Background: Mutations in the viral genome of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can enhance its pathogenicity by affecting its transmissibility, disease severity, and overall mortality in human populations. In addition to mutations within the coding region of SARS-CoV-2 structural proteins, there have been reports of mutations in other SARS-CoV-2 proteins that affect virulence, such as open reading frame 3a (ORF3a), which is involved in viral replication. The expression of ORF3a in host cells activates cell death signaling, leading to tissue damage, which affects the severity of COVID-19. The ORF3a-Q57H variant is the most frequent and recurrent variant of ORF3a and is likely associated with increased transmissibility but lower mortality in the 4th epidemic wave of COVID-19 in Hong Kong. Computational structural modeling predicted that the Q57H variant destabilizes the protein structure of ORF3a, which may result in reduced protein expression in human cells. However, it is still unknown how this mutation affects ORF3a protein function and, if so, whether it can change the severity of host cell damage.

Methods: Plasmids carrying SARS-CoV-2-ORF3a from Wuhan-Hu-1 strain (i.e., wild-type; WT) and its variant Q57H were transiently transfected into HEK293T cells and used for biochemical and cell biological assays.

Results: SARS-CoV-2-ORF3a-Q57H variant exhibits similar protein expression in whole cell lysates compared to WT, but less expression at the plasma membrane. ORF3a-Q57H expression results in less apoptosis in host cells compared to WT via lower activation of the extrinsic apoptotic pathway.

Conclusion: The relatively mild phenotype of the SARS-CoV-2-ORF3a-Q57H variant may result from alterations to ORF3a function by this mutation, rather than its protein expression levels in host cells.