The biology of Coronaviruses
Coronaviruses get their name from the characteristic viral particle (virion) that looks like a corona because small protrusions are formed around their surface. This family of viruses infects various vertebrates, mainly mammals and birds, and it is considered one of the most important causes of viral respiratory infections worldwide.
With the recent discovery of the new coronavirus in 2019 (COVID-19), there are now a total of 7 known coronaviruses that infect humans:
- Human coronavirus 229E (HCoV-229E)
- Human coronavirus OC43 (HCoV-OC43)
- Human coronavirus NL63 (HCoV-NL63)
- Human coronavirus HKU1 (HCoV-HKU1)
- Coronavirus Associated with Severe Acute Respiratory Syndrome (SARS-CoV)
- Middle Eastern Respiratory Syndrome-Related Coronavirus (MERS-CoV)
- New coronavirus COVID-19 (SARS-CoV-2)
Before the SARS-CoV pandemic in 2003, the HCoV-229E coronavirus and the HCoV-OC43 coronavirus were the only known coronaviruses that could infect humans. Following the SARS epidemic, 5 additional coronaviruses were discovered that could infect humans, with the most recent, the new SARS-CoV- 2 in 2019. SARS-CoV and MERS-CoV coronaviruses are highly pathogenic to humans and are associated with high mortality rates.
Coronaviruses belong to the order of Nidovirales, family of Coronaviridae, and subfamily of Orthocoronavirinae. Coronaviruses are covered by an envelope and contain as genetic material positive single-stranded RNA, and they are considered to have the largest genome of all RNA viruses. Two-thirds of the genome at the 5' end encodes viral proteins involved in viral RNA transcription and replication, while one-third at the 3' end encodes viral structural proteins and specific auxiliary proteins.
The major proteins in coronaviruses are called protein S (Spike), protein E (Envelope), protein M (Membrane), and protein N (Nucleocapsid). These proteins (biomarkers) play an important role not only in the diagnosis of the disease but also in the understanding of the pathogenicity profile and ultimately in the research on what molecules can be used for vaccine production and for direct antiviral therapy.
The SARS-CoV and MERS-CoV coronaviruses are thought to have been transmitted to humans by bats. But while coronaviruses have evolved and remained in bat populations for thousands of years, mammalian intermediate hosts such as the civet in the case of SARS-CoV and the camel in the case of MERS-CoV can also become infected and possibly play a role in coronavirus transmission to humans. It is highly suspected that the COVID-19 pandemic started at the Huanan Seafood Market in Wuhan, China. However, some researchers suggest that this market may not be the primary source of viral transmission to humans, as bats are rarely sold in markets in China and they are more often sold directly in restaurants for food.
Coronaviruses mainly infect the upper respiratory tract and the gastrointestinal tract of birds and mammals. The superficial glycoprotein spike (protein S) is a key factor in the virulence of coronaviruses, as it is believed to allow them to attach to host cells. In SARS-CoV, the angiotensin-converting enzyme 2 (ACE2) is the major cellular receptor, and it plays a role in the ability of SARS-CoV to cause upper and lower respiratory tract infections, contributing to mortality. Similarly, the MERS-CoV coronavirus has been shown to bind to dipeptidyl peptidase 4 (DPP4), a protein that has been evolutionarily conserved in species known to host this coronavirus strain. While most respiratory viruses infect ciliated cells (ciliated epithelial cells), DPP4 is expressed in non-ciliated cells in human airways, a factor that appears to play an important role in virus transmission and in high rates of inflammation.
More information on molecular testing for New Coronavirus and antibody testing