CARNEY COMPLEX: USE OF MOUSE MODELS TO UNDERSTAND THE MOLECULAR BASIS OF TUMORIGENESIS

Carney Complex (CNC) is an autosomal dominant, multiple endocrine neoplasia syndrome comprised of spotty skin pigmentation, myxomatosis, endocrine tumors, and schwannomas. The majority of cases are due to inactivating mutations in PRKAR1A, the gene encoding the type 1 A regulatory subunit of the cAMP-dependent protein kinase, PKA.ln order to understand the molecular basis for tumorigenesis associated with PRKAR1A mutations, we have developed conventional and conditional Prkarla knockout mice, as well as primary cell culture models corresponding to these genetic manipulations. As was previously described, homozygous null mice die early during embryogenesis. However, heterozygous mice (the exact genetic model for CNC patients) are born at expected frequencies and are tumor prone, developing neoplasms in cAMP-responsive cell types such as Schwann cells, osteoblasts, and thyrocytes.

In order to understand the basis of tissue-specific tumor formation, we have knocked the gene out of three different tissues: the pituitary gland, the neural crest (Schwann cells), and the heart. Ablation of Prkarl a from the Pit1 lineage of cells in the pitiuitary (which give rise to the cells that produce growth hormone [GHJ, prolactin [PrlJ, and TSH) results in multi-lineage pituitary tumors which arise with a long latency. Biochemical testing revealed only abnormalities of the GH axis, suggesting that activation of PKA in these pituitary lineages causes hyper-proliferation of all cell types, but excess hormone production only by somatotrophs. Removal of Prkarla from the entire neural crest causes perinatal lethality, but targeting of a subset of neural crest cells causes the reproducible formation of Schwann cell tumors within 4-6 months after birth.lntriguingly, ablation of Prkarl a from the developing heart leads to reduced proliferation of cardiomyocytes and embryonic demise from failing, dilated hearts. Although the overall heart morphology was consistent with dilated cardiomyopathy, the hearts also exhibit myxomatous changes suggesting that development of these tumors may in part be an aberrant response to ongoing cardiac damage.

At the biochemical level, removal of Prkar1 a from cells causes enhanced PKA activity, the same effect which has been observed in tumors isolated from CNC patients. Removal of Prkarl a from primary mouse embryonic fibroblasts causes cell immortalization. In other primary cell types, viability is prolonged in culture, but immortalization has not been observed. The enhanced growth and survival of Prkarl a null cells occurs independently of Erk and Akt activation, both in vitro and in vivo. In addition, ablation of Prkarl a appears to cause a change in cell fate, in which cells exhibit signs of epithelial-to-mesenchymal transition, an observation also confirmed in vitro in both mice and human patients.

In summary, we have observed that genetic ablation of Prkarl a causes tumors, confirming the role of this gene as a tissue-specific tumor suppressor. Ongoing work in the lab is focused on identifying the key downstream signaling targets affected by dysregulation of PKA, and determining. a signaling hierarchy that promotes cell survival. By studying how loss of normal PKA regulation causes tumors, we hope eventually to be able to use this knowledge to affect the behavior not only of tumors observed in CNC patients, but in a much broader set of tumors with alterations in PKA or its associated signaling pathways.