Germline Pathogenic Variant in the APC Gene Suggestive of Gardner Syndrome in a Pony.

A 12-year-old pony mare was presented for evaluation of dental disease and nasal discharge. At presentation, clinical signs included bilateral nasal discharge, cutaneous masses, and numerous hard enlargements involving the bones of the skull, maxilla, mandible, and cervical vertebrae. Oral exam revealed advanced dental disease with hard enlargements adjacent to and between numerous cheek teeth. Radiographs and computed tomography confirmed the presence of severe dental disease and proliferative bone lesions disseminated along the skull, hyoid apparatus, and cranial cervical vertebrae. The bony proliferations extended into the subcutis, nasal cavity, paranasal sinuses, orbits, cranial vault, and vertebral canal. Multifocal osteomas were considered the primary differential, and this was confirmed on biopsy of a bony mass. Due to the extent of the lesions and deterioration of the patient's quality of life, euthanasia was elected. On necropsy, multiple osteomas were present on the skull and to a lesser extent the cervical vertebrae. Additional abnormalities included multiple mucosal polyps in the small intestine, epidermal inclusion cysts, and adrenocortical adenomas. These findings resemble those seen in Gardner syndrome, a hereditary disease of people characterized by gastrointestinal polyps and extraintestinal manifestations, including osteomas, epidermoid cysts, adrenal tumors, and dental abnormalities. Historically, Gardner syndrome in people has been considered a separate condition from familial adenomatous polyposis (FAP). Gardner syndrome is now considered a variant of FAP associated with mutations in the adenomatous polyposis coli (APC) gene. Whole genome sequencing and variant discovery in the pony identified multiple unique variants, including a likely pathogenic single base pair insertion leading to a frameshift in APC (ENSECAP00000007276.1:p.Glu1527ArgfsTer9). While osteomas have been infrequently reported in horses, to the authors' knowledge, there have been no cases of Gardner syndrome described in equids. This case is highly suggestive of Gardner-like syndrome in an equid.