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Objective: Most mutations of the arginine vasopressin-neurophysin II (AVP-NPII) gene cause autosomal dominant familial neurohypophyseal diabetes insipidus (adFNDI). Such mutations are predicted to alter the three-dimensional structure of the prohormone, which accumulates in the cell body, ultimately leading to neuronal degeneration and hormonal deficit. In this study we describe the case of a 26-year-old female reporting a long-lasting history of polyuria/polydipsia. The father of the patient was affected by diabetes insipidus and was under desmopressin treatment until the time of his death. Nevertheless, the patient had never been subjected to endocrine evaluation., Design and Methods: Clinical and genetic studies were performed. An 8-h fluid deprivation test plus desmopressin challenge and a 5% saline solution test were performed, in order to confirm the diagnosis. DNA was extracted from peripheral blood lymphocytes and subjected to direct sequencing of the entire coding region of the AVP-NPII gene., Results and Conclusions: Clinical assessment of the patient confirmed the diagnosis of neurohypophyseal diabetes insipidus. Desmopressin treatment was started, which effectively reversed the polyuria/ polydipsia syndrome. Genetic analysis revealed a novel mutation (1665T>A) in exon 2 of the AVP-NPII gene, disrupting one of the disulfide bonds present in the NPII moiety which play a fundamental role in determining the proper folding of the molecule. In summary, in the present study we have described a novel mutation of the AVP-NPII gene, which is consistent with the malfolding/toxicity hypothesis underlying the pathogenesis of adFNDI.

The hormonal precursor pro-ocytocin-neurophysin is activated by selective cleavage at Arg2-Ala13, producing mature ocytocin and neurophysin. To understand the cleavage mechanism better, and in particular the recognition of the cleavage site, it is necessary to characterize the three-dimensional structure of the precursor molecule. Here we combine a variety of experimental data with molecular modeling and dynamics calculations to derive possible precursor conformations. In the models obtained, the N-terminus of the precursor, corresponding to the ocytocin segment, is hydrogen bonded in a pocket of the neurophysin moiety in a similar manner to a crystallographically obtained non-covalent complex between the two molecules. The calculations suggest that although the ocytocin segment is relatively flexible, it adopts a stable, broad loop structure in the vicinity of the cleavage region, which may constitute the structural element recognized by the cleaving enzyme. The calculations also suggest a possible widening of the distance between the two neurophysin domains in the precursor relative to that in the non-covalent neurophysin-ocytocin complex.

The molecular basis of autosomal dominant neurohypophyseal diabetes insipidus, a hereditary deficiency of vasopressin, was determined by nucleotide sequence analysis of the arginine vasopressin-neurophysin-II gene. A C-->T mutation at nucleotide 1761 was detected in one allele of this gene in each affected individual in three generations of one family. This mutant gene encodes a normal arginine vasopressin peptide, but predicts a substitution of leucine for proline at amino acid 24 of neurophysin-II, the arginine vasopressin carrier protein. This mutation was not detected 50 control individuals, thus demonstrating that it is not a common silent genetic polymorphism. The disease arose in the second generation of the studied family, and the chromosome 20 carrying this new mutation was identified by polymorphic CA microsatellite haplotype analysis. The first affected individual inherited this chromosome segment from her mother, who had neither the disease nor this mutation in her somatic cell DNA. Third generation individuals who subsequently inherited this mutation were affected. These data demonstrate that this amino acid substitution in neurophysin-II causes the disease. Two possibilities to explain the mechanism by which clinical deficiency of arginine vasopressin develops even in the presence of one normal arginine vasopressin-neurophysin-II allele are discussed.