Functional heterogeneity of ROMK mutations linked to hyperprostaglandin E syndrome

Background. The renal K+ channel ROMK (Kir1.1) controls salt reabsorption i n the kidney. Loss-of-function mutations in this channel cause hyperprostag landin E syndrome/antenatal Bartter syndrome (HPS/aBS), which is characteri zed by severe renal salt and fluid wasting. Methods. We investigated 10 HPS/aBS patients for mutations in the ROMK gene by single-strand conformation polymorphism analysis (SSCA) and direct sequ encing. To assess the functional consequences, Ba2+-sensitive K+ currents w ere measured in five mutants of the tore region as well as one mutant with truncated C-terminus, using the two-electrode voltage-clamp technique after an injection of mutant cRNA into Xenopus oocytes. Results. Three novel ROMK mutations were identified together with six mutat ions described previously. The mutations were categorized into three groups : (1) amino acid exchanges in the core region (M1-H5-M2), (2) truncation at the cytosolic C-terminus, and (3) deletions of putative promoter elements. While the core mutations W99C, N124K, and I142T led to significantly reduc ed macroscopic K+ currents (1 to 8% of wildtype currents), the A103V and P1 10L variants retained substantial K+ conductivity (23 and 35% of wild-type currents respectively). Coexpression of A103V and P110L, resembling the com pound heterozygous state of the affected individual, further reduced macros copic currents to 9% of the wild-type currents. All mutants in the core reg ion exerted a dominant-negative effect on wild-type ROMK1, The C-terminal f rame shift (fs) mutation (H354fs) did not change current amplitudes compare d with ROMK1 wild type, suggesting that a mechanism other than alteration o f the electrophysiological properties may responsible fur loss of channel a ctivity. Conclusions. Analysis of ROMK mutants linked to HPS/aBS revealed a spectrum of mechanisms accounting for loss of channel function. Further characteriz ation of the molecular defects might be helpful for the development of new therapeutic approaches.