Keywords C-reactive protein - single-nucleotide polymorphisms (SNPs) - metabolic syndrome -
meta-analysis
Introduction
Metabolic syndrome (MetS) is a growing public health problem worldwide. It is defined
as a complex syndrome with coexistence of multiple cardiovascular risk factors consisting
of atherogenic dyslipidemia, central obesity, proinflammatory and prothrombotic states,
hypertension, and hyperglycemia.[1 ]
[2 ] It is associated with endothelial dysfunction, angiogenesis that might lead to increased
risk for diabetes mellitus, insulin resistance, and cardiovascular diseases.[3 ]
Ridker et al reported slight increase in moderate C-reactive protein (CRP) levels
in relation to the number of MetS components in a follow-up study on healthy American
women.[4 ] CRP consists of five 23 kDa subunits which has a crucial role in the human immune
system.[5 ] Also, CRP largely regulated by interleukin 6, has been broadly used to evaluate
various inflammatory states.[6 ]
[7 ] High concentrations of CRP are related with increased risk of the MetS.[2 ] Given that CRP is found to have an important role in MetS, the association between
polymorphisms of inflammatory marker gene and MetS were studied.[3 ] Among various gene association studies, one of the most reported genes is CRP. The
human CRP gene is on chromosome 1q21–1q23, approximately 1.9 KB, and has two exons,
which are linked by a 280 base pair intron.[8 ]
Many recent studies evaluated the genetic association between CRP single-nucleotide
polymorphisms (SNPs) and MetS in different populations.[2 ]
[9 ]
[10 ]
[11 ]
[12 ]
[13 ]
[14 ]
[15 ]
[16 ] Some studies showed the association of CRP polymorphisms,[10 ]
[12 ]
[14 ] whereas some other studies did not.[11 ]
[13 ] Therefore, a comprehensive systematic review and meta-analysis are essential to
evaluate the relationships between CRP polymorphisms and MetS. The present study aimed
to define the association between MetS and the polymorphisms of the CRP gene: rs1800947,
rs3093068, rs1205, and rs3091244.
Methods
Search Strategy
The electronic databases, such as Science Direct, PubMed, Web of Knowledge, Scopus,
and Springer, were systematically searched for all available articles, without any
limitation for publication date, language, article type, access type, and species
until January of 2019. Articles with the following search terms in the titles, abstracts,
or keywords were included: (((“C-reactive protein” OR CRP)) AND (metabolic syndrome
OR MetS)) AND polymorphism. We also improved the search by revising the reference
lists of all of the selected publications and finding supplementary documents related
to these articles.
Inclusion and Exclusion Criteria
For identifying suitable articles, the following criteria were included in this meta-analysis
study. Any article published as an original study that considered the association
between CRP polymorphisms and MetS. The available data for each polymorphism was reported
and also necessary data to estimate the odds ratio (OR) and 95% confidence interval
(CI) was provided. Furthermore, the reviews, animal studies, comments or editorials,
overlapped articles, or studies with insufficient information on genotypes or allele
frequencies were excluded.
Data Extraction
The original information of search articles were extracted by two authors (M.M. and
M.S.) independently using a reliable and standardized method. The following information
was collected from each study: first author, year of publication, origin of the subjects,
study design, variables adjusted for the analysis, and the Hardy–Weinberg equilibrium
value of the overall association for each SNP of CRP gene.
Statistical Analysis
The association between polymorphisms of CRP gene and MetS was evaluated. Statistical
analysis was performed by pooled ORs and associated 95% CIs. The Z test was used for determining the significance of the pooled OR. Statistical analyses
were performed by STATA software (version 12.0; Stata Corp LP, College Station, Texas,
United States). A p -value of < 0.05 was considered as statistically significant. The heterogeneity among
studies was detected using the chi-square test-based Q statistic, and then it was quantified using the I
2 statistic.
The Mantel–Haenszel fixed-effects model was considered when heterogeneity was not
significant (i.e., p > 0.05 and I
2 < 50%). When heterogeneity was significant, random-effects model (the DerSimonian–Laird
method) was used. Finally, for evaluating the existence of heterogeneity among groups,
sensitivity analyses were conducted. In addition to calculating publication bias,
the Begg's and Egger's tests were applied, in which p -values of less than 0.05 indicate significant publication bias.
Results
Characteristics of the Included Studies
Detailed flowchart for this study searching is presented in [Fig. 1 ]. According to the aforementioned search term, 657 potentially appropriate articles
were identified. After removing 26 duplicates, 631 articles remained for the next
step. Among them, 31 papers were selected for reviewing the full text. In total, 27
publications were omitted mainly because they had no relation or meeting abstracts
or reviews. In conclusion, four full-text articles were included in the present meta-analysis
study. The main characteristics of the studies included in this meta-analysis are
listed in [Table 1 ]. As shown in the table, there are four articles containing two case–control studies
from Egypt[10 ] and Iran,[13 ] one cross-sectional study from Taiwan,[12 ] and one cohort study from the United Kingdom.[11 ]
Fig. 1 The methods for selection process.
Table 1
Characteristics of included studies
ID
Authors
Year
Country
SNPs
OR (95% CI)
Study design
Statistical analysis (HWE, p -Value)
1
Tarek et al[10 ]
2013
Egypt
1059 (rs1800947)
GC + CC
3.59 (1.30–9.93)
Case–control
p = 0.001
C allele
3.37 (1.29–8.82)
p = 0.01
2
Hsu et al[12 ]
2010
Taiwan
rs3091244
1.62 (1.05–2.49)
Cross-sectional
p = 0.029
3
Nikpour et al[13 ]
2015
Iran
T allele
rs3091244
1.70 (0.98–2.96)
Case–control
p = 0.059
ID
Study
Year
Country
SNPs
Population (
N event/case) / (
N event/control)
Study design
Statistical analysis (HWE, p -Value)
4
Darya et al[11 ]
2011
UK
rs1205
(618/2100)/(141/1875)
Cohort
p = 0.59
rs3093068
(616/2093)/(141/1875)
p = 0.88
Abbreviations: CI, confidence interval; HWE, Hardy–Weinberg equilibrium; OR, odds
ratio; SNP, single-nucleotide polymorphism.
Meta-Analysis Results
The forest plot for association between CRP gene polymorphism with MetS according
to all the studies is shown in [Fig. 2 ]. Overall, there was no significant association between CRP gene polymorphism and
MetS (OR = 0.92, 95% CI = 0.77–1.10); as shown in [Fig. 3 ], significant heterogeneity was documented (I
2 = 55.4%; p -value = 0.008). We used funnel plot and Egger's test to assess the publication bias.
Based on both funnel plot ([Fig. 3 ]) and Egger's test there was no evidence of publication bias (for all studies: p = 0.44).
Fig. 2 The forest plot of the association between C-reactive protein (CRP) gene polymorphism
with metabolic syndrome (n = 4, all studies).
Fig. 3 The funnel plot of the association between C-reactive protein (CRP) gene polymorphism
with childhood and adolescence metabolic syndrome (n = 4, all studies). OR, odds ratio.
Result of subgroup meta-analysis according to genotype is shown in [Fig. 4 ]. In the GG subgroup, the association of CRP gene polymorphism with MetS was significant
(OR = 0.32, 95% CI = 0.13–0.80, p -value = 0.01) with no significant heterogeneity (I
2 = 0%, p -value = 0.534) ([Fig. 4 ]). It is followed by performing sensitivity analysis in which the individual studies
were removed sequentially to assess their effect on the results. There was no difference
from the initial analysis. Therefore, it is suggested that the results of this meta-analysis
were strong.
Fig. 4 The forest plot of the association between C-reactive protein (CRP) gene polymorphism
with childhood and adolescence metabolic syndrome according to the genotype subgroups.
Discussion
Various investigations have reported the association of CRP polymorphisms and MetS;
however, the findings of previous studies have been controversial. To start, we chose
the CRP gene because there is strong evidence that it is a powerful predictor of incident
MetS events. As a result, the most common variants studied were the following polymorphisms:
rs1800947, rs3093068, rs3091244, and rs1205 of the CRP gene. In the current research
work, to prove whether CRP polymorphism is responsible for the development of MetS,
we conducted a meta-analysis, which included four independent studies. Based on literature
search, this article is the first comprehensive meta-analysis to evaluate the possible
correlations of CRP polymorphisms in MetS.
In adult populations, the association between the CRP rs3091244 and MetS has been
evaluated in several studies.[9 ]
[12 ] In the present study, we assessed the association of different variants of the CRP
gene with MetS. The main results indicated that the CRP polymorphisms including rs1800947,
rs3093068, rs3091244, and rs1205 were not related with MetS. The lack of association
could be described by the limited number of studies. Consequently, it is essential
to increase the number of studies to have a substantial outcome. Additionally, in
subgroup analysis, it was indicated that the GG subgroup of the mentioned CRP gene
polymorphism has significant association with metabolic syndrome.
Some studies reported the association between different populations. For example,
in 2010, Hsu et al demonstrated the correlation of rs3091244 on the risk of MetS in
Taiwanese adults. This study showed that the non-CC genotypes are associated significantly
with MetS after adjustment for smoking, age, sex, and body mass index.[12 ] Similarly, in our study, among the non-CC genotypes only GG had significant association.
But other studies failed to confirm this association.[11 ]
[13 ] Nikpour et al reported that in Iranian children and adolescents, −286C > A > T CRP
polymorphism (rs3091244) is not associated with the increased risk for MetS.[13 ] Furthermore, Gaysina et al did not find association between CRP gene variants rs1205
and rs3093068 with the metabolic syndrome in U.K. population.[11 ]
Some limitations of our meta-analysis study should be noted. First, the limited number
of articles for most of the CRP polymorphisms was available. Second, the languages
of the publications were limited to English. Third, our meta-analysis was based on
evaluations without adjusting the data for population characteristics such as ethnicity,
age, etc. Fourth, due to lack of access to detailed data used in studies, our meta-analyses
are based on the various data of CRP SNPs, and finally, it must be noted that the
study design of included articles in this meta-analysis was different ([Table 1 ]).
Despite these limitations, the current study also has some advantages. First, the
statistical power of the analyses was considerably increased as a large number of
samples were derived from different studies. Second, in our analyses, we investigated
different ethnicity population.
Conclusion
In summary, this meta-analysis demonstrated considerable evidence that there were
no correlations between various polymorphisms of CRP gene (rs1800947, rs3093068, rs3091244,
and rs1205) and the risk of MetS in various populations. Moreover, it is recommended
that future researches into the mutual effects of the environment and genes may improve
existing concept of the associations between polymorphisms of CRP gene and developing
MetS. Further work will help explain the medical and biotechnological implications
of these associations.
