Characterization of amyloid deposits in canine mammary neoplasms

Mammary tumor-associated amyloidosis in dogs has been reported in several studies, but its incidence and predisposing conditions have not been investigated. We examined 252 mammary masses from 183 dogs to determine the prevalence of amyloid deposition and elucidate etiologic factors. Histologically, amyloid deposition was observed in 88 of 252 (35%) mammary masses. Amyloid deposition patterns were categorized into 3 types: stromal deposition (ST), deposition in corpora amylacea (CA), and precipitation within luminal secretions (PR). Of the 88 amyloid deposition cases, 49 were intra-tumoral, especially for ST-type amyloid deposits, which were all intra-tumoral. Immunohistochemistry and mass spectrometry revealed that ST-type amyloid deposits were positive for either serum amyloid A or α-S1-casein, depending on the sample; all CA- and PR-type amyloid deposits were positive for α-S1-casein. Statistically, intra-tumoral ST-type amyloid deposition was significantly more frequent in histologic grade II (<i>p</i> = 0.009) or III (<i>p</i> < 0.001) mammary tumors. In contrast, amyloid deposition in extra-tumoral tissues is significantly associated with aging (<i>p</i> = 0.002). Our findings suggest that the disruption of proteostasis in the canine mammary gland associated with tumorigenesis and aging can readily induce amyloid formation.