Bundibugyo Versus Zaire Ebolaviruses: Examination of the Polymerase Complex as a Therapeutic Target

Ebolaviruses, of the family Filoviridae, are the causative agents of outbreaks of hemorrhagic fever throughout Africa. In eastern Africa, three outbreak causing species of ebolavirus overlap in geographic distribution: Zaire ebolavirus (EBOV), Sudan ebolavirus (SUDV), and Bundibugyo ebolavirus (BDBV). EBOV and BDBV are distinctly different in case-fatality rate, disease course, and clinical presentation in both humans and non-human primates, but the reasons for these differences are unknown. Examination of the growth kinetics indicate that variations in the polymerase complex may be a factor in the differing pathogenicity. The rate of growth for infectious virions, genomic copies, and transcripts was slower for BDBV compared to EBOV. In addition to lower peak viral titers, BDBV infection also resulted in fewer viral transcripts being produced per genome compared to EBOV. An artificial transcription and replication system utilizing a ‘minigenome’ reporter was constructed for the study of BDBV. The minigenome system was used to compare the polymerase complex efficiency with that of EBOV and to screen potential therapeutics. Through the exchange of polymerase complex proteins, it was found that EBOV could more readily accept proteins from another viral species than BDBV. In fact, use of the BDBV NP enhanced transcription of the EBOV minigenome, while the inverse resulted in significantly decreased minigenome expression. The utility of the minigenome system was further shown by screening of small molecule inhibitors, of which the nucleoside analog remdesivir emerged as a lead candidate. Remdesivir was found to be highly effective at inhibiting primary transcription, and delayed treatment required a significantly higher concentration. Use of remdesivir was more effective at inhibiting BDBV than EBOV, likely due to the slower and diminished amount of viral transcription occurring over the course of BDBV infection. Targeting the polymerase complex of ebolaviruses is a viable strategy for therapy but efficacy will be highly dependent on timing and concentration. Future studies should examine modified treatment schedules and doses when care is delayed to overcome the increased polymerase activity that occurs later in infection.