The ongoing coronavirus disease pandemic 2019 (COVID-19) has resulted in more than six million deaths in approximately 500 million cases, although many tens of millions have been left undocumented and undiagnosed, no doubt. The causative agent is severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which arose in December 2019 in Wuhan before spreading to all parts of the world.
Study: Spike SARS-CoV-2 variant and accessory gene mutations alter pathogenesis. Image credit: Fit Ztudio / Shutterstock
The virus has mutated into multiple variants, some with increased pathogenicity and / or immune evasion characteristics. A new prepress impresses on the contribution of these mutations to pathogenicity and viral transmission.
Introduction
SARS-CoV-2 has a ribonucleic acid (RNA) genome made up of open reading frames (ORFs) that encode replicase proteins, structural proteins, and accessory proteins. The replicase gene is located at the 5 ‘end by ORF1a / b, followed by the four structural genes of the spike, envelope, membrane, and nucleocapsid components of the virus.
These genes include accessory proteins that are unique to each family of viruses. These contribute, in one way or another, to the pathogenesis of COVID-19. For example, the interferon signaling pathway triggers antiviral and / or inflammatory responses.
However, SARS-CoV-2 ORF3b interrupts this pathway, while ORF7a blocks the BST2 interferon-stimulated gene (ISG). Similarly, translocation of the STAT1 protein to the nucleus, driven by interferon expression, which modulates ISG expression, opposes viral ORF6.
Virus variants have been studied primarily in terms of ear mutations, as this protein mediates viral binding and entry into target host cells. The spike interacts with the host cell through the angiotensin 2 converting enzyme receptor (ACE2) and is the immunodominant antigen. Mutations in this gene have in some cases been associated with immune evasion.
In the current prepress, published on the bioRxiv * server, researchers analyzed the role of accessory proteins using synthetic genomics assembly technology. They focused on suppressions in ORF3a / b, ORF6, 7a / 7b, and 8 in the ancestral strain of the virus, first examining the effect on in vitro replicative fitness. Subsequently, they analyzed how these affected the pathogenesis of SARS-CoV-2 infection in a mouse model.
They also constructed recombinant spike protein variants in a WA-1 spine. These variants were identical to the Alpha, Beta, and Gamma spike variants and were compared to the ancestral spike protein for replicative fitness and pathogenesis in mice.
What did the study show?
The scientists found that deletions of ORF3a and ORF3b accessory genes (WA-1ΔORF3a / b) reduced the replicability of the virus in mice. Mice infected with suppression virus did not lose as much weight as controls infected with the ancestral strain (WA-1), and the viral load on lung tissue on days 2 and 4 was much lower. This corroborates previous research on this deletion.
However, unlike previous reports of attenuated clinical effects with suppression of ORF7a, ORF7b, and ORF8 in mice, no effect on viral load was reported in the current experiments. This could be due to differences in infectious dose or different strains of mouse.
WA-1ΔORF3a / b also resulted in lower expression of inflammatory cytokines and chemokines than with WA-1 infection, probably due to lower replication levels. Similar downstream effects were observed, as expected, in neutrophil recruitment. Positive regulation of I14 and I15 genes drives a type 2 (Th2) helper T cell response, observed in severe forms of COVID-19.
This is not consistent with the less severe clinical phenotype of WA-1ΔORF3a / b. One possible explanation for the upregulation, rather than the expected drop, of these cytokines could be their association with tissue repair, which is happening earlier in these mice due to a faster elimination of virus.
Adipoq is another gene regulated in these mice and is associated with adiponectin, a hormone that sensitizes tissues to insulin activity. Lower adiponectin levels are associated with severe respiratory failure in COVID-19.
Spike variants introduced into a WA-1 spine were found to have no significant change in replication, but viral load, as measured by viral titer in the supernatant at 72 hours, was lower with the Gamma variant. spike compared to the Gamma. variant.
No differences in weight loss, virus load, or brain titer were found for the other two WA-1 spike variants, compared to the actual variants, in K18-hACE2 mice. However, with BALB / c mice, the Beta spike-WA1 strain produced an attenuated phenotype compared to the Beta variant, whereas with the Alpha spike-WA1 strain, the lung viral titer increased on day 2 with no difference in weight loss.
In both mouse strains, the largest difference was observed with the Gamma spike-WA1 vs Gamma variant, with the former showing a more severe phenotype in terms of weight loss and lung titers than the Gamma variant. In K18-hACE2 mice, there was a trend toward higher viral titers in the brain and higher concentrations of RNA, which are irrelevant.
Pulmonary cytokines and chemokines also showed differences on days 2 and 4, more significantly in the Gamma strain spike-WA1 compared to the Gamma strain. Neutrophil recruitment genes were the most affected, with CXCL5 showing the largest increase in expression. Interestingly, this gene is an important neutrophil attractant in COVID-19 and is implicated as a cause of inflammation.
Nothing like thpok and I15 were regulated downward. Since these lead to CD4 + T cell differentiation, this finding may indicate that the earless WA-1 genes inhibited this pathway and that this inhibition is lost with the full-range Gamma variant.
The latter also shows a loss of interferon antagonism, mediated by earless genes, as Gamma-infected mice had higher levels of gamma interferon in the lungs than Gamma-spike-WA1 mice. This goes against the higher lung titers observed in the latter. This difference, however, was settled on the 4th.
What are the implications?
“This work demonstrates that ORF3a / b plays an important role in host pathogenesis and responses to SARS-CoV-2.” Thus, mutations in the accessory proteins of viral variants, and especially ORF3a, appear to contribute to the pathogenesis of COVID-19 with variants. Both Beta and Gamma variants had ORF3a mutations, and these continue to be found in newer variants, indicating their importance for replicative fitness and transmissibility.
“We interpret these data to suggest that off-spike mutations may be driving critical phenotypes of SARS-CoV-2 infection and disease.” In other words, peak mutations often increase ACE2 binding affinity, enhancing viral entry into cells and immune evasion. In contrast, accessory mutations reduce clinical severity, allowing for prolonged replication and increased viral transmission.
Taken together, this can ensure that the virus continues to adapt with better replicability, transmissibility, and pathogenicity.
* Important news
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guided by clinical practice / health-related behavior, or treated as established information.