Hypertension resistance polymorphisms in ROMK (Kir1.1) alter channel function by different mechanisms

The renal outer medullary K⁺ (ROMK) channel plays a critical role in renal sodium handling. Recent genome sequencing efforts in the Framingham Heart Study offspring cohort (Ji W, Foo JN, O'Roak BJ, Zhao H, Larson MG, Simon DB, Newton-Cheh C, State MW, Levy D, and Lifton RP. Nat Genet 40: 592-599, 2008) recently revealed an association between suspected loss-of-function polymorphisms in the ROMK channel and resistance to hypertension, suggesting that ROMK activity may also be a determinant of blood pressure control in the general population. Here we examine whether these sequence variants do, in fact, alter ROMK channel function and explore the mechanisms. As assessed by two-microelectrode voltage clamp in Xenopus oocytes, 3/5 of the variants (R193P, H251Y, and T313FS) displayed an almost complete attenuation of whole cell ROMK channel activity. Surface antibody binding measurements of external epitope-tagged channels and analysis of glycosylation-state maturation revealed that these variants prevent channel expression at the plasmalemma, likely as a consequence of retention in the endoplasmic reticulum. The other variants (P166S, R169H) had no obvious effects on the basal channel activity or surface expression but, instead, conferred a gain in regulated-inhibitory gating. As assessed in giant excised patch-clamp studies, apparent phosphotidylinositol 4,5-bisphosphate (PIP₂) binding affinity of the variants was reduced, causing channels to be more susceptible to inhibition upon PIP₂ depletion. Unlike the protein product of the major ROMK allele, these two variants are sensitive to the inhibitory affects of a G protein-coupled receptor, which stimulates PIP ₂ hydrolysis. In summary, we have found that hypertension resistance sequence variants inhibit ROMK channel function by different mechanisms, providing new insights into the role of the channel in the maintenance of blood pressure.