The Role of the Canonical Wnt-pathway in the Pathogenesis of Canine Cortisol-Secreting Adrenocortical Tumors

Hypercortisolism is a common disorder in dogs. About 15% of cases of spontaneous hypercortisolism is caused by cortisol-secreting adreno- cortical tumors (ATs). Although knowledge about ATs has expanded in recent years, understanding of the pathogenesis of ATs is still far from complete.

The canonical Wnt-pathway plays a role in cell survival and cell cycle progression and has been shown to be involved in many different tumor types, including human and mouse ATs. Central in this pathway is beta-catenin. In human ATs, mutations in exon 3 of ß-catenin were detected in AT adenomas (13-33%) and carcinomas (33- 38%) resulting in stabilization and nuclear activity of ß-catenin.

In this study, the activity of the canonical Wnt-pathway was investigated by determining mRNA expression of the Wnt target genes cMyc, Axin-2 and Cyclin D1 in 15 normal canine adrenals, and 12 adenomas and 27 carcinomas of dogs with hypercortisolism. Localization of ß-catenin was evaluated by immunohistochemistry (IHC). The coding region of the mRNA of ß-catenin was sequenced in 12 adenomas and 26 carcinomas.

Expression analysis showed a variable activation of the Wnt target genes and stronger staining of ß-catenin by IHC in ATs. In 32% (12/38) of the ATs a mutation was found in ß-catenin: in 9 ATs there was a silent mutation (not resulting in a different amino acid), in one case a neutral mutation (resulting in an amino acid change), in one case a nonsense mutation resulting in a premature stopcodon, and in one case a deletion of 26 basepairs. The amino acid change appeared not to be tumor-associated whereas the stop codon was. 

These findings demonstrate the involvement of the Wnt pathway in the pathogenesis of canine ATs.