AVP-NPII gene mutations and clinical characteristics of the patients with autosomal dominant familial central diabetes insipidus

Investigation of Fibrillar Aggregates Formed by Pathogenic Pre-pro-vasopressin Mutants that Cause ADNDI

CONTEXT

Aggregations of unfolded or misfolded proteins, both inside and outside cells, are implicated in numerous diseases, collectively known as amyloidosis. Particularly, autosomal dominant neurohypophyseal diabetes insipidus (ADNDI) is a rare disease caused by mutations in the AVP-NPII gene, leading to the inability to secrete arginine vasopressin. These misfolded proteins accumulate within the endoplasmic reticulum (ER), causing cellular dysfunction.

OBJECTIVE

This study aimed to investigate the formation of amyloid-like aggregates within the cell resulting from misfolded mutant precursor proteins, which induce disulfide-linked oligomers due to the G45C, 207_209delGGC, G88V, C98X, C104F, E108D-1, E108D-2 and R122H mutations identified by our group in the AVP-NPII gene of ADNDI patients.

METHODS

Deglycosylation studies were performed to analyze the glycosylation patterns of mutant protein precursors. The involvement of these precursors in the ER-related degradation pathway was studied by conducting protease inhibition experiments. Disulfide-linked oligomer analysis determined the oligomerization status of the mutant precursors. Immunofluorescence and electron microscopy studies provided evidence of aggregate structures in the ER lumen. In vitro studies involved bacterial expression and fibril formation in Escherichia coli (E. coli).

RESULTS

Our findings demonstrated that the N-glycan structure of mutant precursors remains intact within the ER. Protease inhibition experiments indicated the involvement of these precursors in the ER-related degradation pathway. Disulfide-linked oligomer analysis revealed homo-oligomer structures in mutations. Immunofluorescence and electron microscopy studies confirmed the presence of aggregate structures in the ER lumen. In vitro studies showed that mutant precursors could form fibril structures in E. coli.

CONCLUSION

Our study may support the idea that ADNDI belongs to the group of neurodegenerative diseases due to the formation of fibrillar amyloid aggregates in the cell.

Central Diabetes Insipidus in Children as a Diagnostic Challenge

Central diabetes insipidus (CDI) is a disorder in the pediatric population resulting from antidiuretic hormone deficiency. The excessive production of dilute urine characterizes it and manifests with polyuria, nocturia, and polydipsia. The diagnostics of CDI is often challenging, especially concerning the underlying condition of the disease. This article highlights the diverse clinical presentation of children with CDI and diagnostic difficulties among patients with polyuria and polydipsia. The article also reviews the etiology, symptoms, diagnostic workup, and management of CDI. We present 4 pediatric patients (aged 3-13.5 years) diagnosed with CDI of different etiology: 1 due to septo-optic dysplasia/optic nerve hypoplasia and 3 due to acquired processes such as Langerhans cell histiocytosis and germ cell tumor in 2 patients. Central diabetes insipidus was the first manifestation of a tumor or granuloma in all presented patients with acquired pathology. The patients sometimes need long-term follow-up to establish the proper final diagnosis.

Functional analyses of three different mutations in the AVP-NPII gene causing familial neurohypophyseal diabetes insipidus

PURPOSE

Familial neurohypophyseal diabetes insipidus (FNDI), a rare disorder, which is clinically characterized by polyuria and polydipsia, results from mutations in the arginine vasopressin-neurophysin II (AVP-NPII) gene. The aim of this study was to perform functional analyses of three different mutations (p.G45C, 207_209delGGC, and p.G88V) defined in the AVP-NPII gene of patients diagnosed with FNDI, which are not included in the literature.

METHODS

For functional analysis studies, the relevant mutations were created using PCR-based site-directed mutagenesis and restriction fragment replacement strategy and expressed in Neuro2A cells. AVP secretion into the cell culture medium was determined by radioimmunoassay (RIA) analysis. Fluorescence imaging studies were conducted to determine the differences in the intracellular trafficking of wild-type (WT) and mutant AVP-NPII precursors. Molecular dynamics (MD) simulations were performed to determine the changing of the conformational properties of domains for both WT and 207-209delGGC mutant structures and dynamics behavior of residues.

RESULTS

Reduced levels of AVP in the supernatant culture medium of p.G45C and p.G88V transfected cells compared to 207_209delGGC and WT cells were found. Fluorescence imaging studies showed that a substantial portion of the mutant p.G45C and p.G88V AVP-NPII precursors appeared to be located in the endoplasmic reticulum (ER), whereas 207_209delGGC and WT AVP-NPII precursors were distributed throughout the cytoplasm.

CONCLUSIONS

The mutations p.G45C and p.G88V cause a failure in the intracellular trafficking of mutant AVP-NPII precursors. However, 207_209delGGC mutation does not result in impaired cellular trafficking, probably due to not having any significant effect in processes such as the proper folding, gain of three-dimensional structure, or processing. These results will provide valuable information for understanding the influence of mutations on the function of the AVP precursor hormone and cellular trafficking. Therefore, this study will contribute to elucidate the mechanisms of the molecular pathology of AVP-NPII mutations.

A Rare Case of Familial Neurogenic Diabetes Insipidus in a 22-Year-Old Man

Objective

Diabetes insipidus (DI) can be classified into 2 types: central/neurogenic DI and nephrogenic DI. Most cases of central DI occur after brain surgery, trauma, tumor, or infection. Here we report a rare case of familial central DI due to a heterozygous AVP gene mutation.

Methods

A case of familial neurogenic DI has been described with thorough clinical, laboratory, and genetic workup. PubMed and Google scholar databases were used for literature discussion.

Results

A 22-year-old man presented with polyuria and polydipsia. He drank about 4 gallons of water everyday and urinated large volumes very frequently. His physical examination was unremarkable. After 2 hours of water-deprivation, his serum sodium level was 147 mmol/L, serum osmolality was 302 mOsm/kg with concurrent urine osmolality of 78 mOsm/kg, vasopressin level was <0.8 pg/mL, and copeptin level was <2.8 pmol/L, suggesting neurogenic DI. His brain magnetic resonance imaging revealed the absence of the posterior pituitary bright spot but a normal anterior pituitary gland. Genetic analysis revealed a nonfunctional heterozygous mutation in the AVP gene. Further questioning revealed that his mother also had the disease and that he had been treated with desmopressin as a child; however, it was later self-stopped. The patient was reinitiated on desmopressin, which improved his symptoms.

Conclusion

Genetic mutations in the AVP gene represent a very rare etiology of DI, and patients with DI respond well to desmopressin treatment.

Differential diagnosis of familial diabetes insipidus

Diabetes insipidus (DI) is a syndrome characterized by the persistent excretion of abnormally large volumes of dilute urine. It can be caused by any of four fundamentally different abnormalities: deficient production of the antidiuretic hormone, arginine vasopressin (AVP) by magnocellular neurons that form the posterior pituitary (hypothalamic DI); impaired renal effects of AVP (nephrogenic DI); reduced AVP secretion due to excessive water intake (primary polydipsia); or degradation of AVP by placental vasopressinase (gestational DI). Each type of DI can be caused or potentiated by other disorders. Hypothalamic and nephrogenic DI can also be caused by mutation of the gene that encodes the AVP prohormone, the AVP-2 receptors in the kidney, or the aquaporin-2 water channels that mediate antidiuresis. Familial hypothalamic DI is usually transmitted in an autosomal dominant mode, but autosomal recessive or X-linked recessive forms also exist. Familial nephrogenic DI is usually transmitted in an X-linked recessive mode but can also be autosomal recessive or dominant. Hence the mode of inheritance does not always indicate the type of DI. Indirect methods of differential diagnosis are also unreliable and the pituitary MRI signal is diminished in both types of familial DI. Thus the determination of plasma AVP and/or the response to desmopressin therapy plus gene sequencing provides the best basis for effective management and family counseling.

Autosomal dominant familial neurohypophyseal diabetes insipidus caused by a novel nonsense mutation in AVP-NPII gene

Familial neurohypophyseal diabetes insipidus (FNDI) is a rare single-gene disorder caused by mutations of the arginine vasopressin-neurophysin II (AVP-NPII) gene. These changes impair the release of vasopressin from the posterior pituitary gland. In the present study, the AVP-NPII gene of a Chinese adult patient with central diabetes insipidus, the patient's symptomatic mother and an asymptomatic sister of the patient was sequenced. Examination of the family history revealed cases of FNDI across four generations. Gene sequencing analysis revealed a novel heterozygous mutation, c.268A>T (p.Lys90Ter), in exon 2 of the AVP-NPII gene, in the patient and the patient's mother, which led to the loss of 6 cysteine residues and aberrant disulfide bonds, which is predicted to alter the mature protein structure. The present study identified a novel heterozygous nonsense mutation of the AVP-NPII gene associated with FNDI, which broadens the spectrum of known mutations associated with this disorder and contributes to the understanding of its molecular basis.

Genetics of Diabetes Insipidus

Diabetes insipidus is a disease characterized by polyuria and polydipsia due to inadequate release of arginine vasopressin from the posterior pituitary gland (neurohypophyseal diabetes insipidus) or due to arginine vasopressin insensitivity by the renal distal tubule, leading to a deficiency in tubular water reabsorption (nephrogenic diabetes insipidus). This article reviews the genetics of diabetes insipidus in the context of its diagnosis, clinical presentation, and therapy.

Two novel mutations in seven Czech and Slovak kindreds with familial neurohypophyseal diabetes insipidus-benefit of genetic testing

Familial neurohypophyseal diabetes insipidus (FNDI) is a rare hereditary disorder with unknown prevalence characterized by arginine-vasopressin hormone (AVP) deficiency resulting in polyuria and polydipsia from early childhood. We report the clinical manifestation and genetic test results in seven unrelated kindreds of Czech or Slovak origin with FNDI phenotype. The age of the sign outset ranged from 2 to 17 years with remarkable interfamilial and intrafamilial variability. Inconclusive result of the fluid deprivation test in three children aged 7 and 17 years old might cause misdiagnosis; however, the AVP gene analysis confirmed the FNDI. The seven families segregated together five different mutations, two of them were novel (c.164C > A, c.298G > C). In addition, DNA analysis proved mutation carrier status in one asymptomatic 1-year-old infant.

CONCLUSIONS

The present study together with previously published data identified 38 individuals with FNDI in the studied population of 16 million which predicts a disease prevalence of 1:450,000 for the Central European region. The paper underscores that diagnostic water deprivation test may be inconclusive in polyuric children with partial diabetes insipidus and points to the clinical importance and feasibility of molecular genetic testing for AVP gene mutations in the proband and her/his first degree relatives.

WHAT IS KNOWN

• At least 70 different mutations were reported to date in about 100 families with neurohypophyseal diabetes insipidus (FNDI), and new mutations appear sporadically. What is New: • Two novel mutations of the AVP gene are reported • The importance of molecular testing in children with polyuria and inconclusive water deprivation test is emphasized.

Genetic forms of neurohypophyseal diabetes insipidus

Neurohypophyseal diabetes insipidus is characterized by polyuria and polydipsia owing to partial or complete deficiency of the antidiuretic hormone, arginine vasopressin (AVP). Although in most patients non-hereditary causes underlie the disorder, genetic forms have long been recognized and studied both in vivo and in vitro. In most affected families, the disease is transmitted in an autosomal dominant manner, whereas autosomal recessive forms are much less frequent. Both phenotypes can be caused by mutations in the vasopressin-neurophysin II (AVP) gene. In transfected cells expressing dominant mutations, the mutated hormone precursor is retained in the endoplasmic reticulum, where it forms fibrillar aggregates. Autopsy studies in humans and a murine knock-in model suggest that the dominant phenotype results from toxicity to vasopressinergic neurons, but the mechanisms leading to cell death remain unclear. Recessive transmission results from AVP with reduced biologic activity or the deletion of the locus. Genetic neurohypophyseal diabetes insipidus occurring in the context of diabetes mellitus, optic atrophy, and deafness is termed DIDMOAD or Wolfram syndrome, a genetically and phenotypically heterogeneous autosomal recessive disorder caused by mutations in the wolframin (WFS 1) gene.

Diabetes insipidus: The other diabetes

Diabetes insipidus (DI) is a hereditary or acquired condition which disrupts normal life of persons with the condition; disruption is due to increased thirst and passing of large volumes of urine, even at night. A systematic search of literature for DI was carried out using the PubMed database for the purpose of this review. Central DI due to impaired secretion of arginine vasopressin (AVP) could result from traumatic brain injury, surgery, or tumors whereas nephrogenic DI due to failure of the kidney to respond to AVP is usually inherited. The earliest treatment was posterior pituitary extracts containing vasopressin and oxytocin. The synthetic analog of vasopressin, desmopressin has several benefits over vasopressin. Desmopressin was initially available as intranasal preparation, but now the oral tablet and melt formulations have gained significance, with benefits such as ease of administration and stability at room temperature. Other molecules used for treatment include chlorpropamide, carbamazepine, thiazide diuretics, indapamide, clofibrate, indomethacin, and amiloride. However, desmopressin remains the most widely used drug for the treatment of DI. This review covers the physiology of water balance, causes of DI and various treatment modalities available, with a special focus on desmopressin.

Diagnosis and Treatment of Hypopituitarism

Hypopituitarism is a chronic endocrine illness that caused by varied etiologies. Clinical manifestations of hypopituitarism are variable, often insidious in onset and dependent on the degree and severity of hormone deficiency. However, it is associated with increased mortality and morbidity. Therefore, early diagnosis and prompt treatment is necessary. Hypopituitarism can be easily diagnosed by measuring basal pituitary and target hormone levels except growth hormone (GH) and adrenocorticotropic hormone (ACTH) deficiency. Dynamic stimulation tests are indicated in equivocal basal hormone levels and GH/ACTH deficiency. Knowledge of the use and limitations of these stimulation tests is mandatory for proper interpretation. It is necessary for physicians to inform their patients that they may require lifetime treatment. Hormone replacement therapy should be individualized according to the specific needs of each patient, taking into account possible interactions. Long-term endocrinological follow-up of hypopituitary patients is important to monitor hormonal replacement regimes and avoid under- or overtreatment.