When the covid-19 epidemic started to advance around the world, a great controversy arose about the origin of the SARS CoV-2 virus, which causes the disease. Some even said that the virus was manipulated or even manufactured in the laboratory. However, a study by researchers from the United States, Scotland and Australia, described in a letter published in the journal Nature Medicine, on March 17, brings evidence that SARS CoV-2 arose from the natural processes of evolution of living beings. The text points out mutations in the virus genome that make it more infectious in humans and that appear randomly during its replication. These changes are imperfect, which makes it unlikely that they have been produced by man.
"Viruses have genomes that are not very large, so it is possible to sequence them entirely in a very reliable way, and to establish comparisons between the different sequences", comments Professor Daniel Lahr, from the Department of Zoology of the Institute of Biosciences (IB) from USP. He did not participate in the study but commented on the article at the request of the Jornal da USP. “The SARS CoV-2 virus, which causes covid-19, has a genome of about 30,000 bases, while the human genome has approximately 3 billion base pairs and the bacterium Escherichia coli, whose use is very common in laboratory experiments , has 4 to 5 million base pairs. ” Bases are the molecular units that make up DNA.
To make the comparisons, the professor says that there are a series of mathematical formulas that determine how the sequences of the genome are related. "All sequences are placed in a large matrix to compare mutations and substitutions in the genome," he explains. "As there are already genetic data on the evolution of a large number of organisms, phylogenetic trees, there are theoretical models that explain how these processes should occur and allow us to make a series of predictions about what happened during the evolutionary history of the virus."
According to Lahr, the researchers, by analyzing the variations of the entire genome of the virus, were able to determine that SARS CoV-2 is very closely related to a virus already described in bats, RATG13. “This means that they have a hypothetical common ancestor”, he highlights, “but the observation of specific parts of the genome indicates similarities that, in comparison with other viruses, give the opportunity to explain evolution events and to identify the most important mutations to infect beings humans".
According to the IB professor, the mutations happen at random, during the replication of the viruses inside the cells. “Although the error rate is very small, thousands of viral genomes are replicated at the same time, over several days, giving rise to random changes”, he reports. "Most of these changes are not viable for the virus, but a small part of them will be potentially adaptive, making the virus able to infect a new host and expand its area of operation."
Knowing that genomes accumulate mutations, scientists were able to find evidence of changes that did not work out and others that may have helped to infect humans more efficiently. “All these indications allowed us to deduce that the general pattern of mutations in SARS CoV-2 corresponds to the existing evolutionary models”, says Lahr. "Thus, the researchers point to two hypotheses for the occurrence of these mutations: one, which would have happened in the animal reservoir of the virus, which is not yet known, and another, that the diversification would have happened after the invasion in humans.
"All of this evidence allowed us to deduce that the general pattern of mutations in SARS CoV-2 corresponds to existing evolutionary models. ”
The professor reports that the study identified a mutation in the SARS CoV-2 genome related to the production of the SPIKE protein, which helps the virus to adhere to the surface of cells and introduce itself into them to replicate. “In humans, the protein binds to a receptor known as ACE2, present in the lung cells, which are invaded in a very intense way, causing the disease”, he describes. "Since this connection, while efficient, is not perfect, it is unlikely that the mutation was produced in the laboratory."
Although the mutation described in the study is similar to that found in the genome of viruses present in the pangolin (a carnivorous mammal of the order Pholidota), Lahr points out that it is necessary to analyze a larger number of samples collected from animals to accurately determine the origin of SARS CoV -2. “The publication demonstrates the importance of understanding the evolutionary path of the virus in order, together with biochemical studies on its introduction into cells, to create future prevention and treatment strategies for covid-19”, he highlights. "Flu vaccines, for example, are produced annually by taking samples and identifying the most infectious virus varieties."