Molecular Basis of Primary Aldosteronism and Adrenal Cushing Syndrome

Abstract

This review reports the main molecular alterations leading to development of benign cortisol- and/or aldosterone-secreting adrenal tumors. Causes of adrenal Cushing syndrome can be divided in 2 groups: multiple bilateral tumors or adenomas secreting cortisol. Bilateral causes are mainly primary pigmented nodular adrenocortical disease, most of the time due to PRKAR1A germline-inactivating mutations, and primary bilateral macronodular adrenal hyperplasia that can be caused in some rare syndromic cases by germline-inactivating mutations of MEN1, APC, and FH and of ARMC5 in isolated forms. PRKACA somatic-activating mutations are the main alterations in unilateral cortisol-producing adenomas. In primary hyperaldosteronism (PA), familial forms were identified in 1% to 5% of cases: familial hyperaldosteronism type I (FH-I) due to a chimeric CYP11B1/CYP11B2 hybrid gene, FH-II due to CLCN-2 germline mutations, FH-III due to KCNJ5 germline mutations, FH-IV due to CACNA1H germline mutations and PA, and seizures and neurological abnormalities syndrome due to CACNA1D germline mutations. Several somatic mutations have been found in aldosterone-producing adenomas in KCNJ5, ATP1A1, ATP2B3, CACNA1D, and CTNNB1 genes. In addition to these genetic alterations, genome-wide approaches identified several new alterations in transcriptome, methylome, and miRnome studies, highlighting new pathways involved in steroid dysregulation.

Keywords: CTNNB1; Cushing syndrome; cAMP; primary aldosteronism.

Figure 1.

Signaling pathways and molecular alterations in adrenal Cushing syndrome. Physiologically, adrenocorticotropic hormone (ACTH) binds to a G protein–coupled receptor, the melanocortin receptor (MC2R), resulting in Gs protein activation. This in turn activates adenylate cyclase (AC) leading to cyclic adenosine monophosphate (cAMP) production. Four cAMP molecules bind to the protein kinase A (PKA) regulatory subunits dimer, which allows the release and activation of the 2 catalytic subunits of PKA. The free catalytic subunits will phosphorylate the transcription factor CREB (cAMP response element-binding protein), stimulating the transcription of several cAMP-dependant genes. Phosphodiesterases (PDE) involved in cAMP degradation are negative regulators of this pathway. Here is the list of molecular alterations disrupting cAMP/PKA signaling pathway which have been reported in the different etiologies of adrenal Cushing syndrome (mentioned in italics). 1. Activating mutation of MC2R (PBMAH) 2. Activating mutation of GNAS1 (germline mutation in McCune-Albright syndrome and somatic mutation in unilateral cortisol-secreting adenoma) 3. Illegitimate G protein–coupled receptors expression (PBMAH) 4. Phosphodiesterase (PDE11A and/or PDE8B)- inactivating mutations (PPNAD and PBMAH) 5. PRKACA duplication (bilateral,adrenal hyperplasia/micronodular and macronodular forms) 6. PRKAR1A-inactivating mutations or deletion (germline mutation and somatic second hit)(PPNAD) 7. Activating somatic mutation of PRKACA (unilateral cortisol-secreting adenoma) Apart from cAMP/PKA pathway: 8. ARMC5 inactivation (germline mutation and somatic second hit) reducing steroidogenesisand adrenocortical cells apoptosis (PBMAH)

Figure 2.

Genetic alterations in adrenal Cushing syndrome and primary aldosteronism. Type of genetic alterations: ^¤ loss of function; * gain of function; ° duplication; ^§ chimeric fusion gene. Gene function: bold black = certain causal gene; gray = causal gene to be confirmed; (brackets black) = causal/modifier gene. FH, familial hyperaldosteronism; PASNA, primary aldosteronism, seizures, and neurological abnormality; PBMAH, primary bilateral macronodular adrenal hyperplasia; PPNAD, primary pigmented nodular adrenal disease.

Figure 3.

Molecular alterations in primary hyperaldosteronism. Physiologically, aldosterone production is stimulated by angiotensin II (AT-II) and extracellular potassium. Indeed, both stimuli induce glomerula cells membrane depolarization, leading to opening of voltage-gated calcium channels. Calcium influx triggers a phosphorylation cascade inducing aldosterone synthase expression and aldosterone production. Here is the list of the molecular alterations responsible for the different forms of primary hyperaldosteronism 1. Expression of the hybrid gene CYP11B1/CYP11B2 (germline alteration in FH-I) making aldosterone production dependent on ACTH regulation and thus responsible for “glucocorticoid-remediable hyperaldosteronism” 2. Gain-of-function mutations in CLCN-2 gene (germline mutation in FH-II), responsible for constitutive membrane depolarization of glomerulosa cells by increased chloride efflux. 3. Mutations in KCNJ5 gene affecting particularly ion selectivity of the K+ channel thus responsible for an increased sodium influx leading to membrane depolarization (germline mutation in FH-III and somatic mutations in APAs). 4. Mutations in CACNA1H (germline mutation in FH-IV) and CACNA1D genes (germline mutation in PASNA syndrome and somatic mutations in APAs) responsible for increased calcium membrane permeability. 5. Somatic mutations in ATP1A1 and ATP2B3 genes responsible for increased permeability for Na+ or H+ resulting in membrane depolarization of cell membrane by due to variants (in APAs) 6. Somatic mutations in CTNNB1 gene responsible of increased CYP11B2 expression (in APAs). ACTH: adrenocorticotropic hormone AT1R: angiotensin receptor; MC2R: melanocortin receptor.