SARS CoV Replication Mechanism
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SARS CoV Replication Mechanism
SARS (Severe Acute Respiratory Syndrome) is a newly emerged infection in humans characterized by fever and pneumonia. This disease may progress rapidly to ARDS (Acute Respiratory Distress Syndrome) with considerable morbidity and mortality. It was first identified in the Guandong Province of China in November 2002, and a major outbreak occurred in Hong Kong in March 2003 (Ref.1). Probable cases have been reported in 31 countries, with extensive ongoing transmission in Taiwan and China, continuing transmission in Hong Kong, and major outbreaks that are now under control in Singapore and Vietnam. There are three different phases of SARS, with symptoms including fever, a dry cough, dyspnea (shortness of breath), headache, and hypoxaemia (low blood oxygen concentration). Typical laboratory findings include lymphopaenia (reduced lymphocyte numbers) and mildly elevated aminotransferase levels (indicating liver damage) (Ref.2). Death may result from progressive respiratory failure due to alveolar damage. The typical clinical course of SARS involves an improvement in symptoms during the first week of infection, followed by a worsening during the second week. Droplets produced by coughing and sneezing spread the SARS virus, but other routes of infection may also be involved, such as faecal contamination.

The causative agent of SARS is a novel Coronavirus (SARS-CoV), which are positive-stranded RNA viruses featuring the largest viral RNA genomes known to date (27 to 31kb) with a 5 cap and 3 poly(A) tail. Many cell surface-associated molecules with diverse sequences, structures, and cellular functions are usurped by coronavirus for use as their receptors. Very recently the functional receptor for the SARS-CoV has been identified as the ACE2 (Angiotensin Converting Enzyme-2) (Ref.7). ACE2 is a homolog of the metalloprotease ACE and is involved in the contraction of smooth vascular muscle and the resulting rise in blood pressure. ACE2 contributes to the ability of the viral protein to orchestrate the process of fusion, supports formation of syncytia due to cell fusion mediated by the interaction with S (Spike), and mediates infection of cells otherwise inefficient for virus replication. Entry of the virus occurs via endocytosis and membrane fusion. The life cycle of a coronavirus starts when the spike protein, which forms the distinctive, eponymous crown that is observed with coronaviruses, interacts with a receptor through its S1 domain (Ref.3). The entry, which is probably mediated by the S2 domain, occurs by membrane fusion. The RNA genome is then released into the cytoplasm where replication takes place. The host translation machinery translates the overlapping ORF1a (Open Reading Frames) and ORF1b by a ribosomal frame-shifting mechanism to produce a single Rep polyprotein, which codes for a viral protease within the polyprotein, called the "Coronavirus Main Protease" Mpro or 3CLpro. This protein is absolutely essential for the virus to make copies of itself and spread the infection. Cleavage by these virally encoded proteinases yields the components that are necessary to assemble the viral replication complex, which synthesizes full-length negative-strand RNA. Initially, the 5 20kb of the (+) sense genome is translated to produce a viral polymerase, which then produces a full-length (-) sense strand. This is used as a template to produce mRNA as a nested set of transcripts, all with an identical 5’ non-translated leader sequence of 72nt and coincident 3 poly (A) ends. An alternative hypothesis proposes that these mRNA molecules are generated by discontinuous transcription during positive-strand synthesis. Each mRNA is monocistronic, the genes at the 5 end being translated from the longest mRNA and so on. These unusual cytoplasmic structures are produced not by splicing (post transcriptional modification) but by the polymerase during transcription. Between each of the genes there is a repeated intergenic sequence that interacts with the transcriptase plus cellular factors to splice the leader sequence onto the start of each ORF. The SARS-CoV genome contains five major ORFs that encode the replicase polyprotein; the S (Spike), E (Envelope), and M (Membrane) glycoproteins; and N (Nucleocapsid) protein in the same order and of approximately the same sizes as those of other coronaviruses (Ref.4). The main function of the S protein is to bind to species-specific host cell receptors and to trigger a fusion event between the viral envelope and a cellular membrane. Much of the species specificity of the initial infection depends upon specific receptor interactions. In addition, the S protein has been shown to be a virulence factor in many different coronaviruses and the S protein is the principal viral antigen that elicits neutralizing antibody on behalf of the host (Ref.2). The M protein is the major component of the virion envelope. It is the major determinant of virion morphogenesis, selecting S protein for incorporation into virions during viral assembly. The N protein joins the new genomic RNA to form new RNPs (Ribonucleoproteins) or the helical nucleocapsid. These RNPs attach to the membrane where S proteins and M proteins have previously assembled. The RNP buds into the lumen of the vesicle and finally the membrane bound RNP and its radiating spikes detach and come to lie free in the lumen as an immature virion. These particles progress up the periphery of the Golgi apparatus, maturing as they do so into a denser and more icosahedral form (Ref.5). The new virus particles collect in large vesicles and are finally released onto the cell surface by exocytosis to start the cycle again. Although the replication cycle occurs in the cytoplasm, the N protein might also target the nucleolus through the nuclear pores where it enhances the life cycle in various ways. Other NSP (Non Structural Proteins) are also produced during replication. Possible binding sites in the SARS-CoV S prote CD13 in tohave also been mapped out by using bioinformatics analysis tools and the result confirmed the bioinformatics predictions, indicating that CD13 is a possible receptor of the SARS-CoV S protein, which may be associated with the SARS infection (Ref.6).

It has been observed that age, absolute lymphocyte count, platelet count, aspartate aminotransferase level, total bilirubin level, LDH (Lactate Dehydrogenase) level, CKMB (Creatine Kinase level-MB), HBDH (Alpha-Hydroxybutyric Dehydrogenase) level, urea level, changes in the urea level, changes in the creatinine level, respiratory rate, degree of hypoxemia, and number of involved lobes during the acute phase are correlated with the prognosis for SARS (Ref.8). A number of antibodies, peptides and small compounds have also been shown to bind to the receptor ACE2. It is possible that some of these may be useful in the treatment of SARS, either by blocking the S-protein-binding site, or by inducing a conformation in ACE2 that is unfavourable to binding or fusion. Alternatively, a soluble form of the receptor itself may slow viral replication in an infected individual. Also, it is likely that a cell line approved for vaccine production and made permissive for viral replication by ACE2 expression will be the most efficient large-scale producer of a whole-killed or attenuated virus for use as a vaccine. Finally, study of the interaction between the SARS-CoV S protein and ACE2 of other animals may provide insights into the origins of the virus. Effective prophylaxis and antiviral therapies are urgently needed in the event of reemergence of the highly contagious and often fatal SARS-CoV infection. In future, if SARS returns as a threat to human health, these studies may contribute to its control.