Functional histopathological markers of aldosterone producing adenoma and somatic KCNJ5 mutations

•Mutations in ion channels and ATPases are found in 54% of APA•Combination of GIRK4 and Dab2 immunostaining provides a new tool for confirming the pathological diagnosis•Low relative expression of GIRK4 in APA is indicative of the presence of KCNJ5 mutations.

The current pathological diagnosis of Aldosterone Producing Adenoma (APA) is limited to the description of nodules and/or hyperplasia in the resected adrenal gland, independent of their functional characteristics. The aim of our study was to characterize histopathological markers to confirm the presence and identify the sites of aldosterone production and to discriminate KCNJ5-related APA. We investigated 18 adrenals with APA and 15 with non-functioning adrenal incidentaloma (NFAI) for expression of Disabled-2 and GIRK4, two markers of zona glomerulosa (ZG), and 77 adrenals with APA with known mutational status for GIRK4 expression. Two-thirds of APA and only one NFAI exhibited both GIRK4 and Disabled-2 membrane staining, allowing to correctly classify 79% of adenomas. Remarkably, 28/32 APA with KCNJ5 mutations exhibited lower GIRK4 expression in APA relative to peritumoral ZG. This was highly specific for KCNJ5 mutations, indicating that GIRK4 immunohistochemistry might be used for initial screening of the somatic mutation status.