Session 1A Presidential Address
Selective sweep of a SARS-CoV-2 mutant, with some hiccups
SARS-CoV-2 originated in China and rapidly evolved to 10 additional subtypes. Two evolved subtypes (A2 and A2a) carry a non-synonymous Spike protein mutation (D614G). We conducted a phylodynamic analysis of over 70,000 SARS-CoV-2 coronaviruses worldwide, sequenced until July 2020, and found that the mutant subtype (614G) outcompeted the pre-existing type (614D). The selective sweep of this mutant was significantly faster in Europe and North-America than in East-Asia. Bioinformatically and computationally, we identified a novel neutrophil elastase (ELANE) cleavage site introduced in the G mutant, near the S1-S2 junction of the Spike protein. We hypothesised that elevation of neutrophil elastase level at the site of infection will enhance the activation of Spike protein thus facilitating host cell entry for 614G, but not the 614D, subtype. The level of neutrophil elastase in the lung is modulated by its inhibitor α1-antitrypsin (AAT). AAT prevents lung tissue damage by elastase. However, many individuals exhibit genotype-dependent deficiency of AAT. AAT deficiency eases host-cell entry of the 614G virus, by retarding inhibition of neutrophil elastase and consequently enhancing activation of the Spike protein. AAT deficiency is highly prevalent in European and North American populations, but much less so in East Asia. Therefore, the 614G subtype is able to infect and spread more easily in populations of the former regions than in the latter region. Our analyses provide a molecular biological and evolutionary model for the higher observed virulence of the 614G subtype, in terms of causing higher morbidity in the host (higher infectivity and higher viral load), than the non-mutant 614D subtype.