Microsatellite instability in canine mammary gland tumors

Background: Genomic instability is a hallmark of cancer and may be required for the accumulation of cancer-causing mutations within cells. One form of genomic instability occurs in tandem nucleotide repeats and is known as microsatellite instability (MSI). Hypothesis: We hypothesized that MSI can be observed in canine mammary gland tumors (MGT) and represents a potential carcinogenic mechanism in dogs. Animals: Thirty-five dogs with MGTs and 9 dogs with other tumors were recruited from the University of Minnesota Veterinary Medical Center and referring veterinary clinics. Methods: A panel of 21 canine microsatellite (MS) markers was amplified by polymerase chain reaction (PCR) from deoxyribonucleic acid obtained from blood and from fresh or formalin-fixed, paraffin-embedded tumor tissues. PCR products were evaluated by using capillary electrophoresis, and the chromatograms were analyzed by using genotyping software. MS genotypes obtained from fresh and formalin-fixed tumor tissues were compared, as were MS genotypes from normal tissue and tumor tissue. Results: Genotypes obtained from formalin-fixed and fresh tissues were identical for all MS in 9 tumors evaluated, suggesting excellent concordance between the 2 sample types. For the 35 canine mammary tumors evaluated, 13 (37%) had stable genotypes; 22 (63%) exhibited aberrations in 1 or 2 MS; and 4 tumors (11%) demonstrated high-level instability, with aberrations in 29 to 61% of MS. Conclusions and Clinical Importance: Although some low-level MSI often is observed, high-level MSI is an infrequent finding in canine mammary tumors. Further evaluations are required to better characterize this phenomenon and to determine its relevance to canine carcinogenesis.