COVID-19 was first reported in December 2019 and SARS-CoV-2, the causative agent of the disease, is believed to have jumped from an animal to humans. Even after about 10 months, scientists are not sure what caused the disease to jump from animals to humans although some previous studies have suggested that it may be a single mutation inside the spike protein. The spike protein is present on the surface of SARS-CoV-2 and interacts with ACE2 receptors on the surface of host cells to gain entry to these cells.
Now, Dr Alejandro Berrio, a postdoctoral associate at Duke University, and his colleagues claim that they have found a number of silent mutations inside the genome of SARS-CoV-2 that explain how the virus jumped from animals to humans and spread so quickly. Silent mutations are changes in the DNA or nucleotide sequence of an organism that do not cause a visible change in the organism. The findings of the study are published in the journal PeerJ.
Cross-species transmission of viruses
Viruses are intracellular parasites that are basically made of nucleic acids wrapped in protein coats, the latter sometimes wrapped in a lipid envelope. They need a living host to survive and multiply. Experts suggest that viruses are pretty skilled in their craft and develop symbiotic relationships with some of their hosts in the long term. In this kind of a relationship, the virus lives inside the host but does not cause disease. Instead, the virus and the host somehow benefit each other.
Humans also have various viruses in their normal microflora. About 8% of the human genome is indicated to be of viral origin, which has come from the various viral diseases that our ancestors had.
While we call it the novel coronavirus, the virus itself is not new at all. It has possibly been living in an animal species for long. Scientists have suggested bats as possible primary hosts. Snakes were initially indicated to be the host but then the disease would have had to pass through a mammalian host to reach humans (since we are mammals). Pangolins were then suggested to be the secondary host (who actually passed the disease to humans) but nothing can be said for sure yet.
Occasionally, while multiplying in their host animal, a viral mutation may make it suitable to jump species and infect another animal (including humans) and that is what is being suggested as the most likely cause of the emergence of COVID-19.
In the latest study, the researchers studied the genome of SARS-CoV-2 along with some other bat and pangolin coronaviruses. They recapitulated the previous findings regarding mutations in the spike protein, suggesting that viruses with these mutations are more likely to thrive (not all mutations in a virus are advantageous). However, the researchers also found other mutations in the virus, specifically in the Nsp4 and Nsp16 regions of the viral genome. These mutations gave the virus an edge over its previous strains and did not affect the proteins of the virus. They changed the way the RNA (the genetic material of SARS-CoV-2) folds and functions inside the human body. As per a news release by the Duke University, these changes in the RNA folding is what may have allowed the virus to transmit from one person to another even before the first person realizes they have the infection.
The authors also suggested that Nsp4 and Nsp16 show up in the body even before the spike protein so these can be a good drug target to fight the virus early on in the infection.
"Viruses are constantly mutating and evolving," said Dr Berrio in a press release, explaining the importance of the findings. He added "So it's possible that a new strain of coronavirus capable of infecting other animals may come along that also has the potential to spread to people, like SARS-CoV-2 did. We'll need to be able to recognize it and make efforts to contain it early.
For more information, read our article on Viral mutations: FAQs.
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