1999 European Regional Conference Proceeding

May 12 - 16, 1999

Line
Conference: 1999 European Regional Conference
Title: V2 vasopressin receptor dysfunction in patients with nephrogenic diabetes insipidus is caused by different molecular mechanisms
Authors: Schulz, Angela; Pasel, Katharina; Volk, HD; Timmermann, Kirsten; Hoeltzenbein, Maria; Jaaskelainen, Jarmo; Gudermann, Thomas; Filler, M.D., Ph.D., FRCPC, Guido; Schoneberg, Torsten
Institutions: University of Leipzig, Institut fur Pharmakologie, Padiatrische Nephrologie, Institut fur Humangenetik, Kuopio University Hospital, Children's Hospital of Eastern Ontario
Line
Abstract • Translation
Loss-of function mutations in the V2 vasopressin receptor (AVPR2) gene have been identified as a molecular basis for X-linked nephrogenic diabetes insipidus (NDI). By screening our NDI patient clientele for mutations within the AVPR2 gene we have identified three novel (I46K, F105V, I130F) and four recurrent (D85N, R106C, R113W, Q225X) mutations. Functional analysis of the three new AVPR2 mutations in second messenger assays, enzyme-immunoassays and ligand-binding studies revealed the defects as NDI-causing mechanisms. The cell surface expression of I46K and I130F was less than 15% of the wild type AVPR2 expression. In concordance with latter findings, the maximum vasopressin-induced cAMP response was dramatically reduced for both mutant AVPR2s (2-fold over basal) as compared with the wild type receptor (8.4-fold over basal). In contrast, the F105V mutation is properly delivered to the cell surface and displayed an unchanged maximum cAMP-response but the mutation interferes with the high affinity vasopressin binding. The impaired ligand binding of F105V is reflected in an about 1000-fold shifted concentration-response curve towards higher vasopressin concentrations (EC50=211 nM). In one NDI case, characterization of the molecular defect revealed a submicroscopic deletion of a 21.5kb genomic fragment encompassing the entire AVPR2 gene and most of the C1 gene locus. The C1 gene at Xq28 encodes a protein assumed to function as a Rho GTPase-activating protein (rhoGAP). In the absence of detailed information about the physiological relevance and specific functions of rhoGAP C1, a thorough clinical and laboratory investigation of the patient was performed. Besides clearly defined NDI symptoms caused by deletion of the AVPR2 gene, no major abnormalities as determined by physical examination, radiography, ultra sound, computed tomographic scan, and extensive analysis of blood chemical, enzyme and hormone values were detected over a period of 16 years. Because of a predominant expression pattern of the C1 gene in hematopoietic cells, we focused on immunologic and hematologic laboratory parameters of the affected boy and the mother who was found to be heterozygous. Differential white-cell counts including lymphocyte typing, determination of lymphokines, cytokines and immunoglobulins, and numerous leukocyte function tests revealed no pathological findings. Based on our observations, the rhoGAP C1 protein is not essential for normal development in the human. Therefore, we postulate that the loss of rhoGAP C1 function is most likely compensated by other members of the GAP family.

Schulz et al., identified three new V2 vasopressin receptor (AVPR2) gene mutations that result in nephrogenic diabetes insipidus (NDI). Two of the mutations, I46K and I130F, could not travel to the cell surface where they must be for proper function. Mutation F105V is able to get to the cell surface, but once there, it is unable to bind with the antidiuretic hormone, vasopressin (VP) with the required high degree of affinity.

The researchers report on one NDI case that resulted from the deletion of the entire AVPR2 gene from the X chromosome. In addition, most of the C1 gene locus on the X chromosome was also missing. The C1 gene encodes for a protein (rhoGAP C1) that is preferentially expressed in white blood cells. The biological function of the rhoGAP C1 protein is poorly understood. Though the researchers extensively analyzed the patient over a 16-year period, they found no related pathology. This led the researchers to conclude that the rhoGAP C1 protein is not essential for normal development in the human. They suggest that the loss of rhoGAP C1 function is compensated by other members of the GAP family and does not require a special therapeutic management of NDI patients with this additional defect.