Lassa virus NP DEDDh 3’-5’ exoribonuclease activity is required for optimal viral RNA replication

Mammarenavirus include several pathogens that can cause severe and fatal zoonotic diseases in humans. Lassa virus (LASV) is the causative agent for Lassa fever (LF) currently endemic in West Africa. There is no approved vaccines and antivirals against LASV infection. Despite the substantial threat of LASV to public health, important questions in the basic biology of LASV still exist. LASV nucleoprotein has a DEDDH 3’- to-5’ exoribonuclease motif (ExoN), which is known to degrade virus-derived double-stranded RNA and thereby is key to LASV evasion of interferon response. Intriguingly, the NP ExoN motif is highly conserved in mammarenaviruses, regardless of pathogenicity, suggesting that NP ExoN has important function in virus life cycle in addition to immune evasion. By using reverse genetics system, we rescued an ExoN-deficient LASV mutant (ExoN- rLASV). Compared to wild-type rLASV, the ExoN- rLASV demonstrated impaired virus growth and decreased viral RNA production in interferon-deficient cells. For the first time, we found that abrogation of LASV ExoN activity led to increased aberrant viral RNA production in infection. Furthermore, NP ExoN deficiency increased LASV sensitivity to mutagenic nucleoside analogue. Our study provided evidence for an additional role of LASV NP ExoN in viral RNA replication besides its recognized function in immune evasion. LASV ExoN activity may be required for optimal viral RNA synthesis and control of aberrant viral RNA and viral RNA mutation.