed as WCHT. ABPM

(SpaceLabs 90217 oscillometric device, Redmond, Was-

hington, USA) confirmed hypertensive BP values ac-

cording to the percentiles o f Soergel et al. 3 in only 48

patients, while diagnosis was not confirmed in 78 pa-

tients. Organic reasons for HT like renovascular or car-

diovascular pathology or endocrine disorders were ex-

cluded and extended diagnostic analysis did not reveal

other feasible reasons for HT in 48 children. All 126

patients (48 HT, 22 female; 78 WCHT, 29 female) were

included in our study and EDTA blood samples or buccal

swabs were taken during a routine visit in order to gen-

erate genomic DNA. Patients were not significantly dif-

ferent regarding age (HT 14.4 ± 2.7 vs. WCHT 13.7 ±

2.5 years), height (HT 153.6 ± 22.6 vs. WCHT 151.5 ±

25.7 cm), weight (HT 57.1 ± 20.4 kg vs. WCHT 56.5 ±

23.9 kg) and BMI (HT 23.17 ± 5.0 vs. WCHT 23.75 ±

5.4 kg/m 2).

Genotyping f or th e ClC-Kb T4 81S polymorp hism was

performed using SSCP and direct sequencing as de-

scribed before 7,8. Results are expressed as mean ±

SEM. Comparison between the distribution of alleles

was performed using a 2-test. A p < 0.05 was consid-

ered statistically significant. The study was approved by

the board of ethics of the University of Duisburg-Essen

and all patients and their parents gave their written con-

sent.

3. RESULTS

A total of 126 patients were diagnosed as WCHT but

diagnosis was confirmed by ABPM in only 48 patients.

HT and WCHT patients were not different regarding

height, weight, and BMI. Systolic and diastolic BP was

significantly increased in hypertensive patients (systolic

daytime141  10 mmHg, diastolic daytime 83  11

mmHg; systolic nighttime 127  8, diastolic nighttime

67  13 mmHg) when compared to 78 WCHT patients

(systolic daytime 115  14 mmHg, diastolic daytime 71

 10 mmHg; systolic nighttime 105  14, diastolic night-

time 60  8 mmHg).

A total of 126 patients were genotyped for the pres-

ence of the ClC-Kb T481S polymorphism (Table 1).

Allelic frequencies for the A- and T-allele in hyperten-

sive and WCHT patients were not significantly different

between both groups (A-allele: HT 0.84 vs. WCHT 0.85;

T-allele: HT 0.16 vs. WCHT 0.15) and did not differ

from data obtained from population studies in the litera-

ture 8 (Table 1). The distribution of all genotypes was

in Hardy-Weinberg equilibrium. When looking more de-

tailed in the group of WCHT patients whose BP exceeds

the 90th percentile, we identified 34 out of 78 patients

Table 1. Allele and genotype frequency distribution of ClC-Kb

T481S.

Genotype HT (n = 48) WCHT (n = 78)

T481T

(homozygous noncarriers) 6 (75%) 58 (74.4%)

T481S

(heterozygote carriers) 9 (18.8%) 17 (21.8%)

S481S

(homozygous carr iers) 3 (6.2%) 3 (3.8%)

Allele

A-allele (n = 81) 0.84 (n = 133) 0.85

T-allele (n = 15) 0.16 (n = 23) 0.15

Allelic frequency of ClC-Kb T481S in WCHTchildren (RR >

90th percentile, n = 34)

A-allele (n = 48) 0.71

T-allele (n = 20) 0.29* vs. 0.15

*p < 0.05, Fisher exa ct te st a nd 2-test

(44%) fulfilling criteria of a borderline HT. In this sub-

group, the allelic frequency was significantly shifted

towards the T-allele (A-allele 0.71 vs. T-allele 0.29; p <

0.05 when compared to 0.15 in all other patients of this

group), suggesting that patients at risk for the develop-

ment of HT express the T-allele more often (Table 1).

4. DISCUSSION

Hypertension is a major risk factor for the development

of cardiovascular and cerebrovascular complications such

as heart attack and stroke 9. Hypertensive end-organ

damage has also been demonstrated in children and there

is a huge scientific and economic interest in identifying

children and adolescents at risk for developing HT in

order to initiate preventive treatment 10. Therefore,

even if HT does not appear to be significant initially,

children with borderline HT should have continuing fol-

low-up of their BP and monitoring for the complications

of HT. Our preliminary data suggest that children with

borderline HT carry the ClC-Kb T481S polymorphism

more often. The T481S polymorphism has been descri-

bed to be the first mutation that leads to a pronounced

gain of channel function and might therefore contribute

to elevated BP 7,8.

We are aware of the limitations of our retrospective

study such as small patients number and single center

design. However, this is the first study to support the in

vitro findings by Jeck and co-workers 7,8 that the

R. Büscher / HEALTH 2 (2010) 682-684

684

ClC-Kb T481S polymorphism detects children at risk for

development of HT while few other studies in adult hy-

pertensive patients d id not identify a positive correlation

so far 11 . Extended analysis and a larger population of

children with marginal elevated BP values are required

to clearly answer this question.

Openly accessible at

It is well known that renal tubular Na+ and Cl- trans-

port mediate BP control by influencing the extracellular

fluid volume 12. Therefore, genes involved in the

regulation of renal salt absorption are of potential inter-

est as candidate genes for the development of renal HT.

Jeck et al. 7,8 demonstrated that the ClC-Kb gene

T481S polymorphism leads to activation of the chloride

channel in the kidney. It is therefore feasible to hypo the-

size that also minor changes in th e handling of renal salt

absorption might lead to elevated BP. Proof of concept

using chloride absorption would be interesting to study.

However, due to the retrospective design of this study

and the lack of established standards in renal tubular

chloride handling in children, we are unable to test this

hypothesis in our study population.

It is important to know that all patients investigated

are European Caucasians which makes the possibility

unlikely that a subpopulation with a different ethnical

background exists Therefore, we can speculate that the

increased prevalence of the T-a llele in patients with bor-

derline HT is feasible and not biased by their genetic

background. However, other co-factors predisposing for

HT such as obesity or metabolic syndrome cannot be

ruled out 13. In this respect i t is interest ing t o no tice that

both, hypertensive and WCHT patients had an increased

BMI exceeding 23 kg/m2 making obesity a likely risk

fac t o r fo r de v e l o p m e n t o f H T in ou r s t u dy population.

In conclusion, children and adolescents with border-

line HT carry the ClC-Kb T481S polymorphism more

often and this genetic variant might predispose for the

development of arterial HT. Larger scale studies in ge-

netically diverse popu lations exclud ing other predispo se-

ing factors of HT a re needed to prove these observations.

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