Unlocking the Role of a Genital Herpesvirus, Otarine Herpesvirus 1, in California Sea Lion Cervical Cancer

SIMPLE SUMMARY: Wild California sea lions (Zalophus californianus) have a high prevalence of urogenital carcinoma. The cancer starts in the sea lion’s genital tract then spreads aggressively to other organs resulting in death. Previous research has identified a herpesvirus, otarine herpesvirus 1 (OtHV1), in the genital tract of most sea lions with urogenital carcinoma, however, this virus has also been found in the genital tracts of sea lions without cancer making its role in urogenital carcinoma ambiguous. Here, tissues from 95 sea lions with and 163 without cancer were tested for OtHV1, the amount of virus was quantified, and viral gene expression was measured. OtHV1 was found in 100% of the sea lions with urogenital carcinoma and there were exceptionally high viral loads and viral gene expression within the genital tumors. Of the sea lions that did not have cancer, 36% tested positive for herpesvirus and they had much lower viral load and no detectable viral gene expression, indicating the herpesvirus was dormant. These findings support that genital herpesvirus plays an integral role in sea lion urogenital carcinoma and suggests there is an underlying trigger or event that causes the virus to induce cancer in some infected sea lions and not others. ABSTRACT: Urogenital carcinoma in California sea lions (Zalophus californianus) is the most common cancer of marine mammals. Primary tumors occur in the cervix, vagina, penis, or prepuce and aggressively metastasize resulting in death. This cancer has been strongly associated with a sexually transmitted herpesvirus, otarine herpesvirus 1 (OtHV1), but the virus has been detected in genital tracts of sea lions without cancer and a causative link has not been established. To determine if OtHV1 has a role in causing urogenital carcinoma we sequenced the viral genome, quantified viral load from cervical tissue from sea lions with (n = 95) and without (n = 163) urogenital carcinoma, and measured viral mRNA expression using in situ mRNA hybridization (Basescope(®)) to quantify and identify the location of OtHV1 mRNA expression. Of the 95 sea lions diagnosed with urogenital carcinoma, 100% were qPCR positive for OtHV1, and 36% of the sea lions with a normal cervix were positive for the virus. The non-cancer OtHV1 positive cases had significantly lower viral loads in their cervix compared to the cervices from sea lions with urogenital carcinoma. The OtHV1 genome had several genes similar to the known oncogenes, and RNA in situ hybridization demonstrated high OtHV1 mRNA expression within the carcinoma lesions but not in normal cervical epithelium. The high viral loads, high mRNA expression of OtHV1 in the cervical tumors, and the presence of suspected OtHV1 oncogenes support the hypothesis that OtHV1 plays a significant role in the development of sea lion urogenital carcinoma.