Engineering a Replication-Competent, Propagation-Defective Middle East Respiratory Syndrome Coronavirus as a Vaccine Candidate

Almazán, Fernando; DeDiego, Marta L.; Sola, Isabel; Zuñiga, Sonia; Nieto-Torres, Jose L.; Marquez-Jurado, Silvia; Andrés, German; Enjuanes, Luis

The 3= third of the MERS-CoV genome contains a set of accessory genes encoding proteins with no similarity to other viral or mammalian known proteins (35) . In general, CoV accessory genes are not essential for virus growth in vitro (36) (37) (38) (39) . The reverse genetics system described in this article was used to study the importance of these proteins in cell culture. MERS-CoV genes 3, 4a, 4b, and 5 were each found to be dispensable for virus replication in tissue cultures. Interestingly, some of the rMERS-CoV viruses recovered from Vero A66 cells contained mutations in the accessory gene genome region that would prevent the expression of any of these genes. Similar results were previously reported for the original MERS-CoV-EMC12 isolate after passage in Vero cells (15) . These data suggested an apparent lack of selection pressure on MERS-CoV accessory genes during passages in cell culture and reinforced the dispensability of these genes for virus growth in vitro.