Partial nephrogenic diabetes insipidus with a novel arginine vasopressin receptor 2 gene variant.

X-linked nephrogenic diabetes insipidus (NDI) is caused by variations in arginine vasopressin receptor 2 (AVPR2). Some patients show partial resistance to arginine vasopressin (AVP). A 19-month-old Japanese boy presented with polydipsia since infancy. His mother had a history of polydipsia during pregnancy, and his maternal granduncle also had polydipsia. Intermediate urine osmolality and markedly high plasma AVP levels were observed in the water deprivation test. Subsequent pitressin administration caused no further elevation in urine osmolality. We diagnosed the patient with partial NDI, initiated therapy with hydrochlorothiazide, and placed him on a low-sodium diet. Although his urine volume decreased by 20-30% after the initiation of therapy, progressive hydronephrosis and growth retardation developed 2 years later. We investigated his genetic background by multiplex targeted sequencing of genes associated with inherited renal diseases, including AVPR2 and aquaporin-2 (AQP2). We identified a hemizygous missense variant in AVPR2 NM_000054:c.371A>G,p.(Tyr124Cys) in the boy and a heterozygous variant in the mother at the same locus. Distinguishing partial NDI from primary polydipsia is difficult because of its mild symptoms. Markedly elevated plasma AVP levels with intermediate urine osmolality may suggest partial NDI, and genetic analysis can be useful for such patients. 2022©The Japanese Society for Pediatric Endocrinology.

Highlights

● We report a partial NDI with a novel AVPR2 variant (NM_000054:c.371A>G,p.(Tyr124Cys)).

● The patient could concentrate urine up to 500 mOsm/kg after water deprivation.

● Markedly elevated plasma AVP with intermediate urine osmolality may suggest partial NDI.

Introduction

Nephrogenic diabetes insipidus (NDI) is a rare inherited disease characterized by the inability to concentrate urine due to renal resistance to the antidiuretic hormone, arginine vasopressin (AVP). There are two forms of inherited NDI: a common (approximately 90%) X-linked recessive form caused by a variant of the vasopressin type 2 receptor (V2R)-encoding gene, arginine vasopressin receptor 2 (AVPR2), and a less common (approximately 10%) autosomal recessive or dominant form caused by a variant of aquaporin-2 (AQP2) (1). Most patients with inherited NDI appear to have complete or nearly complete resistance to the antidiuretic effect of AVP, with a urine osmolality of less than 300 mOsm/kg (2). However, some patients with NDI have mild phenotypes that present with intermediate urine osmolality. These mild forms, called partial NDI, are difficult to distinguish from primary polydipsia. To date, more than 200 variants of AVPR2 have been identified in cases with X-linked NDI (3), and several AVPR2 variants have been associated with a mild phenotype (4). Here, we report a case of partial NDI with a novel AVPR2 variant. The patient was able to concentrate urine up to 500 mOsm/kg and had markedly elevated plasma AVP levels after water deprivation.

Patient and Methods

A 19-month-old boy presenting with polydipsia was referred to our hospital. The mother became aware of his polydipsic tendency from six months of age. He got up several times during the night to drink water, and drank more than 2 liters of water recently. He was born at the 40th gestational week as the second of two children of non-consanguineous parents. His birth weight was 3,282 g (+ 0.2 SD) and birth height was 51.0 cm (+ 0.9 SD). He did not have a history of dehydration or recurrent fever. The mother had an episode of polydipsia during pregnancy, and the maternal granduncle also had polydipsia; however, they had not been diagnosed. The sister, father, and other relatives did not have polydipsia. His neurodevelopment was appropriate for his age. His height was 80.2 cm (−0.4 SD) and weight was 9.9 kg (−0.7 SD). No growth retardation was observed prior to admission (Fig. 1). The initial investigation revealed that the plasma AVP, urine, and plasma osmolality were 15.7 pg/mL, 86 mOsm/kg, and 279 mOsm/kg, respectively. His serum electrolytes, creatinine, glucose, and HbA1C levels were normal (Table 1). Brain magnetic resonance imaging showed a normal pituitary gland and posterior lobe with high intensity on a T1-weighted image. Ultrasound revealed no hydronephrosis or dilation of the lower urinary tract. The volumes of 24-h drinking water and urine volume during hospitalization were 2,880 mL and 3,300 mL (7,221 mL/m2/d), respectively. After 7 h of water deprivation, urine osmolality reached 506 mOsm/kg, and his weight had decreased by 6.6%. The concurrent plasma osmolality and AVP were 294 mOsm/kg and 65.1 pg/mL, respectively. Subsequent exogenous vasopressin (Pitressin) loading showed no further increase in urine osmolality (Table 2).

Fig. 1.

Growth chart and therapeutic dosages of drugs.

Table 1.

Laboratory data

Table 2

. Water deprivation and arginine vasopressin (AVP) loading test

On the basis of these findings, the patient was diagnosed with NDI. Therapy was initiated with hydrochlorothiazide (2 mg/kg/d) and a low sodium diet (< 3 g/d), but his sodium intake remained higher than desired. The urine volume decreased by 20–30% with a gradual increase in hydrochlorothiazide up to 6 mg/kg/d. Spironolactone (1 mg/kg/d) was added when mild hypokalemia became evident at 2 yr and 11 mo of age (Table 1). Ultrasound showed bilateral hydronephrosis (Society for Fetal Urology grade: right 2, left 1) after 2 yr of therapy. His height growth velocity slightly decreased as his recent height was 94.2 cm (−1.3 SD), while his weight was 13.8 kg (−0.8 SD) (Fig. 1).

DNA analysis

After obtaining informed consent from the parents and approval from the institutional ethical committee, multiplex targeted sequencing of 95 genes associated with renal inherited disease, including AVPR2 and AQP2, was performed on peripheral blood-derived DNA from the patient and his mother, as described in a previous publication (5).

All procedures were performed in accordance with the 1964 Helsinki Declaration and the 2003 Japanese Ethical Guidelines for Clinical Research, as well as their later amendments.

Ethical Consideration

The parents provided informed consent for genetic testing and approved the clinical and genetic data of the boy and his mother for publication. This study was approved by the institutional ethical committee.

Results

The analysis revealed a hemizygous missense variant in AVPR2 (NM_000054:c.371A>G,p.(Tyr124Cys)) in the boy and a heterozygous variant in the mother at the same locus. Variants were confirmed using Sanger sequencing of all exons and exon-intron boundaries (Fig. 2). This variant has not been registered in either ClinVar (https://www.ncbi.nlm.nih.gov/clinvar/) or the Human Genome Mutation Database (www.hgmd.org) and is very rare as it is absent in the public databases of minor allele frequencies, such as gnomAD (v2.1.1, https://gnomad.broadinstitute.org/) and 8.3KJPN, which include 8,300 Japanese reference genomes (https://jmorp.megabank.tohoku.ac.jp/202102/). In silico prediction scores for the variants also supported the interpretation of pathogenicity (SIFT 0: deleterious, Polyphen2 1: damaging, CADD_phred 24, MCAP 0.386, and REVEL 0.743). Clinical interpretation of the variant by the American College of Medical Genetics and Genomics guidelines (6) is “Uncertain Significance” (PM1, PM2, PP3), which supports the possibility of pathological significance (7). Considering all our observations, we considered this variant to be pathogenic. No variants in AQP2 or other genes associated with renal-inherited diseases were detected.

Fig. 2

. Result of arginine vasopressin receptor 2 (AVPR2) sequencing. The sequence chromatogram of AVPR2 in the patient and mother.

Discussion

We report a case of partial NDI with a novel AVPR2 variant, Tyr124Cys (Y124C). The amino acid, Y124, is highly conserved among mammals, according to the UCSC Genome Browser (https://genome.ucsc.edu/). We hypothesized the following mechanism in which the Y124C V2R molecule decreases water reabsorption. The majority of missense variants in the AVPR2 gene have been reported to generate misfolded V2R proteins located in the endoplasmic reticulum (ER), leading to the abnormal expression of V2R on the cell surface (8). The Y124C variant produces an additional cysteine residue, indicating that it may generate an additional inter- or intra-disulfide bond that disrupts proper folding in the ER and translocation to the membrane. Furthermore, Y124 has been shown to be a binding site for AVP by computerized 3D analysis of molecular docking (9). Therefore, we speculate that the binding efficiency of AVP would be reduced, and the urine concentration would be attenuated even if AVPR2 was properly expressed on the membrane to some extent in this case.

Most patients reported to have inherited NDI have a phenotype characterized by early dehydration episodes, such as fever or hypernatremia, observed as early as the first week of life (1); however, a few NDI patients with mild symptoms and intermediate urine osmolality have been reported (4, 10,11,12,13,14). Our patient had a mild phenotype with a missense variant (Y124C), whereas Bichet et al. had reported a patient with a nonsense variant at the same locus (Y124*) who showed a lifelong history of polydipsia and unconcentrated urine despite the administration of desmopressin (DDAVP), suggesting a complete form (15). Based on a MEDLINE search, 19 AVPR2 variants (A37P, D85N, V88M, R104C, R106C, Y128S, L161P, W200R, G201D, T273M, F287L M311V, N317K, N317S, N321Y, P322S, S329R, S333del, and the splice variant c.276A>G) were associated with a mild phenotype (3, 4, 10,11,12,13,14). Bockenhauer et al. reported the phenotypic diversity in six patients with V88M variants. Four of these six patients demonstrated a substantial increase in the urine concentration after the administration of DDAVP, while two did not show any response. The in vitro analysis suggested that this diversity may be attributed to both the cell surface expression of V88M-V2R and the AVP-binding affinity, which is affected by the variant (16). Partial NDI has been rarely reported as mild symptoms may be misdiagnosed as primary polydipsia or nocturnal enuresis. The variants associated with partial NDI may become more evident if genetic analyses of milder cases are performed more frequently.

Interestingly, the mother had a history of polydipsia during pregnancy. A few symptomatic females with heterozygous AVPR2 variants have been described. A Japanese nationwide survey on NDI identified 10 symptomatic females among 65 patients with AVPR2 variants (10). Furthermore, 16 of 64 Japanese female heterozygotes exhibited some degree of polydipsia or polyuria (17). Phenotypic expression in females has been attributed to skewed X chromosome inactivation, which has been demonstrated in some cases (18). This variant may be associated with polydipsia in the mother during pregnancy.

Partial NDI may often be missed or misdiagnosed as primary polydipsia or nocturnal enuresis due to the mild symptoms. In our case, the water deprivation test showed that urine osmolality was > 500 mOsm/kg, whereas the plasma AVP levels were disproportionally high when compared with the plasma osmolality (Fig. 3) (2, 19). We suspected NDI and identified a novel AVPR2 missense variant. Various complications occur in NDI, such as mental retardation, failure to thrive, and urinary tract disorders. Early diagnosis is of great importance in preventing these complications. In NDI, plasma AVP level tends to be elevated due to the insensitivity of the collecting tubule. Careful attention to not only the urine osmolality, but also the plasma AVP levels is warranted when diagnosing NDI.

Fig. 3.

The plasma AVP levels and osmolality of this case. The plasma AVP levels are shown as black dots. The reference range of AVP levels is shown in the gray area (19).

Conclusions

We identified a novel AVPR2 missense variation that caused partial NDI. We found that in addition to urine osmolality, plasma AVP levels are important for the diagnosis of partial NDI. Moreover, genetic analysis of AVPR2 or AQP2 can be useful for evaluating patients with intermediate urine osmolality and elevated plasma AVP levels.

Conflict of Interests

The authors declare no conflict of interest.