Molecular analysis of the cyclic AMP-dependent protein kinase A (PKA) regulatory subunit 1A (PRKAR1A) gene in patients with Carney complex and primary pigmented nodular adrenocortical disease (PPNAD) reveals novel mutations and clues for pathophysiology: augmented PKA signaling is associated with adrenal tumorigenesis in PPNAD

Abstract

We studied 11 new kindreds with primary pigmented nodular adrenocortical disease (PPNAD) or Carney complex (CNC) and found that 82% of the kindreds had PRKAR1A gene defects (including seven novel inactivating mutations), most of which led to nonsense mRNA and, thus, were not expressed in patients' cells. However, a previously undescribed base substitution in intron 6 (exon 6 IVS +1G-->T) led to exon 6 skipping and an expressed shorter PRKAR1A protein. The mutant protein was present in patients' leukocytes and tumors, and in vitro studies indicated that the mutant PRKAR1A activated cAMP-dependent protein kinase A (PKA) signaling at the nuclear level. This is the first demonstration of an inactivating PRKAR1A mutation being expressed at the protein level and leading to stimulation of the PKA pathway in CNC patients. Along with the lack of allelic loss at the PRKAR1A locus in most of the tumors from this kindred, these data suggest that alteration of PRKAR1A function (not only its complete loss) is sufficient for augmenting PKA activity leading to tumorigenesis in tissues affected by CNC.

Figure 1

Pedigrees of seven families with a heterozygous mutation of PRKARIA gene. Half-filled squares and circles represent heterozygous mutated male and female patients, respectively. Open squares and circles represent unaffected patients. The individuals who were studied in each generation are numbered. Question marks (?) represent individuals of unknown affection status. The proband is indicated by the arrow. The results of nondenaturing polyacrylamide gel electrophoretic analysis of mutated exons are shown below the three pedigrees. The presence of heteroduplexes (upper bands), which are formed by the combination of mismatched alleles, confirms heterozygosity for mutated patients. Migration of the PCR products of family CNC02 shows abnormal bands only for proband with a heterozygous 13-bp deletion in exon 7. The upper bands correspond to heteroduplexes, the lower bands to normal and mutant alleles.

Figure 2

A heterozygous G→A transversion in exon 1B gives rise to a novel ATG translation initiation codon. A, Pedigree of family CNC07. The proband is indicated by the arrow. B, Sequencing of PRKAR1A exon 1B revealed a heterozygous mutation that creates an upstream, out-of-frame ATG codon in the RIα1B mRNA within a consensus sequence for translation initiation. This AUG codon is classified as a strong start site on the basis of the A and G, respectively, at position −3 and +4 (Kozak 1997). According to the scanning model of eukaryotic translation, the novel ATG should initiate translation of a truncated protein and decrease translation from the wild-type start codon (Kozak 1999), as administrated for CDKN2A gene (Liu et al. 1999). The location of mutant ATG codon is shown with a horizontal band. The mutation is indicated by an arrow.

Figure 3

An exon 6 splice-site mutation leads to an exon 6 skipping in family CNC04. A, Pedigree of family CNC04. The subjects who were studied in each generation are numbered. The question mark (?) represents an individual of unknown affection status. The proband (II.1) is indicated by the arrow. She presented with a severe form of Carney complex and died of a pancreatic adenocarcinoma with rapidly growing liver metastasis. Her unaffected mother was also investigated for the presence of the mutation. Only the proband and her affected son were heterozygous for the intron 6 splice mutation. The proband’s sequence trace of the sense strand showed a G→T transversion in the splice donor site of intron 6. B, Migration, on a 1% agarose gel, of RT-PCR products from transformed lymphocytes and PPNAD from family CNC04’s proband. After gel extraction and purification, RT-PCR products were directly sequenced. Top, Schematic representation of the gene organization of coding exons giving rise to the mature RIα mRNA. Regions that encode functional domains are denoted. The localization of the primers used for the RT-PCR is indicated with arrows. Bottom, Schematic representation of exon 6 skipping mRNA. C, RT-PCR products from transformed lymphocytes from patient carrying the exon 6 IVS del(−9→−2) mutation. No change is obvious after 4 h of treatment with 100 μg/ml cycloheximide (X) compared to the vehicle (C).

Figure 4

Identification of a deleted form of RIα subunit in proband of family CNC04 and absence of LOH except for the pancreatic adenocarcinoma. A, Western blotting of RIα subunit in 5 μg protein lysates from transformed lymphocytes of Carney complex patients with mutation in PRKAR1A gene and three control subjects. Only the CNC patient with an exon 6 skipping mRNA presents a shortened form of the protein. Size markers are indicated at left. B, Western blotting analysis of 20 μg total cellular proteins prepared from lymphocytes and five different tumors of family CNC04’s proband. Two bands are present in all tissues of the patient except for the pancreatic adenocarcinoma where the wild-type RIα is lost. In the schwannoma, RIα-Δ184–236 was present after longer exposure. The human adrenal cell line H295R is shown as a control for RIα expression. Size markers are indicated at left. C, LOH analysis was performed using DNA samples from peripheral blood and four tumors from the same patient. Seven markers located around PRKAR1A were used, including PRKAR1A(CA)n which was located within the 5′ region of the gene. The five informative loci demonstrated LOH only in the tumor DNA of the pancreatic adenocarcinoma (a finding consistent with Western blotting data).

Figure 5

Transcriptional activity of the RIα-WT and RIα-Δ184–236 on the somatostatin CRE promoter stimulated by forskolin. A, COS cells were transiently cotransfected with a reporter plasmid expressing luciferase under the control of the somatostatin CRE sequence and equivalent molar quantity of pREP4 expression vectors for HA-RIα-WT and HA-RIα-Δ184–236. Cells were left untreated or were stimulated in the presence of forskolin and IBMX. Experiments were repeated five times with triplicate samples. The figure shows the mean of the five experiments. All error bars represent standard error of the mean; * denotes significance at P<.05; ** denotes significance at P<.01. The stimulation factor is reported to the transcription level observed in the cells without transfected RIα. B, Control immunoblotting analysis of whole-cell extracts of HA-RIα expression. Anti-HA antibodies were used in Western blotting with total lysates from empty vector, HA-RIα-WT, or HA-RIα-Δ184–236 COS transfected cells.