Frequent protein kinase A regulatory subunit A1 mutations but no GNAS mutations as potential driver in sporadic cardiac myxomas.

PURPOSE: Cardiac myxomas (CMs) are the second most common benign primary cardiac tumors, mainly originating within the left atrium. Approximately 5% of CM cases are associated with Carney Complex (CNC), an autosomal dominant multiple neoplasia syndrome often caused by germline mutations in the protein kinase A regulatory subunit 1A (PRKAR1A). Data concerning PRKAR1A alterations in sporadic myxomas are variable and sparse, with PRKAR1A mutations reported to range from 0% to 87%. Therefore, we investigated the frequency of PRKAR1A mutations in sporadic CM using next-generation sequencing (NGS). Additionally, we explored mutations in the catalytic domain of the Protein Kinase A complex (PRKACA) and examined the presence of GNAS mutations as another potential driver. METHODS AND RESULTS: This study retrospectively collected histological and clinical data from 27 patients with CM. First, we ruled out the possibility of underlying CNC through clinical evaluations and standardized interviews for each patient. Second, we performed PRKAR1A immunohistochemistry (IHC) analysis and graded the reactivity of myxoma cells semi-quantitatively. NGS was then applied to analyze the coding regions of PRKAR1A, PRKACA, and GNAS in all 27 cases. Of the 27 sporadic CM cases, 13 (48%) harbored mutations in PRKAR1A. Among these 13 mutant cases, six displayed more than one mutation in PRKAR1A. Most of the identified mutations resulted in premature stop codons or affected splicing. In PRKAR1A mutant CM cases, the loss of PRKAR1A protein expression was significantly more common. In two cases with missense mutations, protein expression remained preserved. Furthermore, a single mutation was detected in the catalytic domain of the protein kinase A complex, while no GNAS mutations were found. CONCLUSION: We identified a relatively high frequency of PRKAR1A mutations in sporadic CM. These PRKAR1A mutations may also represent an important oncogenic mechanism in sporadic myxomas, as already known in CM cases associated with CNC.

Superficial Angiomyxomas Frequently Demonstrate Loss of Protein Kinase A Regulatory Subunit 1 Alpha Expression: Immunohistochemical Analysis of 29 Cases and Cutaneous Myxoid Neoplasms With Histopathologic Overlap.

Superficial angiomyxomas (SAMs) are benign cutaneous tumors that arise de novo and in the setting of the Carney complex (CC), an autosomal dominant disease with several cutaneous manifestations including lentigines and pigmented epithelioid melanocytomas. Although most SAM do not pose a diagnostic challenge, a subset can demonstrate histopathologic overlap with other myxoid tumors that arise in the skin and subcutis. Traditional immunohistochemical markers are of limited utility when discriminating SAM from histopathologic mimics. Since protein kinase A regulatory subunit 1 alpha (PRKAR1A) genetic alterations underlie most CC cases, we investigated whether SAM demonstrate loss of PRKAR1A protein expression by immunohistochemistry. In our series, 29 SAM, 26 myxofibrosarcoma, 5 myxoid dermatofibrosarcoma protuberans, 11 superficial acral fibromyxomas, and 18 digital mucous cysts were characterized. Of the 29 SAM examined in this study, 1 was associated with documented CC in a 5-year-old girl. SAM tended to arise in adults (mean 49.7 y; range: 5 to 87 y). Loss of PRKAR1A was seen in 55.2% of cases (16/29) and had a male predilection (87.5%, 12/16). PRKAR1A-inactivated SAM demonstrated significant nuclear enlargement (100%, 16/16 vs. 23.1%, 3/13), multinucleation (81.3%, 13/16 vs. 23.1%, 3/13), and presence of neutrophils (43.8%, 7/16 vs. 0%, 0/13). In contrast, PRKAR1A was retained in all cases of myxofibrosarcoma (100%, 26/26), myxoid dermatofibrosarcoma protuberans (100%, 5/5), superficial acral fibromyxomas (100%, 11/11), and digital mucous cyst (100%, 18/18). Taken together, PRKAR1A loss by immunohistochemistry can be used as an adjunctive assay to support the diagnosis of SAM given the high specificity of this staining pattern compared with histopathologic mimics.

Cyclic AMP-dependent protein kinase catalytic subunit A (PRKACA): the expected, the unexpected, and what might be next.

Protein kinase A (PKA) or cyclic-AMP (cAMP)-dependent kinase was among the first serine-threonine kinases to be molecularly and functionally characterized. For years, it was investigated as the enzyme that mediates cAMP functions in almost all cell systems and organisms studied to date. Despite PKA's critical role in signaling and the long history of investigations of cAMP in oncogenesis (dating back to the 1970s), it was not until relatively recently that PKA defects were found to be directly involved in tumor predisposition. First, PKA's main regulatory subunit, PRKAR1A, was found to be mutated in Carney complex, a genetic syndrome that predisposes to heart tumors (cardiac myxomas) and a variety of other lesions of the endocrine system, including the adrenal cortex, and several cancers, including liver carcinoma. Then, PKA's main catalytic subunit, PRKACA, was found to be mutated in sporadic adrenal tumors and fibrolamellar liver carcinoma. Not surprisingly, therefore, a new research study published in The Journal of Pathology showed PRKACA mutations in sporadic cardiac myxomas. The real question is what other pathologies will be found to be due to PRKACA (or other PKA subunit) defects. The possibilities abound and may show the way for a totally new class of medications that target cAMP signaling to be useful in fighting the corresponding tumors. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

Microinsertions in PRKACA cause activation of the protein kinase A pathway in cardiac myxoma.

Cardiac myxoma is the most common cardiac tumour. Most lesions occur sporadically, but occasional lesions develop in patients with Carney complex, a syndrome characterized by cardiac myxoma, spotty pigmentation, and endocrine overactivity. Two-thirds of patients with Carney complex harbour germline mutations in PRKAR1A, which encodes the type I regulatory subunit of protein kinase A (PKA). Most studies have not found a mutation in PRKAR1A in sporadic cardiac myxoma cases. Recent studies identified frequent mutations in PRKACA, which encodes the catalytic subunit of PKA, in cortisol-secreting adrenocortical adenoma cases. To determine whether the PRKACA mutation is involved in the tumourigenesis of cardiac myxoma, we performed Sanger sequencing of 41 specimens of sporadic cardiac myxoma to test for the presence of mutations in the coding regions and intron-exon boundaries of PRKACA. Mutations were identified in four cases (9.7%). In contrast to the point mutations identified in adrenocortical adenoma, all mutations were in-frame microinsertions of 18-33 bp clustered in exons 7 and 8. The mutated PRKACA proteins lost their ability to bind to PRKAR1A, and thereby lead to constitutive activation of the PKA pathway. Together with previous reports of PRKAR1A mutations in syndromic cardiac myxoma, our study demonstrates the importance of the PKA pathway in the tumourigenesis of cardiac myxoma. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Left atrial myxoma presenting as pulmonary embolism: potential role of heme oxygenase-1.

We present the case of a patient with left atrial myxoma that presented with pulmonary embolism. The patient did not have any intracardiac communication between right and left sides of the heart. Using thrombelastography, the patient was determined to have an abnormally large velocity of plasma thrombus growth and strength with reduced vulnerability to lysis. Critically, increased carboxyhemoglobin concentrations were present, likely secondary to hemolysis from the tumor and engagement of systemic heme oxygenase-1. It was determined that the patient's plasmatic hypercoagulability was in part due to carboxyhemefibrinogen formation via a thrombelastographic method. In addition to circulating hypercoagulability, the patient also had an area of chronic venous stasis in his left ankle that had not changed for over a decade prior to this thrombophilic episode. In conclusion, we present the first case of paradoxical pulmonary embolism in the presence of a left atrial myxoma, potentially secondary to a combination of hemolysis, heme oxygenase-1 up-regulation, systemic hypercoagulability/hypofibrinolysis, and regional venous stasis.

Inhibitory effect of atorvastatin on the cell growth of cardiac myxomas via the PTEN and PHLPP2 phosphatase signaling pathway.

Insulin-like growth factor 1 (IGF-1) is a molecule with strong proliferative effects, and statins have been reported to exhibit antitumor effects based on clinical and experimental studies. However, their effects on cardiac myxoma (CM) cells and the underlying signaling mechanism(s) are largely unknown. Therefore, we investigated whether the protein/lipid phosphatases and tensin homolog deleted on chromosome ten (PTEN) and pleckstrin homology domain leucine-rich repeat phosphatase 1 and 2 (PHLPP1 and 2) are involved in the proliferative effect of IGF-1 on CM cells and the pharmacological impact of atorvastatin. The activity of PTEN and PHLPPs was determined using specific substrate diC16PIP3 and pNPP. We found that IGF-1 enhanced CM cell proliferation and inhibited both PTEN and PHLPP2 activity in a concentration- and time-dependent manner. Atorvastatin acted counter to IGF-1 and reversed the above effects mediated by IGF-1. Both IGF-1 and atorvastatin did not affect the activity of PHLPP1 and the protein expression of the three phosphatases. The results suggest that IGF-1 may exert its proliferative effects by negatively regulating the PTEN/PHLPP2 signaling pathway in CM cells, and atorvastatin may be a potential drug for the treatment of CM by enhancing the activity of PTEN and PHLPP2.

Expression of MMP-2 and MMP-9 in odontogenic myxoma in a child: report of a clinical case.

BACKGROUND: Odontogenic myxoma (OM) is a benign, locally invasive, non-metastasizing neoplasm of the jaw bones. Despite the benign nature of these lesions, there is a high rate of recurrence and the current recommended therapy, depending on the size and behaviour of the lesion, can vary from curettage with peripheral ostectomy, segmental resection up to radical resections for more aggressive lesions. OM is a rare tumour which occurs predominantly in the third decade of life and it is rare in children. Matrix metalloproteinases (MMPs) are a family of extracellular endopeptidases responsible for the degradation and remodelling of extracellular matrix, they are known to be involved in the progression and invasiveness of many types of tumour. MMPs have been studied in OM because of their well-known role in extracellular matrix degradation, tumour invasion and recurrence. CLINICAL CASE REPORT: We report a case of OM in a 6-year-old boy. A conservative excision was accomplished. The mass was excised without affecting the mandibular bone and the inferior alveolar nerve. Curettage and removal of the first right inferior molar were performed. After 6-month follow-up, no evidence of recurrence was found. EXPERIMENTAL DATA: We investigated the expression of MMP-2 and MMP-9 in this case of OM in a child. RT-PCR showed the expression of both MMP-2 and MMP-9 mRNAs. Immunohistochemistry showed a weak MMP-2 protein expression while MMP-9 protein was not detected. CONCLUSION: In this case of OM in a child, we report lack of recurrence after excision associated with low MMP-2 protein expression and absence of MMP-9. We believe it is worthy to deeply investigate the relationship between MMPs expression and OM behaviour with the aim to use MMPs as prognostic and/or therapeutic markers in OM.

Effects of LDL lipids on activity of group IIA secretory phospholipase A2.

Effects of phosphatidylcholine, oxidized phosphatidylcholine, sphyngomyelin, cholesterol, and cholesterol esters incorporated in LDL on activity of group IIA secretory phospholipase A2 from human cardiac myxoma were studied. Liposomes containing radioisotope-labeled phosphatidylethanolamine served as the substrate for group IIA secretory phospholipase A2. Oxidized phosphatidylcholine significantly stimulated activity of group IIA secretory phospholipase A2, while phosphatidylcholine in the same concentrations did not modify enzyme activity. Sphyngomyelin incorporated in LDL inhibited group IIA secretory phospholipase A2 activity. Cholesterol and cholesterol esters virtually did not modify enzyme activity. The results indicate that LDL phospholipids and their oxidized forms can be involved in regulation of group IIA secretory phospholipase A2. Study of the mechanisms regulating the proinflammatory group IIA secretory phospholipase A2 can promote the development of new approaches to the diagnosis and treatment of inflammatory processes.

Clinical and molecular genetics of Carney complex.

Carney complex (CNC) is a rare multiple familial neoplasia syndrome that is characterized by multiple types of skin tumors and pigmented lesions, endocrine neoplasms, myxomas and schwannomas and is inherited in an autosomal dominant manner. Clinical and pathologic diagnostic criteria are well established. Over 100 pathogenic variants in the regulatory subunit type 1A (RI-A) of the cAMP-dependent protein kinase (PRKAR1A) have been detected in approximately 60% of CNC patients, most leading to R1A haploinsufficiency. Other CNC-causing genes remain to be identified. Recent studies provided some genotype-phenotype correlations in CNC patients carrying PRKAR1A-inactivating mutations, which provide useful information for genetic counseling and/or prognosis; however, CNC remains a disease with significant clinical heterogeneity. Recent mouse and in vitro studies have shed light into how R1A haploinsufficiency causes tumors. PRKAR1A defects appear to be weak tumorigenic signals for most tissues; Wnt signaling activation and cell cycle dysregulation appear to be important mediators of the tumorigenic effect of a defective R1A.

Opposite effects of native and oxidized lipoproteins on the activity of secretory phospholipase A(2) group IIA.

Elevated circulating level and activity of secretory phospholipase A(2) group IIA (sPLA(2)(IIA)) are associated with the development of adverse cardiovascular events. The mechanisms of sPLA(2)(IIA) activity regulation in human blood serum so far remain obscure. We have suggested that the enzyme activity is influenced by circulating lipoproteins. The activity of sPLA(2)(IIA) was examined in whole serum of healthy individuals and after removal of lipoproteins from it. The effects of different classes of native and oxidized lipoproteins on sPLA(2)(IIA) in blood serum were compared with their effects on purified sPLA(2)(IIA). Activity of sPLA(2)(IIA) was not detected in whole serum despite the high concentration of the enzyme. However after lipoproteins had been removed from the serum, the lipoprotein-depleted serum displayed sPLA(2)(IIA) activity which was proportional to the amount of sPLA(2)(IIA) in it. Native LDL, HDL and VLDL+IDL inhibited the activity of both purified sPLA(2)(IIA) and the enzyme activity in lipoprotein-depleted serum. By contrast, oxidized LDL, HDL and VLDL+IDL significantly stimulated the activity of purified and serum sPLA(2)(IIA) and enhanced the release of fatty acids from the substrate. The data indicate that native and oxidized lipoproteins regulate catalytic activity of sPLA(2)(IIA). Activation of sPLA(2)(IIA) by oxidized lipoproteins may be regarded as one of the mechanisms of atherosclerosis development.

The Carney complex gene PRKAR1A plays an essential role in cardiac development and myxomagenesis.

Cardiac myxomas are the most common primary tumors of the heart, although little is known about their etiology. Mutations of the protein kinase A regulatory subunit gene PRKAR1A cause inherited myxomas in the setting of the Carney complex tumor syndrome, providing a possible window for understanding their pathogenesis. We recently reported that cardiac-specific knockout of this gene causes myxomatous changes in the heart, although the mice die during gestation from cardiac failure. In this review, we discuss these findings and place them in the larger understanding of how protein kinase A dysregulation might affect cardiac function and cause myxomagenesis.

[The effect of LDL on the activity of proinflammatory secretory phospholipase A2 (IIA) depends on the degree of their oxidation].

Phospholipase A2 group IIA which is secreted in inflammation [secPLA2(IIA)] does not always exhibit catalytic activity, although being present in patients' serum. The mechanisms regulating the enzyme activity in peripheral blood have not been studied in sufficient detail. In this study we examined the effects of native and oxidized LDL with varied degree of oxidation on secPLA2(IIA) from human heart myxoma. The degree of LDL oxidation was evaluated from the amount of conjugated dienes and lysophosphatidylcholine. Liposomes containing radio-labelled phosphatidylcholine were used as a substrate in the secPLA2(IIA) activity assay. Native LDL isolated from serum of healthy subjects inhibited secPLA2(IIA) in a dose-dependent manner. Minimally and moderately oxidized LDL in which < 40 % phosphatidylcholine was hydrolysed to lysophosphatidylcholine activated secPL2(IIA). Strongly oxidized LDL in which > 40 % phosphatidylcholine was hydrolysed to lysophosphatidylcholine inhibited the enzyme. Thus, our findings indicate that the characteristics of circulating lipoproteins are changed by their oxidation. Minimal and moderate oxidation of LDL results in activation of secPLA2(IIA), while strong oxidation causes inhibition of the enzyme. Since inflammation leads to an increase in secPLA2(IIA) secretion and LDL oxidation, the results obtained can be used for the diagnostics of inflammatory processes and provide more insight into molecular mechanisms underlying the development of atherosclerosis.

Analysis of GNAS1 and PRKAR1A gene mutations in human cardiac myxomas not associated with multiple endocrine disorders.

Cardiac myxomas are rare tumors that usually occur as sporadic lesions or,more rarely, in the familial form,mostly in the context of Carney complex (CNC). The molecular basis for the development of cardiac myxomas is unclear. However, somatic activating mutations in the GNAS1 gene (the gsp oncogene) are detected in the myocardium ofMcCune-Albright syndrome patients while germ-line mutations in the PRKAR1A gene are associated with CNC and familial myxomas. We investigated the presence of activating missense mutations in the GNAS1 gene as well as of inactivating mutations in PRKAR1A in 29 sporadically occurring cardiac myxomas. No gsp and no PRKAR1A mutations were found by direct sequencing of PCR products amplified from tumoral DNA. This is the first study including a large series of sporadic, isolated cardiac myxomas and showing that these cardiac neoplasms do not share the same mutations found in familial forms.

Immunohistochemical expression of matrix metalloproteinases 1, 2, and 9 in odontogenic myxoma and dental germ papilla.

The aim of this study was to compare the immunohistochemical expression of matrix metalloproteinases (MMPs) 1, 2, and 9 in odontogenic myxomas and dental germ papillae. Twelve cases of odontogenic myxoma and eight tooth germ specimens were selected for analysis of the immunohistochemical expression and the pattern of distribution of MMPs 1, 2, and 9 in extracellular matrix (ECM), as well as of the number of MMP-positive cells. MMP-2 was expressed only in the ECM of myxomas (p<0.05). No significant difference was observed between ECM immunoreactivity for MMP-9 in myxomas and dental papillae (p>0.05). MMP-1 immunoreactivity was detected in most myxoma cases at a proportion similar to that observed in dental papillae (p>0.05). A significant difference was observed in the number of immunoreactive cells in myxomas (p<0.05), MMP-1 being present at higher proportions than MMPs 2 and 9. There was a gradient in the expression of MMPs in the ECM and in neoplastic cells of odontogenic myxomas, with higher immunoreactivity to MMP-1 and lower immunoreactivity to MMP-9. Taken together, our results suggest the existence of a coordinated mechanism between MMPs 1, 2, and 9 that aimed at the efficient degradation of extracellular matrix in odontogenic myxomas.

[Phospholipase A2, group IIa, as an earlier unknown immunohistological marker of cardiac myxoma].

Phospholipase A2, group IIA, gene expression has been analyzed in primary heart tumors. High expression has been demonstrated through several ways: reverse-transcriptase chain polymerase chain, Northern blotting hybridization at the RNA level and immunoblotting, immunohistochemical assay at the protein level. Human cardiac myxoma exhibits highly positive phospholipase A2, group IIA, immunophenotype (100% positive cases). The immunophenotype is unique among human primary cardiac tumors. Phospholipase A2, group IIA, can be proposed as a tissue marker for pathological examination after heart tumor resection.

Caspase-3-dependent apoptosis in cardiac myxoma: not associated with human papillomavirus or Epstein-Barr virus.

Cardiac myxoma is the most common tumor of the heart, has a variable clinical presentation and immunohistochemical profile. Viral infections, such as herpes simplex virus, human papillomavirus (HPV), and Epstein-Barr virus (EBV), may play an important role in the causes of cardiac myxoma. This investigation will demonstrate caspase-3-dependent apoptosis in cardiac myxoma without HPV or EBV infection. This study included 15 patients with cardiac myxoma, who were treated with surgical excision of the lesion. Data were collected on detailed clinical parameters. Terminal deoxynucleotidyl transferase nick-end labeling assay, electrophoresis, and caspase-3 immunohistochemical studies were performed to characterize apoptosis. Genechip containing 39 subtypes was used to elucidate HPV; and polymerase chain reaction to detect LMP-1 gene of EBV. The patient population comprised of eight (53%) women and seven (47%) men. The mean age of patient participants was 45 years, with an age range of 30-70 years. All patient cases were sporadic myxomas rather than familial myxomas. The patient presentations included dyspnea (53%), asymptomatic (27%), stroke (7%), chest pain (7%), and fever (7%). All lesions were located in the left atrium. The individual patient cases of myxoma did not differ in location or clinical event in terms of pathological scores, such as vascular proliferation, inflammation, cellularity, hyaline, calcification, or thrombosis. Cardiac myxoma is characterized by apoptosis through caspase-dependent pathway. HPV or EBV was not detected in any of the study patient samples. In conclusion, no viral genomes of HPV or EBV were detected in these 15 patients. This study demonstrates that caspase-3-dependent apoptosis in cardiac myxoma is not dependent on concurrence of previous HPV and/or EBV infection.

Increased NADPH-diaphorase activity in canine myxomatous mitral valve leaflets.

Comparable pathological changes in the mitral valve have been described in dogs, pigs and human patients with myxomatous mitral valve disease (MMVD), i.e., primary mitral valve prolapse. The progressive myxomatous changes are probably a response to repeated impact on the leaflets, and endothelial stress or damage probably plays a central role in the pathogenesis. Little, however, is known about the vasoactive substances that mediate the subendothelial changes. The aim of this study was to investigate the expression of nitric oxide synthase (NOS) in canine mitral valve leaflets and to relate the findings to MMVD changes. The mitral valve was taken post mortem from 12 dogs (six males and six females) and a whole valve NADPH (the reduced form of nicotinamide-adenine dinucleotide phosphate) diaphorase (NADPH-d) reaction was performed. Macroscopical (semiquantitative) and microscopical (computer image analysis) evaluations of the staining due to NADPH-d activity were performed at four specific areas of the valve and related to microscopical signs of MMVD and gross signs of thickening or prolapse, or both. Macroscopically, the NADPH-d colour grade was correlated with the degree of MMVD (P=0.01). In addition, endothelial NADPH-d staining intensity was correlated with macroscopical signs of disease (P=0.004) as well as with collagen degeneration (P=0.008) and deposition of mucopolysaccharides (P=0.02). Age, gender and specific area of the valve did not seem to influence the NADPH-d activity. In conclusion, increased NADPH-d activity, suggesting increased NOS expression, was found in areas of the mitral valve with myxomatous changes. This indicates that nitric oxide (NO) may play a role in the pathogenesis of MMVD in dogs.

Mutations of the gene encoding the protein kinase A type I-alpha regulatory subunit (PRKAR1A) in patients with the "complex of spotty skin pigmentation, myxomas, endocrine overactivity, and schwannomas" (Carney complex).

Carney complex (CNC) is a familial multiple neoplasia syndrome associated with abnormal skin and mucosal pigmentation. The complex has features overlapping those of McCune-Albright syndrome (MAS) and the other multiple endocrine neoplasias (MENs). CNC is inherited as an autosomal dominant trait, and the responsible genes have been mapped by linkage analysis to loci at 2p16 and 17q22-24. Because of its unusual biochemical features (e.g., paradoxical responses to various endocrine signals) and its clinical similarities to MAS, genes implicated in cyclic nucleotide-dependent signaling, including GNAS1 (which is responsible for MAS), had been considered likely candidates for causing CNC. The gene encoding the protein kinase A (PKA) type I-alpha regulatory subunit (RI alpha), PRKAR1A, had been mapped to 17q22-24; loss-of-heterozygosity (LOH) analysis using polymorphic markers from this region revealed consistent changes in tumors from patients with CNC, including those from one family previously mapped to 17q22-24. Investigation of a polymorphic site within the 5' of the PRKAR1A gene showed segregation with the disease and retention of the allele bearing the disease gene in CNC tumors. Mutations of the PRKAR1A gene were also found to have occurred de novo in sporadic cases of CNC; no mutations were found in kindreds mapping to 2p16. Thus, genetic heterogeneity in CNC was confirmed; in total, 41% of all patients with CNC had mutations in the PRKAR1A gene. All mutations were frameshifts, insertions, and deletions that led to nonsense mRNA and premature termination of the predicted peptide product. Functional studies in CNC tumors suggested that inactivating mutations of the PRKAR1A gene led to nonsense mRNA decay (the mutant peptide product was not present) and were associated with dysregulated PKA activity, increased responsiveness to cAMP, and excess of type-II PKA activity. We conclude that the PRKAR1A gene, coding for the RIalpha subunit of PKA, a critical cellular component of a number of cyclic nucleotide-dependent signaling pathways, is mutated in a subset of patients with CNC. In their tumors, there is LOH of the normal allele, suggesting that normal RI-alpha may have tumor suppression function in the tissues affected by CNC. An excess of type-II PKA activity was present in affected tissues, which may be responsible for the apparent tumorigenicity of PRKAR1A mutations in endocrine tissues.

The essential role of RI alpha in the maintenance of regulated PKA activity.

Cloning of the individual regulatory (R) and catalytic (C) subunits of the cAMP-dependent protein kinase (PKA) and expression of these subunits in cell culture have provided mechanistic answers about the rules for PKA holoenzyme assembly. One of the central findings of these studies is the essential role of the RI alpha regulatory subunit in maintaining the catalytic subunit under cAMP control. The role of RI alpha as the key compensatory regulatory subunit in this enzyme family was confirmed by gene knockouts of the three other regulatory subunits in mice. In each case, RI alpha has demonstrated the capacity for significant compensatory regulation of PKA activity in tissues where the other regulatory subunits are expressed, including brain, brown and white adipose tissue, skeletal muscle, and sperm. The essential requirement of the RI alpha regulatory subunit in maintaining cAMP control of PKA activity was further corroborated by the knockout of RI alpha in mice, which results in early embryonic lethality due to failed cardiac morphogenesis. Closer examination of RI alpha knockout embryos at even earlier stages of development revealed profound deficits in the morphogenesis of the mesodermal embryonic germ layer, which gives rise to essential structures including the embryonic heart tube. Failure of the mesodermal germ layer in RI alpha knockout embryos can be rescued by crossing RI alpha knockout mice to C alpha knockout mice, supporting the conclusion that inappropriately regulated PKA catalytic subunit activity is responsible for the phenotype. Isolation of primary embryonic fibroblasts from RI alpha knockout embryos reveals profound alterations in the actin-based cytoskeleton, which may account for the failure in mesoderm morphogenesis at gastrulation.

Activated interstitial myofibroblasts express catabolic enzymes and mediate matrix remodeling in myxomatous heart valves.

BACKGROUND: The mechanisms of extracellular matrix changes accompanying myxomatous valvular degeneration are uncertain. METHODS AND RESULTS: To test the hypothesis that valvular interstitial cells mediate extracellular matrix degradation in myxomatous degeneration by excessive secretion of catabolic enzymes, we examined the functional characteristics of valvular interstitial cells in 14 mitral valves removed for myxomatous degeneration from patients with mitral regurgitation and in 11 normal mitral valves obtained at autopsy. Immunohistochemical staining assessed (1) cell phenotype using antibodies to alpha-actin (microfilaments), vimentin and desmin (intermediate filaments), smooth muscle myosin (SM1), and SMemb (a nonmuscle myosin produced by activated mesenchymal cells) and (2) the expression of proteolytic activity using antibodies to collagenases (matrix metalloproteinase [MMP]-1, MMP-13), gelatinases (MMP-2, MMP-9), cysteine endoproteases (cathepsin S and K), and interleukin-1beta, a cytokine that can induce secretion of proteolytic enzymes. Although interstitial cells in normal valves stained positively for vimentin, but not alpha-actin or desmin, cells in myxomatous valves contained both vimentin and alpha-actin or desmin (characteristics of myofibroblasts). Moreover, cells in myxomatous valves strongly expressed SMemb, MMPs, cathepsins, and interleukin-1beta, which were weakly stained in controls. Nevertheless, interstitial cells in both groups strongly expressed procollagen-I mRNA (in situ hybridization), suggesting preserved ability to synthesize collagen in myxomatous valves. CONCLUSIONS: Interstitial cells in myxomatous valves have features of activated myofibroblasts and express excessive levels of catabolic enzymes, without altered levels of interstitial collagen mRNA. We conclude that valvular interstitial cells regulate matrix degradation and remodeling in myxomatous mitral valve degeneration.