Genetically modified mouse models to fight COVID-19

Soumya Sarathi Ganguly, Indian Institute of Technology, Kharagpur

There is a dire need for animal models in which vaccines against coronavirus disease or therapeutics can be tested. Besides, to understand the molecular mechanism of SARS-CoV-2 infection and pathogenesis. Several animal models, like cats, ferrets, hamsters, and monkeys are susceptible, but there are a few challenges like accessibility for most researchers. The most commonly used animal in the laboratory for biomedical research in mice, which were used to develop many vaccines and therapeutics for other human diseases. But in this case, the challenge is the susceptibility of mice to SARS-CoV-2, which can be overcome by producing genetically modified mice model which are susceptible to mice.

Over the decades, several Genetically engineered mouse models (GEMMs) were used in SARS studies, which express hACE2 used by SARS-CoV-2 for entry into host cells. These models have some limitations like they express mouse ortholog of ACE2 (mACE2), which occupy a portion of the pool of cell surface ACE2, and secondly, these transgenic expression cassettes are driven by different promoters and are expressed at different levels in various types of cells. To simulate the comorbid conditions, these models are not ideal for the researchers.

One of the ideal GEMMs is Knock out: Knock in (KO: KI) design, in which the mouse ACE2 is deleted, and human ACE2 is inserted. This is generated by deleting the region between the start and stop codon of mouse Ace2 and inserting 2.4 kb hAce2 cDNA. This design has additional regulatory elements that enhance mRNA stability and translational efficacy, producing high hAce2 protein levels than physiological levels. Instead of knocking out mAce2, certain amino acids can be mutated required for hAce2 binding to the spike protein of SARS-CoV-2. 

Transmembrane protease, Serine 2 (TMPRSS2), is a protease that helps in cleaving hAce2 and spike protein, leading to the efficient entry of viral cells into host cells. Co-expression of TMPRSS2 in KI mouse models is also proposed to achieve infection as well as pathogenesis to levels similar to humans. Knocking-in CRE-activatable or tetracycline induced expression cassettes can also be a method of developing GEMMs. This is done by a reengineered reporter or existing mouse lines. A mouse model in which CRE-activatable cassettes are knocked into the mAce2 locus is also generated.

Robust CRISPR-based strategies or i-GONAD (improves genome editing via oviductal nucleic acid delivery) can be used for the insertion of hAce2 or KI mice. Injection of a larger number of zygotes for KIs is required due to longer insertion sizes as insertion efficiency decreases with an increase in the insertion cassette’s size. Models expressing soluble hAce2 and FURIN protease can be generated in the future for performing extensive research in COVID-19 as many pathogenicity mechanisms are still left undiscovered. This can extend the list of GEMMs, and these pre-clinical models are expected to prove valuable for understanding COVID-19 therapeutics and vaccines.

Also read: Homonymous Hemianopsia: The vision in fragments

Reference: Gurumurthy, C., Quadros, R., Richardson, G., Poluektova, L., Mansour, S., and Ohtsuka, M., 2020. Genetically modified mouse models to help fight COVID-19. Nature Protocols,.

https://doi.org/10.1038/s41596-020-00403-2

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