A cAMP-specific phosphodiesterase (PDE8B) that is mutated in adrenal hyperplasia is expressed widely in human and mouse tissues: a novel PDE8B isoform in human adrenal cortex.

Bilateral adrenocortical hyperplasia (BAH) is the second most common cause of corticotropin-independent Cushing syndrome (CS). Genetic forms of BAH have been associated with complex syndromes such as Carney Complex and McCune-Albright syndrome or may present as isolated micronodular adrenocortical disease (iMAD) usually in children and young adults with CS. A genome-wide association study identified inactivating phosphodiesterase (PDE) 11A (PDE11A)-sequencing defects as low-penetrance predisposing factors for iMAD and related abnormalities; we also described a mutation (c.914A > C/H305P) in cyclic AMP (cAMP)-specific PDE8B, in a patient with iMAD. In this study we further characterize this mutation; we also found a novel PDE8B isoform that is highly expressed in the adrenal gland. This mutation is shown to significantly affect the ability of the protein to degrade cAMP in vitro. Tumor tissues from patients with iMAD and no mutations in the coding PDE8B sequence or any other related genes (PRKAR1A, PDE11A) showed downregulated PDE8B expression (compared to normal adrenal cortex). Pde8b is detectable in the adrenal gland of newborn mice and is widely expressed in other mouse tissues. We conclude that PDE8B is another PDE gene linked to iMAD; it is a candidate causative gene for other adrenocortical lesions linked to the cAMP signaling pathway and possibly for tumors in other tissues.

In vitro studies of novel PRKAR1A mutants that extend the predicted RIα protein sequence into the 3'-untranslated open reading frame: proteasomal degradation leads to RIα haploinsufficiency and Carney complex.

BACKGROUND: Carney complex (CNC) is a multiple endocrine neoplasia syndrome due to inactivating mutations in the PRKAR1A gene that codes for type Iα regulatory (RIα) subunit of protein kinase A. Most PRKAR1A mutations are subject to nonsense mRNA decay (NMD) and, thus, lead to haploinsufficiency. METHODS AND SETTING: Patient phenotyping for CNC features and DNA, RNA, protein, and transfection studies were carried out at a research center. RESULTS: We describe in unrelated kindreds with CNC four naturally occurring PRKAR1A mutations (1055del4, 1067del4ins5, 1076delTTins13, and 1142del4) that are predicted to escape NMD because they are located in the last coding exon of the gene. The phenotype of CNC was not different from that in other patients with the condition, although the number of patients was small. Each of the mutations caused a frameshift that led to a new stop codon into the 3' untranslated open reading frame, predicting an elongated protein that, however, was absent in patient-derived cells. After site-directed mutagenesis, in vitro transcription, and cell-free translation experiments, the expected size mutant proteins were present. However, when the mutant constructs were transfected in adrenal (NCI-295), testicular (N-TERA), and embryonic (HEK293) cells and despite the presence of the mutant mRNA, Western blot analysis indicated that there were no longer proteins. The subsequent application of proteasome inhibitors to cells transfected with the mutant constructs led to the detection of the aberrant proteins, although a compound that affects protein folding had no effect. The wild-type protein was also decreased in both patient-derived cells and/or tissues as well as in the in vitro systems used in this study. CONCLUSIONS: This was the first demonstration of proteasomal degradation of RIα protein variants leading to PRKAR1A haploinsufficiency and CNC, adding protein surveillance to NMD in the cellular mechanisms overseeing RIα synthesis. In agreement with the molecular data, CNC patients bearing PRKAR1A defects that extend the open reading frame did not have a different phenotype, although this has to be confirmed in a larger number of patients.

Functional phosphodiesterase 11A mutations may modify the risk of familial and bilateral testicular germ cell tumors.

Inactivating germline mutations in phosphodiesterase 11A (PDE11A) have been implicated in adrenal tumor susceptibility. PDE11A is highly expressed in endocrine steroidogenic tissues, especially the testis, and mice with inactivated Pde11a exhibit male infertility, a known testicular germ cell tumor (TGCT) risk factor. We sequenced the PDE11A gene-coding region in 95 patients with TGCT from 64 unrelated kindreds. We identified 8 nonsynonymous substitutions in 20 patients from 15 families: four (R52T, F258Y, G291R, and V820M) were newly recognized, three (R804H, R867G, and M878V) were functional variants previously implicated in adrenal tumor predisposition, and one (Y727C) was a known polymorphism. We compared the frequency of these variants in our patients to unrelated controls that had been screened and found negative for any endocrine diseases: only the two previously reported variants, R804H and R867G, known to be frequent in general population, were detected in these controls. The frequency of all PDE11A-gene variants (combined) was significantly higher among patients with TGCT (P = 0.0002), present in 19% of the families of our cohort. Most variants were detected in the general population, but functional studies showed that all these mutations reduced PDE activity, and that PDE11A protein expression was decreased (or absent) in TGCT samples from carriers. This is the first demonstration of the involvement of a PDE gene in TGCT, although the cyclic AMP signaling pathway has been investigated extensively in reproductive organ function and their diseases. In conclusion, we report that PDE11A-inactivating sequence variants may modify the risk of familial and bilateral TGCT.

Protein kinase A effects of an expressed PRKAR1A mutation associated with aggressive tumors.

Most PRKAR1A tumorigenic mutations lead to nonsense mRNA that is decayed; tumor formation has been associated with an increase in type II protein kinase A (PKA) subunits. The IVS6+1G>T PRKAR1A mutation leads to a protein lacking exon 6 sequences [R1 alpha Delta 184-236 (R1 alpha Delta 6)]. We compared in vitro R1 alpha Delta 6 with wild-type (wt) R1 alpha. We assessed PKA activity and subunit expression, phosphorylation of target molecules, and properties of wt-R1 alpha and mutant (mt) R1 alpha; we observed by confocal microscopy R1 alpha tagged with green fluorescent protein and its interactions with Cerulean-tagged catalytic subunit (C alpha). Introduction of the R1 alpha Delta 6 led to aberrant cellular morphology and higher PKA activity but no increase in type II PKA subunits. There was diffuse, cytoplasmic localization of R1 alpha protein in wt-R1 alpha- and R1 alpha Delta 6-transfected cells but the former also exhibited discrete aggregates of R1 alpha that bound C alpha; these were absent in R1 alpha Delta 6-transfected cells and did not bind C alpha at baseline or in response to cyclic AMP. Other changes induced by R1 alpha Delta 6 included decreased nuclear C alpha. We conclude that R1 alpha Delta 6 leads to increased PKA activity through the mt-R1 alpha decreased binding to C alpha and does not involve changes in other PKA subunits, suggesting that a switch to type II PKA activity is not necessary for increased kinase activity or tumorigenesis.