In a new bioRxiv study, a multinational group of authors identified a novel, primate-specific isoform of angiotensin-converting enzyme 2 (designated as deltaACE2 or dACE2) receptor, shown to be induced in various human cell types by interferons and viruses instead of ACE2. This finding has substantial implications for our understanding of coronavirus disease (COVID-19) pathology.
Figure 1. dACE2 is a novel primate-specific and virally-induced isoform of ACE2.
The ongoing COVID-19 pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which spreads person-to-person via close contact. The angiotensin-converting enzyme-2 (ACE2) receptor was found to mediate the viral entry into human cells.
Importantly, ACE2 was proposed to belong in the interferon-stimulated gene (ISG) group, based on its inducible expression when cells are treated with interferons or infected by viruses that induce interferon response (most notably with influenza virus).
Consequently, this raised concerns about its potential role in potentially increasing SARS-CoV-2 infection, as well as the safety and side effect profile of interferon-based treatments that are currently being touted for COVID-19.
The hijacking of the physiological host tissue-protective machinery that is guarded by ACE2 was proposed as a mechanism through which SARS-CoV-2 could infect more cells. Therefore, it was essential to identify factors affecting ACE2 expression in normal physiological processes and during viral infections, such as in COVID-19.
In this new paper, a research group from the US, Germany and Canada aimed to explore the interferon-induced expression of ACE2 in more depth and elucidate its exact role in SARS-CoV-2 infection.
Exploring expression patterns
To appraise the extent to which interferons induce the expression of ACE2 in human cells, the researchers used the existing RNA-sequencing dataset of a breast cancer cell line infected with Sendai virus, known to be a strong inducer of interferons and ISGs.
The novel ACE2 transcript that was found was designated as deltaACE2 (dACE2). Of the one hundred vertebrate species with genomic sequences available through the University of California Santa Cruz (UCSC) Genome Browser, it was shown that dACE2 is highly conserved only in primates.
To address head-on whether interferons were responsible for the induced expression of dACE2, the research group performed expression analysis in primary normal human bronchial epithelial cells and human intestinal (ileum and colon) organoid cultures.
To investigate whether dACE2 expression can be induced by RNA viruses (known as potent inducers of the interferon response), these authors de novo quantified the expression of ACE2 and dACE2 exons (i.e., genes encoding a part of the final mature RNA) in several public RNA-sequencing datasets of virally infected human respiratory epithelial cells.
Finally, they explored the expression patterns of dACE2 in various human tissues. Once it was established that dACE2 is an ISG in multiple human cells under various conditions, this study group tested whether SARS-CoV-2 could also induce its expression.
Only dACE2 expression is inducible by interferons
The study authors stated, "Our discovery of dACE2, a primate-specific version of ACE2, demonstrates that it is dACE2 and not ACE2 that is induced by IFNs and viruses, including SARS-CoV-2.”
Nonetheless, dACE2 did not appear to bind SARS-CoV-2 spike protein or influence the binding of ACE2 in the conducted in vitro experiments – assuring that ISG-type induction of dACE2 does not have the propensity increase viral entry.
In this paper, dACE2 expression was common in tumors and particularly enriched in lung tumors of bronchial origin, where the adequate function of ACE2 is indispensable for protection from virus-induced tissue damage.
The study authors further explained in their bioRxiv paper, "By analyses in multiple human cell types and tissues, we showed that expression of dACE2, but not ACE2, is inducible by interferons (type I, II and III) and viruses that induce interferon responses.”
Although all interferon types appeared to be potent inducers of dACE2 expression in the laboratory conditions, interferon-gamma might be a principal driver of dACE2 expression in tissues with substantial infiltration by immune cells, such as in virally infected tissues or tumors.
Improved understanding of COVID-19 pathology
In a nutshell, this study has demonstrated that ACE2 expression is not inducible by interferons. However, it would be essential to explore the exact effects of ACE-inhibitors and angiotensin receptor blockers on dACE2 expression to assess this risk accurately.
While our experiments did not yet identify a clear functional role for dACE2, the absence in non-primate species and the high degree of dACE2-Ex1c sequence conservation over 43 million years of primate evolution argues for a potentially important role of dACE2.”
These results reconcile current knowledge on ACE2 expression and suggest that the ISG-type induction of dACE2 in conditions with high levels of interferon (i.e., tumor microenvironment, treatments or viral co-infections) is unlikely to affect the cellular entry of SARS-CoV-2 and, in turn, promote the infectious process.
In conclusion, this is a first report of the discovery and functional annotation of dACE2, an interferon-inducible isoform of ACE2 receptor. This is important not only for understanding susceptibility and outcomes of COVID-19, but also for informing future treatment strategies.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.