Evolutionary insights into the furin cleavage sites of SARS-CoV-2 variants from humans and animals

The SARS-CoV-2 spike protein Q677P/H mutation and furin cleavage site (FCS) have been shown to affect cell tropism and virus transmissibility. Here, we analyzed the frequency of Q677P/H and FCS point mutations in 1,144,793 human and 1042 animal spike protein sequences and from those of the emergent variants B.1.1.7, B.1.351, P.1, B.1.429 + B.1.427, and B.1.525, which were deposited in the database of the GISAID Initiative. Different genetic polymorphisms, particularly P681H and A688V, were detected in the FCS, mainly in human isolates, and otherwise, only pangolin and bat sequences had these mutations. Multiple FCS amino acid deletions such as Δ⁶⁸⁰SPRRA⁶⁸⁴ and Δ⁶⁸⁵RSVA⁶⁸⁸ were only detected in eight and four human isolates, respectively. Surprisingly, deletion of the entire FCS motif as Δ⁶⁸⁰SPRRARSVA⁶⁸⁸ and Δ⁶⁸⁰SPRRARSVAS⁶⁸⁹ was detected only in three human isolates. On the other hand, analysis of FCS from emergent variants showed no deletions in the FCS except for spike P681del, which was detected in seven B.1.1.7 isolates from the USA. Spike Q677P was detected only once in variant, B.1.1.7, whereas Q677H was detected in all variants, i.e., B.1.1.7 (n = 1938), B.1.351 (n = 28), P.1 (n = 9), B.1.429 + B.1.427 (n = 132), and B.1.525 (n = 1584). Structural modeling predicted that mutations or deletions at or near the FCS significantly alter the cleavage loop structure and would presumably affect furin binding. Taken together, our results show that Q677H and FCS point mutations are prevalent and may have various biological effects on the circulating variants. Therefore, we recommend urgent monitoring and surveillance of the investigated mutations, as well as laboratory assessment of their pathogenicity and transmissibility.