Genetic Investigation of Covid 19 in Lung Disease

SUMMARY The COVID-19 pandemic due to the SARS-CoV-2 virus has resulted in millions of deaths worldwide. The majority of COVID-19 deaths are caused by lung disease characterized by alveolar filling and severe hypoxemia. Descriptive studies of human patients, animal models and cultured cells have supported numerous pathogenic mechanisms for COVID-19 lung disease, including direct epithelial cell infection, vascular cell infection and thrombosis, and acute respiratory distress syndrome. Despite intense investigation, these hypotheses remain unproven due to a lack of cellular functional evidence for causality. SARS-CoV-2 infection requires viral binding of the human ACE2 (hACE2) cell surface protein, and wild-type virus cannot bind mouse ACE2. Existing hACE2- expressing mice either drive disease through non-endogenous transgenes that do not permit conditional analysis or fail confer severe illness after infection with SARS-CoV-2. Thus, powerful mouse genetic approaches have not yet been harnessed to test COVID-19 pathogenic mechanisms. To address this gap in knowledge we have generated new mouse genetic models that (i) express hACE2 from the mouse Ace2 locus at levels sufficient to confer lethal disease and hypoxia like that observed in human patients, (ii) permit Cre-mediated loss of hACE2 expression to functionally identify cells required to confer COVID-19 disease, and (iii) permit-Cre mediated gain of hACE2 expression to functionally identify cells sufficient for COVID-19 disease. Our preliminary studies identify both epithelial cell infection and vascular disease associated with extensive intravascular thrombosis in the lungs of hACE2 knockin animals. We therefore hypothesize that COVID-19 lung disease arises due to synergistic infection and/or dysfunction of both lung epithelial and lung vascular cells. To test this central hypothesis we will (i) use established Cre-expressing transgenes to test the requirement(s) for epithelial and vascular cell types during COVID-19 lung disease, (ii) use lung slice explants from human and hACE2 knockin mouse lungs to map the cells infected by SARS-CoV-2 virus, and (iii) compare acute and chronic lung responses to infection by the influenza and SARS-CoV-2 viruses to identify the pathogenic mechanisms that underlie the exceptional lethality of COVID-19 lung disease. These studies are expected to yield a functional and integrated understanding of the events that underlie COVID-19 lung disease and provide a solid scientific foundation for the development of novel therapeutic approaches.