Identification of novel amyloidosis in dogs: α-S1-casein acquires amyloidogenicity in mammary tumor by overexpression and N-terminal truncation

<jats:p> Mammary tumor–associated amyloidosis (MTAA) in dogs is characterized by amyloid deposition in the stroma of mammary adenoma or carcinoma; however, the amyloid precursor protein remains unknown. We attempted to identify an amyloid precursor protein and elucidated its etiology by characterizing 5 cases of canine MTAA. Proteomic analyses of amyloid extracts from formalin-fixed paraffin-embedded specimens revealed α-S1-casein (CASA1) as a prime candidate and showed the N-terminal truncation of canine CASA1. Both immunohistochemistry and immunoelectron microscopy showed that amyloid deposits or fibrils in MTAA cases were positive for CASA1. Reverse transcription-polymerase chain reaction and quantitative polymerase chain reaction revealed the complete mRNA sequence encoding CASA1, whose expression was significantly higher in the amyloid-positive group. The recombinant protein of the N-terminal–truncated canine CASA1 and the synthetic peptides derived from canine and human CASA1 formed amyloid-like fibrils in vitro. Structural prediction suggested that the N-terminal region of CASA1 was disordered. Previously, full-length CASA1 was reported to inhibit the amyloidogenesis of other proteins; however, we demonstrated that CASA1 acquires amyloidogenicity via excessive synthesis followed by truncation of its disordered N-terminal region. By identifying a novel in vivo amyloidogenic protein in animals and revealing key mechanistic details of its associated pathology, this study provides valuable insights into the integrated understanding of related proteopathies. </jats:p>