High expression of ACE2 receptor of 2019-nCoV on the epithelial cells of oral mucosa

The COVID-19 pandemic, caused by the novel coronavirus 2019-nCoV, presented a global health crisis. Early symptoms include fever, cough, and fatigue, potentially leading to severe respiratory failure. Research identified angiotensin-converting enzyme 2 (ACE2) as the primary receptor for 2019-nCoV entry into cells. Previous studies demonstrated high ACE2 expression in various organs, including lung, esophagus, intestine, and kidney, suggesting these as potential high-risk infection sites. This study aimed to investigate ACE2 expression in the oral mucosa to assess its susceptibility to 2019-nCoV infection, exploring a potential route of transmission.

Several studies have established ACE2 as the primary receptor for 2019-nCoV, similar to its role in SARS-CoV and HCoV-NL63 infections. Research utilizing single-cell RNA sequencing (scRNA-seq) has mapped ACE2 expression across various human tissues. High expression has been reported in type II alveolar cells of the lung, esophageal and intestinal epithelial cells, cholangiocytes, and other cell types in the kidney and bladder. However, ACE2 expression and its distribution within the oral mucosa remained largely uninvestigated before this study.

The researchers employed a multi-pronged approach. First, they analyzed publicly available bulk RNA-seq data from The Cancer Genome Atlas (TCGA) and Functional Annotation of The Mammalian Genome Cap Analysis of Gene Expression (FANTOMS CAGE) datasets to assess ACE2 expression across various organs, including different sites within the oral cavity. They then employed in-house generated single-cell RNA-seq data from oral tissues (tongue, buccal mucosa, and gingiva) to identify ACE2-expressing cells and determine their proportion within different cell types. Data preprocessing involved quality control, filtering, and normalization steps before cell type identification using marker genes and dimensionality reduction techniques like UMAP. ACE2 expression was then analyzed across cell types and different oral sites.

Analysis of bulk RNA-seq data from TCGA and FANTOM5 CAGE datasets revealed ACE2 expression in the oral mucosa. Further analysis within the oral cavity showed a tendency towards higher ACE2 expression in the oral tongue compared to other sites (base of tongue and floor of mouth), although statistical significance was limited by sample size (p = 0.062). Single-cell RNA-seq analysis confirmed ACE2 expression in oral tissues, with a significant enrichment in epithelial cells (93.38% of ACE2-positive cells). Specifically, 95.86% of ACE2-positive cells were located in the oral tongue, indicating a higher expression in the tongue compared to buccal and gingival tissues.

The findings demonstrate the presence of ACE2 in the oral mucosa, particularly at a high concentration within the tongue epithelium. This indicates that the oral cavity, especially the tongue, represents a potential entry point for 2019-nCoV infection. The high expression in epithelial cells, which are directly exposed to the external environment, increases the risk of viral entry. These findings highlight the need for further investigation into the role of the oral cavity in COVID-19 transmission and inform the development of preventative strategies in dental practice and daily life. Future studies should focus on investigating the viral load and infectivity in oral secretions to understand transmission dynamics better.

This study provides evidence for the expression of ACE2 in the oral mucosa, particularly within the tongue epithelium, suggesting a potential high-risk site for 2019-nCoV infection. Further research is needed to fully elucidate the role of the oral cavity in COVID-19 transmission and to inform the development of effective prevention and treatment strategies.

The study's limitations include a relatively small sample size in the bulk RNA-seq analysis, particularly for the oral cavity sub-sites, which may have affected the statistical power to detect differences in ACE2 expression among those sites. Furthermore, the study focused on ACE2 expression and didn't directly measure viral infectivity in oral tissues.