Characterization of a novel somatic KCNJ5 mutation delI157 in an aldosterone-producing adenoma

Objective: Adrenal aldosterone-producing adenomas (APAs) are an increasingly recognized cause of primary aldosteronism, and somatic mutations within the KCNJ5 gene encoding an inwardly rectifying K+ channel (also called GIRK4 or Kir3.4) have been identified by several groups including our own. We identified the previously noted G151R and L168R mutations in the region of a selectivity filter of the channel as well as a previously unreported 3-base deletion, delI157. Here, we report the functional properties of KCNJ5 channels carrying this novel delI157 mutation. Methods: The delI157 mutation was introduced into wild-type KCNJ5 sequences to allow its expression in both H295R cells and Xenopus oocytes to study its expression and electrophysiology, respectively. Results: In the adrenal cell line H295R, the delI157 mutant expresses and traffics normally to the cell surface. However, the current-voltage behavior of the mutant in oocytes is distinct from wild-type channels and mimics closely other selectivity filter mutations. In particular, its ability to support substantial current when extracellular K+ is replaced by Na+. We also report for the first time that the mutants have reduced sensitivity to the KCNJ5 inhibitor tertiapin-Q that binds to the external vestibule of the channel pore. Conclusion: This novel KCNJ5 mutation behaves like the three selectivity filter mutations previously reported in APAs depolarizing the cell and showing reduced cation selectivity. The reduced sensitivity to tertiapin-Q suggests that the abnormal Na+ permeability of these selectivity mutations does indeed reflect structural changes around the mouth of the ion channel.