Introduction: Alterations of the cAMP signaling pathway are described in adrenal tumors causing Cushing syndrome, specifically mutations in the gene coding for the protein kinase A (PKA) catalytic subunit alpha (PRAKCA) in cortisol producing adenomas (CPA) with overt Cushing syndrome.
Materiel and Methods: Eight CPAs without PRKACA mutations were analyzed by whole exome sequencing. Direct sequencing of PRAKCB encoding for the catalytic subunit beta (Cβ) of the PKA was performed in 21 others. Interaction of WT or mutant Cβ with the PKA-regulatory subunits was studied by bioluminescence resonance energy transfer and surface plasmon resonance analysis. To study the PKA activity, phosphorylation of the synthetic PKA-substrat Kemptide was analyzed in cellulae in HEK293 cells transfected with a combination of RIα and Cβ and in vitro with purified proteins.
Results: A PRKACB somatic mutation p.S54L was found in one CPA, from a 41-year-old patient presenting with a severe form of Cushing syndrome. Direct sequencing of PRKACB did not show any other mutations in additional samples. Functional studies demonstrated loss of interaction between the mutant Cβ and the regulatory subunit type 1 (RIα, RIβ). After stimulation by cAMP or forskolin, dissociation of Cβ and RIα was faster and stronger with the mutant than with the WT Cβ. In addition, basal PKA activity was higher for the mutant catalytic than the wild-type while the maximal activity after stimulation was lower.
Conclusions: PRKACB is a new gene responsible for at least one CPA, mutated in the somatic state. This finding suggests that the Cβ subunit of the PKA, too, may have specific functions in the adrenal cortex. Particularly, the role of the residue Ser54 (as was previously suggested) may be very important for PKA function in the adrenal cortex.
20 - 23 May 2017
European Society of Endocrinology