A novel mutation in the coding region for neurophysin-II is associated with autosomal dominant neurohypophyseal diabetes insipidus.

OBJECTIVE: Autosomal dominant neurohypophyseal diabetes insipidus (ADNDI) is a rare cause of diabetes insipidus, in which AVP serum levels are insufficient. AVP is synthesized along with neurophysin-II (NPII) as an AVP-NPII precursor polypeptide in the hypothalamus. After proteolytic cleavage during axonal transport, AVP and NPII are reassembled and stored loosely bound to each other in the posterior pituitary until both are released into the circulation. In this study, we investigated the genetic basis of ADNDI in a German kindred with 10 affected members spanning three generations.
DESIGN: Genomic DNA was isolated from peripheral blood leucocytes. The entire coding region of the AVP-NPII gene of one of the affected persons was amplified by polymerase chain reaction (PCR) and subjected to nucleotide sequence analysis. Sequencing results were confirmed by restriction enzyme analysis of PCR products. PATIENTS: Six affected and two unaffected members of a family with ADNDI and 54 unrelated healthy control subjects were studied.
RESULTS: The index patient was found by direct sequencing to be heterozygous for a G to T transversion at nucleotide position 1884 (exon 2) of the AVP-NPII gene. This mutation introduced a new recognition site for the restriction enzyme Ava II, which was used to test for the presence of the mutation in other family members and in control subjects. The mutation was detected in all family members with ADNDI, but was not found in unaffected family members or in control subjects. The mutation encodes a valine in place of the normal glycine at amino acid 65 of NPII, which is known to be highly conserved during evolution.
CONCLUSIONS: In this family, the autosomal dominant neurohypophyseal diabetes insipidus phenotype cosegregates with a point mutation in a region of the AVP-neurophysin-II gene which codes for the carboxy-terminal domain of neurophysin-II. Although the altered amino acid is not directly involved in AVP binding, the mutation might lead to conformational changes that impair the dimerization of neurophysin-II molecules. This could in turn affect the AVP binding affinity of neurophysin-II or might interfere with the transport of the AVP-neurophysin-II precursor in the AVP-producing cells of the hypothalamus.