Immunohistochemical Expression of MSH2 and MSH6 Mismatch Repair Proteins and Their Pathological Correlations in Canine Primary Pulmonary Carcinoma.

DNA mismatch repair (MMR) proteins have been investigated as biomarkers in human oncology, but their role in canine PPCs has not been characterized. Immunohistochemistry for MCK, TTF-1, MSH2, and MSH6 was performed and evaluated in relation to clinicopathological parameters. Affected dogs had a median age of 10.6 years. Histologically, 16 tumors were classified as adenocarcinoma and one as adenosquamous carcinoma, with papillary architecture predominating. MCK and TTF-1 immunoreactivity was detected in all cases, supporting epithelial and pulmonary differentiation when considered with histopathological and available clinical findings. Absent MSH2 immunoreactivity was observed in 17.6% of cases, whereas absent MSH6 immunoreactivity was observed in 11.8%. MSH2 expression showed a significant inverse association with tumor volume (p = 0.006), whereas no significant associations were identified between MSH6 expression and the evaluated clinicopathological parameters. As only MSH2 and MSH6 were assessed and no microsatellite instability or molecular analyses were performed, the observed abnormalities cannot be interpreted as definitive evidence of functional MMR deficiency. These findings provide preliminary data on MSH2 and MSH6 expression in canine PPCs and support further investigation using comprehensive MMR assessment and molecular approaches.