Results

Public bulk RNA-seq dataset analysis
NA-seq profile data of 13 organs including 695 para-carcinoma normal tissues as control from public TCGA were obtained for our analysis, and Fig. 1a showed that ACE2 could be expressed in various organs, the mean expression of different organs could be found in Table 1. According to the mean expression of ACE2, the mucosa of oral cavity could express ACE2, and the results were validated by the data of normal tissues from the FANTOM5 CAGE dataset (Fig. 1b).
Fig. 1 Bulk RNA-seq analysis of public datasets. a Violin plot of ACE2 expression in para-carcinoma normal tissues from TCGA, colored by organs. b Bar plot of ACE2 expression in normal tissues from FANTOM5 CAGE dataset, colored by organs. c Bar plot of ACE2 expression of para-carcinoma normal tissues from TCGA in different oral sites, colored by oral sites; d Boxplot of ACE2 in two kinds of oral sites from TCGA, colored by oral sites
Table 1 Sample size and ACE2 expression of para-carcinoma normal tissues in different organs
Sample size ACE2 mean expression Standard deviation of expression
Intestine 51 9.50 1.183
Kidney 129 9.20 2.410
Stomach 35 8.25 3.715
Bile duct 9 7.23 1.163
Liver 50 6.86 1.351
Oral cavity 32 6.23 1.271
Lung 110 5.83 0.710
Thyroid 59 5.65 0.646
Esophagus 11 5.31 1.552
Bladder 19 5.10 1.809
Breast 113 4.61 0.961
Uterus 25 4.37 1.125
Prostate 52 4.35 1.905
To investigate the ACE2 expression on mucosa of oral cavity, we looked into the ACE2 expression in different oral sites. According to the site information provided by the TCGA, among the 32 adjacent normal tissues, 13 tissues located in the oral tongue, 2 tissues located in the base of tongue, 3 tissues located in the floor of mouse, and 14 tissues did not definite the site and were just put into the category of oral cavity. The mean expression distribution of different sites was shown in Fig. 1c. When we combined the base of tongue, floor of mouth and oral cavity as other sites, and compared them with oral tongue, we found the obvious tendency that the mean expression of ACE2 was higher in oral tongue (13 tissues) than others (19 tissues) (Fig. 1d), while may due to the limitation of the sample size, the p value was not significant (P = 0.062).

Single cell RNA-seq analysis of oral tissues
Single cell RNA-seq was utilized for four oral tissues, and the data was analyzed to confirm the above results and assess the cell type-specific expression of ACE2. After the data preprocessing (shown in section “Materials and methods”), 22 969 cells were acquired and 7 cell types were identified (Fig. 2a), including epithelial cells (marker genes including SFN, KRT6A, and KRT10), fibroblasts (marker genes including FAP, PDPN, COL1A2, DCN, COL3A1, COL6A1), T cells (marker genes including CD2, CD3D, CD3E, and CD3G), macrophages (marker genes including CD163, CSF1R, CD68, and FCGR2A), mast cells (marker genes including CMA1, MS4A2, TPSAB1, TPSB2), B cells (marker genes including SLAMF7, FCRL5, and CD79A) and endothelial cells (marker genes including PECAM1, VWF, and ENG). The heatmap of main cell markers across the cell types can be found in Fig. 2b.
Fig. 2 Single cell RNA-seq analysis of oral tissues from independent dataset. a Seven-cell types were identified by the cell markers; cells were clustered by UMAP method. b Heatmap of cell markers for identifying cell types. c Scatter plots of all the cells with ACE2 expression. d Violin plot of ACE2 expression across different oral sites. e Violin plot of ACE2 expression across different cell types. f Scatter plots of tongue epithelial cells with ACE2 expression
According to Fig. 2c, d, we confirmed the ACE2 was expressed in oral tissues (0.52% ACE2-positive cells), and higher in oral tongue than buccal and gingival tissues (95.86% ACE2-positive cells located in oral tongue). Figure 2e shows that the ACE2-positive cells could be found in oral tissues including epithelial cells (1.19% ACE2-positive cells), T cells (<0.5%), B cells (<0.5%), and fibroblast (<0.5%), and the ACE2 was highly enriched in epithelial cells, of which 93.38% ACE2-positive cells belong to epithelial cells (Fig. 2f). The above results indicated that the ACE2 could be expressed on the epithelial cells of the oral mucosa and highly enriched in tongue epithelial cells.