Middle East Respiratory Syndrome Virus Pathogenesis

Abstract

Coronaviruses (CoVs) are enveloped RNA viruses that infect birds, mammals, and humans. Infections caused by human coronaviruses (hCoVs) are mostly associated with the respiratory, enteric, and nervous systems. The hCoVs only occasionally induce lower respiratory tract disease, including bronchitis, bronchiolitis, and pneumonia. In 2002 to 2003, a global outbreak of severe acute respiratory syndrome (SARS) was the seminal detection of a novel CoV (SARS-CoV). A decade later (June 2012), another novel CoV was implicated as the cause of Middle East respiratory syndrome (MERS) in Saudi Arabia. Although bats might serve as a reservoir of MERS-CoV, it is unlikely that they are the direct source for most human cases. Severe lines of evidence suggest that dromedary camels have been the major cause of transmission to humans. The emergence of MERS-CoV has triggered serious concerns about the potential for a widespread outbreak. All MERS cases were linked directly or indirectly to the Middle East region including Saudi Arabia, Jordan, Qatar, Oman, Kuwait, and UAE. MERS cases have also been reported in the later phases in the United Kingdom, France, Germany, Italy, Spain, and Tunisia. Most of these MERS cases were linked with the Middle East. The high mortality rates in family-based and hospital-based outbreaks were reported among patients with comorbidities such as diabetes and renal failure. MERS-CoV causes an acute, highly lethal pneumonia and renal dysfunction. The major complications reported in fatal cases are hyperkalemia with associated ventricular tachycardia, disseminated intravascular coagulation, pericarditis, and multiorgan failure. The case-fatality rate seems to be higher for MERS-CoV (around 30%) than for SARS-CoV (9.6%). The combination regimen of type 1 interferon + lopinavir/ritonavir is considered as the first-line therapy for MERS. Antiviral treatment is generally recommended for 10 to 14 days in patients with MERS-CoV infection. Convalescent plasma therapy has shown some efficacy among patients refractory to antiviral drugs if administered within 2 weeks of the onset of the disease.

Fig. 1

Schematic of the replication cycle of Middle East respiratory syndrome coronavirus (MERS-CoV). MERS-CoV binds to dipeptidyl peptidase 4 (DPP4) on the host cell through its receptor-binding domain (RBD) in the S1 subunit of the spike (S) glycoprotein, which leads to virus–cell fusion and the release of genomic RNA into the cytoplasm. Initially open reading frame 1a (ORF-1a) and ORF-1b are translated into polyproteins, polyprotein 1a (pp1a) and pp1ab, respectively, which are cleaved by the virus-encoded proteases papain-like protease (PLpro) and 3C-like protease (3CLpro) into 16 mature nonstructural proteins (nsps). The proteins involved in replication and transcription are gathered into replication-transcription complexes (RTCs) that associate with double-membrane vesicles (DMVs) derived from the endoplasmic reticulum (ER). The genomic RNA contains adenylate uridylate (AU)-rich sequences called transcription regulation sequences (TRSs). If the TRSs are recognized by RTCs, then RNA of subgenomic length for transcription will be generated, otherwise a full-length template RNA of genomic length for replication will be synthesized. The newly produced genomic RNAs are encapsidated in the nucleocapsid (N) proteins in the cytoplasm and then transported to the ER–Golgi intermediate compartment (ERGIC) for further assembly. The S, membrane (M), and envelope (E) proteins are inserted into the membrane of the rough ER (RER), from where they are transported to the ERGIC to interact with the RNA-encapsidated N proteins and assemble into viral particles. The budded vesicles containing mature viral particles are then transported to the cell surface for release after maturation in the Golgi bodies. Double-stranded RNAs (dsRNAs) are partially generated during viral replication. The 4a competes with Toll-like receptor 3 (TLR3) and retinoic acid-inducible gene I product (RIG-I)-like helicases (RIG-I and melanoma differentiation-associated protein 5 [MDA5]) to bind to dsRNAs and evades the host immune response. (Adapted from Durai P, Batool M, Shah M, Choi S., Middle East respiratory syndrome coronavirus: transmission, virology and therapeutic targeting to aid in outbreak control. Exp Mol Med 2015;47:e181; doi:10.1038/emm.2015.76 under Creative Commons CC-BY license (Creative Commons Attribution 4.0 International License.)