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Subunit Gene of ENaC (1627delG and 1570-1G
A) in One Sporadic Japanese Patient with a Systemic Form of Pseudohypoaldosteronism Type 1
Department of Endocrinology and Metabolism (M.A., K.T., Y.A.), Kanagawa Childrens Medical Center, Yokohama 232-8555; and Department of Pediatrics (S.A., J.N., T.T., K.F.), Hokkaido University School of Medicine, Sapporo 060-8638, Japan
Address correspondence and requests for reprints to: Masanori Adachi, M.D., Department of Endocrinology and Metabolism, Kanagawa Childrens Medical Center, Minami-ku, Mutsukawa, 2-138-4, Yokohama 232-8555, Japan. E-mail: DZF01210{at}nifty.ne.jp
| Abstract |
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, ß, and
) of the epithelial
sodium channel (ENaC). To investigate the molecular basis of one
sporadic Japanese patient with a systemic form of PHA1, we determined
the nucleotide sequence of the genes of every subunit of ENaC of this
patient. The patient was found to be a compound heterozygote for one
base deletion in exon 12 (1627delG) in combination with 1570-1
GA
substitution at the 5' splice acceptor site of intron 11 in the
subunit gene of ENaC. The 1627delG mutation altered a reading frame,
resulting in a premature stop codon in exon 12. Messenger RNA from the
allele harboring the splice site mutation was not identified by RT-PCR.
In conclusion, two novel mutations in the
subunit gene of ENaC
caused systemic PHA1 in the sporadic Japanese patient. Identification
of the molecular basis of PHA1 is helpful for early diagnosis and
understanding the pathophysiology of the disease. | Introduction |
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Recently, the molecular basis of both forms of PHA1 has been clarified.
The systemic PHA1 is caused by the loss-of-function mutations in
,
ß, and
subunit genes for the amiloride-sensitive epithelial
sodium channel (ENaC) (8, 9, 10), and the renal PHA1 is
caused by heterozygous mutations of the mineralocorticoid receptor gene
(11).
ENaC is proposed to form a heterotrimer composed of three homologous
subunits (
, ß, and
) (12). Each subunit of ENaC
has highly conserved two hydrophobic domains (H1M1 and H2M2), in which
M1 and M2 represent membrane-spanning regions, H1 and H2 hydrophobic
regions, intracellular C- and N-terminals, and a large extracellular
loop (Fig. 1A
) (12). Sodium
transport mediated by this channel is the rate-limiting step for sodium
absorption by the epithelial cells that line the distal renal tubule,
the distal colon, the ducts of the salivary and sweat glands, and the
lung epithelium (13). Thus, the mutations of each subunit
gene of ENaC result in the failure of sodium absorption in every target
organ, leading to the systemic PHA1.
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subunit gene of ENaC.
To our knowledge, this is the first case of PHA1 caused by compound
heterozygous mutations of the
subunit gene of ENaC.
| Subjects and Methods |
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The patient was born at full term and was the second product of healthy, unrelated Japanese parents. The entire course of the pregnancy and the delivery were uneventful. The patients birth weight and length were 3.8 kg and 50 cm, respectively. He was referred to our neonatal unit at 7 days of age because of frequent vomiting and severe dehydration. Neither unusual skin pigmentation nor ambiguous genitalia were present.
The initial examination revealed marked hyponatremia (Na, 116 mEq/L),
hyperkalemia (K, 8.6 mEq/L), and metabolic acidosis (base excess, -8.0
mEq/L). His plasma aldosterone concentration was extremely elevated
[29,740 pmol/L (normal range for neonates, 5602,800)], as was his
PRA [192 mg/L·h (normal range, 0.217.5)]. Other endocrine
findings were all within normal range, including adequate serum
cortisol response (1,290 nmol/L). His renal function was also normal.
Based on these clinical and laboratory findings, he was diagnosed as
having the systemic form of PHA1. Since then, he has been treated with
salt supplementation (NaCl 6.0 g daily) and dietary potassium
restriction. Up to the age of 7 yr he required frequent hospital
admissions for iv hydration. Also, he had frequent lower respiratory
tract illnesses characterized by chest congestion and cough often
associated with fever and wheezing. These respiratory symptoms became
less severe and less frequent with advancing age. This patient was the
only clinically affected member among his family, and his elder sister
was healthy (Fig. 1B
).
PCR and direct sequencing of the
, ß, and
subunit genes of
ENaC
Informed consent was obtained from the parents for DNA analysis.
Genomic DNA was prepared from white blood cells of the patient and his
parents using standard techniques. The exons and exon-intron boundaries
of genes for the
, ß, and
subunits of ENaC were amplified by
PCR using oligonucleotide primers described by Chang et al.
(9) with a modification (G11 forward,
5'-TTCCTGTGTGAGGCCAACTTGG-3' for amplification of exon 12 of the
subunit gene). Each PCR using a Perkin-Elmer/Cetus Thermal
Cycler (Perkin-Elmer Corp., Norwalk, CT) was performed as:
30 cycles of 60 sec at 94 C for denaturation, 60 sec at 62 C for
annealing, and 60 sec at 72 C for polymerization. Each PCR product was
purified by 2% NuSieve (FMC Bioproducts, Rockland, ME) gel
electrophoresis, and then both strands of products were directly
sequenced using an automated DNA sequencer with Taq DyeDeoxy
sequencing reagents (PE Applied Biosystems, Inc., Foster
City, CA), as described previously (14).
RNA extraction, RT-PCR, and complementary DNA (cDNA) direct sequencing
A lymphoblastoid cell line from the patient was established
following Epstein-Barr virus transformation by standard methods.
Total RNA was extracted using the RNAzol B kit (Tel-Test,
Inc., Friendswood, TX). Specific antisense- primed cDNAs were
synthesized from 200250 ng total RNA in a final volume of 20 mL using
a GeneAmp Thermostable rTth Reverse Transcriptase RNA PCR Kit
(Perkin-Elmer Corp., Foster City, CA). Using all the
reactant containing cDNA, the first PCR was carried out with a set of
primers 5'-CAAAGACCTGAACCAGAGATCC-3' (primer A) and
5'-GAGCTCATCCAGCATCTGG-3' (primer B) according to the manufacturers
protocol (Fig. 2A
). After
chloroform-ethanol precipitation, the first PCR products were subjected
to the second PCR reaction with nested primer C
(5'-GATCCATCATGGAGAGCCCA-3') and primer B. Primers B and C were
designed to encompass the 3' end of exon 11 and the entire exon 12 of
the
subunit gene of ENaC, with a predicted product size of 407 bp
(Fig. 2A
). Conditions for the second PCR were 40 cycles of 60 sec at 94
C, 60 sec at 60 C, and 120 sec at 72 C. The second PCR products were
visualized by 2% NuSieve gel electrophoresis. The cDNA band of the
patient was purified using Wizard PCR Preps DNA Purification System
(Promega Corp., Madison, WI), and then both strands of
product were directly sequenced using an automated DNA sequencer, as
mentioned above.
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Additionally, oligo d(T)16 (Perkin-Elmer Corp.) was also used for priming the RT reaction as the manufacturers protocol, followed by the amplification with primers A and B, and then with primers C and B.
| Results |
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, ß, and
subunit genes of
ENaC
PCR-direct sequencing of the entire coding regions of the
and
ß subunit genes of ENaC failed to identify any mutations. However, we
identified one base deletion (1627delG) in exon 12 (Fig. 1C
) and a G to
A transition, which occurred at the terminal acceptor splice site,
flanking the 5' end of exon 12 (1570-1G
A) (Fig. 1D
) in the
subunit gene of ENaC. The former one-base deletion (1627delG) altered
an open reading frame, resulting in a premature stop codon 166
nucleotides downstream in exon 12. Family analysis revealed that the
mother carried the 1627delG mutation and the father carried the
1570-1G
A mutation as a heterozygote, respectively (Fig. 1B
).
Detection of
subunit gene mRNA in lymphoblastoid cell line by
RT-PCR
Because the alignment of AG at the -2 and -1 positions of the
acceptor splice site is well conserved in eukaryotes (15, 16), it is assumed that an abnormal splicing may have occurred
because of 1570-1G
A mutation. To assess the effect of this mutation,
we performed RT-PCR experiments [one with oligo d(T)16
primer and those with two different sets of specific primers] with the
lymphoblastoid cell line. In each experiment, the resulting cDNA band
from the patient was identical in size to that from the normal control
lymphoblastoid cell line (Fig. 2B
). Direct sequencing of the patients
bands revealed only 1627delG mutation (Fig. 2C
). These findings
indicated that mRNA was transcribed from only the maternal allele, not
from the paternal allele, because of the splicing junction mutation in
the last intron.
| Discussion |
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In this study, we identified two novel heterozygous mutations (1627delG
and 1570-1G
A) in the
subunit gene of ENaC. Family analysis
identified that the mother carried the heterozygous 1627delG mutation
and the father carried the heterozygous 1570-1G
A mutation,
indicating autosomal recessive inheritance.
Until now, several mutations in either the
, ß, or
subunit
gene of ENaC were identified in the patients with systemic PHA1. In the
subunit gene of the ENaC, mutations including one or two base
deletions leading to frameshift, two different premature stop codons,
and deletion of exons 3 and 4 have been described (8, 9, 17). All these mutations are thought to produce truncated
protein, resulting in the loss of function of the channel.
Additionally, one missense mutation in the M2 domain was reported,
recently (17). In the ß subunit gene of the ENaC, one
missense (G37S) and two deletion mutations accompanying frameshift have
been identified (8, 9). In the
subunit gene of the
ENaC, only one mutation of the 3' splice site mutation (318-1G
A) has
been reported in familial patients (Ref. 10 ; Fig. 1A
).
This mutation caused abnormal splicing resulting in the substitution of
conserved amino acids and grossly truncated C-terminal
(10).
The 1627delG mutation in our patient is located in M2 domain and
produces a premature stop codon 166 nucleotides downstream (Fig. 1A
).
Therefore, this mutation eliminates the most of M2 domain and
C-terminal intracellular domain, resulting in the loss of channel
function. In RT-PCR reaction there exists the expected PCR fragment,
including 1627delG mutation. Because of the presence of the premature
stop codon, one would expect an absence of cDNA due to mRNA instability
or a truncated cDNA. However, it has been reported that introduction of
the premature stop codon does not always affect the mRNA transcription
in several cases (18, 19, 20). Thus, our results of RT-PCR
obtaining expected fragments could be explained.
The other mutation, 1570-1G
A, is located in a terminal exon splice
junction. In general, any mutations in consensus splice-site sequences
yield to exon skipping and to the activation of cryptic splice sites at
a lesser frequency, causing human disease (21, 22, 23).
However, mutation of a terminal exon splice junction is rare. Otterness
et al. (24) reported a mutation with the G
A
transition at the last nucleotide of the final intron/exon junction in
the thiopurine methyltransferase gene, in human thiopurine intorelance.
In their study, mRNA of thiopurine methyltransferase from the mutated
allele was not identified by RT-PCR analysis, presumably due to mRNA
instability resulting either from activation of a cryptic splice site
within intron or from creation of a novel splice site. Similarly, mRNA
from the allele harboring the 1570-1G
A mutation was not detected in
our study. Thus, we presume that this mutation might cause aberrant
splicing and inhibit the normal mRNA transcription.
In conclusion, we reported the first sporadic patient with the systemic
form of PHA1 caused by compound heterozygous mutations of the
subunit gene of ENaC. Identification of the molecular basis of this
disorder is helpful for early diagnosis, clinical management, and
understanding the pathophysiology of PHA1.
| Acknowledgments |
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Received March 1, 2000.
Revised August 29, 2000.
Accepted September 13, 2000.
| References |
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subunit of the epithelial
sodium channel gene in three pseudohypoaldosteronism type 1 families. Nat Genet. 13:248250.[CrossRef][Medline]
-subunit of the epithelial sodium
channel. J Pediatr. 135:739745.[CrossRef][Medline]
, ß, and
subunits of the epithelial sodium channel involved in amiloride block
and ion permeation. J Gen Physiol. 109:1526.This article has been cited by other articles:
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