This study searched a rare and aggressive type of cancer in dogs and humans, the breast carcinosarcoma. Both clinical and pathological traits of mammary carcinosarcomas in dogs are similar to humans, such as infrequent occurrence, fast tumor growth, and unfavorable prognosis when compared to carcinomas. Other possible alterations include chromosomal abnormalities that can be useful for the identification of tumoral cells and diagnosis. The aim of this study was to compare the chromosomal features of peripheral lymphocytes and tumor cells in a mammary carcinosarcoma of a 14-year-old female Poodle. Chromosomes were analyzed from 210 metaphases by conventional Giemsa staining, C-banding, and base-specific fluorochrome staining with chromomycin A3 (CMA3+) and DAPI. Of the 105 blood cells, 56.3% followed the standard karyotype of dogs (2n = 78). In contrast, the carcinosarcoma cells showed high chromosomal numbers (104 to 153), divided into 80% hypertriploid (118 to 136 chromosomes), 10.5% hypotetraploid (137 to 153 chromosomes), 5.7% hypotriploid (104 to 116 chromosomes), and 3.8% triploid cells (117 chromosomes). Among the aneuploid cells identified, we highlighted the trisomy of pair 1 and X chromosome once these elements were easily recognized in karyotype because of their size (pair 1) or differential morphology. Heterochromatin in normal cells was restricted to the pericentromeric region of all chromosomes while few C-bands were observed in tumor cells. This apparent loss of heterochromatin in neoplastic cells was supposed to favor centric fusion among formerly acrocentric chromosomes. Fluorochrome staining reinforced this hypothesis once GC-rich segments (CMA3+) were identified on 10 chromosomes from normal cells (2n = 78) whereas carcinosarcoma metaphases had up to 11 chromosomes bearing CMA3 signals in spite of their remarkable high chromosomal numbers. We concluded that, like in humans, the carcinosarcoma in dogs caused genome instability that eventually led to structural and numerical chromosomal aberrations. Besides, this study reinforced the importance of cytogenetic studies in dogs as a reference material for human cancer studies, especially in rare cases, since it is possible to increase knowledge about the characteristics of breast neoplasms in which there is a little availability of similar cases for comparative studies.
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