Genetic susceptibility of COVID-19: a systematic review of current evidence

Introduction While COVID-19 pandemic continues to spread worldwide, researchers have linked patterns of traits to poor disease outcomes. Risk factors for COVID-19 include asthma, elderly age, being pregnant, having any underlying diseases such as cardiovascular disease, diabetes, obesity, and experiencing lifelong systemic racism. Recently, connections to certain genes have also been found, although the susceptibility has not yet been established. We aimed to investigate the available evidence for the genetic susceptibility to COVID-19. Methods This study was a systematic review of current evidence to investigate the genetic susceptibility of COVID-19. By systematic search and utilizing the keywords in the online databases including Scopus, PubMed, Web of Science, and Science Direct, we retrieved all the related papers and reports published in English from December 2019 to September 2020. Results According to the findings, COVID-19 uses the angiotensin-converting enzyme 2 (ACE2) receptor for cell entry. Previous studies have shown that people with ACE2 polymorphism who have type 2 transmembrane serine proteases (TMPRSS2) are at high risk of SARS-CoV-2 infection. Also, two studies have shown that males are more likely to become infected with SARS-CoV-2 than females. Besides, research has also shown that patients possessing HLA-B*15:03 genotype may become immune to the infection. Conclusion Combing through the genome, several genes related to immune system’s response were related to the severity and susceptibility to the COVID-19. In conclusion, a correlation was found between the ACE2 levels and the susceptibility to SARS-CoV-2 infection.

SARS-CoV-2 utilizes molecules to enter the cells, such as angiotensin-converting enzyme 2 (ACE2) to attach to the receptor-binding domain (RBD) and type 2 transmembrane serine proteases (TMPRSS2) to cleave the spike (S) protein [15][16][17]. SARS-CoV-2 S-protein cleavage by TMPRSS2 initiates viral entry and also helps Page 2 of 12 SeyedAlinaghi et al. Eur J Med Res (2021) 26:46 the virus escape the immune system [15]. The hidden receptor-binding domain attached to ACE2 enhances immune evasion by weakening the immune surveillance. SARS-CoV-2 S-protein cleavage by TMPRSS2 is also responsible for immune escape besides initiating the viral entry [15]. Hence, genetic differences among such molecules responsible for cellular entry might alter the observed responses among different individuals [18,19]. Various cells in the body express ACE2 to different extents; and therefore, numerous organs of the body can be invaded by the virus, such as the lungs and the heart [18,[20][21][22]. Additionally, cell entry is reinforced in the presence of TMPRSS2 in ACE2-positive cells by cleaving the viral Spike protein, which can result in its activation for membrane fusion [23][24][25]. There are reports that not only genetic differences can cause certain races and populations to be more affected by this virus, but also they might contribute to gender-specific differences [26].
Higher testosterone levels enhance TMPRSS2 levels and might cause higher disease susceptibility in males [27,28]. To make matters even more important, the differences in severity observed in children compared to older adults may be in parts due to the different expressions of genetic components, such as TMPRSS2 [29]. As mentioned above, various genetic patterns might alter the risk of infection, viral entry, immune response to the virus, COVID-19 severity, and death. Therefore, we aim to review the published literature on the genetic aspects of susceptibility to COVID-19 and discuss the above-mentioned statements. Recognizing these points might help us determine at-risk groups and take specific measures to support them.

Methods
This study aims to review the current evidence of genetic susceptibility to COVID-19 carried out in September 2020. The authors studied the probable genetic vulnerability to novel Coronavirus . In this study, the preferred reporting items for systematic reviews and meta-analyses (PRISMA) checklist is used as a criterion to ensure the accuracy and reliability of the results.

Data sources
By systematic search and utilizing the keywords in the online databases including Scopus, PubMed, Web of Science, and Science Direct, we retrieved all related papers and reports published in English from December 2019 to September 2020. To convey our search strategy, we considered different combinations of keywords in the following orders: 1. "COVID-19" or "SARS-CoV-2" or "Novel Coronavirus" or "2019-nCoV" or "Coronavirus" (title/abstract).

Study selection
Two independent investigators screened and selected the studies with the most relevant titles and abstracts. Then, the full text of the extracted papers was reviewed, and based on the eligibility criteria, the most relevant papers were selected. The peer-reviewed original papers published in English that satisfied the eligibility criteria were considered in the final report.
Our exclusion criteria for this study, whereas the following measures: • Papers focusing on non-human studies including in vitro observations or researches concentrating on animal experiments, or discussing COVID-19 without a satisfying detail, and papers without referring to the keywords of this study. • Papers whose full texts were not accessible. • Any results were suspicious of duplication in databases.

Data extraction
After the summarization process, data including type of article (e.g., case reports), information of authors, country of origin, publication date, sample size, gender, age, and genetic susceptibility data were transferred into an organized sheet. Two independent investigators gathered this information. Finally, the other authors cross-checked all the selected papers to make sure no overlap or duplications exist in the content.

Quality assessment
As previously mentioned, we considered the preferred reporting items for systematic reviews and meta-analyses (PRISMA) checklist to ensure the quality and authenticity of selected studies. The quality and consistency of the articles and the probable risk of bias were evaluated by two independent researchers. A third independent researcher was recruited to resolve the issue in any situation where differences in thoughts were encountered. In the end, the full text of selected articles was read and the significant findings of each article were extracted.

Results
Using the described search strategy, we retrieved 148 full-text articles. After an initial review of retrieved resources, 69 duplicates were removed, and the title and abstract of the remaining 79 articles were reviewed.
Applying the selection criteria, 58 articles were excluded, and only 21 articles met the inclusion criteria and were included in the final review ( Fig. 1). According to the findings, COVID-19 uses the ACE2 receptor for cell entry. A review of articles showed individuals with ACE2 polymorphism along with TMPRSS2 are more vulnerable to COVID-19. Two studies have shown that males are more likely to become infected with SARS-CoV-2 than females. Besides, research have also shown that patients possessing HLA-B*15:03 genotype may become immune to the infection (Table 1).
ACE2 is a receptor for SARS-CoV-2. Also, it can facilitate the post-infection downstream processes including inflammatory responses [33]. ACE2 receptors are ubiquitous and widely expressed in the heart, vessels, gut, lung, kidney, testis, and brain [20,22,58]. SARS-CoV-2 enters into the cell through binding of the viral S-protein to the ACE2 receptors [15,17]. The ACE2 receptors diminish the unfavorable effects of angiotensin II in several ways [58]. First, they can degrade angiotensin II and reduce its deleterious effect, and second, by generating angiotensin (1-7) they perform counter-regulatory effects to those of angiotensin II [59]. Several conditions including older age, diabetes, hypertension, and heart diseases like heart failure can lead to ACE2 deficiency [58]. Given the above information, ACE2 deficiency is supposed to play a pivotal role in the pathogenesis of SARS-CoV-2 infection which favors the progression of inflammatory and thrombotic processes [58].
As mentioned earlier, binding SARS-CoV-2 to ACE2 is facilitated by a RBD located within the S-protein of the virus. There are 25 different ACE2 gene variants in the Leiden open variation database (LOVD) [37]. Two specific ACE2 alleles (i.e., rs73635825 and rs143936283) showed a low binding affinity of the entire SARS-CoV-2 S-protein, so viral attachment decreased and potential resistance to infection was reported [37]. According to recent studies, genetic polymorphisms in the ACE2 gene were detected in populations, which modulate the SARS-CoV-2 affinity spike protein and worsen pulmonary and systemic injury by accelerating lung damage in COVID-19 patients, which enhanced vasoconstriction, inflammation, oxidation, coagulation, capillary permeability, fibrosis, and apoptosis in the alveolar cells [37,60]. Single-nucleotide polymorphism (SNPs) in ACE2 is associated with different diseases. Therefore, SNPs can impact gene expression, which might influence disease severity. Moreover, COVID-19 is referred to the state of cytokine storm which results in an excessive release of cytokines that lead to multiple organ damage. Such response cytokine storm was showed in ACE2-positive cells. Thus, differences in response to COVID-19 associated with genetic polymorphism within the genes responsible for the synthesis of cytokines and pro-inflammatory mediators along with ACE2 [61].
Male express higher ACE2 levels in their type II pneumocytes; and therefore, their lung samples may express threefold higher levels of ACE2 compared to the females [53][54][55][56][57]. In addition, the ACE2 expression in Asian populations is higher than African-American and Caucasian populations [31,34]. As the evidence showed, the conversion of angiotensin II to angiotensin (1-7) by ACE2 was higher in males than females, indicating over expression of ACE2 in males [54]. As ACE2 is encoded by a gene located on the X chromosome and males express more ACE2 than females it could be considered that depending on the allele expressed by females, they could be considered of lower sensitivity against the most severe adverse effects of the infection [54].
As thoroughly discussed above, ACE2 protein at the surface of lung alveolar epithelial cells allows infection of the respiratory tract with SARS-CoV-2 [54]. In the lung, SARS-CoV-2 mainly infects alveolar epithelial cells, resulting in respiratory symptoms, and SARS-CoV infects mainly pneumocytes and macrophages. ACE2 expression protects from lung injury and is down-regulated by SARS-S. The expression of ACE2 also manifests itself beyond the lung. Since ACE2 is a functional receptor for SARS-CoV-2, researchers started to carefully consider the safety and potential effects of antihypertension therapy with ACE inhibitors or angiotensin-receptor blockers in patients with COVID-19. Besides, diabetes mellitus and hypertension are known as the most common comorbidities in COVID-19; both these conditions are controlled by ACE2. Earlier researchers were concerned on whether patients with COVID-19 and hypertension who are taking an ACE inhibitor (ACEI) or angiotensin-receptor blocker (ARB) medication should seek another antihypertensive drug [11,41]. However, novel studies refuted the association between ACEI/ARB use and the COVID-19 disease severity; and therefore, physicians should not have concerns regarding prescribing these medications during the pandemic [62,63]. On the other hand, a TMPRSS2 inhibitor can be a treatment option by blocking the cell entry, since an inhibitor of TMPRSS2 can block SARS-CoV-2 infection of lung cells [18,41].
ACE2 can cause SARS-CoV-2 infection in the host cells, while also causing damage to the myocardium; however, the specific mechanisms have not yet been precisely discovered. Some studies indicated that ACE2 polymorphisms were in association with cardiovascular conditions by modifying the angiotensinogen-ACE2 interactions, asp.Arg514Gly in the African/African-American people. Particularly, 54% (33/61) and 39% (24/61) of deleterious variants in ACE2 appear in non-Finnish European (EUR) and African/African-American (AFR) populations, respectively. One of the reasons for the difference in genetic susceptibility to COVID-19 on one hand, and risk factors like high-risk male patients with cancer, on the other hand, can be the presence of a high number of polymorphisms in TMPRSS2 [e.g., p.Val160Met (rs12329760) which is an expression quantitative trait locus (eQTL)]. Moreover, the comparative genetic analyses implied that ACE2 genomic variants play critical roles in susceptibilities to COVID-19 and its associated cardiovascular conditions by altering the AGT-ACE2 pathway. Also, the developmental regulation of TMPRSS2 may make infants and children immune from COVID-19. The prevalent polymorphisms in ACE2 or TMPRSS2 lead to the use of personalized treatments (i.e., hydroxychloroquine and camostat) for COVID-19. However, the evidence refuted the effectiveness of these treatments and ruled out their use as logical treatments [64]. Also TMPRSS2 variants and resulting expression influence COVID-19 severity [26,65]. For example, endometrium carries low susceptibility to COVID-19 due to the low expression of TMPRSS2 and ACE2. However, expression of the genes related to SARS-CoV-2 infection varies with menstrual cycle phase and age, with older women expressing higher ACE2 levels [30].
Last but not least, the similarity of the chemokine receptor protein encoded by the CCR5 gene for HIV has been compared to ACE2 for SARS-CoV. The target of HIV infection is CCR5 gene, and genetic variations in CCR5 can result in the different rates of this infection. ACE2 can play the role of an efficient or inefficient receptor for SARS-CoV if the amino acid remains are altered. Several ACE2 variants could have an important influence on the association between ACE2 and S-protein in SARS-CoV. Therefore, this gene variation can justify the gender differences and different susceptibility and disease severity to SARS-CoV infection in different racial groups [36].
In addition, some evidence indicates that the entry of SARS-CoV-2 into the cell is facilitated by the polymorphisms of the cellular proteases in the presence of furin [65]. The furin can be connected to the SARS-CoV-2, and the S-protein might be cleaved by TMPSS2 with the collaboration of furin. This function results in the entry of the virus into the respiratory tract and also in an increased risk of infection. Despite the aforementioned procedures, an association between furin and COVID- 19 has not yet been discovered [49].

Conclusion
Combing through the genome, a relation has been found between the severity and susceptibility of COVID-19 and some genes associated with the immune system's response. In conclusion, a correlation was found between the ACE2 levels and the susceptibility to SARS-CoV-2 infection. Therefore, in SARS-CoV-2, as well as SARS-CoV, ACE2 has been recognized as a functional receptor. ACE2 seems to be crucial in outcomes of COVID-19, and the efficiency of ACE2 usage is a significant element of SARS-CoV transmissibility. SARS-2-S uses ACE2 (SARS-CoV receptor) to enter the cells and the serine protease TMPRSS2 for S-protein preparing to the entrance.