Comparative PRKAR1A genotype-phenotype analyses in humans with Carney complex and prkar1a haploinsufficient mice

Tumor development: haploinsufficiency and local network assembly

According to the current models, tumor development is a continuous process of mutation accumulation, leading to several intermediate phenotypes and final phases of autonomy, unlimited growth and metastasis. One of the most important events in that process is the initial destabilization of cellular pathways that subsequently allow mutations to accumulate. The mechanisms involved in that stage are not clear. In principle, the estimated very low mutation frequency in human or mouse cells would suggest that accumulating the required number of mutations for tumor development should be a statistically unlikely event. However, this theory is contradicted by the high incidence of cancers. Here we discuss the role of protein haploinsufficiency as a contributor to the initial phases of tumor development, and suggest possible mechanisms that might be involved in that process.

Molecular genetics of adrenocortical tumours, from familial to sporadic diseases

Adrenal masses can be detected in up to 4% of the population, and are mostly of adrenocortical origin. Adrenocortical tumours (ACTs) may be responsible for excess steroid production and, in the case of adrenocortical cancers, for morbidity or mortality due to tumour growth. Our understanding of the pathogenesis of ACTs is more limited than that for other tumours. However, studies of the genetics of ACTs have led to major advances in this field in the last decade. The identification of germline molecular defects in the hereditary syndrome responsible for ACTs has facilitated progress. Indeed, similar molecular defects have since been identified as somatic alterations in sporadic tumours. The familial diseases concerned are Li-Fraumeni syndrome, which may be due to germline mutation of the tumour-suppressor gene TP53 and Beckwith-Wiedemann syndrome, which is caused by dys-regulation of the imprinted IGF-II locus at 11p15. ACTs also occur in type 1 multiple endocrine neoplasia (MEN 1), which is characterized by a germline mutation of the menin gene. Cushing's syndrome due to primary pigmented nodular adrenocortical disease (PPNAD) has been observed in Carney complex patients presenting inactivating germline PRKAR1A mutations. Interestingly, allelic losses at 17p13 and 11p15 have been demonstrated in sporadic adrenocortical cancer and somatic PRKAR1A mutations have been found in secreting adrenocortical adenomas. More rarely, mutations in Gs protein (gsp) and the gene for ACTH receptor have been observed in ACTs. The genetics of another group of adrenal diseases that can lead to adrenal nodular hyperplasia -- congenital adrenal hyperplasia (CAH) and glucocorticoid-remediable aldosteronism (GRA) -- have also been studied extensively. This review summarizes recent advances in the genetics of ACTs, highlighting both improvements in our understanding of the pathophysiology and the diagnosis of these tumours.

Adrenal pathophysiology: lessons from the Carney complex

The Carney complex (CNC) is a dominantly inherited syndrome responsible mainly for spotty skin pigmentation (lentiginosis), endocrine overactivity, and cardiac myxomas. Adrenocorticotropic hormone independent Cushing's syndrome due to primary pigmented nodular adrenocortical disease (PPNAD) is a main characteristic of CNC. PPNAD is a very rare cause of Cushing's syndrome due to a primary bilateral adrenal defect that can be also observed in some patients without other CNC manifestations nor familial history. One of the putative CNC genes, located on 17q22-24, has been identified as the regulatory subunit R1A of protein kinase A (PRKAR1A). Heterozygous inactivating mutations of PRKAR1A have been reported initially in about 45% of the CNC index cases and could be found in about 80% of the CNC families presenting mainly with Cushing's syndrome. PRKAR1A is a key component of the cyclic AMP signaling pathway that has been implicated in endocrine tumorigenesis and could, at least partly, function as a tumor suppressor gene. Interestingly, patients with isolated PPNAD and no familial history of CNC can also present a germline de novo mutation of PRKAR1A. Somatic mutations of PRKAR1A have been found in PPNAD as a mechanism of inactivation of the wild-type allele, in a patient already presenting a germline mutation, and in a subset of sporadic secreting adrenocortical adenomas with clinical, hormonal, and pathological features quite similar to PPNAD. This review will summarize the recent findings on CNC from the perspective of the pathophysiology of adrenal Cushing's syndrome and PPNAD.

Molecular and immunohistochemical investigation of protein kinase a regulatory subunit type 1A (PRKAR1A) in odontogenic myxomas

Odontogenic myxomas are rare benign neoplasms affecting the jaw. Myxomas of bones and other sites occur as part of Carney complex (CNC), a multiple neoplasia syndrome caused by mutations in the PRKAR1A gene, which codes for the regulatory subunit of protein kinase A (PKA). In the present study, 17 odontogenic myxomas from patients without CNC were screened for PRKAR1A mutations and PRKAR1A protein expression by immunohistochemistry (IHC). Mutations of the coding region of the PRKAR1A gene were identified in 2 tumors; both these lesions showed no or significantly decreased immunostaining of PRKAR1A in the tumor compared to that in the surrounding normal tissue. One mutation (c.725C>A) led to a nonconservative amino acid substitution in a highly conserved area of the gene (A213D); the other was a single base-pair deletion that led to a frameshift (del774C) and a stop codon 11 amino acids downstream of the mutation site; both tumors were heterozygous for the respective mutations. Of the remaining tumors, 7 of the 15 without mutations showed almost no PRKAR1A in the tumor cells, whereas IHC showed that the protein was abundant in nontumorous cells. We concluded that PRKAR1A may be involved by its down-regulation in the pathogenesis of odontogenic myxomas caused by mutations and/or other genetic mechanisms. Of the sporadic, nonfamilial tumors associated with PRKAR1A mutations, the odontogenic type was the first myxomatous lesion found to harbor somatic PRKAR1A sequence changes.

Tumor suppressor genetics

The observation that mutations in tumor suppressor genes can have haploinsufficient, as well as gain of function and dominant negative, phenotypes has caused a reevaluation of the 'two-hit' model of tumor suppressor inactivation. Here we examine the history of haploinsufficiency and tumor suppressors in order to understand the origin of the 'two-hit' dogma. The two-hit model of tumor suppressor gene inactivation was derived from mathematical modeling of cancer incidence. Subsequent interpretations implied that tumor suppressors were recessive, requiring mutations in both alleles. This model has provided a useful conceptual framework for three decades of research on the genetics and biology of tumor suppressor genes. Recently it has become clear that mutations in tumor suppressor genes are not always completely recessive. Haploinsufficiency occurs when one allele is insufficient to confer the full functionality produced from two wild-type alleles. Haploinsufficiency, however, is not an absolute property. It can be partial or complete and can vary depending on tissue type, other epistatic interactions, and environmental factors. In addition to simple quantitative differences (one allele versus two alleles), gene mutations can have qualitative differences, creating gain of function or dominant negative effects that can be difficult to distinguish from dosage-dependence. Like mutations in many other genes, tumor suppressor gene mutations can be haploinsufficient, dominant negative or gain of function in addition to recessive. Thus, under certain circumstances, one hit may be sufficient for inactivation. In addition, the phenotypic penetrance of these mutations can vary depending on the nature of the mutation itself, the genetic background, the tissue type, environmental factors and other variables. Incorporating these new findings into existing models of the clonal evolution will be a challenge for the future.

A mouse model for the Carney complex tumor syndrome develops neoplasia in cyclic AMP-responsive tissues

Carney complex is an autosomal dominant neoplasia syndrome characterized by spotty skin pigmentation, myxomatosis, endocrine tumors, and schwannomas. This condition may be caused by inactivating mutations in PRKAR1A, the gene encoding the type 1A regulatory subunit of protein kinase A. To better understand the mechanism by which PRKAR1A mutations cause disease, we have developed conventional and conditional null alleles for Prkar1a in the mouse. Prkar1a(+/-) mice developed nonpigmented schwannomas and fibro-osseous bone lesions beginning at approximately 6 months of age. Although genotype-specific cardiac and adrenal lesions were not seen, benign and malignant thyroid neoplasias were observed in older mice. This spectrum of tumors overlaps that seen in Carney complex patients, confirming the validity of this mouse model. Genetic analysis indicated that allelic loss occurred in a subset of tumor cells, suggesting that complete loss of Prkar1a plays a key role in tumorigenesis. Similarly, tissue-specific ablation of Prkar1a from a subset of facial neural crest cells caused the formation of schwannomas with divergent differentiation. These observations confirm the identity of PRKAR1A as a tumor suppressor gene with specific importance to cyclic AMP-responsive tissues and suggest that these mice may be valuable tools not only for understanding endocrine tumorigenesis but also for understanding inherited predispositions for schwannoma formation.

Clinical phenotypes and molecular genetic mechanisms of Carney complex

Carney complex is a familial multiple neoplasia disorder with characteristic features such as cardiac and cutaneous myxomas and spotty pigmentation of the skin. Clinical genetic analyses have shown that Carney complex is transmitted in an autosomal dominant way and can present with a wide array of other tumours, such as psammomatous melanotic schwannoma, testicular Sertoli-cell tumours, and pituitary adenomas. Molecular genetic studies show that mutations in the PRKAR1A gene, encoding the R1alpha regulatory subunit of cyclic-AMP-dependent protein kinase A, are the cause of Carney complex in most patients. Investigation of genetically engineered animal models confirms the role of PRKAR1A as a tumour suppressor and has begun to elaborate mechanisms underlying tumorigenesis in this disorder. Further genetic studies in human beings have highlighted novel variant phenotypes, such as congenital contractures, which are potentially associated with Carney complex, and have identified alternative genetic pathways to cardiac tumorigenesis, including mutation of the MYH8 gene that encodes perinatal myosin.

The lentiginoses: cutaneous markers of systemic disease and a window to new aspects of tumourigenesis

Familial lentiginosis syndromes cover a wide phenotypic spectrum ranging from a benign inherited predisposition to develop cutaneous lentigines unassociated with systemic disease, to associations with several syndromes carrying increased risk of formation of hamartomas, hyperplasias, and other neoplasms. The molecular pathways involved in the aetiology of these syndromes have recently been more clearly defined and several major cellular signalling pathways are probably involved: the protein kinase A (PKA) pathway in Carney complex (CNC), the Ras/Erk MAP kinase pathway in LEOPARD/Noonan syndromes, and the mammalian target of rapamycin pathway (mTOR) in Peutz-Jeghers syndrome and the diseases caused by PTEN mutations. Here we discuss the clinical presentation of these disorders and discuss the molecular mechanisms involved. The presence of lentigines in these diseases caused by diverse molecular defects is probably more than an associated clinical feature and likely reflects cross talk and convergence of signalling pathways of central importance to embryogenesis, neural crest differentiation, and end-organ growth and function of a broad range of tissues including those of the endocrine, reproductive, gastrointestinal, cardiac, and integument systems.