Prognostic Significance of Kit Receptor Tyrosine Kinase Dysregulations in Feline Cutaneous Mast Cell Tumors

Feline cutaneous mast cell tumors (FeCMCTs) are characterized by variable biological behavior. Development of multiple nodules and potential visceral involvement, along with inconsistency of conventional prognostic aids, justify uncertainty in differentiating benign from malignant forms. c-Kit proto-oncogene activating mutations have been reported in feline mast cell tumors (MCTs), but their prognostic relevance was not investigated. This study was performed on FeCMCTs with variable clinical outcome to assess whether Kit cytoplasmic immunohistochemical labeling can be regarded as indicative of c-Kit mutations and to evaluate the relationship between Kit dysregulation and survival. Twenty-four cats diagnosed with a primary cutaneous MCT were enrolled. Kit immunohistochemical pattern and c-Kit (exons 8, 9, 11) mutational status were assessed in 34 tumor samples. Risk factors affecting survival were a number of mitoses greater than 5 per 10 HPFs (P = .017) and cytoplasmic Kit labeling (P = .045). Increased mitotic activity was associated with Kit cytoplasmic expression (P = .01). c-Kit encoding mutations were present in 19 (56%) tumors (exon 8, 19%; exon 9, 71%; exon 11, 10%), however, they were not significantly related to protein expression and they had no influence on prognosis. Additionally, in 6 of 9 (67%) cats, multiple nodules from the same cat had different mutational statuses. Mutations in the fifth immunoglobulin-like domain of Kit occur frequently in FeCMCT, but they are variably associated with aberrant protein expression and do not appear to be strictly correlated with biological behavior. These findings need to be confirmed in larger series, and exploration of further genomic regions of c-Kit is warranted.