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Modern Rheumatology, 00, 2021, 1–5
DOI: https://doi.org/10.1093/mr/roab052
Advance access publication date: 1 September 2021
Original Article
Urticaria and increased risk of rheumatoid arthritis: a
two-sample Mendelian randomisation study in
European population
Downloaded from https://academic.oup.com/mr/advance-article/doi/10.1093/mr/roab052/6360891 by guest on 10 December 2021
Xue Yua , Ming-Gang Dengb , Zi-Ying Tangc and Zhi-Jiang Zhanga,*
a
Department of Epidemiology, School of Public Health, Wuhan University, Wuhan, China
b
Department of Epidemiology and Health Statistics, School of Public Health, Wuhan University, Wuhan, China
c
Department of Public Health, School of Public Health, Wuhan University, Wuhan, China
*Correspondence: Zhi-Jiang Zhang; zhang22968@163.com; Department of Epidemiology and Health Statistics, School of Public Health, Wuhan University, 115
Donghu Road, Wuhan 430071, China.
ABSTRACT
Background: In recent years, a growing body of observational studies suggest that urticaria is associated with a higher risk of rheumatoid
arthritis (RA). However, the causal association between urticaria and RA remains unknown.
Objective: To investigate the causal relationship of urticaria and RA in European populations by Mendelian randomisation (MR) approach.
Methods: We conducted two-sample MR analyses. Eleven single-nucleotide polymorphisms associated with urticaria were used as instrumental
variables. The summary data on urticaria were derived from FinnGen Data Freeze 2. The summary data on RA were obtained from a published
meta-analysis using European samples. Four MR methods were applied to the MR estimates. Three heterogeneity tests, including Cochran’s Q
test, single variant analysis, and leave-one-out variant analysis, were used. The pleiotropy and horizontal pleiotropy among instrumental variables
were assessed with MR-Egger regression intercept, MR pleiotropy residual sum and outlier global test, and PhenoScanner.
Results: The MR analysis suggested that urticaria was causally associated with RA (odds ratio = 1.114, 95% confidence interval = 1.024–1.211,
p = .011). No genetic pleiotropy or horizontal pleiotropy was revealed by MR-Egger regression intercept and MR pleiotropy residual sum and
outlier global test. The sensitivity analysis results were relatively robust.
Conclusions: The MR analysis suggested there was sufficient evidence to indicate urticaria is the cause of RA.
KEYWORDS: Mendelian randomisation; urticaria; rheumatoid arthritis; genetics; causal relationship
Introduction urticaria patients have autoimmune diseases, which include
Rheumatoid arthritis (RA) is an autoimmune disease charac- RA [15].
terised by joint involvement and inflammatory arthritis [1]. Whether the relationship between urticaria and RA is
The common extra-articular manifestations involve malaise, causal remains unclear because of the reverse causation and
fever, weight loss, and skin symptoms [2, 3]. It is estimated confounding bias in observational studies. Mendelian ran-
that 23.7 million people worldwide are suffering from RA domisation (MR) is a widely used epidemiological method
[4]. RA imposes considerable individual burden and socioe- that can use genetic variants as instrumental variables (IVs)
conomic burden on patients [5, 6]. Early diagnosis of RA to explore whether there is a causal relationship between the
is paramount since that may prevent disease development or exposure and the outcome [16]. As alleles are independently
even stop the disease [7]. As a result, early identification of assigned randomly at meiosis, MR can limit both typical con-
risk factors for RA is the key to preventing and treating RA. founding factors and reverse causation, providing stronger
Urticaria is a common disease, which is characterised evidence of causal inference [17]. Herein, we assess the
by wheals and angioedema [8]. The available data suggest causal direction of the association between urticaria and RA
urticaria can cause a decrease in the quality of life and increase in European ancestry population using the two-sample MR
societal costs of health hazards [9, 10, 11, 12]. In recent years, analysis.
a growing body of evidence suggests that urticaria has been
linked to autoimmune diseases, including RA [13]. One reg-
istry study has revealed that chronic urticaria may develop Materials and methods
into RA. And a retrospective cohort study has reported that All participants provided written informed consent for
the incidence of RA was higher in patients with urticaria than participation in the study as approved by the ethical commit-
those without urticaria [14]. A follow-up of 100 individuals tees of each of the institutional review boards. Our analyses
with chronic urticaria found it was estimated that 21% of in the study were based on publicly available genome-wide
Received 19 March 2021; Accepted 29 July 2021
© Japan College of Rheumatology 2021. Published by Oxford University Press. All rights reserved.
For permissions, please e-mail: journals.permissions@oup.com2 Yu et al.
association study (GWAS) summary statistics, and no addi- the individual IV. Then, we used the two-sample MR to assess
tional ethical approval and informed consent were required. the potential causal relationship of urticaria on RA, includ-
ing four MR methods: inverse variance weighted (IVW),
Instrumental variable selection weighted median (WM), MR-Egger, and MR pleiotropy resid-
For a causal interpretation of MR analysis to be valid, three ual sum and outlier (MR-PRESSO) [21–24]. We used the
key assumptions must be satisfied (Figure 1). Briefly, (1) these IVW method, which assumes all IVs must satisfy the three
genetic variants should be robustly associated with urticaria, key assumptions of MR method. Other MR approaches, i.e.
(2) be not associated with any confounders of the urticarial MR-Egger, WM, and MR-PRESSO, were also compared with
RA, and (3) influence RA only through the effect of urticaria. the IVW method. The MR-Egger method gives an unbiased
We selected single-nucleotide polymorphisms (SNPs) associ- estimate if genetic pleiotropy is present. The WM method
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ated with urticaria from GWAS summary statistics. First, orders the MR estimates of each IV weighted for the inverse
since few loci identified by urticaria GWAS have achieved of their variance, and this method assumes that at least half
a genome-wide significance, we selected 74 SNPs with sug- of the IVs are effective. MR-PRESSO test removed outliers to
gestive genome-wide significance (p < 1 × 10−5 ). Second, we reduce heterogeneity and the effect of horizontal pleiotropy.
clumped these 74 SNPs to remove the linkage disequilibrium We performed heterogeneity tests, including Cochran’s Q
(r2 = 0.01, kb = 5000). A total of 11 independent SNPs associ- test [25], single-variant analysis, and leave-one-out variant
ated with urticaria were identified. Third, the R2 value of each analysis [26]. MR-PRESSO was used to identify outliers. And
SNP was used to assess the proportion of variance in urticaria. we used MR-Egger regression intercept to examine the direc-
F-statistic was used to estimate the instrument strength of each tional pleiotropy. Besides, the MR-PRESSO global test was
SNP [18]. Last, we used proxy SNPs to replace the unavail- used to examine the horizontal pleiotropy among IVs. In
able SNPs. Palindromic variant would not be excluded if it addition, we investigated pleiotropy by looking up each SNP
would not incur ambiguity. in PhenoScanner (www.phenoscanner.medschl.cam.ac.uk/)
[27, 28] with the settings p < 1 × 10−5 and inclusion of proxy
variants in linkage disequilibrium (LD) (r2 > 0.8) to evaluate
Data sources
the risk factors for RA, i.e. smoking [29], silica [30], vitamin
The GWAS summary data of urticaria were derived from D [31], and body mass index [32]. Also, we inputted the IVs
FinnGen Data Freeze 2 (https://finngen.gitbook.io/docume associated with urticaria to determine the associations with
ntation.GWASID:finn-a-L12_URTICARIA). FinnGen Data metabolites and disease traits. Finally, the obtained results of
Freeze 2 has been released to public on 14 January 2020. It MR methods and leave-one-out variant analysis are visualised
consists of 96,499 individuals, 16,152,119 variants, and 1122 in the form of forest plot and scatter plot.
endpoints. The GWAS summary data of urticaria included The result of MR analysis was presented as odds ratio (OR)
96,018 (1860 cases and 94,158 controls) samples of Euro- with 95% confidence interval (CI). All analyses were per-
pean ancestry. All reported genomic coordinates were in formed by the TwoSampleMR packages (Version 0.5.5) and
HG19/GRCh37. MR-PRESSO packages (Version 1.0) with R (Version 4.0.3).
The data on RA were accessed from a GWAS meta-analysis
using European samples [19]. This GWAS meta-analysis was
conducted in a total of >100,000 subjects (29,880 RA cases Results
and 73,758 controls) by evaluating ∼10 million SNPs. All RA
We included 11 SNPs as IVs of urticaria in this MR analysis
cases fulfilled the 1987 criteria of the American College of
explaining about 0.24% of the variance in the risk of urticaria.
Rheumatology for RA diagnosis [20]. In order to minimise
The data of 11 urticaria-associated SNPs are presented in
population stratification, only European continental ancestry
Supplementary Table S1. The median value of F-statistic was
groups were included in our analysis. The summary statistics
21 (range 20–23). Since all F-statistics were greater than 10,
data for RA in European ancestry included 2843 cases and
suggesting that IVs were strong predictors of endogenous vari-
5540 controls. Biobank Japan Project provides this summary
ables. Two SNPs (rs79654727 and rs17281228) were not
statistics.
presented in the RA dataset. And one palindromic variant
(rs17391162) was not excluded since it would not incur ambi-
Statistical analysis guity while inferring the strand. No proxy SNP was identified.
First, the Wald ratio (the ratio of SNP-outcome estimate to Besides, we found no SNPs associated with the risk factor of
SNP-exposure estimate) was used to derive MR estimates of RA. Finally, a total of 9 SNPs were available for this MR
analysis (Supplementary Table S2).
The results between urticaria and RA obtained from the
MR analysis were listed in Table 1. The result of the IVW
method suggested that urticaria was associated with the
increased risk of RA (OR = 1.114, 95% CI = 1.024–1.211,
Table 1. MR estimates of urticaria on RA from four methods.
Method IVs (SNPs) OR (95%CI) p
MR-Egger 9 1.089 (0.879–1.349) .460
WM 9 1.125 (1.002–1.262) .047
IVW 9 1.114 (1.024–1.211) .011
MR-PRESSO 9 1.114 (1.024–1.211) .035
Figure 1. Schematic of the MR study and key assumptions.Urticaria and increased risk of rheumatoid arthritis 3
Downloaded from https://academic.oup.com/mr/advance-article/doi/10.1093/mr/roab052/6360891 by guest on 10 December 2021
Figure 2. Forest plot of urticaria-associated SNPs potential effects on RA. This figure displays the forest plot of pooled MR estimates and individual
estimate between IV and the risk for RA. The spots represent the MR effect size for urticaria on rheumatoid arthritis.
Table 2. The heterogeneity across all instrumental variables (Cochran’s Q
test).
Method Q Q_df Q_pval
MR-Egger 8.038 7 .329
IVW 8.096 8 .424
IV and the risk for RA. Figure 3 shows a scatter plot of
MR results.
The Cochran’s Q test suggested that the effect esti-
mates across all IVs were no heterogeneity (QEgger = 8.038,
pEgger = .329, QIVW = 8.096, pIVW = .424, Table 2). The
MR-Egger regression intercept showed no directional
pleiotropy for the association between urticaria and RA
(MR-Egger = 0.005, p = .828). Likewise, MR-PRESSO global
Figure 3. Scatter plot of urticaria-associated SNPs potential effects on test showed no horizontal pleiotropy among individual SNPs
RA. These black dots represent the individual IV association with urticaria of urticaria for RA (p = .455). MR-PRESSO did not iden-
risk plotted against the individual IV association with RA. Vertical and tify any outliers for IVs. The results obtained from the
horizontal lines present the 95% CI of OR for each IV. The slope of lines
phenoscanner are set out in Supplementary Table S3. Out of
represents the estimated causal effect of MR methods.
the nine SNPs variants genetically associated with urticaria,
four SNPs were located in the intron region of the fol-
p = .011), and the result was supported using the MR- lowing genes, PIP5K1B, AKAP13, KDM4C, and C12orf75;
PRESSO method (OR = 1.114, 95% CI = 1.024–1.211, two SNPs were located in the intergenic region of RP11-
p = .035). The WM result was in agreement with the 173M1.5 and CD200R1L; one SNP was located missense of
result of the IVW method (OR = 1.125, 95% CI = 1.002– the MUM1 gene; also, one SNP was located downstream of
1.262, p = .047). Although the MR-Egger method showed the CSN3 gene; and one SNP was located 3_prime_UTR of
broader CIs due to lower statistical power but sug- the CBLB gene. Most of the SNPs had no known disease
gested the same direction of effect (OR = 1.089, 95% associations. Analysis using PhenoScanner in the metabolites
CI = 0.879–1.349, p = .460). We calculated the individual option showed that none of the nine SNPs was associated
and pooled MR estimates. Figure 2 displays the forest plot with any metabolites. Further analysis using PhenoScanner
of pooled MR estimates and individual estimate between in the diseases and traits option showed that none of the4 Yu et al.
nine SNPs were associated with any risk factor for RA. analysis. Second, the databases are European ancestry; thus,
One SNP, rs1676583, was associated with unspecified t-cell our findings may not be extrapolated to other races. Third,
lymphomas, but the results of leave-one-out sensitivity anal- data are limited on GWAS for specific types of urticaria.
ysis (Supplementary Figure S1) did not suggest a significant Whether any specific type of urticaria has a causal relationship
influence. with RA warrants further investigation. Last, the non-linear
association of urticaria and RA could not be evaluated as
the MR analysis assumes a linear association between the
Discussion exposure and outcome.
Previous studies have investigated the link between urticaria
and RA. Some studies support the view that RA is one of the Conclusions
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most common manifestations of urticaria-like lesions [33–35]. These results suggest that urticaria increases the risk of RA in
For example, one registry-study has revealed that chronic Europeans. Further exploration of the molecular mechanisms
urticaria may develop into RA [36]. And a retrospective is needed to account for the relationship between urticaria and
cohort study has reported that the incidence of RA was higher RA.
in patients with urticaria than those without urticaria [14]. A
follow-up of 100 individuals with chronic urticaria found it
was estimated that 21% of urticaria patients have autoim- Acknowledgements
mune diseases, which include RA [15]. Besides, some studies The authors sincerely appreciate Biobank Japan Project and
have reported patients of urticaria lesions associated with FinnGen study related investigators for sharing GWAS sum-
arthritis [37, 38]. Together, these studies provide important mary statistics.
insights into a possible epidemiological link between urticaria
and RA. These results are in accord with our findings.
However, these results must be interpreted with caution Supplementary data
because observational studies might cause spurious associa- Supplementary data is available at Modern Rheumatology
tions and present residual confounders [39]. As a result, we online.
used the two-sample MR analysis to assess whether there is
a causal association between urticaria and RA in European
ancestry. Conflict of interest
There was no evidence of horizontal pleiotropy or hetero- None declared.
geneity across instrumental SNPs via MR analysis. Therefore,
we give priority to the results of IVW method, which is
without regard to the intercept of the regression [40]. It is
Funding
noteworthy that our study found urticaria can elevate the This work was financially supported by the National Natural
risk of RA. These results confirm a causal relation between Science Foundation of China (grant number 81641123) and
urticaria and RA. Taken together, these results suggest that the Fundamental Research Funds for the Central Universities
patients in urticaria are easier to get RA. Given the causal (grant number 2042017kf0193).
link, we recommended that RA prevention, management, and
treatment should be enhanced for urticaria prevention. Authors’ contributions
One biological mechanism may explain the increased risk
of RA associated with urticaria. Mast cells are the primary Z.J.Z. designed the study and edited the manuscript. X.Y.
effector cell of urticarial, and they play an ongoing role in the performed the statistical analysis and drafted the manuscript.
arthritic process [41, 42]. However, further work is required M.G.D. and Z.Y.T. reviewed and edited the manuscript. All
to better explore the mechanisms underlying urticaria and RA. authors contributed to the article and approved the submitted
There are several strengths of this study. First, this study version.
reported for the first time a causal relationship between
urticaria and RA using a two-sample MR approach. And References
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