Is COVID-19 Worsening? – The Effects of the D614G Mutation
by Belinda Nzadi
It has been over one year since the declaration of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, and the virus has made several strides since its identification. As of 17 June 2021, there have been 177 million cases of coronavirus disease of 2019 (COVID-19) and 3.83 million deaths worldwide, with several countries having entered their third wave, such as South Africa.
SARS-CoV-2 has undergone several mutations to its genetic material, some of which have appeared to be advantageous resulting in the variant becoming dominant, one such topical mutation is the substitution of aspartic acid residue mutating to a glycine residue at position 614 (D614G) of the SARS-CoV-2 spike protein. This mutation was revealed in May 2020 and said to have been rare before March of that same year, but within a month of its identification it was so common, it occurred in over 74% of published sequences.
This mutation increased in frequency globally as well as within individual regions during outbreaks, suggesting that this mutation increased the fitness of the virus as opposed to having occurred as a result of genetic drift or founders effect (the loss of genetic variation when a small subset of a large population establish a new population). The manner in which the mutation conferred an advantage for the virus is what was still undetermined, and as a result, Plante et al sought to determine the effect of this mutation on the viral spread and vaccine efficacy.
The researchers engineered the D614G mutation in the USA-WA1/2020 SARS-CoV-2 strain and performed experiments in cell culture, the main human airway tissue and a hamster infection model.
They discovered that the D614G mutation had no effect on viral replication when examined in cell culture using monkey kidney epithelial cells, however, in the human lung epithelial cell line (Calu-3) as well as in a primary human airway culture model, the viral replication was enhanced.
They then infected Syrian golden hamsters with either D614 or G614 viruses to evaluate the mutation’s relevance in-vivo and discovered that the viral loads were much higher in hamsters infected with the G614 virus than those with D614 virus in the upper airways.
The researchers then conducted a competition experiment, whereby hamsters were infected with both D614 and G614 viruses and quantified the relevant amounts of their respective RNAs by RT-PCR and Sanger sequencing, and after this experiment, they discovered that the G614 virus consistently had an advantage over the D614 virus. The competition experiment was also conducted in a primary human airway culture model, and in all circumstances, the G614 virus outcompeted the D614 virus, even when the airway tissue was infected with more D614 virus than the G614 virus.
The stability of the D614 and G614 viruses was also determined by measuring the decay of infectivity over time at different temperatures and it was discovered that the G614 retained higher infectivity at all temperatures in comparison to D614, suggesting the D614G mutation increases the stability of SARS-CoV-2.
Lastly to evaluate the potential effect on vaccine efficacy, the investigators took a panel of sera from hamsters infected with the D614 virus and measured the neutralisation titres and discovered that the D614G mutation could potentially confer higher susceptibility to serum neutralisation.
This study confirmed the hypothesis that the D614G mutation enhances viral replication in the upper respiratory tract and discovered that it potentially offers more stability to SARS-CoV-2 and increases the virus’ susceptibility to neutralisation by antibodies. The advantages the mutation has offered to the virus could result in more adverse effects in a shorter period of time, however the increased susceptibility to antibodies offers some hope for mitigating its potential harmful effects.
- Plante JA, Liu Y, Liu J, Xia H, Johnson BA, Lokugamage KG, Zhang X, Muruato AE, Zou J, Fontes-Garfias CR, Mirchandani D, Scharton D, Bilello JP, Ku Z, An Z, Kalveram B, Freiberg AN, Menachery VD, Xie X, Plante KS, Weaver SC & Shi PY. 2020. Spike mutation D614G alters SARS-CoV-2 fitness. Nature. 592:116-121