Discussion
Asthma is a common chronic pulmonary disease, and
the incidence of
asthma has increased in the last few decades
[27]. With the increased incidence of asthma,
new preventive strategies and therapies for asthma are urgently needed
to further reduce the morbidity and mortality of
asthma. Of particular note is the potential causal role of aging
in the asthma pathogenesis [28, 29]. Several relevant studies have
identified the altered expression of aging-related genes (such as TP53
and FOXO3) in respiratory diseases [30, 31]. The polymorphism of
transcription factor FOXO3 was confirmed to regulate the overactivation
of mast cells, down-regulation of anti-inflammatory factors and
production of cytokines during the pathogenesis of COPD and asthma
[32]. FOXO3 deficiency has been confirmed to play an important role
in regulating lung inflammation of COPD/emphysema, which has emerged as
a new approach to address the development of pulmonary inflammatory
diseases [33]. Similarly, TP53 has been implicated
in COPD pathogens is by mediating the senescence of multiple lung
cells, and the overexpression of TP53 also could promote the progression
of emphysema in COPD patients [31, 34].
Not only that, as a stable epigenetic marker, aging-related CpG sites
were either hypo- or hyper-methylated in COPD and other aging-related
diseases [35, 36]. Our previous research identified that DNAm was
involved in regulating the expression of 9 aging-related genes in
peripheral venous blood of COPD patients [18]. As asthma and COPD
have similar even overlapping clinical phenotypes in chronic
inflammation and decreased lung function. In this study, we further
explored the methylation change of the previous screened aging-related
genes in peripheral venous blood of asthma patients. Indeed,
the association between these screened 9 aging-related genes and asthma
have been extensively studied by previous literatures [37-44]. AREG,
E2F1, FOXO3,
HDAC1, MMP2, TGFB1 and TP53 have been confirmed as crucial signaling
molecules in asthma [30, 45-51]. Although ATG3 is a key central
regulator in autophagy induction during aging [52], and NUF2 is
closely related closely associated with lung cell senescence [53],
their specific role in asthma has rarely been studied. The differential
expression of ATG3, FOXO3, NUF2 and TP53 in asthma patients were also
aligned with former studies [30, 53-55]. In addition, excessive
secretion of AREG in the airway after acute asthma attack promote airway
remodeling [51]. However, AREG is downregulated in peripheral blood
of elderly asthma patients, which may be attributed to the
different disease stages. It is particularly worth noting
that the decreased expression of E2F1 in asthma patients is consistent
with what we have previously observed in COPD patients [18], which
is different from that in lung cancer patients [55]. One possible
reason is the specificity of the sample tissue and pathogenic genes
in different diseases. MMP2, as a member of the matrix
metalloproteinase family, shows an increasing trend in the acute and
chronic phases of lung disease. Our results observed the increased
expression of MMP2 in asthma patients which is consistent with previous
literatures [56].
Additionally, we identified the methylation status of the 9
aging-related genes in asthma patients. Most DMSs of asthma patients
were hypermethylated, which was consistent with the differential
expression of mRNA, indicating that DNA methylation regulating gene
expression is related to aging. Moreover, except for ATG3, HDAC1, and
TGFB1, correlation analysis showed that the expression of the
aging-related genes in peripheral blood of asthma patients was
associated with pulmonary function parameters (FEV1%,
FEV1, FVC, PEF, FEF75,
FEF50, FEF25). It is known that TGFB1
was a key cytokine that directs airway remodeling [57] and HDAC1
played a critical role in the pathogenesis of asthma [58]. This
partial difference may be due to the presence of single nucleotide
polymorphism in asthma [59]. Chr16:55514392 located in the promoter
region has a regulatory effect on gene expression, which is inversely
associated with lung function index (FVC) [60]. Interestingly,
Chr16:55514437 is also located at the transcription initiation site, but
the specific molecular mechanism which regulate gene expression still
needs further study [60]. Furthermore, there were 9 asthma-related
CpG sites on the CpG islands of the differential aging-related genes.
The ROC curve and PCA analysis of methylation level showed that all the
9 DMSs could be used as potential biomarkers to distinguish asthma from
HCs. Most notably, the methylation rate of either single DMS or total 9
DMSs in asthma patients were significantly higher than that of HCs. As
population size and ethnicity may influence the methylation level, we
assumed that a methylation marker
holds promise for better biomarker of asthma. Our analysis of the 9 DMSs
methylation mutation rate also produced a better ROC specificity and
sensitivity, suggesting that the combinatorial DMSs had a great
potential to predict asthma. BALF (IL-25 and IL-33, etc.), induced
sputum (eosinophils, Th2 cells, etc.) and airway remodeling could all be
used as an useful indicators for asthma diagnosis [61, 62]. However,
the detect of DNAm in peripheral blood has greater advantage of
widespread access to samples and simple operation. Not only that, DNAm
is an important cause of asthma exacerbation, the specific role of
allergens and environmental exposure on the epigenetic modification
during the exacerbation of asthma also deserved more attention [63].
Although our study provides potential value for diagnosis and treatment
of asthma assessment, there are also some limitations. Firstly, asthma
can be divided into different phenotypes which may have differential
epigenetic modification. Besides, our previous work is not comprehensive
enough to screen all the aging-related genes. Moreover, the sample size
is relatively small.