The Poly (<scp>ADP</scp>‐Ribose) Polymerase‐1 Enzyme Is Overexpressed in Various Solid Canine Tumours: An Immunohistochemical Study

The enzyme poly (ADP-ribose) polymerase-1 (PARP-1) is crucial for cellular regulation and DNA repair. Its immunohistochemical overexpression is known in various human neoplasms, but it was not yet a topic of veterinary research. Formalin-fixed paraffin-embedded canine samples of 15 controls and 34 tumours were immunohistochemically tested for PARP-1 expression. Controls included five skin samples with mast cells, five oral mucosa samples and five thyroid glands. Tumours included 18 mast cell tumours (MCTs), 10 oral squamous cell carcinomas (SCCs) and six follicular thyroid carcinomas. A board-certified veterinary pathologist defined the optimal region for the blind PARP-1 evaluation, assessed by two independent veterinary PhD students. Positive nuclei were evaluated by the immunoreactivity score (IRS) and quick score (QS) and, for both scores, the averages of the two observers were used for statistical analysis. In all MCTs, 6/10 SCCs and all thyroid carcinomas as well as four thyroid controls a nuclear expression was observed. A cytoplasmic granular staining was visible in all dermal mast cells and in 11/18 MCTs due to non-specific antibody uptake. No PARP-1 was expressed in 11/15 controls.Compared to the controls, thyroid carcinomas significantly overexpressed PARP-1 when calculated by IRS and QS (p = 0.003 and p = 0.005, respectively). The latter also applied to the MCTs (p = 0.001). A significantly higher PARP-1 IRS and QS were observed in thyroid carcinomas (p = 0.003, p = 0.005) and MCTs (p = 0.003, p = 0.012) compared to oral SCCs. The immunohistochemical PARP-1 overexpression in these tumours invites further research to assess its potential as a therapeutic target.