RaTG13 Sarbecovirus

• 05 Aug 2020

Why in News

According to a recent study published in Nature Microbiology, the novel coronavirus (SARS-CoV-2) has been circulating unnoticed in bats for decades.

• Bats have been the ‘primary reservoirs’ for SARS-CoV-2, which is likely to have diverged from closely related bat viruses called the RaTG13 sarbecovirus, 40-70 years ago.

Key Points

• The current study confirms an earlier Chinese study and also suggests the probable time of divergence of the two viruses.
  
  • According to that Chinese study, SARS-CoV-2 is most closely related to RaTG13, which was isolated from a horseshoe bat in Yunnan province in 2013.
  • Based on the nearly 96% genome sequence identity between SARS-CoV-2 and RaTG13, it was held that an origin in bats is probable for the Covid-19 outbreak.
• SARS-CoV-2 has not arisen from recombination of any sarbecovirus.
  
  • Recombination: It is a process by which pieces of deoxyribonucleic acid (DNA) are broken and recombined to produce new combinations of alleles (forms of a gene). This recombination process creates genetic diversity at the level of genes that reflects differences in the DNA sequences of different organisms.
• The ability of the spike protein in the virus to bind to Angiotensin-Converting Enzyme 2 (ACE2) human receptors had emerged within bats and is an ancestral trait shared with bat viruses and not one acquired recently via recombination.
• The results of the study suggest the presence of a ‘single lineage’ circulating in bats with properties that allowed it to infect human cells.
  
  • This was also the case with the bat sarbecoviruses related to the 2002 Severe Acute Respiratory Syndrome (SARS) lineage.
• Methodology:
  
  • The researchers analysed the evolutionary history of SARS-CoV-2 using genomic data on sarbecoviruses.
  • They employed three approaches to identify regions in the virus that had not undergone recombination and that could be used to reconstruct its evolution.
  • All approaches suggest that RaTG13 and SARS-CoV-2 share a single ancestral lineage and estimate that SARS-CoV-2 genetically diverged from related bat sarbecoviruses in 1948, 1969 and 1982.
• Challenging Assumptions:
  
  • The study challenged the idea of pangolins serving as an intermediate host where the virus would have acquired its ability to infect human cells thus facilitating the jump into humans.
    
    • It is possible that pangolins could have been a medium for the transmission to humans, but there is no evidence that pangolins facilitated adaptation to humans by being an intermediate host.
    • Current evidence is consistent with the idea that the virus evolved in bats and enabled itself to replicate in the upper respiratory tract of both humans and pangolins.
  • Snakes were thought to have served as intermediate hosts of SARS-CoV-2 as well but the present study has found no evidence to support this hypothesis.
• Concerns:
  
  • The long divergence period raises the possibility of other undocumented virus lineages circulating in horseshoe bats that have the potential to jump from bats to humans.
  • Different and yet unstudied bat sarbecoviruses that have descended from the SARS-CoV-2/RaTG13 common ancestor form a clade with properties to infect many different mammals, including humans.
  • Due to the lack of better sampling, it is impossible to estimate whether or how many of these additional lineages exist.
  • The diversity and dynamic process of recombination amongst lineages in the bat reservoir makes identifying viruses difficult.
  • Many species of bat harbour several viruses which can cross over to new hosts. With the disruptions of their habitats, humans will face more such threats.

Way Forward

To identify the viruses and their lineages, there is a need for a global network of real-time human disease surveillance systems for pathogen identification and characterisation so that pandemic-like situations can be avoided.

Source: TH