Epstein-Barr virus (EBV) is a ubiquitous human oncovirus. Previous studies by us and others have indicated that pet dogs frequently encounter EBV or EBV-related viral infection. In this study, we explored whether EBV is involved in canine malignancies in dogs. EBV-specific BamHI W sequence was detected by polymerase chain reaction (PCR) in 10 of 12 canine tumor specimens, including 8 of 10 oral tumors. Using reverse transcription-PCR, gene expressions of latent membrane protein 1 (LMP 1) and BamHI H rightward reading frame 1 (BHRF1) were identified in 8 and 7 of 12 specimens, respectively. A novel LMP1 variant, T0905, was predominant in 5 canine tumor specimens and found to exist in EBV positive human BC-2 cells. Another LMP1 variant, T0902, was similar to human tumor variant JB7. The BHRF1 sequence identified from these canine tumors was identical to that of the B95-8 viral strain. LMP1 protein and EBV-encoded RNA (EBER) were detected by immunohistochemistry and fluorescent in situ hybridization, respectively, in several tumors, particularly in tumor nests of oral amelanotic melanomas. Furthermore, EBV-like virions adopting a herpesvirus egress pathway were detected in a canthal fibroblastic osteosarcoma and an oral amelanotic melanoma.In conclusion, we report the expressions of BHRF1 transcript (a viral anti-apoptotic protein), LMP1 (a viral oncoprotein) transcript and protein, EBER (a viral oncogenic RNA), and EBV-like virions in multiple canine tumors. The identity of BHRF1 and the resemblance of LMP1 variants between canine and human tumors indicate either a close evolutionary relationship between canine and human EBV, or the possibility of zoonotic transmission.
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