An array of SARS-CoV-2 virus variants have been isolated, propagated and used in in vitro assays, in vivo animal studies and human clinical trials. Observations of working stocks of SARS-CoV-2 suggest that sequential propagation in Vero cells leads to critical changes in the region of the furin cleavage site, which significantly reduce the value of the working stock for critical research studies. Serially propagating SARS-CoV-2 in Vero E6 cells leads to rapid increases in genetic variants while propagation in other cell lines (e.g. Vero/hSLAM) appears to mitigate this risk thereby improving the overall genetic stability of working stocks. From these observations, investigators are urged to monitor genetic variants carefully when propagating SARS-CoV-2 in Vero cells.

The function of the SARS-CoV-2 accessory protein p6, encoded by ORF6, is not fully known. Based upon its similarity to p6 from SARS-CoV, it may play a similar role, namely as an antagonist of type I interferon (IFN) signaling. Here we report the sequencing of a SARS-CoV-2 strain passaged six times after original isolation from a clinical patient in Hong Kong. The genome sequence shows a 27 nt in-frame deletion (Δ27,264-27,290) within ORF6, predicted to result in a 9 aa deletion (ΔFKVSIWNLD) from the central portion of p6. This deletion is predicted to result in a dramatic alteration in the three-dimensional structure of the resultant protein (p6 Δ22-30 ), possibly with significant functional implications. Analysis of the original clinical sample indicates that the deletion was not present, while sequencing of subsequent passages of the strain identifies the deletion as a majority variant. This suggests that the deletion originated ab initio during passaging and subsequently propagated into the majority, possibly due to the removal of selective pressure through the IFN-deficient Vero E6 cell line. The specific function of the SARS-CoV-2 p6 N-terminus, if any, has not yet been determined. However, this deletion is predicted to cause a shift from N-endo to N-ecto in the transmembrane localization of the SARS-CoV-2 p6 Δ22-30 N-terminus, possibly leading to the ablation of its native function.

SARS-CoV-2 pathogenesis, vaccine, and therapeutic studies rely on the use of animals challenged with highly pathogenic virus stocks produced in cell cultures. Ideally, these virus stocks should be genetically and functionally similar to the original clinical isolate, retaining wild-type properties to be reliably used in animal model studies. It is well-established that SARS-CoV-2 isolates serially passaged on Vero cell lines accumulate mutations and deletions in the furin cleavage site; however, these can be eliminated when passaged on Calu-3 lung epithelial cell lines, as presented in this study. As numerous stocks of SARS-CoV-2 variants of concern are being grown in cell cultures with the intent for use in animal models, it is essential that propagation methods generate virus stocks that are pathogenic in vivo. Here, we found that the propagation of a B.1.351 SARS-CoV-2 stock on Calu-3 cells eliminated viruses that previously accumulated mutations in the furin cleavage site. Notably, there were alternative variants that accumulated at the same nucleotide positions in virus populations grown on Calu-3 cells at multiple independent facilities. When a Calu-3-derived B.1.351 virus stock was used to infect hamsters, the virus remained pathogenic and the Calu-3-specific variants persisted in the population. These results suggest that Calu-3-derived virus stocks are pathogenic but care should still be taken to evaluate virus stocks for newly arising mutations during propagation.

The recent emergence of Zika virus (ZIKV) has been concentrated in the Caribbean, Southeastern United States, and South- and Central America; resulting in travel-based cases being reported around the globe. As multi-disciplinary collaborations are combatting the ZIKV outbreak, the need to validate the sequence of existing strains has become apparent. Here, we report high-quality sequence data for multiple ZIKV strains made publicly available through the National Institutes of Health- (NIH) funded biorepository, BEI Resources (www.beiresources.org). Next-generation sequencing, 3′ rapid amplification of cDNA ends (RACE), and viral genome annotation pipelines generated GenBank sequence records for 16 BEI Resources strains. Minor variants, consensus mutations, and consensus insertions/deletions were identified within the viral stocks using next-generation sequencing (NGS) and consensus changes were confirmed with Sanger sequencing. Bioinformatics analyses of the sequencing results confirm that the virus stocks available to the scientific research community through BEI Resources adequately represent the viral population diversity of ZIKV.

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