Eliezer J. Holtzman

A molecular defect in the vasopressin V2-receptor gene causing nephrogenic diabetes insipidus

Fundamental to the maintenance of water balance in humans is the rate at which the kidneys excrete free water, which is primarily regulated by arginine vasopressin. The antidiuretic action of argin...

Fetal RhD genotyping in fetal cells flow sorted from maternal blood

OBJECTIVE The aim of this study was to determine the accuracy of noninvasive fetal RhD genotyping by fetal cell isolation from maternal blood. STUDY DESIGN Candidate fetal cells from 18 pregnant women (one twin gestation) were flow-sorted. Polymerase chain reaction amplification of a 261 bp fragment of the RhD gene was performed on sorted fetal cells. The presence of amplified product was considered predictive of the Rhd-positive genotype in the fetus. RESULTS Sixteen of the 19 fetal RhD genotypes were correctly predicted in fetal cells isolated from maternal blood (10 were Rh positive, 6 were Rh negative). In 3 cases no amplification products were detected in RhD-positive fetuses. The association between presence of the fragment and RhD-positive genotype was significant (p=0.003, Fishers exact test). CONCLUSIONS Noninvasive prenatal diagnosis of the fetal RhD genotype is feasible. Absence of amplification products in the reaction requires confirmation that fetal material is present. Improvements in fetal cell purity and yield should increase diagnostic accuracy, although the current protocol has a positive predictive value of 100% and a negative predictive value of 67%.

Renal Epithelial Protein (Apx) Is an Actin Cytoskeleton-regulated Na+ Channel

Apx, the amphibian protein associated with renal amiloride-sensitive Na+ channel activity and with properties consistent with the pore-forming 150-kDa subunit of an epithelial Na+ channel complex initially purified by Benos et al. (Benos, D. J., Saccomani, G., and Sariban-Sohraby, S. (1987) J. Biol. Chem. 262, 10613-10618), has previously failed to generate amiloride-sensitive Na+ currents (Staub, O., Verrey, F., Kleyman, T. R., Benos, D. J., Rossier, B. C., and Kraehenbuhl, J.-P. (1992) J. Cell Biol. 119, 1497-1506). Renal epithelial Na+ channel activity is tonically inhibited by endogenous actin filaments (Cantiello, H. F., Stow, J., Prat, A. G., and Ausiello, D. A. (1991) Am. J. Physiol. 261, C882-C888). Thus, Apx was expressed and its function examined in human melanoma cells with a defective actin-based cytoskeleton. Apx-transfection was associated with a 60-900% increase in amiloride-sensitive (Ki = 3 μM) Na+ currents. Single channel Na+ currents had a similar functional fingerprint to the vasopressin-sensitive, and actin-regulated epithelial Na+ channel of A6 cells, including a 6-7 pS single channel conductance and a perm-selectivity of Na+:K+ of 4:1. Na+ channel activity was either spontaneous, or induced by addition of actin or protein kinase A plus ATP to the bathing solution of excised inside-out patches. Therefore, Apx may be responsible for the ionic conductance involved in the vasopressin-activated Na+ reabsorption in the amphibian kidney.

Mutations in the vasopressin V2 receptor gene in two families with nephrogenic diabetes insipidus.

Congenital nephrogenic diabetes insipidus (CNDI) is a rare X-linked disorder in which the renal collecting duct is unresponsive to arginine vasopressin, and thus, the urine is consistently hypotonic to plasma. As a result, affected individuals are unable to concentrate urine and suffer from episodes of severe dehydration and hypernatremia. Recently, the association between arginine vasopressin V2 receptor gene mutations and CNDI has been demonstrated. In this report, two additional novel molecular defects of the arginine vasopressin V2 receptor gene in CNDI families are described. In one family, the affected individual demonstrated a G-->T transversion causing a nonsense mutation in codon 231. This mutation results in a glutamic acid becoming a termination codon, causing premature termination and truncation of the encoded receptor protein. This mutation causes a NciI site within the gene to be abolished and a BsaWI site to be created. In the second family, affected individuals showed a 28-basepair duplicating insertion in the very beginning of exon 2 down-stream of the splice acceptor site. It was hypothesized that an insertion mutagenesis mechanism involves the formation of a stem-loop structure within the newly synthesized DNA strand, followed by a slipped mispairing. This may be a general mechanism for the deletion or insertion of repeated sequences within the genome. Recent data show that G-protein-coupled receptors are susceptible to many different mutations that often result in the loss of function, causing a similar clinical phenotype.

Nephrogenic diabetes insipidus: causes revealed.

In congenital NDI, the failure of the renal tubules to respond to antidiuretic hormone is caused by mutation of the arginine vasopressin receptor gene. Two dozen different mutations have been identified to date--all with the same clinical consequences. Several causes of acquired NDI, of which lithium is the most common, are also discussed.