{'fraction_non_alphanumeric': 0.037494429944617734, 'fraction_numerical': 0.031064994589089057, 'mean_word_length': 5.163201255394272, 'pattern_counts': {'":': 0, '<': 5, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Association of Fucosyltransferase 2 Gene Variant with Inflammatory Bowel Diseases: A Meta-Analysis

Background Ulcerative colitis (UC) and Crohn’s disease (CD) are the 2 main type of inflammatory bowel diseases (IBDs). Several studies have been conducted to investigate the association of fucosyltransferase 2 gene (rs601338) variant with UC and CD, but the results were inconsistent. Here, we performed a meta-analysis to clarify this issue based on a relatively larger sample size. Material/Methods A systematic literature search was conducted in PubMed, Embase, CNKI, and Chinese Wangfang databases up to 31 May 2018. Meta results were synthesized by using crude odds ratio with 95% confidence interval. Heterogeneity, sensitivity analysis, subgroup analysis, and publication bias were assessed using STATA 11.0 software. Results A total of 8 relevant studies including 3874 IBDs patients (1872 UC cases, 2002 CD cases) and 5445 controls were included for meta-analysis. We found a significant association between rs601338 A allele and risk of IBDs in the Chinese population (OR=2.35, 95%CI=1.66~3.34, P=0.001), but not in whites. Stratified by disease type, we found a significant association between rs601338 polymorphism with CD and UC in the Chinese population, but not in the white population. In addition, funnel plot and Egger’s linear regression test suggests no publication bias in all genetic models. Conclusions Fucosyltransferase 2 gene (rs601338) polymorphism is associated with susceptibility to IBD, UC, and CD in the Chinese population, but these results might not be generalizable to other ethnic populations. Further well-designed studies are needed to confirm these findings.


Background
Inflammatory bowel diseases (IBDs) are a group of chronic and nonspecific inflammatory diseases that mainly comprise ulcerative colitis (UC) and Crohn's disease (CD). Clinical features in both disorders include diarrhea, abdominal pain, weight loss, and increased risk of developing colorectal cancer [1]. The precise etiology of IBDs is not yet fully understood, but numerous studies have suggested that multiple factors, including heredity, environment, infection, and immunity, which interact each other, are involved in the development and exacerbation of the diseases [2]. With the wide application of genome-wide association studies (GWASs) and candidate gene association studies, many susceptibility loci for the predisposition of CD have been identified in the populations of northern European origin, such as NOD2, ATG16L1, and IRGM [3][4][5][6][7][8]. More than 30 single-nucleotide polymorphisms are definitively known to be associated with CD, although these loci account for only a minority of the genetic variance to CD in this population [9].
In humans, Fucosyltransferase (FUT) 2 gene is located in the q13 region of chromosome 19 and encodes a specific fucosyltransferase enzyme. FUT2 enzyme can catalyze transfer of the sugar fucose to acceptor substrates and plays a vital role in the synthesis of histo-blood group antigens (HBGA), including ABH and Lewis (a) antigens [10]. In the past decade, an increasing number of studies has been conducted to explore the association between FUT2 (rs601338) gene polymorphism and susceptibility to IBDs in different populations [11][12][13][14][15][16][17][18]. However, the role of genetic variations on FUT2 gene (rs601338) in the progression of UC and CD remained unclear. These inconsistent results indicate the need for further analysis of the effect of rs601338 polymorphism on UC and CD risks.
To date, no meta-analysis has been conducted to determine the correlation of FUT2 gene (rs601338) polymorphism with UC and CD risk. Thus, the aim of this study was to clarify the relationship between FUT2 (rs601338) gene variant with UC and CD risk by performing a meta-analysis.

Identification of eligible studies
A comprehensive literature search was performed in the following databases: PubMed, EMBASE, CNKI, and Chinese Wangfang, up to 31 May 2018. The search terms and keywords were: "polymorphisms or variants", "fucosyltransferase 2 or FUT2", and "ulcerative colitis or Crohn's disease or inflammatory bowel disease". The research subjects were limited to humans, and the language was limited to English or Chinese. References from retrieved papers were also examined for additional studies.

Inclusion criteria
Studies were included in the meta-analysis if they met the following criteria: (1) study on the genetic relationship of fucosyltransferase 2 gene (rs601338) with ulcerative colitis or Crohn's disease or inflammatory bowel disease; (2) case-control study design; (3) genotype distributions were available for both cases and controls to calculate an OR and its 95%CI. Exclusion criteria were as follows: (1) abstract, case report, editorial comment, and review; (2) repeated publication; (3) studies with insufficient genotypic data; (4) studies performed on animal models.

Quality score assessment
The Newcastle-Ottawa scale was used to evaluate the quality of studies. The scale consists of 3 aspects: selection, comparability, and exposure, with a maximum score of 9 [19]. Any disagreements were adjudicated by a third reviewer. A total score of £3, 4-6, ³7 was considered to indicate low-, medium-, and high-quality studies, respectively.

Data extraction
Two investigators independently and carefully extracted available data from each eligible study. The following details were extracted from all eligible studies: (1) first author's name, (2) year of study publication, (3) origin of participants, (4) race of included subjects, (5) source of control population, (6) sample size of cases and controls, and (7) results of the Hardy-Weinberg equilibrium (HWE) test for the controls. Discrepancies were resolved by discussion within our research team.

Statistical analysis
The effect sizes were calculated using odds ratios (ORs) and 95% confidence interval (95%CI) to evaluate the association between the fucosyltransferase 2 gene (rs601338) polymorphism and inflammatory bowel disease, ulcerative colitis, and Crohn's disease risk. Pooled ORs were calculated by using 4 genetic models: allelic model (A vs. G), dominant model (AA+AG vs. GG), recessive model (AA vs. AG+GG), and homozygous model (AA vs. GG).
Heterogeneity among the eligible studies was analyzed by the chi-square test based on the Q statistic, with significant heterogeneity considered to be present at P value <0.10 [20]. The I 2 statistic test was used to quantify the heterogeneity across individual studies (P<0.10 and I 2 >50% suggested heterogeneity).
The fixed-effects model on the Mantel-Haenszel method was used to estimate the pooled OR if I 2 <50%; otherwise, the random-effects model on the DerSimonian and Laird method was used. Heterogeneity was also quantified by the I 2 test, the values of I 2 in 0-25%, 26-50%, and 50-100% were considered as lower, moderate, and high heterogeneity, respectively.
We evaluated the potential publication bias by using the Begg's funnel plot and the Egger's linear regression tests [21,22], which measures funnel plot asymmetry on the natural logarithm scale of the effect size. One-way sensitivity analysis was used to assess which study has a significant impact on the stability of results.
All statistical analyses were performed using STATA version 11.0 software (STATA Corporation, College Station, TX). A 2-sided P value of less than 0.05 was considered statistically significant.

Characteristics of eligible studies
As Figure 1 shows, the selection process of the studies involved in this meta-analysis was performed according to PRISMA flow diagram. We found 44 potentially relevant papers from databases (16 in PubMed, 17 in Embase, 8 in CNKI, and 3 in Chinese Wangfang). Among these, 19 duplicates were removed, and another 14 studies were removed due to irrelevant topics, reviews, and not about IBDs or fucosyltransferase 2 gene. The remaining 11 studies underwent full publication review, during which 3 studies were excluded due to insufficient data for calculating OR and 95%CI. Finally, a total of 8 studies were included in this meta-analysis. The basic characteristics of these studies are shown in Table 1.
Further subgroup analysis was carried out by grouping the data based on the population (7 studies in Chinese and 3 studies in whites). The results showed significant associations between FUT2 gene (rs601338) polymorphism and risk for IBDs in allelic model (A vs. G: OR=2.35, 95%CI=1.66~3.34, P<0.001) and dominant model (AA+AG vs. GG: OR=2.53, 95%CI=1.76~3.63, P<0.001), but not in recessive and homozygous models in Chinese populations. We did not find any significant association between FUT2 gene (rs601338) polymorphism and risk for IBD in all genetic models in whites.  Figure 3).
Because there was only 1 study in a white population, it was excluded from subgroup analysis.

FUT2 gene (rs601338) variant with CD
The detailed results of the association between FUT2 gene (rs601338) variant and risk of CD are shown in Table 3 29, P=0.003), but not in recessive and homozygous models in Chinese populations. We did not find any significant association between FUT2 gene (rs601338) polymorphism and risk for CD in all genetic models in whites.

Sensitive analysis and publication bias
Sensitive analysis was conducted to determine if our results were substantially affected by any individual study. Our results suggested that the pooled effects were not significantly influenced by the omission of any individual study. Begg's and Egger's tests were performed in all comparisons except for comparisons of FUT2 gene (rs601338) polymorphism and risk for CD in all genetic models in Chinese and white populations because only 2 studies were included in these comparisons. Begg's funnel plots showed the shape was symmetrical, and Egger's linear regression analysis indicated no publication bias (Tables 2, 3 and Figure 4).

Discussion
The associations between FUT2 genes and susceptibility to IBDs in different populations have been widely reported.
However, the outcomes of these studies vary from study to study. Hu et al. found that mutant A allele and AG genotype of FUT2 gene (rs601338) were significantly increased in CD patients in Chinese populations [14], but they failed to find any association between rs601338 of FUT2 and UC severity [17]. Aheman et al. [12] observed that rs601338 was statistically correlated with UC risk in Chinese populations, and AA homozygous frequencies of rs601338 were lower in the UC groups than in the controls ( Table 3. Meta-analysis of the association of fucosyltransferase 2 (rs601338) polymorphism with ulcerative colitis and Crohn's disease.
R -odds ratio; CI -confidence interval. P a -P value for between-study heterogeneity; P b -P value for test of the association between rs601338 polymorphism with ulcerative colitis and Crohn's disease.
showed that the A allele of rs601338 was associated with UC and UC+CD, indicating that FUT2 secretor status may play a role in IBD in the Finnish population. Wu et al. [18] showed that mutant A allele and genotype of FUT2 rs601338 were significantly increased in CD patients from southeast China. However, there were no statistically significant differences in allele and genotype frequencies of FUT2 gene in UC patients compared to controls.
The inconsistency among different studies seems to be mainly due to the relatively small sample size of most studies and the different populations studied, and there was inadequate statistical power to detect a slight association between FUT2 genes and susceptibility to IBDs. Other possible explanations include diverse study designs and methods, differences in race and geography, and publication bias. Meta-analysis is an effective statistical method that pools the results of independent studies to get comprehensive results. It has been widely used in evaluating the relationship between candidate genes and complex diseases with genetic predisposition.
This systematic review and meta-analysis was performed to determine the association between rs601338 polymorphism and susceptibility to IBDs. The results demonstrated that rs601338 polymorphism was associated with IBD in allelic and dominant genetic models in Chinese, but not in whites, in all genetic models. After being stratified by disease type, similar results were also presented in CD. In UC, this association was only observed in the Chinese population. The different results presented in different populations illustrates that FUT2 genes polymorphisms with IBDs susceptibility might be associated with race. Of note, the results of rs601338 polymorphism with CD risk in whites in this meta-analysis were inconsistent with the findings of the previous GWAS replication study (rs601338 not covered in this GWAS, but covered in the replication study), which used a confirmatory cohort of 1174 cases and 357 controls and demonstrated that FUT2 rs601338 is associated with CD in whites [11]. The cause of this inconsistency may be population differences (genetic and/or microbial), but it is also not unlikely that our study lacked power to identify an association of FUT2 with CD. A larger follow-up study of CD in the white population will help elucidate the true effect of FUT2 variation in CD risk.
Although the etiology and pathology of IBDs are unknown, a previous study showed that imbalances in the species of bacteria that live in our guts may be linked to the development of diseases [23]. FUT2 is the key enzyme determining the presentation of HBGA antigens in intestinal mucosal tissue [24]. HBGA, including ABH antigens and Lewis antigens, act not only as binding sites for intestinal microbes such as Helicobacter pylori, Campylobacter jejuni, norovirus, and rotavirus, but also are a carbon source for microbes, including Escherichia coli [25][26][27]. The FUT2 gene that codes for an alpha-(1,2)-fucosyltransferase is a Golgi stack membrane protein involved in creation of a precursor of the H antigen, which forms the basis of A and B antigen synthesis [28]. FUT3 encodes alpha-(1,3/4)-fucosyltransferase, which is required to synthesize Lewis a antigens, and mostly utilizes the H antigen, determined by FUT2, as a substrate to synthesize Lewis b antigen [29]. The intestinal microbiota can be influenced by FUT2 and FUT3, which control presentation of HBGA on the gastrointestinal mucosa and in bodily secretions [30]. FUT2 gene polymorphisms influence presentation of HBGA antigens in the gastrointestinal mucosa and their secretion, and further affect the microbiota composition in the human intestine [31].
Although an advantage of this meta-analysis is pooling goodquality individual studies with relatively large sample sizes for a comprehensive result, but several limitations should be addressed when interpreting our results. Firstly, we included relevant articles published only in English and Chinese, so language bias may exist in this study. Secondly, most of the studies are conducted in Chinese populations, and there were few studies in the white subgroup analyses, which could have led to insufficient statistic power to detect weak relationships. Thirdly,   the onset of IBD is affected by risk factors such as age, sex, genetic variants, and exposure to environmental factors and their interactions. However, only gene polymorphisms were considered in this study. The effects of gene-gene and geneenvironment interactions on the initiation and development of the disease need to be further studied.

Conclusions
Our meta-analysis provides statistical evidence that the FUT2 gene (rs601338) polymorphism is associated with IBD, UC, and