key: cord-0785217-0jpuaug0 authors: Yildirim, Zafer; Sahin, Oyku Semahat; Yazar, Seyhan; Bozok Cetintas, Vildan title: Genetic and epigenetic factors associated with increased severity of Covid‐19 date: 2021-03-01 journal: Cell Biol Int DOI: 10.1002/cbin.11572 sha: 7c08c546853c3cfc2e6a5db493f1dc6f7fd6dc2e doc_id: 785217 cord_uid: 0jpuaug0 Since December 2019, a new form of severe acute respiratory syndrome (SARS) from a novel strain of coronavirus (SARS coronavirus 2 [SARS‐CoV‐2]) has been spreading worldwide. The disease caused by SARS‐CoV‐2 was named Covid‐19 and declared as a pandemic by the World Health Organization in March 2020. Clinical symptoms of Covid‐19 range from common cold to more severe disease defined as pneumonia, hypoxia, and severe respiratory distress. In the next stage, disease can become more critical with respiratory failure, sepsis, septic shock, and/or multiorgan failure. Outcomes of Covid‐19 indicate large gaps between the male–female and the young–elder groups. Several theories have been proposed to explain variations, such as gender, age, comorbidity, and genetic factors. It is likely that mixture of genetic and nongenetic factors interplays between virus and host genetics and determines the severity of disease outcome. In this review, we aimed to summarize current literature in terms of potential host genetic and epigenetic factors that associated with increased severity of Covid‐19. Several studies indicated that the genetic variants of the SARS‐CoV‐2 entry mechanism‐related (angiotensin‐converting enzymes, transmembrane serine protease‐2, furin) and host innate immune response‐related genes (interferons [IFNs], interleukins, toll‐like receptors), and human leukocyte antigen, ABO, 3p21.31, and 9q34.2 loci are critical host determinants related to Covid‐19 severity. Epigenetic mechanisms also affect Covid‐19 outcomes by regulating IFN signaling, angiotensin‐converting enzyme‐2, and immunity‐related genes that particularly escape from X chromosome inactivation. Enhanced understanding of host genetic and epigenetic factors and viral interactions of SARS‐CoV‐2 is critical for improved prognostic tools and innovative therapeutics. According to the World Health Organization (WHO) guideline, Covid-19 severity is classified into four groups: mild, moderate, severe, and critical disease (Zu et al., 2020) . Most of the infected patients experience mild or moderate disease and do not need medical assistance. However, approximately 15% of cases develop severe disease and 5% progress to critical status and require intensive care unit, which results in a major burden on the healthcare systems (Bulut & Kato, 2020; Epidemiology Working Group for NCIP Epidemic Response., 2020). Surprisingly, some patients are asymptomatic and never develop symptoms of Covid-19, even though infected with severe acute respiratory syndrome (SARS)-CoV-2 (SARS-CoV-2; Lavezzo et al., 2020) . Asymptomatic patients play a significant role in the rapid spread of the Covid-19 pandemic, as asymptomatic infected individuals may also become the infectious source of SARS-CoV-2. The prevalence of asymptomatic cases is approximately 40%-45% (Oran & Topol, 2020) . Some progress rapidly to critical disease (Meng et al., 2020) . Age, gender, and comorbidity are found to be associated with poor clinical outcomes. Patients older than 75 years old have more severe disease and a higher risk of death Feng et al., 2020; Niederman et al., 2020; . Gender also plays a pivotal role in the severity of Covid-19 outcomes. Despite the infection rates being similar for both males and females, the average case fatality rate is 1.7 times higher in males and contributes to male-biased death (Scully et al., 2020) . Proposed hypotheses to explain this gender differences include lifestyle choices, social factors, and use of steroid hormones (Gagliardi et al., 2020) . The percentage of comorbidities, especially diabetes and hypertension, are higher in the severe and critical cases ). Yet, these risk factors do not explain the severity of Covid-19, especially in healthy young cases. Evidence from previous human viral infections shows that host genotype determines predisposition or response to infection. For example, C-C chemokine receptor gene (CCR5) mutations are associated with resistance to HIV-1 (Quillent et al., 1998) , some human leukocyte antigen (HLA) alleles have modulator effects on the viral load, especially for EBV, HIV, and hepatitis C (Agostini et al., 2018; Julg et al., 2011) , and deficiencies of antiviral interferons (IFNs) cause severe viral disease (Lim et al., 2019) . It was suggested that common variants in multiple loci, moderately rare variants, and gene environment interactions form the basis of genetic predisposition to Covid-19 (Darbeheshti & Rezaei, 2020) . In this review, we described the potential host genetic and epigenetic factors that associated with increased severity of Covid-19. Understanding these factors is important to improve genotype-based individual therapies and vaccines, especially for high-risk groups for Covid-19. Angiotensin I-converting enzyme (ACE) and angiotensin-converting enzyme-2 (ACE2) are homolog genes that regulate physiological homeostasis of renin-angiotensin system (RAS) (Figure 1 ). SARS-CoV-2 uses the ACE2 receptors to invade the target cells (Hoffmann et al., 2020b) . ACE and ACE2 regulate each other's expression levels in several ways (Ghafouri-Fard et al., 2020) . Therefore, investigation of genetic variants of these genes can make us understand why Covid-19 is more severe in some individuals. A common polymorphism of the ACE gene is characterized by insertion (I) or deletion (D) of a 287-bp Alu repeat and DD allele is associated with higher ACE levels (Gemmati et al., 2020) . The importance of the ACE/ACE2 balance for Covid-19 progression and therapy has been widely discussed (Gemmati et al., 2020; Sriram & Insel, 2020; Tseng et al., 2020; Zamai, 2020) . ACE and ACE2 regulate the RAS system to balance the local vasoconstrictor/proliferative and vasodilator/antiproliferative actions (Gemmati et al., 2020) . Dysregulation of ACE/ACE2 balance might increase tissue damage, proliferation, fibrosis, thrombosis, or inflammation. Therefore, it was speculated that in addition to ACE2, ACE genotype may affect the clinical outcomes of the Covid-19 (Delanghe et al., 2020b; Zheng & Cao, 2020) . Delanghe et al. (2020b) found that the prevalence of infections was inversely correlated with the D allele frequency in 25 European countries. They obtained similar findings for 33 countries in Europe, North Africa, and the Middle East, reporting that increasing D allele frequency correlated to a reduction in prevalence but an increase in mortality from Covid-19 (Delanghe et al., 2020a) . Contrary to these results, increased frequency of the I/I genotype was inversely correlated with susceptibility to SARS-CoV-2 infection and consequent mortality in European and Asian countries (Yamamoto et al., 2020) . Similarly, an ecological study reported that increased I/I genotype frequency was significantly associated with decreased Covid-19 mortality in 25 countries from different geographical regions of the world (Aung et al., 2020) . The frequency of D allele was significantly associated with the number of SARS-CoV-2-infected cases/million in the Asian populations (Pati et al., 2020) . Mortality rates and D allele also showed a significant positive correlation in the Asian populations, indicating higher levels of ACE are deleterious in Covid-19 patients (Pati et al., 2020) . European population has a higher incidence of SARS-CoV-2 infection and higher mortality rates (Yamamoto et al., 2020) . As ACE DD genotype frequency is higher in the European population than Asian, it is likely that higher mortality rates in European population might be explained by ethnic differences in allele frequency of ACE I/D polymorphism. 2.2 | Angiotensin-converting enzyme-2 ACE2 is the master regulator of the RAS and responsible for cleaving Angiotensin I and II into peptides Angiotensin 1-9 and Angiotensin 1-7, respectively. Both peptides are key elements involved in 2 | C Cell ell B Biology iology I International nternational regulation of cardiovascular physiology (Donoghue et al., 2000; Gheblawi et al., 2020) . SARS-Cov-2 infection downregulates ACE2 expression and disrupts homeostatic and protective functions of ACE2, which can result in inflammation (Verdecchia et al., 2020) . ACE2 is expressed in most human tissues at different levels (Bibiana et al., 2020; . As SARS-CoV-2 uses the ACE2 receptor to attack the alveolar epithelial cells, it is speculated that ACE2 expression level in different tissues could uncover the potential genetic susceptibility to Covid-19 infection Zou et al., 2020) . Single-cell RNA sequencing of lung tissue cell types showed that subsegmental bronchial branches include ACE2-expressing transient secretory cell population, and interestingly, 40% of these cells are co-expressing the transmembrane serine protease-2 (TMPRSS2), which is involved in S protein priming and making these cells potentially vulnerable for SARS-CoV-2 infection (Lukassen et al., 2020) . F I G U R E 1 Genetic variants and epigenetic factors associated with severe Covid-19. SARS-CoV-2 uses ACE2, TMPRSS2, and furin for entry to cytoplasm. The innate immune response signaling cascade starts with the recognition of pathogen-associated molecular patterns (PAMPs) by endosomal toll-like receptors (TLRs) and the others. Sensing of the pathogen-derived molecules triggers the cell signaling cascades to the induction of IFNs and other proinflammatory cytokines. IFN signaling induces a large set of IFN-stimulated genes (ISGs). HLA, 3p21.31, and 9q34.2 loci are significantly associated with Covid-19 severity. Epigenetic mechanisms including methylation, histon acethylation, and X chromosome inactivation (XCI) also affect Covid-19 outcomes by regulating IFN signaling, ACE2 expression, and immunity-related genes that particularly escape from XCI. ACE2, angiotensin-converting enzyme-2; HLA, human leukocyte antigen; IFN, interferon; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; TMPRSS2, transmembrane serine protease-2 YILDIRIM ET AL. | 3 The ACE2 gene includes key regulatory elements for chromatin modification and transcription factors, which provides epigenetic and hormonal regulation of expression levels. Association of the ACE2 expression with age, gender, ethnic groups, and body mass index was reported in many tissues . The association with age was the strongest and higher in Asian females, followed by sex, and then ethnic groups. Age-dependent ACE2 gene expression in nasal epithelium was investigated in a cohort of 305 individuals, and it was found that younger children (aged <10 years) have the lowest ACE2 levels (Bunyavanich et al., 2020) . Therefore, it was suggested that the lower risk among children might due to age-dependent lower expression of ACE2 (Bunyavanich et al., 2020 ACE2 genotype is associated with the protein structure and binding affinity, serum concentration, and circulating angiotensin levels (Chen et al., 2018 . Investigation of the host entry machinery genes (ACE2, CtsB, CtsL, and TMPRSS2) showed that the frequencies of the rare variants are significantly different among the populations and might potentially be decisive in SARS-CoV-2 entry (Darbani, 2020) . Furthermore, Darbani (2020) Among the natural ACE2 coding variants, 17 were found at the important S protein-binding positions (Hussain et al., 2020) . Whereas most variants showed an identical binding affinity, rs143936283 (E329G) and rs73635825 (S19P) alleles showed noticeable variations in their intermolecular interactions. Therefore, it was speculated that rs143936283 and rs73635825 alleles may confer resistance against the SARS-CoV-2 attachment with the human ACE2 receptor (Hussain et al., 2020) . Molecular dynamic simulations demonstrated that K26R and I468V variants may affect binding characteristics to S protein through increasing the binding free energy and decreasing the binding affinity . K26R variant is mutated more frequently in Non-Finnish Europeans, whereas I468V is mutated in East Asians . Potential variants that impact protein stability were identified from whole-exome sequencing (WES) data of 6930 Italian control individuals (Benetti et al., 2020 . In silico analyses for ACE2-S1 protein-binding dynamics also revealed some conflicting results (Benetti et al., 2020; Darbani, 2020; Hou et al., 2020; Hussain et al., 2020; Mohammad et al., 2020) . Further functional studies and genotype analyses of larger severe/critical patients would shed light on the role of ACE2 genotypes in Covid-19. Cell entry mechanism of SARS-CoV-2 involves a cell surface receptor to bind and spike protein priming by cellular proteases, which facilitates membrane fusion. TMPRSS2 was the first protease that was associated with the S protein priming. Subsequent studies have shown that different proteases may also play a role in the entry mechanisms of SARS-CoV-2. Hoffmann et al. (2020b) studied the TMPRSS2 and endosomal cysteine proteases cathepsin B and L (CatB/L). TMPRSS2 and CatB/L prime S protein; however, CatB/L activity is dispensable for viral spread and pathogenesis. On the contrary, Ou et al. (2020) reported that cathepsin L is critical for SARS-CoV-2 entry. Furin is also reported as another protease that cleaves the spike protein at the S1/S2 site, which is essential for S protein-mediated cell-cell fusion and entry into human lung cells (Hoffmann et al., 2020a) . 3.1 | Transmembrane serine protease-2 TMPRSS2 gene is localized on chromosome 21 and encodes a serine protease enzyme that is involved in cleavage and activation of the SARS-CoV-2 spike protein during membrane fusion (Hoffmann et al., 2020b) . As TMPRSS2 is an androgen-responsive serine protease, it was hypothesized that males might have higher TMPRSS2 expression in the lung, which contributes to pathogenesis of SARS-CoV-2 and explains the higher mortality in males . However, the overall expression of TMPRSS2 did not show a remarkable increase in males . TMPRSS2 gene variations have been associated with several diseases including viral infections. Therefore, it was suggested that genotyping studies of Covid-19 patients could aid in understanding the impact of TMPRSS2 variations on clinical outcomes (Stopsack et al., 2020; Strope et al., 2020) . In silico analyses showed that 21 single-nucleotide polymorphisms can affect the function and structure of TMPRSS2 by influencing the protein folding, posttranslational modifications, splicing, and microRNA (miRNA) function (Paniri et al., 2020) . Among these, rs12329760 and rs875393 may create de novo a pocket protein or a donor site, silencer, and broken enhancer motifs, whereas rs12627374 can affect a wide spectrum of miRNAs profile (Paniri et al., 2020) . Regulatory variants (rs713400, rs11910678, rs77675406, and rs112657409) of TMPRSS2 were found to influence the expression level that could be involved in the SARS-CoV-2 infection (Senapati et al., 2020) . The rare variants and polymorphisms of TMPRSS2 were compared between Europeans and East Asians using an exome and SNP array data from a large Italian cohort . Exonic variant p.Val160Met (rs12329760) and distinct haplotypes (rs8134378, rs2070788, rs9974589, rs7364083), predicted to induce higher levels of TMPRSS2, were more frequent in the Italian than in the East Asians. It was suggested that these variants might explain the sex bias in Covid-19 severity and higher mortality rates in Italy Irham et al. (2020) reported that the frequencies of TMPRSS2upregulating variants (rs2070788, rs464397, rs469390, and Low-producing or high-producing IL-6 polymorphisms were related clearance of viral infection, sustained virologic response, and attenuated adaptive immune response (Barrett et al., 2001; Bogdanović et al., 2016; Nattermann et al., 2007) . A recent meta-analysis showed that mean IL-6 concentrations were dramatically increased in patients with cytokine release syndrome, sepsis, and acute respiratory distress syndrome unrelated to Covid-19 (Leisman et al., 2020) . However, the association of IL-6 polymorphisms with pathogenesis and severity of Covid-19 has not been elucidated. Kirtipal and Bharadwaj (2020) TLRs are a family of innate immune receptor molecules known as pattern recognition receptors (PRRs) (Takeda & Akira, 2015) . The HLA system consists of a group of proteins that play a crucial role in the antigen presentation and genetic diversity. HLA is the most polymorphic genetic locus in humans that enables the immune system to cope with all the different pathogens that will be encountered. The HLA region consists of more than 240 genes and they are divided into three classes, namely I, II, and III (Dendrou et al., 2018) . HLA Classes I and II molecules present antigenic peptides to T cells and enable the immune system to discriminate between self and nonself. If HLA peptides have increased binding specificities to the SARS-CoV-2 peptides, it would provide better T cell response to virus infection. Therefore, few studies have been carried out to find out the relationship between HLA genotype and disease severity. The frequencies of the HLA-C*07:29 and B*15:27 alleles were significantly higher in Covid-19 patients (W. . Nguyen et al. (2020) hypothesized that HLA genotypes may differentially induce the T-cell-mediated antiviral response and could potentially alter the severity of disease. In-silico analysis revealed potential alleles that could be associated with more severe infection. In particular, HLA-B*46:01 allele has the fewest predicted binding peptides for SARS-CoV-2. Therefore, HLA-B*46:01 allele is predicted to present the fewest SARS-CoV peptides, which is consistent with Lin's study (Lin et al., 2003) . However, HLA-B*15:03 allele has the greatest capacity to present highly conserved SARS-CoV-2 peptides, so HLA-B*15:03 carriers may enable cross- protective T-cell-based immunity SLC6A20 expression was found in the lung pneumocytes (Uhlen et al., 2015) . SLC6A20 functionally interacts with ACE2 receptor, which might explain its relationship with SARS-CoV-2 (Vuille-dit-Bille et al., 2015). To clear the significance of these loci in Covid-19 disease, Katz et al. (2020) utilized proteomic profiling and genetic information from three cohorts to identify proteins influenced by these loci. C-X-C motif chemokine (CXCL)-16 and teratocarcinoma growth factor-1 proteins were found to have a quantitative trait in the 3p21.31 locus. CXCL16 and CXCR6 are chemokine and receptor pair whose genes are within the 3p21.31 locus. CXCL16 and CXCR6 are associated with localization of tissue-resident memory CD8 T cells to the airways and LPS-induced acute lung injury (Tu et al., 2019; Wein et al., 2019) , which are therefore suggested as potential Covid-19 mediators (Katz et al., 2020) . The association signal at locus 9q34.2 coincided with the ABO locus. Comparing with the non-A blood carriers, group A carriers are associated with the higher risk for acquiring Sars-CoV-2 infection. Likewise, infection risk is lower in O carriers, compared with the non-O blood carriers Zhao et al., 2020) . A moderately increased infection rate was reported among non-O blood types and Rh-positive individuals (Zietz & Tatonetti, 2020) . Also, intubation risk was found to be increased in AB and B groups and decreased in A and Rh-negative groups (Zietz & Tatonetti, 2020) . Two genome-wide association studies of severe Covid-19 re- ported that blood group A shows higher risk than other blood groups, whereas O group displays a protective effect as compared with other blood groups Severe Covid-19 GWAS Group et al., 2020) . It is still uncertain how blood types influence the outcome of Covid-19. Proteomic profiling analysis of ABO locus showed that variants in the ABO locus are associated with levels of CD209/DC-SIGN (Dendritic cell-specific intercellular adhesion molecule-3-Grabbing Non-integrin) (Katz et al., 2020) , which is a binding protein for SARS-CoV (Yang et al., 2004) . It was suggested that ABO locus may confer its risk by modulating DC-SIGN, which is a binding site for SARS-CoV-2. Another hypothesis is that ABO blood group influences the glycosyltransferase activity and risk of venous thromboembolism, which is frequent in severe Covid-19 cases, and can lead to severe outcomes (Ibrahim-Kosta et al., 2020; Porfidia et al., 2020) . Apolipoprotein E (APOE) is a polymorphic gene locus and ε2/ε3/ε4 alleles code for three major isoforms in plasma. ApoE ε4 (ApoE4) allele is associated with atherosclerosis, cardiovascular disease, shortened longevity, and Alzheimer's disease (Smith, 2000) . H. showed that APOE enhances the entry and infectivity of the SARS-CoV-2. They suggested that SARS-CoV-2 entry sites are cholesterol-dependent, and furin priming of SARS-CoV-2 is also sensitive to cholesterol and likely protects against severe Covid-19 symptoms in low cholesterol (H. . It was hypothesized that the APOE4 genotype may predict the severity of Covid-19 (Finch & Kulminski, 2020; Goldstein et al., 2020; Monteiro-Junior, 2020) . Kuo et al. (2020a) found that individuals homozygous for APOE4 were more likely to be positive for Covid-19, and thus severe disease. The authors published a second report that shows genome-wide significance for the association of APOE4 genotype with Covid-19 positivity and mortality (Kuo et al., 2020b) . It had been suggested that APOE modulates Covid-19 response by regulating proinflammatory pathways in a genotype-dependent manner (Kasparian et al., 2020) . Apolipoprotein 1 (APOL1) G1 and G2 variants were associated with the excess risk of nondiabetic kidney disease in African Americans (Wasser et al., 2012) . G1 variant codes for two amino acid substitutions that nearly always occur together, S342G and I384M (rs73885319 and rs60910145], whereas G2 (rs71785313) is a sixbase pair deletion leading to loss of amino acid residues 388N and 389Y. Case studies have reported three patients with acute kidney injury after Covid-19 infection (Kissling et al., 2020; Larsen et al., 2020; Peleg et al., 2020) . These cases were homozygous for APOL1 G1 variant (A342G and I348M) and showed upregulated chemokine expressions . Therefore, collapsing glomerulopathy in Covid-19 patients was associated with high-risk APOL1 variants . could be involved in SARS-CoV-2 internalization (Senapati et al., 2020) . DPP9 locus was also associated with severe Covid-19 in a GWAS (Pairo-Castineira et al., 2020) and the COVID-19 HGI report (covid19hg.org, October 2020). Klaassen et al. (2020) responses by polycomb repressive complex 2-mediated H3K27me3 at the promoter region of certain IFN-stimulated genes (Menachery et al., 2014) . It is not yet known whether SARS-CoV-2 uses the same epigenetic mechanisms, but potential epigenetically regulated factors have been reported, and it would be worth exploring these to find out successful treatment strategies as hypothesized by Ayaz and Crea (2020) . Trained immunity consists of the innate cell populations with "epigenetic scar," which is a pattern of exposed enhancers and promoters of host-defence genes (Mantovani & Netea, 2020) . The effects of trained immunity on the protection and progression of the Covid-19 have been considered in terms of BCG vaccination (Gursel & Gursel, 2020; Netea et al., 2020; O'Neill & Netea, 2020) . Arts et al. Single-cell RNA sequencing data of the alveolar type II cells was analyzed to find out age-related expression differences and mechanistic connection of Covid-19 severity outcomes (Abouhashem et al., 2020) . Superoxide dismutase 3 (SOD3, EC-SOD3) was identified as the top-ranked gene that was most downregulated in the elderly. The potential role of epigenetic mechanisms including histone deacetylation at the gene promoter indicated age-dependent epigenetic downregulation of SOD3 (Roman et al., 2017) . Therefore, different epigenetic patterns in elderly patients might predispose them to severe Covid-19. Epigenetic-targeted therapies would provide more efficient therapies for Covid-19, as El Baba and Herbein (2020) discussed. ACE2 locus contains epigenetic regulation marks of histone acetylation and methylation . Histone methyltransferase enzyme EZH2 inhibits ACE2 expression via H3K27 trimethylation in human embryonic stem cells . ACE2 DNA methylation profiles of human lung tissues were highly variable in both men and women. In addition, females were significantly hypomethylated as compared with males (Corley & Ndhlovu, 2020) . Pinto et al. (2020) showed that ACE2 was highly expressed in critical Covid-19 cases. Pathway enrichment and ChIP-seq analyses revealed that several of the genes positively associated with ACE2 were regulated by KDM5B, specific histone acetylation (H3K27ac), and histone methylation (H3K4me1 and H3K4me3) dynamics . Thus, the authors suggested that ACE2 might be epigenetically upregulated in the lungs of severe Covid-19 patients . Another study confirmed that ACE2 expression is regulated by DNA methylation dynamics, which increases Covid-19 susceptibility and severity in lupus patients (Sawalha et al., 2020) . Whole-genome DNA methylation analysis showed significant hypomethylation in the ACE2 gene, which consequently increased ACE2 messenger RNA expression in lupus patients (Sawalha et al., 2020) . ACE2 nasal DNA methylation reflected differences by gender, ethnicity, and biological aging (Cardenas et al., 2020) . It was suggested that ACE2 hypomethylation in nasal epithelium among black males may cause increased SARS-CoV-2 infectivity and severity, depending on the increased ACE2 receptors (Cardenas et al., 2020) . It is important to discuss some points about ACE2 expression, whether it is harmful or beneficial for Covid-19 severity. ACE2 has multiple physiological roles in maintaining homeostasis in humans. As ACE2 is protective against organ damage, hypertension, diabetes, cardiovascular disease, and inflammation, a higher ACE2 expression could be associated with better prognosis, probably due to reduced hyperinflammation Devaux et al., 2020; El Baba & Herbein, 2020; Verdecchia et al., 2020) . According to this proposition, we cannot explain male-biased severity of SARS-CoV-2 with ACE2 expression, as females upregulate ACE2 using several mechanisms such as hypomethylation, escaping X chromosome inactivation (XCI) and estrogen-dependent signaling (Tukiainen et al., 2017; Foresta et al., 2020; Forsyth & Anguera, 2021 9.2 | X chromosome inactivation XCI is the best example of the epigenetic gene regulation in females to equalize expression of the X-linked genes between the sexes also known as dosage compensation. Xist RNA, repressive histone modifications, the histone variant macroH2a, and DNA methylation mechanisms participate in the inactive X (Xi) chromosome development process (Galupa & Heard, 2018) . However, 30% of X-linked genes escape XCI and are expressed from both X chromosomes C Cell ell B Biology iology I International nternational (Tukiainen et al., 2017 (Gabriele et al., 2020) . Hyper-responsiveness of the female immune system is due to XCI escaper immunity-related genes (Marquez et al., 2020) . As we indicated before, TLR7 receptor functions are very important in viral infections. Localization of TLR7 and -8 on the X chromosome give genetic advantage to female immune system, as escaping XCI provides altered expression levels (de Groot & Bontrop, 2020) . This might be a reason for TLR7 loss-of-ffunction variants being lethal in men (van der Made et al., 2020). Moreover, it was reported that female dendritic cells exhibit altered TLR7-mediated IFN-α production, depending on XCI and oestrogens (Laffont et al., 2014) . It is very interesting that ACE2 gene is located on the X chromosome and it is likely to escape inactivation (Foresta et al., 2020) . It was reported that ACE2 is a tissue-specific escape gene that showed moderate male-biased expression in lungs and higher male-biased expression in the small intestine (Tukiainen et al., 2017) . Considering that sex hormones also influence the regulation of ACE2, it is very difficult to evaluate the real effect of XCI on the severity of Covid-19 through ACE2 expression. Studies on the effect of ACE2 genotype rather than expression itself might be more useful to understand the male-biased mortality. ACE2 is the first contact point for the RBM domain of SARS-CoV-2, which carriers 16 residues to bind. SARS-CoV RBD residues are in contact with 20 residues of ACE2 protein (Lan et al., 2020) . As females are heterozygous for ACE2, they can assemble heterodimers in the presence of XCI. Theoretically, SARS-CoV-2 RBM domain cannot perfectly bind to all heterodimer ACE2 receptors, allowing unbound ACE2 to have protective functions in females . However, men are hemizygous and assemble only homodimers. So, they are lacking the organ protective functions of ACE2. We summarized studies involving analyses of the potential genetic and epigenetic factors that might be associated with severe out- (Yousefzadegan & Rezaei, 2020) . Another report from a large family cluster suggested a genetic predisposition due to apparent familial clustering of severity (Ikitimur et al., 2020) . These examples suggest that underlying genetic and epigenetic factors can lead to severe Covid-19 outcomes. Genetic variants of the SARS-CoV-2 entry mechanism-related genes, host immune response-related genes, and other potential genetic loci would explain inherited predispositions and mechanisms that underlying severe Covid-19. A global project has been started to discover the pathogenesis of unexplained, severe Covid-19 in young patients . In addition, "Covid-19 Host Genetics Initiative" was started to bring the human genetics community together to identify the genetic determinants of Covid-19 susceptibility, severity, and outcomes (Initiative, 2020) . In this review, we just focused on genetic and epigenetic factors, so all other factors such as age, comorbid disease, and lifestyle were excluded. To review all literature, we used some preprint papers that have not been peer-reviewed, which might be the other limitation of our study. The genetic landscape of an individual plays a pivotal role in the immune response and disease outcomes. 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