Confirmation of canine acanthomatous ameloblastoma using RAS Q61R immunohistochemical staining of formalin-fixed paraffin-embedded tissues.

Differentiating canine acanthomatous ameloblastoma (CAA) from oral squamous cell carcinoma (OSCC) based on routine histopathology can be challenging. We have previously shown that more than 95% of CAAs harbor an p.Q61R somatic mutation, while OSCCs carry either wild-type alleles or other MAPK pathway activating mutations (e.g., p.Q61L, p.V595E). Given that p.Q61R mutations are highly prevalent in CAA, we hypothesized that a RAS Q61R-specific rabbit monoclonal antibody may be a useful tool for confirmation of CAA by immunohistochemical (IHC) staining. In the present study, we assessed IHC staining of archived formalin-fixed and paraffin-embedded biopsy samples with a diagnosis of CAA ( = 23), using a RAS Q61R-specific rabbit monoclonal antibody (SP174) and an automated IHC stainer. Negative control samples consisted of p.Q61R mutation-negative OSCC tumors with either a known p.Q61L mutation ( = 1), p.V595E mutation ( = 4), or wild-type corresponding alleles ( = 3). We found that all 23 CAAs showed diffuse and strong membranous RAS Q61R immunoreactivity (100% sensitivity), while none of the 8 OSCCs showed immunoreactivity (100% specificity). The data supports the use of RAS Q61R-specific rabbit monoclonal antibody for diagnostic IHC confirmation of CAA and ruling out OSCC in dogs.