Introduction
Congenital nephrogenic diabetes insipidus (NDI) is a rare disease caused by genetic
mutations in AVPR2 or AQP2 (1). AVPR2 is located at the Xq28 locus, and it encodes
arginine
vasopressin receptor 2 (AVPR2). Mutations in AVPR2 have been associated
with X-linked NDI. AQP2 is located at the 12q13.12 locus, and it encodes
the water transporter aquaporin-2. Mutations in AQP2 result in autosomal
NDI. Here, we describe a male infant with a novel AVPR2 variant who was
referred to our hospital due to delayed head control.
Case Report
A 5-mo-old boy was referred to the Department of Child Neurology in Okayama University
Hospital (Okayama, Japan) due to delayed head control. He was born to non-consanguineous
parents. His birth weight was 3630 g (+ 1.39 SD) and birth length was 50.8 cm (+ 0.86
SD).
He showed no signs of asphyxia. A recurrent fever of unknown origin was observed beginning
at 2 mo of age. Poor weight gain was observed at 3 mo of age. Notably, he did not
take any
medications.
At his first visit to our hospital, the patient’s length was 62.6 cm (–1.55 SD) and
weight
was 6190 g (–1.82 SD). His heart rate was 128 bpm, and his body temperature was 36.8°C.
He
was dehydrated, and his head control was incomplete. Laboratory examination revealed
that
his sodium level and serum osmolarity were elevated, and his urinary osmolarity and
specific
gravity were noticeably low. Head magnetic resonance imaging scanning revealed that
high
posterior lobe intensity was absent in T1-weighted images. However, the pituitary
stalk was
intact, and no other abnormalities were identified. These findings were indicative
of
diabetes insipidus. Water deprivation test with vasopressin challenge test resulted
in no
urine osmolarity increase and no urine volume decrease. Moreover, the AVP level at
admission
was extremely high (114 pg/mL; reference range; 0.0–4.2 pg/mL). Based on these findings,
we
diagnosed the patient with congenital NDI. His urine volume decreased, his weight
increased,
and head control was achieved after initiation of hydrochlorothiazide.
Assessment of the patient’s family history revealed that his mother had polydipsia
and
polyuria from infancy until the present day (Fig.
1A
Fig. 1.
A: Family tree of our patient (arrow). Black square and black circle indicate family
members with polydipsia and polyuria, respectively. Arrow indicates our patient. B:
Results of mutation analysis. Our patient showed hemizygous duplication of four base
pairs (c.990_993 dup CAGC; single bold line indicates CAGC). His older brother did
not
show this duplication. His mother was heterozygous for the wild-type allele and the
four-base pair duplication (dotted line).
). She also had increased levels of sodium of unknown origin during each of her three
pregnancies. Furthermore, the patient’s maternal grandfather, maternal great-grandfather,
and maternal aunt had histories of polyuria and polydipsia. We suspected that our
patient
had familial NDI and, thus, conducted a genetic analysis.
Mutational Analysis
Genetic analysis was approved by the ethical committee of Okayama University Hospital
and
conducted in accordance with the 1975 Declaration of Helsinki and subsequent amendments.
Informed consent for genetic analysis was obtained from the patient’s mother.
We identified a hemizygous four-base pair duplication in exon 3 of AVPR2
(c.990_993 dup CAGC, Fig. 1B). This four-base pair
duplication results in early termination (p.Val332Gln Fs26X). This variant was previously
unreported, and it is not present in the 1000 Genomes Project databases
(http://www.internationalgenome.org/1000-genomes-browers), the Human Genetic Variation
Database (http://www.hgvd.genome.med.kyoto-u.ac.jp), or the Exome Aggregation Consortium
Server (http://exac.broadinstitute.org). This variant is considered to be likely pathogenic
based on ACMG criteria (PS1, PM2 and PM4). We suspected that the patient’s mother
was
heterozygous for the wild-type allele and the four-base pair duplication. Thus, an
amplicon
obtained from the mother was cloned into a PCR® 4-TOPO® vector using
the TOPO® TA Cloning® Kit for Sequencing (Thermo Fisher Scientific,
Waltham, MA, USA), and each transformant was, subsequently, sequenced. The results
confirmed
heterozygosity for the wild-type allele and the four-base pair duplication. Since
the
patient’s mother suspected that the patient’s older brother had polydipsia, we also
analyzed
the patient’s older brother, However, the patient’s brother did not carry the four-base
pair
duplication. Notably, we did not analyze AQP2 in our patient, and we did
not perform genetic analysis for his father, older sister, maternal aunt, maternal
grandfather, or maternal great-grandfather.
Discussion
We identified a novel AVPR2 variant in a patient with familial congenital
NDI. Duplication variants are relatively rare among X-linked NDI patients: insertion
AVPR2 variants were found in only about 5% of large cohort (1) and Japanese patients
(2).
There are five classes of loss of function mutations in AVPR2 (1). Class I mutations
result in unstable mRNA, which
undergoes nonsense-mediated RNA decay. Class II mutations cause misfolding of receptors,
and
these mutant receptors remain in the endoplasmic reticulum. Class III mutations also
cause
receptor misfolding. Class III receptors reach the plasma membrane and react with
AVP, but
subsequent interactions with G proteins and cAMP production are impaired. Class IV
mutations
also result in misfolded receptors that reach the plasma membrane, but result in incomplete
AVP binding. Class V mutations cause mis-sorting to incorrect cellular compartments.
A
duplication of four bases in exon 3 of AVPR2 causes early termination.
However, this is the final exon, which might avoid nonsense-mediated RNA decay (3).
Residue 332 is located in an intracellular domain that
does not bind to AVP. Therefore, the variant in our patient may lose its function
via a
non-class I mechanism. Functional analysis is needed to clarify the pathophysiology
of this
variant.
Congenital NDI is generally identified by the presence of DI symptoms like polyuria,
polydipsia, fever, and poor weight gain (2). The
patient whose case is reported here was referred to the department of Child Neurology
in our
hospital because of delayed head control, poor weight gain, and fever. After treating
his
dehydration, he gained head control and his development proceeded normally. We consider
delayed motor development in infancy to be an important clinical symptom of congenital
NDI.
The patient’s mother had a heterozygous AVPR2 variant. She demonstrated a
high sodium level during each of her pregnancies, but her sodium levels were normal
before
pregnancy and after birth for all children. In gestational DI, urinary volume increases
during pregnancy because of increased clearance of AVP by placental vasopressinase,
a
cystine aminopeptidase that strongly degrades AVP (4).
Although inactivation of the X chromosome was not analyzed in the patient’s mother,
AVP
resistance and degradation by vasopressinase, might have contributed to her high sodium
levels during pregnancy.
In conclusion, we identified a novel AVPR2 variant in a patient with
familial congenital NDI. Common symptoms of DI were observed, including polyuria,
polydipsia, poor weight gain, and fever, and delayed motor development was also observed
during infancy.
Conflict of Interest
The authors declare that they have no conflict of interest.