Disease or Genetic Predisposition
6.2 Genetic Variants Relating to Virus Binding and Entry
Receptors
6.2.1 Angiotensin-Converting Enzyme (ACE)
Angiotensin-converting enzyme 1 (ACE1) and 2 (ACE2) genes occur on chromosomes 17q and Xp, respectively. These homologous genes encode the essential enzymes that are part of the renin-angiotensin system with a 42% identical protein sequence. ACE1 regulates blood pres- sure and the balance of fluids and salts by target- ing angiotensin Ι. ACE2, located on the outer surface of cells, modulates the levels of angio- tensin II. Also, this protein has been established as a functional receptor for the spike glycopro- tein of SARS-CoV-1 and SARS-CoV-2 (Li et al.
2003).
6.2.1.1 ACE1
ACE1 is highly expressed on microvascular endothelium and is probably involved in angiop- athy of the lung. It has been assumed that ACE1
variants could be associated with the progression of SARS-CoV-1 (Itoyama et al. 2004). This gene has an insertion (I) or a deletion (D) polymor- phism in the 287-bp Alu repeat sequence in intron 16 (rs1799752). The D allele demonstrates an enhanced ACE1 expression and activity (Rigat et al. 1990).
The association of this polymorphism with pathological states such as myocardial infarction has been documented (Baudin 2002). Concerning SARS-CoV-1 infection, the D allele was statisti- cally associated with hypoxemic status in Vietnamese patients (Itoyama et al. 2004). In Chinese patients, individuals with the DD geno- type had higher mortality of acute lung injury (Lu et al. 2011). Moreover, a meta-analysis of ten related studies shows evidence supporting a causal association between D allele and pneumo- nia vulnerability, and that individuals with the I/I genotype have a high chance to survive from acute respiratory distress syndrome (Wang et al.
2015). However, based on another study, there is no significant association between the ACE1 I/D polymorphism and poor outcomes after SARS- coronavirus infection (Chan et al. 2005).
6.2.1.2 ACE2
The large-scale structural analyses of an atomic- level investigation have revealed a strong interac- tion between the receptor-binding domain (RBD) of both SARS-CoV-1 and SARS-CoV-2 spike protein with ACE2 as their host receptor, which is responsible for human-to-human transmission (Letko and Munster 2020). A high ACE2 expres- sion has been detected in different cell types, such as epithelial cells of the tongue, alveolar cells, enterocytes, a subpopulation of macro- phages, and small intestine (Xu et al. 2020). The spike protein binds to ACE2, and then a trans- membrane protease (TMP) cleaves the S1 and S2 subunits of the spike protein that is critical for the virus entry into target cells. Therefore, genetic variation in ACE2 and/or TMP that could affect its expression, protein conformation, and stabil- ity are potential genetic predisposing factors to COVID-19. The recent systematic analysis of coding-region variants in the ACE2 gene, as well as using the GTEx database, has revealed that
some SNPs are associated with a higher RNA expression of ACE2, which might cause different susceptibility to COVID-19 among patients (Cao et al. 2020).
Studies have investigated the significance of ACE2 variants in susceptibility and vulnerability of individuals to SARS-CoV-1. Generally, 30%
of SARS-CoV-1-infected patients required inten- sive care admission, and interestingly males infected were more likely to manifest severe symptoms (Booth et al. 2003). Moreover, the mortality rates have been estimated at 21.9% and 13.2% of male and female patients, respectively (Karlberg et al. 2004). This evidence points to the intriguing coincidence that the ACE2 gene is located on the X chromosome.
The assessment of variants in the protein- coding region of ACE2 in a large human cohort revealed some polymorphisms that either likely protect or predispose individuals to the virus (Stawiski et al. 2020). However, the importance of variants in the human ACE2 gene for suscepti- bility to both SARS-CoV-1 and SARS-CoV-2 has not been comprehensively examined. Chiu et al.
reported no associations between ACE2 common genetic variants and SARS-CoV-1 susceptibility or outcome (Chiu et al. 2004).
6.2.2 CD147
A transmembrane glycoprotein referred to as CD147/Basigin, which belongs to the immuno- globulin superfamily, is involved in virus infec- tion and tumor development. This protein is highly expressed in infected and inflamed tissues.
CD147 has a functional role in facilitating the SARS-CoV-1 invasion of the host cells.
Interestingly, it has been presumed that SARS nuclear (N) protein interacts with CD147 located on the membrane of the endoplasmic reticulum (ER), thereby facilitating the virus particles in attaching to and budding into the ER (Chen et al.
2005). Recently, it was documented that the SARS-CoV-2 spike protein also binds to CD147 and facilitates the viral invasion (Wang et al.
2020b). Therefore, both SARS-CoV-1 and
SARS-CoV-2 are expected to invade host cells via CD147.
There is no specific evidence for the role of CD147 polymorphisms in predisposition to coro- navirus infection. However, a functional poly- morphism of the CD147 gene has been reported in acute coronary syndrome (ACS) in several independent studies. The rs8259 AA genotype shows a significant association not only with the high expression of CD147 mRNA and protein but also with ACS patients (Yan et al. 2015). It pro- vided evidence for the possible role of rs8259 for the individual difference in the susceptibility to viral infection and the severity of the disease.
Although ACE2 has been widely known as a critical receptor involved in virus invasion, anti- ACE2 antibodies may lead to severe adverse events, since ACE2 is widely expressed in a vari- ety of tissues (Tipnis et al. 2000). On the other hand, Meplazumab, a humanized anti-CD147 antibody, has shown safety in preclinical research (Wang et al. 2020b). Therefore, it has been sug- gested that blocking CD147 might effectively inhibit the viruses from invading host cells.
6.2.3 Dipeptidyl Peptidase 4 (DPP4) DPP4, also known as CD26, is a type-II trans- membrane protein that is expressed in various tis- sues, such as the intestine, respiratory tract, liver, kidney, and fibroblasts of the lung. Moreover, DPP4 is expressed on different immune cells, including dendritic cells, macrophages, NK cells, activated T cells, and B cells (Shao et al. 2020). It is involved in the immune response and auto- inflammatory reaction by modulating the produc- tion of chemokines and cytokines (Shao et al.
2020). The significant association of the genetic variants in the DPP4 gene with some human dis- eases has been reported, such as diabetes (Ahmed et al. 2016) and myocardial infarction (Aghili et al. 2012).
The MERS-CoV, which causes Middle East respiratory syndrome (MERS), recognizes DPP4 as the primary receptor (Raj et al. 2013).
Interestingly, four DPP4 polymorphisms (rs1229319529, rs780673235, rs764879525,
rs763246423) have been documented to reduce the binding efficiency of MERS-CoV spike pro- tein to DPP4 (Kleine-Weber et al. 2020). It has been suggested that the DPP4 polymorphisms affect the course of MERS-CoV infection.
Recently, a high-affinity binding between SARS-CoV-2 spike protein and DPP4 has been reported, using bioinformatics approaches and protein docking based on crystal structures (Li et al. 2020). However, the affinity of the SARS- CoV- 2 spike protein with human DPP4 (−34.8 kcal/mol) is lower than its affinity with ACE2 (−39.2 kcal/mol). It is well known that the coronavirus spike protein can be associated with a wide range of molecules on cell surfaces. These molecules could play the role of co-receptors or attachment factors, which play a predominant role in viral entry (Cui et al. 2019). It is conceiv- able that DPP4 acts as a co-receptor for SARS- CoV- 2, though ACE2 is considered the primary receptor for SARS-CoV-2. Hence, the host sus- ceptibility to SARS-CoV-2 might be influenced by protein interactions between the spike protein and cell-surface co-receptors. Moreover, DPP4 inhibitors have been recently proposed as a potential therapeutic strategy for COVID-19 infection (Strollo and Pozzilli 2020).
6.2.4 Transmembrane Serine Protease 2 (TMPRSS2)
As mentioned above, the coronavirus spike gly- coprotein, which is embedded in the viral enve- lope, facilitates membrane fusion through mediating receptor recognition. The spike protein consists of two subunits, S1 and S2, which are involved in binding to the receptor and fusion with the cellular membrane, respectively. After membrane fusion, the coronavirus genome is delivered into the cellular cytoplasm to transla- tion and replication.
TMPRSS2 proteolytically primes the fusion proteins of the human influenza virus (Böttcher et al. 2006), metapneumovirus (Shirogane et al.
2008), and coronavirus (Glowacka et al. 2011).
The cleavage of the coronavirus spike glycopro- tein by TMPRSS2 produces the fusion- catalyzing
forms, which can boost or can be critical for viral infectivity (Bergeron et al. 2005; Iwata- Yoshikawa et al. 2019). Interestingly, a recent in vitro study indicated that Camostat Mesylate, a clinically proven TMPRSS2 inhibitor, partially blocks SARS-CoV-2 entry into cells (Hoffmann et al. 2020).
Some variants in the 3′ UTR region of the TMPRSS2 gene show a significant impact on its expression in the lung. Different inter-population frequencies of these variants are described. The haplotype with higher expression of TMPRSS2 is characterized by three SNPs (rs2070788, rs9974589, rs7364083), whose minor allele fre- quencies (MAFs) are significantly increased in Italians compared with East Asians (Asselta et al.
2020). These observations suggest a role of TMPRSS2 polymorphisms in modulating COVID-19 severity, which, however, need addi- tional experimental investigations.