An exogenous retrovirus isolated from koalas with malignant neoplasias in a US zoo.

Leukemia and lymphoma account for more than 60% of deaths in captive koalas (Phascolarctos cinereus) in northeastern Australia. Although the endogenizing gammaretrovirus koala endogenous retrovirus (KoRV) was isolated from these koalas, KoRV has not been definitively associated with leukemogenesis. We performed KoRV screening in koalas from the San Diego Zoo, maintained for more than 45 y with very limited outbreeding, and the Los Angeles Zoo, maintained by continuously assimilating captive-born Australian koalas. San Diego Zoo koalas are currently free of malignant neoplasias and were infected with only endogenous KoRV, which we now term subtype "KoRV-A," whereas Los Angeles Zoo koalas with lymphomas/leukemias are infected in addition to KoRV-A by a unique KoRV we term subtype "KoRV-B." KoRV-B is most divergent in the envelope protein and uses a host receptor distinct from KoRV-A. KoRV-B also has duplicated enhancer regions in the LTR associated with increased pathology in gammaretroviruses. Whereas KoRV-A uses the sodium-dependent phosphate transporter 1 (PiT1) as a receptor, KoRV-B employs a different receptor, the thiamine transporter 1 (THTR1), to infect cells. KoRV-B is transmitted from dam to offspring through de novo infection, rather than via genetic inheritance like KoRV-A. Detection of KoRV-B in native Australian koalas should provide a history, and a mode for remediation, of leukemia/lymphoma currently endemic in this population.