Functional heterogeneity of ROMK mutations linked to hyperprostaglandin E syndrome.
BACKGROUND
The renal K(+) channel ROMK (Kir1.1) controls salt reabsorption in the kidney. Loss-of-function mutations in this channel cause hyperprostaglandin E syndrome/antenatal Bartter syndrome (HPS/aBS), which is characterized 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 sequencing. To assess the functional consequences, Ba(2+)-sensitive K(+) currents were measured in five mutants of the core 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 mutations 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 reduced macroscopic K(+) currents (1 to 8% of wild-type currents), the A103V and P110L variants retained substantial K(+) conductivity (23 and 35% of wild-type currents, respectively). Coexpression of A103V and P110L, resembling the compound heterozygous state of the affected individual, further reduced macroscopic currents to 9% of the wild-type currents. All mutants in the core region exerted a dominant-negative effect on wild-type ROMK1. The C-terminal frameshift (fs) mutation (H354fs) did not change current amplitudes compared with ROMK1 wild type, suggesting that a mechanism other than alteration of the electrophysiological properties may responsible for loss of channel activity.


CONCLUSIONS
Analysis of ROMK mutants linked to HPS/aBS revealed a spectrum of mechanisms accounting for loss of channel function. Further characterization of the molecular defects might be helpful for the development of new therapeutic approaches.
Jeck
N
N
Department of Pediatrics, Philipps-University, Marburg, Germany.
Derst
C
C
Wischmeyer
E
E
Ott
H
H
Weber
S
S
Rudin
C
C
Seyberth
H W
HW
Daut
J
J
Karschin
A
A
Konrad
M
M
D000818:Animals; D001477:Bartter Syndrome / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D002675:Child, Preschool; D018389:Codon, Nonsense; D017931:DNA Primers / Q000235:genetics; D005091:Exons; D005260:Female; D008040:Genetic Linkage; D006801:Humans; D066298:In Vitro Techniques; D007223:Infant; D008297:Male; D009154:Mutation; D020125:Mutation, Missense; D009865:Oocytes / Q000378:metabolism; D010375:Pedigree; D015221:Potassium Channels / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D024661:Potassium Channels, Inwardly Rectifying; D011458:Prostaglandins E / Q000378:metabolism*; D017384:Sequence Deletion; D013577:Syndrome; D014162:Transfection; D014981:Xenopus
D018389:Codon, Nonsense; D017931:DNA Primers; C086989:KCNJ1 protein, human; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D011458:Prostaglandins E