key: cord-0048717-xl4pzu2b
authors: Tang, Hongtao; Lu, Xiaosheng; Qie, Shuyan; Xi, Jianing
title: Thoughts on detecting tissue distribution of potential COVID-19 receptors
date: 2020-08-12
journal: nan
DOI: 10.2217/fvl-2020-0136
sha: 1479c29f0cbd9c4ebfa5b688a7ee0aa06c114ca8
doc_id: 48717
cord_uid: xl4pzu2b

Aim: As a novel coronavirus, coronavirus disease 2019 (COVID-19) has caused a global epidemic. Many clinical data show that COVID-19 can not only cause viral pneumonia but also damage a variety of target organs. Materials & methods: We searched some open online datasets, such as Gene ORGANizer, the Human Protein Atlas and Pubmed, to explore the tissue distribution of potential COVID-19 receptors (ACE2, CD209, CLEC4M and BSG) in the human body. Results: The above potential COVID-19 receptors were highly expressed in the lungs, intestine, kidney, liver, heart, testis, placenta, hematopoietic tissue and nerve tissue. Conclusion: It is speculated that they may be potentially high-risk organs susceptible to COVID-19 infection. It is expectant to provide some help for further research in the future.

Gene ORGANizer (http://geneORGANizer.huji.ac.il/) is a kind of phenotypic tool, which directly links human genes to the organ systems they affect. It uses the two largest databases of human genetic diseases and phenotypic associations: human phenotypic ontology and DisGeNET, to establish more than 150,000 gene-organ associations and link more than 7000 genes to more than 150 anatomical sites [11] . For example, we know that mutations in the HOXA2 gene can lead to congenital microtia with dysplasia of the external ear (OMIM ID: 612290) [12] . By using this tool to search, we can relate HOXA2 to the following organ systems: external ear, ear, head, body surface system, head and neck regions and ectoderm.

In this article, we will detect the tissues distribution of potential COVID-19 receptors (ACE2, DC-SIGN, L-SIGN and CD147; the genotypes are ACE2, CD209, CLEC4M and BSG; Gene ID are 59272, 30835, 10332 and 682 respectively) in the Gene ORGANizer, the Human Protein Atlas (HPA) and Pubmed Gene database and compare these outputs. On the one hand, the purpose is to test the effectiveness of the application of the tool 'Gene ORGANizer' in the relationship between genes and organs. On the other hand, it is expected to provide early warning signals for the potentially involved organs of COVID-19 to a certain extent and help clinicians to better understand and deal with the related complications of COVID-19.

Gene ORGANizer can be retrieved in two ways: 'browse' and 'ORGANize'. 'Browse' allows you to see the organ systems associated with a particular gene. 'ORGANize' can get the results of the enrichment or depletion of a list of genes expressed in the organ systems. Furthermore, 'Confident' represents that the association is derived from human phenotypic ontology and DisGeNET2 databases; 'Tentative' means that this association is derived from one of the databases or based on observations of nonhuman organisms, such as rats and mice. 'Typical' represents the phenotype that happened in more than 50% of the patients with the disease [11] .

This time, we only need to use 'Browse' in this tool to retrieve the result of a single gene. Enter four potential receptor genes 'ACE2, CD209, CLEC4M and BSG' or Gene ID respectively in the 'Browse' column of the Gene ORGANizer (http://geneORGANizer.huji.ac.il/browse/). Then select the 'organ' item in the returned outputs, to get the results of related organs in the database.

The tissue atlas in the Human Protein Atlas (www.proteinatlas.org/) shows the expression and localization of human proteins across tissues and organs, based on deep sequencing of RNA (RNA-seq) from 37 major different normal tissue types and immunohistochemistry on tissue microarrays containing 44 different tissue types. Its' RNA expression overview shows RNA-data from three different sources, respectively: internally generated HPA RNA-seq data, RNA-seq data from the Genotype-Tissue Expression (GTEx) project and CAGE data from FANTOM5 project, as well as the consensus data set which is based on a combination of all three sources [13] .

Pubmed Gene database (www.ncbi.nlm.nih.gov/gene/) contains the gene expression in Homo sapiens (human) from the project named HPA RNA-seq normal tissues. They were performed of tissue samples from 95 human individuals representing 27 different tissues, to determine tissue-specificity of all protein-coding genes.

Gene ORGANizer data In the results of Gene ORGANizer (Figure 1 ), we find that three organs 'blood vessels, heart and kidney' are associated with ACE2 ( Figure 1A) , while CD209 has 13 linked organs 'blood vessels, brain, head, intestine, kidney, large intestine, lungs, lymph nodes, small intestine, spinal column, spinal cord, ureter and urinary bladder' ( Figure 1B) . BSG is bound with 'blood vessels and heart' ( Figure 1D ), but there are no organs linked to CLEC4M ( Figure 1C ).

According to the results obtained in the HPA and Pubmed Gene database, Figure 2 shows the similar tissues distribution of these genes in the human body, as shown in Figure 3 respectively. ACE2 is expressed in many organs, especially the small intestine and duodenum, followed by kidney, testis, gall bladder, heart, adipose tissue, epididymis, liver and other organs, while a trace distribution is also found in the lung tissue (Figures 2A & 3A) . CD209 is distributed in most human organs, including lymph nodes, adipose tissue, placenta, small intestine, heart, epididymis, duodenum, urinary bladder, liver, cerebellum, etc ( Figures 2B & 3B) . The expression of CLEC4M is mainly concentrated in the liver, lymph nodes, placenta, ovary, lungs and testis ( Figures 2C & 3C specificity of BSG is low and it is expressed in almost all human tissues, especially in the heart, cerebral cortex, placenta, colon, testis and kidney ( Figures 2D & 3D) . In previous studies on these genes, the distribution of the above four receptors in human organs and tissues has been supplemented. ACE2 is highly expressed in alveolar epithelial cells, intestinal enterocytes, vascular endothelial cells and smooth muscle cells. It is expressed more in renal tubular epithelial cells than in glomeruli and also exists in the nasal cavity, oral mucosa and nasopharynx [14] , endocardial cells and cardiomyocytes [15] . CD209 is enriched on the surface of dendritic cells located in the thymus, tonsil, spleen and lymph nodes, followed by mucosal surface and dermis, as well as some specific macrophages, such as Hofbauer cells in the placenta, Kupffer cells in hepatic sinusoids and alveolar macrophages [16, 17] . CLEC4M is mainly located in lymph nodes, placental capillaries, hepatic sinusoids, endothelial cells of the lungs [16] , alveolar cells, capillaries in lamina propria of ileal villi and intestinal Peyer collecting lymph nodes [17] . BSG is present on the surface of epithelial cells, endothelial cells, hematopoietic cells (monocytes, granulocytes, T lymphocytes, erythrocytes and platelets) and tumor cells in most organs and it is highly expressed in heart, skeletal muscle, kidney, testis, thymus, bone marrow, fetal liver and lungs [18, 19] .

Parts of them are summarized in the following Table 1 . 

As we all know, RNA-seq could convert the sample of mRNA into cDNA and sequence on a high-throughput platform. The primary data obtained from RNA-seq are mapped, assembled into expression summaries and normalized. Then, we can get the statistical test of differential expression [20] . In this article, we use the existing public RNA-seq expression data of potential COVID-19 receptors genes and obtain their specific tissue distribution according to different standards of three databases. These standards can be found on their online websites. It is precisely because of these RNA-seq data of COVID-19 that we could speculate on the results of these potentially involved organs. According to the above results, four potential invasive receptor genes (ACE2, CD209, CLEC4M and BSG) of COVID-19 are co-expressed in the lungs, liver, placenta and testis. Except for CLEC4M, the receptor L-SIGN, the other three genes are expressed in the heart, intestine, kidney, gall bladder, blood vessels and partly in hematopoietic cells and brain tissue. Among these four genes, we pay attention to the distribution of ACE2, which has been identified as an aggressive receptor. From these results, we could speculate that the potentially high-risk organs which are vulnerable to COVID-19 infection may be lungs, intestine, kidney, testis, heart, liver, placenta, hematopoietic tissue and nerve tissue.

Referring to the clinical characteristics of existing COVID-19 cases, Fang et al. [5] systematic retrospective study data included 3470 patients, 'their most common clinical symptoms were fever (83%), cough (61%), fatigue (37.9%), increased sputum (28.7%), dyspnea (14.5%)', all of which were consistent with the manifestations of viral pneumonia. Second, 'there were fewer patients with gastrointestinal symptoms, such as anorexia, nausea and vomiting (8.9%) and diarrhea (6.1%)'. Their common complications included acute respiratory distress syndrome (ARDS; 8.9%) and acute renal failure (2.1%). According to the fact that COVID-19 can be transmitted through the respiratory tract and digestive tract, these findings all indicate that COVID-19 has the ability to invade the lungs, intestines and kidney, which is consistent with our results. In addition, studies on the relationship between COVID-19 and renal function show that it is common for patients to have elevated levels of proteinuria, plasma creatinine and blood urea nitrogen [21] . In another study by Guan et al. [22] , laboratory indicators such as LDH (41.0%), creatine kinase (13.7%), ALT (21.3%) and AST (22.2%), may indirectly indicate the damage of heart and liver after the virus invasion. However, based on the distribution of receptors in endocardial cells, cardiomyocytes and vascular endothelial cells of the heart, it is speculated that if the virus could attack the heart, it may cause the degeneration and necrosis of cardiomyocytes, exfoliation of endothelial cells, subendocardial ischemia and coronary atherosclerosis, leading to cardiac dysfunction and even cardiovascular diseases.

Apart from the large amount of clinical data mentioned above, in other aspects, although the distribution of these potential receptors is also abundant in testis, placenta and hematopoietic tissue, little attention has been paid to whether COVID-19 will cause unknown damage to these organs or affect blood safety and invade the nervous system.

Concerning the testis, we wonder if the virus has the opportunity to directly attack the testis through its receptors or produce a secondary autoimmune response, which leads to viral orchitis causing swelling, fever, pain and other local inflammatory manifestations. Then it may be possible to cause sexual dysfunction, male infertility and even testicular tumors in the long run [23] . However, these symptoms and diseases are often ignored by the patients and may not be diagnosed at the correct time of their infection. Or the results can only be obtained through the researches of a small number of pathological specimens. Only in a pathological study in 2006, orchitis was confirmed as one of the SARS-related complications. By observing the autopsy specimens of the testis from six patients with SARS, the team found that SARS's infection would cause the infiltration of immune cells and cytokines in testis, thus destroying germ cells and causing orchitis [24] . Second, previous studies on the distribution of ACE2 in testis, through HPA, GTExportal [25] , Gene Expression Omnibus and Sequence Read Archive [26] , have confirmed that ACE2 receptors are abundantly expressed in testicular spermatogonia, Leydig cells and Sertoli cells. In accordance with us, it is speculated that the testis may be a potential target for COVID-19 infection, and if the virus could attack the spermatogenic cells of the testis, then there is a possibility that sexual transmission could also be achieved.

In terms of special groups like pregnant women, if COVID-19 can invade the placenta through its potential receptors, it will affect the placental development of these pregnant women, resulting in adverse pregnancy outcomes such as poor placental position, premature rupture of membranes, fetal growth restriction, preterm delivery, neonatal infection, perinatal death and so on [27] . However, there are few clinical data in this field and only a few studies have failed to find the potential vertical transmission of COVID-19 and the possibility of causing the above-mentioned potential events, such as Chen's analysis [28] of the characteristics of nine infected pregnant women in Wuhan area, the Wei's team's research [29] on maternal and neonatal outcomes and Huang's study [30] on placental pathology. Nevertheless, considering the particularity of pregnant women and newborns, pregnant women are more susceptible to be infected with respiratory pathogens in physiology. For that reason, they should be regarded as key groups for the prevention and management of COVID-19 [31] .

Furthermore, in the view of the expression of receptors on hematopoietic cells, if COVID-19 could affect the blood system or its components fall off into the blood, it may induce the contamination of blood products and lead to limited blood transmission. Some studies have shown that the viral RNA could be detected by real-time PCR in the plasma, called RNAaemia [32] . Although this does not represent the infectious virus in the blood, it is still worthy of our vigilance. It has been reported that only one case of aplastic anemia was infected by receiving blood products from COVID-19 patients [33] . In addition, with the emergence of more and more asymptomatic patients, there is also the increasing possibility of the potential spread by their blood products [34] . Therefore, it is necessary to strengthen the detection of blood donation and the management of blood products among residents at high-risk of COVID-19. Besides, there is no definite confirmation as to whether COVID-19 could invade the nervous system. Only studies have shown that β-CoVs, such as SARS-CoV and MERS-CoV, which is homologous to COVID-19, has the ability of neurophagia [35] . And some critically ill patients have manifestations of acute cerebrovascular disease and disturbance of consciousness, while a few have symptoms of the peripheral nervous system such as hypogeusia and hyposmia [36] .

In summary, although we have obtained the potential high-risk organs susceptible to COVID-19 infection by exploring the tissue distribution of potential COVID-19 receptors in the human body and summarizing the existing studies, it still needs further verification and correction through clinical practice and pathological studies. We expect that our findings could provide some help for further research in the future.

By comparing the results retrieved by Gene ORGANizer with those in the HPA and Pubmed, we realize that they are quite different and the results of this tool are not comprehensive. It is supposed that there may be the following reasons. First of all, the diseases involved in the tool are mainly hereditary originated from the two databases. It is based on these genetic diseases or phenotypes to associate genes with the organs they affect. When a gene is normally or abnormally expressed in an organ, if there is no known occurrence of genetic diseases or phenotypes, the association between the gene and the organ will not be reflected in this tool. Besides, the database version of the tool ends in September 2016. It comes naturally that the genetic diseases or phenotypes discovered after that time, as well as genes and their affected organs, will not be updated in the search results of the tool. Therefore, when we need to find out the relationship between genes and their organs, that is, the expression of genes in organs, the utilization of this tool is quite limited. This tool is a useful tool for us only when we need to study the relationship among genes, genetic diseases/phenotypes and organs.

Through these open online datasets, we obtained the result that the potential COVID-19 receptors are highly expressed in the lungs, intestine, kidney, liver, heart, testis, placenta, hematopoietic tissue and nerve tissue. It is speculated that they may be potentially high-risk organs susceptible to COVID-19 infection. It is expectant to provide some help for further research in the future.

• A novel coronavirus named COVID-19 has caused a global epidemic.

• Many clinical data show that COVID-19 can not only cause viral pneumonia but also damage a variety of target organs. • We searched Gene ORGANizer, the Human Protein Atlas and Pubmed, to explore the tissue distribution of potential COVID-19 receptors (ACE2, CD209, CLEC4M and BSG). • The potential COVID-19 receptors are highly expressed in the lungs, intestine, kidney, liver, heart, testis, placenta, hematopoietic tissue and nerve tissue. • It is speculated that they may be potentially high-risk organs susceptible to COVID-19 infection.

Author contributions J Xi contributed in conceptualization, methodology, software. H Tang did data curation, writing-original draft preparation. X Lu and S Qie did writing-reviewing and editing.

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The authors thank B Pan for putting forward the idea of using the tool named 'Gene ORGANizer' and writing assistance.

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.No funded writing assistance was utilized in the production of this manuscript.