Introduction
Coronaviruses are a large family of single-stranded RNA viruses, which cause illnesses ranging from mild cold to more severe diseases such as Severe Acute Respiratory Syndrome (SARS)-CoV and Middle East Respiratory Syndrome (MERS-CoV) [1]. COVID-19 was first reported in Wuhan, China, and subsequently spread worldwide. As of February 2021, there have been more than a hundred million cases of COVID-19 worldwide and nearly two and a half million deaths [2].
The entry of SARS-CoV-2, the agent that causes COVID-19, into the cell occurs by binding viral spike proteins to Angiotensin-Converting Enzyme 2 (ACE2) receptors of the host membrane. It was suggested that increase susceptibility to Covid-19 infection is associated with the expression of the target ACE2 receptor in the epithelium exposed to the virus [3]. The low infection and complication level is a fact among children. Serum ACE levels in children are higher than in adults and ACE2 receptor gene expression in the nasal epithelium that is the first point of contact for SARS-CoV-2, was age-dependent, lowest in younger children and increasing with age into adulthood [4]. It was also suggested that the lower risk among children is due to the lower expression of the ACE2 receptor [5]. Epidemiological data show that a majority of pediatric COVID-19 cases showed mild to moderate clinical features, and only a few developed severe or critical diseases (0.6% and 0.3%, respectively) [6]. Total 44 672 cases, only 1% is under 10 years old and in China [7]. Also lower ACE2 receptor expression in children, highlighting the importance of ACE and ACE2 receptor expression level in Covid19 severity.
A counter-regulatory relationship between ACE2 and ACE located on opposite axes in the Renin-Angiotensin System (RAS) was reported. ACE plays important role in converting angiotensin I to angiotensin II and ACE2 is a negative regulator of the RAS and counterbalances the function of ACE. The lungs are primary organs for ACE receptor expression and generating circulating angiotensin II. RAS plays a role in the pathogenesis of pulmonary hypertension and fibrosis, which are common chronic lung diseases. Recent studies show that RAS also plays important role in acute lung diseases, particularly acute respiratory distress syndrome (ARDS) [8].
A common 287 base pair insertion/deletion (I/D) polymorphism has been reported in intron 16 of the ACE gene and is known to be associated with serum levels of circulating ACE. Serum ACE concentrations were reported to be significantly higher in subjects with the D/D genotype compared to the I/D and I/I genotypes [9]. Considering the opposite effect between ACE and ACE2, decreased ACE2 receptor gene expression is strongly related to an increase in ACE expression. So it could be hypothesized that having a D allele for ACE I/D polymorphism affects the clinical course of the Covid-19 by decreasing the ACE2 receptor level.
Besides, it has been shown that patients with chronic obstructive pulmonary disease (COPD) have increased gene expression of ACE2 receptor in bronchial epithelial cells in the lower respiratory tract, and also smoking has been shown to increase both the expression and activity of ACE-2 in the airways. While up-regulation of ACE-2 is beneficial in protecting the host against acute lung injury, this makes individuals more susceptible to coronavirus infections that use this receptor to enter epithelial cells [10,11].
In the changes in ACE2 receptor gene expression level, genetic variations play a very important role as well as environmental factors. Of the many polymorphisms identified in the ACE2 receptor gene, rs2106809 and rs2285666 are particularly remarkable. A bioinformatics tool called Human Splicing Finder (HSF) (v.3.0) predicts that rs2106809 polymorphism, intronic SNP found in the ACE2 receptor gene, might create an intronic-exonic splicing enhancer site (ESE). It was suggested that the splicing efficiency of the ACE2 receptor gene may be influenced by the creation of these enhancer motifs [12]. It was also found that ACE2 rs2106809 CC or CT genotype carriers had higher circulating ACE2 receptor levels compared to TT genotype carriers [13].
G8790A (rs2285666) polymorphism is at the fourth base of the third intron and situated in the intron adjoined to the exon, suggesting, this locus could alter mRNA alternate splicing and affect ACE2  receptor gene expression. It has also been reported that this polymorphism shows a strong linkage disequilibrium with the other SNPs (rs1978124 intron 1 and rs714205 intron 16) in the ACE2 receptor gene [14].
The relationship between ACE2 gene expression and Covid-19 has been shown in the literature, however, most of the studies conducted to investigate the role of ACE receptor gene polymorphisms are in-silico analyzes or epidemiological studies. Also, to the best of our knowledge, there is no study investigating the relationship between ACE I/D and ACE2 receptor gene rs2106809 and rs2285666 polymorphism and Covid-19 severity. In this study, it is aimed to investigate whether the course of the disease (mild, moderate, and severe) is predictable by determining the genotypes of ACE I/D and ACE2 receptor gene polymorphism.