The World Health Organization (WHO) announced the reclassification of the COVID-19 outbreak as a global pandemic on March 11, 2020. With the rapid spread of the disease across the globe, countries worldwide have joined the war against the SARS-CoV-2 virus. It is of great significance to study the interaction mechanisms between human coronaviruses (hCoVs) and host molecules, and also explore the pathogenic process of viruses to provide a wholistic guide for developing clinical treatments, vaccines, and antiviral drugs. Towards this end, the generation of optimized research models will promote the efficient translation of potential therapeutics from the laboratory to human trials.
The clinical manifestations of diseases caused by three kinds of hCoVs - SARS-CoV, SARS-CoV-2 and MERS-CoV - all exhibit severe acute respiratory disease. In order to study this kind of disease effectively, mouse models need to show similar pathological changes after the initial infection, and may also present severe respiratory diseases, as in humans.
Compared to other research model animals, the mouse has the advantages of lower cost, easy breeding, faster reproduction, and more litters – providing several generations on a reduced timespan, ideal for such time-sensitive studies. However, there are certain species differences between mice and humans that influence viral pathology, resulting in some human-infecting viruses being unable to directly infect mice, or that no obvious symptoms are exhibited in mice even after in vivo virus replication. Given that the genetic background of inbred mice is consistent, editing the mouse genes can overcome the shortcomings of wild-type mice to a certain extent. Such genetic engineering approaches help to elucidate the role of the host’s specific genes in viral pathogenesis, providing a mechanism for research across a range of applications - from investigating preliminary genetic targets, through evaluating the safety and effectiveness of vaccines and drugs.
According to construction strategy and methods, mouse models of SARS-CoV infection fall into three main classifications:
1. Direct infection ofelderly inbred mice with human-susceptible SARS-CoVs
In order to explore a mouse model that is more suitable for simulating the infection of SARS-CoV in the elderly, there are studies adopting BALB/c, C57BL/6, and 129S6 inbred wild-type (WT) mice in 12-14 month age range for experiments. These studies clarified for the first time that the genetic and age characteristics of different hosts can significantly affect the pathogenicity of SARS-CoV infection.
2. Apply gene-editing technology: knockout related genes in mice, or transfer human host cell virus-binding receptors (such as ACE2) into mice.
Regarding the construction strategy of a hACE2 humanized mouse model, researchers are currently using different (tissue-specific or widely expressed) promoters and expression vectors constructed by the hACE2 gene, to obtain a randomly inserted transgenic mouse model by prokaryotic injection. The results of such studies using ACE2 humanized mouse models indicate that the level of hACE2 expression is directly related to the severity of the disease. Although these mouse models all showed respiratory epithelial cell infections from exposure to SARS-CoV, mice were found to die with encephalitis.
3. Adapt wild-type SARS-CoVs to mice by repeated exposure to obtain a more mouse-adapted viral pathogenesis, thereby establishing a virus-infected mouse model that can present obvious clinical phenotypes.
Researchers repeatedly inoculated SARS-CoV clinical strains in the nasal cavity of BALB/c mice (6-weeks-old) for 15 times until obvious clinical symptoms (such as weight loss, etc.) arose in wild-type mice. Through comparing the adapted lethal virus strains obtained from different passages, it is helpful to analyze the correlation between the variation of severe respiratory infections caused by the virus and the specific mutations of its proteins. This approach provides insights for further research on the role of adaptive mutations in specific SARS-CoV protein interactions between the virus and the hosts.
Comparisons of both nucleic and amino acid sequences have confirmed a high similarity between SARS-CoV-2 and SARS-CoV - approximately 80% and 76%, respectively. Additionally, SARS-CoV-2 interacts with the host through the human ACE2 receptor on the cell surface to initiate infection, causing clinical manifestations similar to SARS-CoV infection (though not exactly the same). Due to the similarities between these two viruses, the research on SARS-CoV may provide an important reference point for SARS-CoV-2 research. It should be noted that the reported mouse models of SARS-CoV infection constructed by gene-editing technology are all implemented using the strategy of randomly introducing the hACE2 gene along with the corresponding promoter. Such random integration strategies are often related to new symptoms that are difficult to explain, such as the severe neurological damage that causes death of infected mice. On the contrary, severe respiratory failure is the main reason for the death of patients infected with SARS-CoV. Furthermore, it is now believed that multiple organ failure caused by severe cytokine storms (such as a significant increase in IL-6, etc.) may also be the main cause of death in COVID-19 among humans. For these reasons, it is vital to choose a suitable mouse model construction strategy that can better reflect the clinical characteristics of human pathogenic viruses & their related diseases.
As a global provider of professional genetically modified animal model services - Cyagen’s research and development (R&D) team is prioritizing the development of improved hACE2 humanized models catered to the current needs of studying SARS-CoV-2 infections. We are hoping to provide researchers in the field of biomedical research with a selection of ideal mouse models for researching the pathogenic mechanisms of SARS-CoV-2 infection and expedite the development of efficient vaccines and antiviral drugs.