Discussion
ACE2 has been established as the functional host receptor for
SARS‐CoV‐2, responsible for the current devastating worldwide pandemic
of coronavirus disease 2019 (COVID‐19) [16]. COVID-19 displays
symptoms ranging from a mild cold to cardio-respiratory failure that may
lead to death [17,18]. Both ACE and ACE2 receptor genes have
numerous genetic variations including functional polymorphism of
insertion/deletion (Ins/Del) for ACE and rs2106809 and rs2285666
polymorphisms for ACE2 receptor gene. The del allele shows higher ACE
activity and polymorphisms in the ACE2 receptor gene affect circulating
ACE2 receptor levels [12,13]. Considering the role of ACE in
Covid-19 pathogenesis and the variation in disease severity, ACE I / D
and ACE2 receptor gene variants have attracted the attention of
researchers. However, the studies were limited to in silico analyzes and
epidemiological studies. To the best of our knowledge, our work is the
first study including wet lab analysis for investigating ACE I/D, ACE2
rs2106809, rs2285666 polymorphisms. Unfortunately, no association was
found between these polymorphisms and the severity of symptoms of
Covid-19.
Delanghe et all conducted a multiple regression analysis model to
compare the prevalence and mortality of the COVID-19 infection from a
number of European countries and frequency data of the geographical
variation of I/D polymorphism in the ACE gene. Differently, the
authors reported a negative correlation between the frequency of the D
allele of the ACE Ins/Del polymorphism and prevalence and
mortality rates of COVID-19 in 33 countries. It should be noted that
this study does not involve wet lab analysis and the results conflicted
with eastern Asian populations data. It was suggested a converse
association between ACE D allele frequency and prevalence of Covid-19.
Considering that the D allele frequency is lower in Asian populations
than in European populations, the mortality rate and prevalence of
Covid-19 in the Asia population is expected to be higher. However, the
prevalence and mortality of COVID-19 in Europe are higher than in Asia
[19,20]. On the contrary to Delangheet all, Pati et al. suggested a
significant positive correlation of D allele of ACE polymorphism with
SARS-CoV-2 infection and mortality rate in their epidemiological
investigation [21].
Hatami et al. conducted a meta-analysis study and showed the recovery
rate significantly increased with the I/D allele frequency ratio.
However, this study provides an ecological perspective but does not
provide a direct clinical relevance between the COVID-19 and ACE I/D
polymorphism [22].
In two different populations, genetic variants in the ACE2 receptor gene
were analyzed by whole-exome sequencing in hospitalized COVID-19
patients. Differently from our study, they investigated the relation of
Covid19 with coding-region variants in the ACE2 receptor gene. These
studies also provided no strong evidence that ACE2 receptor gene
variants are a consistent association with COVID-19 severity.
The ACE2 receptor gene sequencing showed no coding sequence
variants that could explain an increased risk of developing COVID-19.
Similarly, in our study, no relationship was found between ACE-2
receptor gene intron variants and Covid19 severity [23,24].
Our results showed that ACE I/D, ACE2 rs2106809, rs2285666 polymorphisms
have no role in the severity of Covid-19. Novelli et al. (2020) also
reported that ACE2 receptor gene coding-region variants have no effects
on covid-19 severity [23]. Taking that ACE2 receptor gene expression
may affect the susceptibility of individuals to infection into
consideration, we speculate that genetic variations in the non-coding
regions of the ACE2 receptor gene or in other non-coding DNAs that
control the expression levels of ACE genes may have a potential role in
the severity of the disease.
A growing number of recent findings point out that epigenetic mechanisms
such as DNA methylation and histone modifications, play key roles to
control gene expression. A recent analysis of public genomic and
transcriptomic data outlined the role of histone methylation to
regulate ACE2 receptor gene transcription [25]. Further
regulation occurs at the mRNA level. From putative microRNA-binding
sites identified in vitro, Lambert et al. (2014) demonstrated that
miR-421 downregulates the ACE2 receptor gene [26]. Besides
undergoing post-translational modifications by glycosylation and
phosphorylation, the ACE2 receptor gene is also post-translationally
regulated [27]. Therefore, variations in other genes related to
epigenetic mechanisms of ACE2 receptor gene expression may have a
potential role in the severity of Covid-19.
Second, mortality or severity of disease in COVID-19 patients might be
linked to excessive production of proinflammatory cytokines leads to
ARDS aggravation and widespread tissue damage resulting in multi-organ
failure and death, not linked to increased ACE2 receptor expression.
Wang et al. showed that most of the severely ill patients had viral
shedding in a variety of tissues for 20-40 days after onset of disease,
while the majority of mildly ill patients had viral shedding restricted
to the respiratory tract and had no detectable virus RNA 10 days after
onset [28]. Similarly, Liu et al. (2020) reported that mild cases
were found to have an early viral clearance [29]. These results may
indicate that the nasopharyngeal SARS-CoV-2 RNA load is higher in the
severe group, or they may be related to the immune response of the host.
In the current literature impairment of SARS-CoV-2 clearance is related
due to genetic and viral features, enhanced levels of interferons,
neutrophil extracellular traps and pyroptosis, and possible other
unknown mechanisms were reported [30]. Besides, the patients from
the severe group showed elevated levels of inflammatory cytokines (such
as IL-2, IL-2R, RIL-6, IL-8, and IL-10), significant enhancement in
coagulation parameters (such as D-dimer, prothrombin time, and
fibrinogen), and increases in myocardial injury indicators. Therefore,
inflammatory release, coagulation dysfunction, and myocardial injury
correlate with disease severity and rise throughout the course of the
disease [31,32]. Thus, the severity of COVID-19 may be related to
variations in genes encoding proinflammatory cytokines or other genes
that are suggested to be associated with the occurrence of severe
COVID-19.
In conclusion, our study does not support the hypothesis that ACE I/D,
ACE2 rs2106809, rs2285666 polymorphisms are related to Covid-19
severity. Considering that little is known about the genetic basis of
the difference of Covid-19 severity and analysis of genetic
polymorphisms reveals important information, the possible relation
between ACE2 rs2106809, rs2285666 polymorphisms, and Covid-19 severity
required to be investigated. However, in order to explain the personal
variations in Covid-19 pathogenesis, further studies are needed to
investigate variations in other genes related to epigenetic mechanisms
of ACE2 receptor gene expression and variation in genes encoding
proinflammatory cytokines, coagulation indicators.