key: cord-0782231-h4grfgnr
authors: Pujhari, Sujit; Paul, Sanjeeta; Ahluwalia, Jasmina; Rasgon, Jason L.
title: Clotting disorder in severe acute respiratory syndrome coronavirus 2
date: 2020-10-06
journal: Rev Med Virol
DOI: 10.1002/rmv.2177
sha: 2939f25437c2f94cd8d20fff19812f9567d1a061
doc_id: 782231
cord_uid: h4grfgnr

The severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) is a novel human respiratory viral infection that has rapidly progressed into a pandemic, causing significant morbidity and mortality. Blood clotting disorders and acute respiratory failure have surfaced as the major complications among the severe cases of coronavirus disease 2019 (COVID‐19) caused by SARS‐CoV‐2 infection. Remarkably, more than 70% of deaths related to COVID‐19 are attributed to clotting‐associated complications such as pulmonary embolism, strokes and multi‐organ failure. These vascular complications have been confirmed by autopsy. This study summarizes the current understanding and explains the possible mechanisms of the blood clotting disorder, emphasizing the role of (1) hypoxia‐related activation of coagulation factors like tissue factor, a significant player in triggering coagulation cascade, (2) cytokine storm and activation of neutrophils and the release of neutrophil extracellular traps and (3) immobility and ICU related risk factors.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a newly emerged positive sense RNA virus belonging to the family of betacoronaviruses. 1, 2 Members of this family of coronaviruses have crossed the species barrier, adapted to humans and get transmitted effectively from person to person through the respiratory route. So far, humankind has witnessed seven different human coronaviruses with varying incubation times, degrees of transmissibility, and disease severity, which are ordered by mortality rate as MERS-CoV > SARS-CoV > SARS-CoV-2 > HKU1 ≃ NL63 ≃ OC43 ≃ 229E. [3] [4] [5] [6] Among these, SARS-CoV-2 is unique with a relatively prolonged incubation time. 2 In addition to the acute respiratory failure associated with this virus, disturbances in haemostatic balance have emerged as a key issue in moderately and severely ill patients. These disturbances can result in hypercoagulability disseminated intravascular coagulation (DIC), which can contribute to organ failure, stroke, and heart and kidney complications. [7] [8] [9] [10] Here, we briefly review and outline the current knowledge on the progression of COVID-19, and how multifactorial pathologies and molecular responses influence the coagulation system and the fibrinolytic system during COVID-19.

Typically, SARS-CoV-2 infection runs a course of illness that is over by 20 days post infection. Initial infection can manifest a range of clinical symptoms, including dry cough, sore throat, fever, malaise, myalgias, gastrointestinal symptoms such as anorexia, nausea and diarrhoea. 11, 12 Some patients also present a temporary loss of taste and smell. 13 If the infection progresses, by the end of second week and early third week, patients exhibit shortness of breath or dyspnoea, a range of haematological irregularities such as lymphopenia and neutrophilia, coagulation abnormalities such as pulmonary embolism (PE), blood thickening, and strokes and, rarely, neurological symptoms. 14, 15 Patients with severe disease can develop a condition called "cytokine storm", in which cytokines, released by lymphocytes, monocytes and alveolar macrophages that encounter virus, contribute to damaging inflammatory responses. Damage to the liver and other virus induced factors, indicated by an increase in D-dimers, combined with the cytokine storm, leads to altered blood coagulation factors and DIC. The terminal stages of SARS-CoV-2 infection in patients that die often include acute respiratory distress syndrome (ARDS), stroke, myocardial injury, and multiorgan function damage. 15 

Patients with COVID-19 present with evidence, most commonly in the form of elevated levels of D-dimer, of activation of the coagulation system. More than 70% of the deaths related to COVID-19 are associated with deregulation of the mechanisms that control blood clotting. Blood thickening and clotting are important to prevent excessive loss of blood due to injury. In infection, localized clotting or systemic clotting are part of the innate immune response to limit the spread of the pathogen. 16 However, this clotting response might be associated with harmful effects. When released into the blood stream, a blood clot or thrombus can block the arteries supplying oxygenated blood, resulting in an embolism and death of the oxygenstarved tissue. 17 When coagulation is insufficiently controlled, DIC may evolve, resulting in a clinical syndrome that involves both widespread microvascular thrombosis (referred to as microthrombosis) from excessive clotting and enhanced bleeding from depletion of clotting factors. 18 

Alveoli, small thin-walled bulb-like structures, are the structural and functional units of the lungs where blood gets oxygenated through a very thin interstitial space between the capillary and alveoli. The oxygenated blood returns to the heart, where it is pumped to other parts of the body. In ARDS due to SARS-CoV-2 infection or other lung infection or injury, the lungs become inflamed and fluid fills the interstitial space, the capillaries become leaky, and the alveoli fill with proteinaceous liquid that prevents oxygen exchange ( Figure 1 ). The resulting hypoxic condition necessitates artificial respiration or assisted breathing. The hypoxic environment reduces the blood anticoagulation and activates pro-coagulation factors that may promote hypoxia-induced thrombosis. 19 Animal studies showed that, under hypoxic conditions such as that caused by infection, tissue factor (TF) is produced by the endothelial and subendothelial smooth muscle cells of the vasculature and leukocytes of the lungs. [20] [21] [22] Early growth response-1 (Egr-1), a cellular mediator, stimulates the transcription of the gene encoding TF via vascular endothelial growth factor, initiating the local procoagulant response. 23 Homozygous Egr-1-null mutant mice placed in a hypoxic environment show no change in TF abundance in the lungs. 24 TF initiates the clotting process through thrombin formation. Thrombin is a serine protease that catalyses coagulationrelated reactions and converts soluble fibrinogen into insoluble fibrin. 25 Hypoxia also reduces the abundance of protein S (PS), a natural anticoagulant produced primarily in the liver. 26 The amount of PS is inversely correlated with the amount of hypoxia inducible factor 1 (HIF1), a transcriptional regulator stabilized under hypoxic conditions. This inverse relationship establishes a molecular link between hypoxia and thrombosis. Cells respond to hypoxia through HIF1, a dimeric transcription factor composed of HIF1α and HIF1β. In the cytoplasm, HIF1α is continuously degraded in an O 2 -dependent manner. 27 Patients deficient in PS, with the Factor V Leiden mutation, have reduced endogenous antithrombotic activity and are vulnerable to enhanced fibrin deposition at hypoxemic sites. 28 Thus, we hypothesize that the COVID-19-induced hypoxia may cause a PS deficiency, which elevates thrombotic risk.

In severe cases of COVID-19, D-dimer, a small protein fragment that is a fibrin degradation product of the clot-dissolving process, is significantly increased, and this protein is a reliable marker of disease severity. 29 As a sensitive marker of coagulation and fibrinolysis, D-dimer abundance is also useful in diagnosing deep venous thrombosis or PE. 30 induce macrophages to secrete IL1β, which in turn enhances NET formation, creating a self-amplifying loop. 44 Electrostatic interactions between the NET histones and platelet phospholipids activate the blood coagulation pathway. 45 Neutrophil elastase (a serine protease) is one of the key enzymes that contributes to NET formation. 46 We hypothesize that this enzyme also promotes the coagulation process by digesting inhibitors of coagulation, such as antithrombin III and TF pathway inhibitor. Excessive NET formation can trigger a cascade of inflammatory reactions that can destroy surrounding tissues, facilitate microthrombosis, and result in permanent organ damage, including the vital pulmonary, cardiovascular and renal systems.

Elevated levels of NETs accelerate thrombosis in arteries and veins. [47] [48] [49] NETs are also detected in a severe form of dengue fever, DHF, which also involves multiorgan failure and is associated with fatal outcomes. 50 Also, integrating antiviral treatment with promising antiviral candidates, such as hydroxychloroquine and remdesivir, that target SARS-CoV-2 entry and viral replication, could speed recovery times and perhaps even reduce death rates. Future observations and experimental studies will play a vital role in portraying a clear picture in understanding the physiological, molecular and cellular signalling pathways that contribute to SARS-CoV-2-related thrombus formation.

The authors declare no conflict of interest.

Sujit Pujhari conceived the idea. Sujit Pujhari and Sanjeeta Paul did the literature search, drew the figures and wrote the manuscript. Sujit Pujhari, Sanjeeta Paul, Jasmina Ahluwalia and Jason L. Rasgon edited the manuscript. All the authors discussed and approved the manuscript.

https://orcid.org/0000-0001-5856-5328

Jason L. Rasgon https://orcid.org/0000-0002-4050-8429

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