Severe acute respiratory disease COVID-19 is a result of infections with the coronavirus SARS-CoV-2 first reported in the Chinese city Wuhan, Province Hubei, in December 2019 and has since 11 March 2020 been designated as a pandemic by WHO. The origin of the virus is still under investigation although some studies suggest that it was transferred from another species, possibly pangolins 1 or bats 2 . At the end of April 2020 the number of globally confirmed cases of COVID-19 exceeded 3 million and recorded deaths beyond 200,000 in the real-time statistics of the Johns Hopkins University 3 . Due to many unreported and asymptomatic cases, the infection fatality rate (IFR) is difficult to determine however Verity et al. estimate approximately 0.66% (0·39-1·33) in China 4 . The age-associated IFR is approximately 7.8% for those above 80 years 4 . Drugs have been repurposed for stabilizing COVID-19, but these are not effective therapies 5 , examples include such as remdesivir (Ebola) 6, 7 , hydroxychloroquine (Malaria) 6, 7 and nelfinavir (HIV) 8 . Another treatment option is to indirectly immunize individuals with plasma from convalescent COVID-19 patients 9 . Further approaches aim at mimicking the human virus cell entry receptor ACE2 2 with human recombinant soluble ACE2 (hrsACE2) 10 . The cell entry receptor ACE2 associates with transmembrane proteases which prime the spike protein of the virus. Hoffmann et al. assigned this task to the protein TMPRSS2 11 which they identified in the predecessor virus SARS-CoV-2 and assume it is the same for SARS-CoV-2. The protease can be inhibited by existing compounds to interrupt further propagation of the virus in the human host.
Here, we describe a meta-analysis focussing on the transcriptome data from human lung epithelial cells including samples infected with SARS-CoV-2 from a study described by Blanco Melo et al. 12 . We directed the exploration to co-expression with the known CoV-2 receptor ACE2. The analysis revealed a signature consisting of 72 genes significantly co-expressed with ACE2 either with positive or negative Pearson correlation. Of the transmembrane serine proteases, the most significantly coexpressed with ACE2 was TMPRSS4, suggesting it to be a putative druggable target. 
Building on the knowledge about ACE2 as receptor of the SARS-CoV-2 virus we set out to indentify genes with highly correlated expression with ACE2 with the aim of elucidating the molecular mechanisms underlying COVID-19. Figure 2 shows the genes most significantly (Bonferoni-adjusted p < 1E-11) correlated (red to yellow in last column) or anti-correlated (green) with ACE2 (full table in suppl. Table 3 ). The transmembrane serine protease 4 (TMPRSS4) is one of the most significantly correlated (r=0.9142, p=4.59E-20) with ACE2 therefore implying a major role in priming the CoV-2 spike protein. CXCL17 (r=0.9273, p=1.1E-21), ABCA12 (r=0.9256, p=1-92E-21) and ATP10B (r=0.9193, p=1.14E-20) had marginally higher correlation with ACE2 while another transmembrane protease TMPRS11E (r=0.9121, p=7.91E-20) had a slightly lower correlation. The expression of CXCL17 is probably due to a reaction on the infection by chemo-attracting macrophages 13 . A The role of ATP binding cassette subfamily A member 12 (ABCA12) is not fully elucidated with respect to COVID-19 but assumed to transport lipid via lipid granules to the intracellular space and transporting specific proteases -in the case of harlequin ichtyosis associated with desquamation 14 . Current knowledge on ATP10B is scarce. However, Wilk et al. (Table 3 in their publication) report Atp10b gene expression levels as highly inversely (negative) correlated with influenza gene expression changes in infected C57BL/6J mice 15 . In Figure 3a a cluster analysis and gene expression heatmap of the 72 most significantly (Bonferoni-adjusted p < 1E-11) correlated and anti-correlated genes with ACE2 shows close clustering of the serine protease TMPRSS4 with ACE2. Also in this analysis of 72 genes SARS-CoV-2 (red color bar) cluster together with mock-infected SARS lung epithelial cells separated from the other lung cells (blue color bar indicates control). In the heatmap presented in Figure 3b , TMPRSS family members TMPRSS11D/E and TMPRSS4 show close clustering with ACE2 but also TMPRSS2 and TMPRSS13 have similar expression in all experiments, especially in the SARS-CoV-2 infected samples.
In order to investigate the functionality of genes interacting with ACE2 we filtered genes correlated with ACE2 by Bonferoni-adjusted p-value < 0.05 and Pearson correlation coefficient > 0.6. 1891 genes fulfilled these criteria and were subjected to over-representation analysis of KEGG pathways 16 
Analogously to the positively correlated genes we also examined the negatively correlated genes with ACE2 by filtering for Bonferoni-adjusted p-value < 0.05 and Pearson correlation coefficient < -0.6. 1993 genes passed these filtering criteria and were subjected to over-representation analysis of KEGG pathways 15 . The most significantly over-represented pathways in the 1993 genes negatively correlated with ACE2 (Figure 4b , suppl. 17 . This would be a defense against the attempts of the virus to recruit the host's DNA repair and homologous recombination mechanisms 18 .
We furthermore assessed the GOs over-represented in the 1891 genes positively and the 1993 genes negatively correlated with ACE2. Table 2 shows a selection of significant GOs from all three categories Biological Process (BP, Figure 5a ), Cellular Component (CC) and Molecular Function (MF), suppl. 19 . In the GO-CCs, the terms intracellular and membrane-bounded organelle are most significant. In the GO-MFs, the terms metal ion binding and protein binding emerge as most significant due probably reflecting the binding of the virus proteins to the host cells. For the full gene lists associated with these terms refer to suppl. Tables 5 and 6.
Immune system associated GOs co-regulated with ACE2 Table 3 and Figure 5b show GOs (all Biological Processes) related to the immune system over- p=0.0079). CXCL17 was the gene with the highest correlation with ACE2 and is involved in immune system process -as described above by chemo-attracting macrophages 13 . Among the most significantly ACE2-correlated genes was IFI16 which is associated with several immune system GOs and known as regulator of the interferon response to viruses 20 and will be described in more detail in the next section about Protein interaction networks. Also in the protein interaction network of the most significantly ACE2-correlated genes was the interleukin 20 receptor B (IL20RB) which appeared in several GOs listed in Table 3 . With respect to viruses, there is meagre knowledge on IL20RB, however a study reported over-expression in the pneumonia causing avian influenza A H7N9 virus 21 . Table 4 and Figure 5c show GOs (all Biological Processes) related to inflammation and ROS overrepresented in genes correlated with ACE2. The GO, positive regulation of inflammatory response (p=0.0039) would imply that an inflammatory process is induced which finally leads to apoptosis -as demonstrated by the GO inflammatory cell apoptotic process (p=0.042). The virus receptor ACE2 is involved in the positive regulation of reactive oxygen species metabolic process (p=0.0185). Induction of ROS by a respiratory virus and subsequent inflammation has been reported Jamaluddin et al. 22 .
We further restricted the set of ACE2-correlated or -anti-correlated genes by drastically filtering with Bonferoni-adjusted p < 1E-11 in order to construct a human readable protein interaction network of the most significant proteins ( is part of it and among the most significantly correlated genes to ACE2. The connection of ACE2 to calmodulin 1 (CALM1) is based on a publication by Lambert et al. 25 in which they show that CALM1 interacts with the corona virus receptor ACE2 and inhibits shedding of its ectodomain 25 . CALM1 inhibitors in turn can revert this process so that the ACE2 ectodomain is shed partially mediated by a metalloproteinase 25 . The direct connection between CALM1 and LIMA1 was found in a large-scale interactome study 26 . The involvement of the hub gene Fanconi anemia complementation group C (FANCC), although not experimentally proven, might reflect recruitment of DNA repair and homologous recombination mechanisms from the host by the virus.
A pivotal result of this meta-analysis is the transmembrane serine protease TMPRSS4 which was one of the most significantly correlated genes with ACE2. Additionally TMPRSS11E (r=0.9121, Bonferonicorrected p=1.09E-15) and TMPRS11D (r=0.9018, Bonferoni-corrected p=1.32E-14) from the same gene family were found more significant than TMPRSS2 which however was still significantly correlated with ACE2 (r=0.767, Bonferoni-corrected p=1.79E-6). The assignment of the SARS-CoV-2 spike protein priming functionality to TMPRSS2 was based on the assumption that it would be the same as for its predecessor SARS-CoV 11 . However, this assumption must not necessarily hold true.
See the heatmap in Figure 3b which illustrates expression of transmembrane serine proteases. The highly significant co-expression of TMPRSS4 with ACE2 and its relevant role as transmembrane serine protease has enabled us to hypothesize that TMPRSS4 might also be involved in priming the SARS-CoV-2 spike protein. We therefore anticipate that inhibitors of TMPRSS4, TMPRS11D and TMPRS11E -besides those for TMPRSS2 -could be a promising subject of further research.
In this meta-analysis we compared RNA-seq data of lung cells infected with SARS-CoV-2 and other lung cells with particular focus on correlated gene expression with the known SARS-CoV-2 receptor gene ACE2. We found a signature of genes positively or negatively correlated with ACE2 amongst which the most outstanding was the transmembrane serine protease TMPRSS4. In a recent publication Hoffmann et al 11 . inferred from the knowledge that the preceding virus SARS-CoV uses ACE2 as receptor for entry and the serine protease TMPRSS2 for spike protein priming that the new virus SARS-CoV-2 11 would do the same. While the involvement of the receptor ACE2 appears to be established 1,10 the use of TMPRSS2 for spike protein priming appears not fully settled as Hoffmann et al. still have to concede "that SARS-CoV-2 infection of Calu-3 cells was inhibited but not abrogated by camostat mesylate" (serine protease inhibitor with activity against TMPRSS2) 11 . The high significance (r=0.9142, p=4.59E-20) in our co-expression analysis with ACE2 suggests that TMPRSS4 is a considerable candidate for spike protein priming. However, TMPRSS4 is closely related to TMPRSS2 which both can proteolytically cleave the hemagglutinin of influenza viruses 27 working best together with cathepsin B/L inhibitors. We propose to investigate the effect of TMPRSS4 inhibitors in further research. As TMPRSS4 has been implicated in the invasion and metastasis of several cancers it has been also considered as target for cancer therapy for which a modest inhibitory effect of the above mentioned inhibitors in TMPRSS4-overexpressing SW480 cells was reported 30 .
Interestingly, also TMPRSS2 is connected to epithelial carcinogenesis as consistently overexpressed in prostate cancer 31 , and later a gene fusion of TMPRSS2 and ERG was reported as the predominant molecular subtype of prostate cancer 32 , where TMPRSS2 however only contributes untranslated sequence 33 . Assuming that co-expression with ACE2 is an indication that TMPRSS4 may prime the SARS-CoV2 spike protein we suggest that further development and testing of more effective TMPRSS4 inhibitors in in vitro and in vivo models could support the translation into clinical settings.
However, we have to state the limitation that this study is a meta-analysis based solely on transcriptome and not proteome data.
Besides the identification of TMPRSS4, we found several significantly over-represented GOs and 34 . TMPRSS2 has been described as a mediator of ACE2-coupled endocytosis in the first SARS-CoV 35 and by previous publication also for SARS-CoV2 11 . However, we identified high levels of co-expression between ACE2 and TMPRSS4 and other members of the TMPRSS family and hypothesize that any of these additional family members might have the same function as the well described TMPRSS2. As a consequence, we propose that besides TMPRSS2 also other TMPRSS family members could be targets of pharmaceutical intervention warranting further research. After entering the cell, SARS-CoV-2 RNA is released, replicated and packaged again. Drugs can target the viral protease and the polymerase needed for replication 36 . Replication can further be inhibited by interferon and interferon-stimulated genes (ISG) 17 which we also found evidence for in negatively correlated replication pathways (e.g. DNA replication and homologous recombination). This depends on a healthy immune response and may be impaired in individuals with a weak immune system due to age or underlying diseases. The virus is packaged and released into the extracellular space where it can be attacked by macrophages chemo-attracted by CXCL17 13 . Also T-Cells can be involved in the immune response. We found evidence for their activation in GO analysis by associated interleukins IL1 and IL7. It is tempting to speculate that the severity of the clinical manifestations such as the acute respiratory failure and also failure in other organs depend on the state of the immune system which decreases with age or diseases such as diabetes. The involvement of ACE2 in the renin-angiotensin system as antagonist of ACE in regulating blood pressure via Angiotensin II, vasoconstriction, dilation and its protective role against lung injury 37 are additional factors which correlate with age. This is confirmed by reports from Wadmann et al. 38 about Centers for Disease Control and Prevention (CDC) data from 14 U.S. states that 50% hospitalized COVID-19 patients had pre-existing high blood pressure 38 . In their study about ACE2 in the preceeding SARS-CoV virus Imai et al. 37 found that ACE2 protects against lung injury and acid-induced lung injury in a Ace2-knockout mouse can be improved by an inhibitor of the Angiotensin II receptor AT1 37 We conclude, that our meta-analysis of RNA-Seq data of lung cells partially infected with SARS-CoV-2 identified the transmembrane serine protease TMPRSS4 as one of the most significantly correlated genes with the virus receptor ACE2. We propose that inhibitors of TMPRSS family members TMPRSS4, TMPRSS11D and TMPRSS11E besides TMPRSS2 are worthwhile testing in vitro and if that turns out to be successful and toxicity can be excluded potentially for in vivo studies. As clinicians, pathologists and scientists are struggling to get to grips with and understanding of the damage wrought by SARS-CoV-2 as it invades the body, we hope that our analyses and dataset will contribute to a better understanding of the molecular basis of COVID-19.
Next-generation sequencing datasets measured in RNA-Seq experiments with lung cells (GSE147507:
Illumina NextSeq 500; GSE146482: Illumina NovaSeq 6000; GSE85121: Illumina HiSeq 2500) were downloaded from NCBI GEO ( Table 1 , suppl. Table 1 ). These datasets were provided along with studies by Blanco-Melo et al. 12 (GSE147507) and Staudt et al. 41 (GSE85121). A final publication related to the dataset GSE146482 is yet to materialize. From accession no. GSE85121 only small airway epithelium brushing cells were used but alveolar macrophages were excluded while from accession no. GSE147507 only human epithelial and adenocarcinoma lung cells were used but ferret cells and updates after March 24 were excluded and from accession no. GSE146482 only control epithelial cells were used but graphene oxide treated cells were excluded. After exclusion of the above mentioned datasets not fitting the target cell type, 49 samples remained useful.
After the excluded samples had been filtered from the downloaded RNA-Seq data data was imported into the R/Bioconductor environment 42 . Read counts from accesion nos. GSE147507 and GSE146482 were converted to FPKM (fragments per kilobase of exon model per million reads mapped) using trancript lengths downloaded from ENSEMBL (version GRCh38, p13). Batch effects were removed with the package sva 43 employing method ComBat 44 . Normalization was performed via the voom method 45 . Pearson correlation coefficients between samples were calculated with the R-builtin method cor. Dendrograms were drawn employing the dendextend package 46 filtering genes for high coefficient of variation above the 75% quantile.
The Pearson correlation of the normalized gene expression values for all samples was calculated between the gene ACE2 and each other gene. The method cor.test was applied to calculate the p-value for the t-test for Pearson correlation. The p-value was Bonferoni-corrected by division by the number of genes and additionally adjusted via the Benjamini-Hochberg method 47 . Correlated genes were filtered with a very restrictive criterion (Bonferoni-adjusted-p < 1E-11) -in order to get human readable numbers of genes for heatmap and network generation-and conventional criteria (r > 0.6, Bonferoniadjuste-p < 0.05) for positively correlated genes and (r < 0.6, Bonferoni-adjusted-p < 0.05) for negatively correlated genes.
The R package GOstats 48 was employed for over-representation analysis of positively and negatively correlated genes with the CoV-2 receptor gene ACE2. KEGG pathway annotations which had been downloaded from the KEGG database 16 in March 2018 were used for testing overrepresentation of the positively and negatively ACE2-correlated genes via the R-builtin hypergeometric test.
Dot plots showing the p-value of the hypergeometric test, the ratio of significant genes to all genes in the pathway and the number of significant genes per pathway were plotted via the package ggplot2 49 .
A human protein interaction network was constructed in a similar manner as we described in our previous publication 50 . However, here only direct interactors and no further interactors of interactors were extracted from the Biogrid database version 3.4.161 51 using the restrictively filtered (Bonferoniadjusted-p < 1E-11) genes significantly correlated or anti-correlated with ACE2 gene expression. The network was reduced to the n=30 nodes with most interactions and was plotted via the R package network 52 showing original genes in green and BioGrid-derived interactors in red.
WW analysed the data and wrote the manuscript. JA supervised the work, co-wrote the manuscript and gave the final approval.
The authors declare no competing interests. 
2) as set Figure 7 : Scheme of SARS-CoV-2 infection. The coronavirus SARS-CoV-2 docks at the receptor ACE2 on the membrane of the human epithelial cell. Transmembrane serine proteases TMPRSSx mediate SARS-CoV-2 cell entry via ACE2. TMPRSS2 was reported for this in the first SARS-CoV a by previous publication also for SARS-CoV-2 but we hypothesize that due to co-expression with ACE2, TMPRSS4 and other members of the TMPRSS family may well perform this task. We sugges that inhibitors of TMPRSS4 and other TMPRSS family members might have therapeutic potential. Upon entry into the cell, viral RNA is released, replicated and packaged again. Replication can be inhibited by interferon and interferon stimulated genes (ISG) what we also saw in negatively correlat replication pathways (e.g. DNA replication and homologous recombination). This indicates a healthy immune response and may be impaired in persons with a weak immune system due to age or disease The packaged virus is released from the cell and can be attacked by macrophages chemo-attracted by CXCL17 -or T-cells for which we found evidence for by GO analysis and by associated interleukins IL1 and IL7. It is tempting to speculate that the severity of the clinical manifestations such as the acu respiratory failure and also failure in other organs depends on the quality of the immune system decreasing with age or diseases such as diabetes. The involvement of ACE2 in the renin-angiotensin system as antagonist of ACE in regulating blood pressure via Angiotensin II (Ang-II), vasoconstrictio dilation and its protective role against lung injury are additional factors which correlate with age. and gest lated thy se. by ins cute in ction,