Nephrogenic diabetes insipidus: Functional analysis of new AVPR2 mutationsidentified in Italian families

The aim of this study was to identify loss-of-function mutations of the V2 vasopressin receptor gene (AVPR2) in Italian patients affected by X-linked nephrogenic diabetes insipidus (NDI). Mutations were found in 15 of the 18 unrelated families investigated: nine of these mutations were previously un known, including two affecting residues located in regions known to be impo rtant for determining the pharmacologic properties of the receptor, which w ere therefore functionally investigated. The first (A84D) involves a residu e located near an aspartic acid (D85) that is highly conserved in all G pro tein-coupled receptors and that is believed to play a role in the process o f their isomerization into functionally active and inactive states. The pre sent study indicates that this mutation not only affects receptor folding i n such a way as to lead to its retention inside the intracellular compartme nts but, as expected, also has profound effects on its binding and coupling properties. The second was a mutation of a tryptophan located at the begin ning of the first extracellular loop (W99R) that greatly impaired the bindi ng properties of the receptor and had a minor effect on its intracellular r outing. Molecular analysis of the first extracellular loop bearing this mut ation suggests that this residue plays a fundamental role in stabilizing th e peptide/ receptor interactions responsible for the high-affinity binding of agonists to the V2 receptor.