Correlation between the geographical origin of Helicobacter pylori homB-positive strains and their clinical outcomes: a systematic review and meta-analysis

Background In general, all virulence factors of Helicobacter pylori (H. pylori) are involved in its infections. However, recent studies have shown that the homB gene is one of the virulence genes that affects the severity of the clinical results of this bacterium. Methods The main purpose of this study was to investigate the relationship between the presence of homB gene in H. pylori and the progression of its infection to peptic ulcer and gastric cancer. In the present study, we conducted a systematic search to collect all articles related to the effect of homB-positive strains on clinical outcomes. Finally, 12 eligible studies according to our criteria were included in this meta-analysis and the effect of homB gene on gastric ulcer and gastric cancer diseases was evaluated by summary odds ratio (OR). Results Current results showed that the homB-positive strains significantly increase the risk of peptic ulcer (OR 1.36; 1.07–1.72 with 95% CIs), especially in western countries (OR 1.61; 1.20–2.14 with 95% CIs). Moreover, we observed a positive association between the homB gene and risk of gastric cancer (OR 2.16; 1.37–3.40 with 95% CIs). In addition, based on subgroup analysis, it was found that the presence of this gene in H. pylori strains increases the risk of gastric cancer in the Asian population (OR 3.71; 1.85–7.45 with 95% CIs). Conclusions Overall, in the present study we found that homB gene is responsible for the progressing of primary infection to severe complications, in particular peptic ulcer in western countries and gastric cancer in Asian countries.


Background
Helicobacter pylori (H. pylori), formerly known as Campylobacter pyloridis, is a gram-negative, microaerophilic, helical, and motile (lophotrichous flagella) bacterium that colonizes the gastric sub-mucosa of more than 50% of the world's population [1]. Infection with this pathogen generally occurs in childhood and may continue asymptomatic for life [2]. However in 15-20% of infected people, the primary infection progresses to worse conditions such as peptic ulcer disease (PUD), duodenal ulcer (DU), gastric ulcer (GU), gastric adenocarcinoma, and mucosa-associated lymphoid tissue (MALT) lymphoma; PUD rarely occurs in children [3][4][5]. According to the literature, interactions between bacteria, the host genome, and environmental conditions play a decisive role in the development of primary infection to severe clinical outcomes [6]. Although the main role of some Keikha and Karbalaei BMC Gastroenterol (2021) 21:181 virulence genes such as vacA and cagA is well known, the effect of other virulence factors on bacterial pathogenesis is unclear and needs further study [7][8][9][10]. The H. pylori genome encodes about 1,100 genes, of which 500-600 are strain-specific genes and in turn contribute in various clinical outcomes [11,12]. Outer membrane proteins (OMPs) are among the most divergent proteins in this bacterium, encoded by 4-5% of the bacterial genome [13]. The hom genes are known as a small paralogous family of adhesion proteins and are distinguished from other OMPs by the signal sequence and hydrophobic motif located in the C-terminal domain [14]. The hom family consists of four classes homA, homB, homC, and homD, so that homA and homB are encoded by one locus, while each of homC and homD is encoded by a distinct locus [14,15]. Despite 90% similarity between homA and homB, studies show that the distribution of each is specific in each geographic area, so that homA is a diagnostic marker for East Asian strains, and homB has global distribution [15,16]. Recently, the relationship between homB and severe clinical outcomes has attracted much attention; the product of homB, HomB is a virulence factor that contributes to several bacterial activities such as biofilm formation, antibiotic resistance, delivery of CagA from type 4 secretion system (T4SS), induction of IL-8 production, gastritis, corpus atrophy, and persistent colonization [17][18][19][20]. Oleastro et al. first showed a significant association between the homb-positive strains and the progression of infection to PUD in Portuguese children [21]. In another study, they found that the ability of homB knockout mutant strains to bind to gastric epithelium was significantly reduced compared to homB-positive strains [22]. Jung et al. found a positive correlation between the simultaneous presence of cagA and homB genes in East Asian strains; they showed that having two copies of homB gene could increase the risk of PUD [23]. In contrast, the homA gene is correlated with non-ulcer dyspepsia (NUD), in other words, it appears that there is no significant relationship between homA and gastritis and corpus atrophy [16,19]. In the present meta-analysis, we evaluated the association between homB-positive strains of H. pylori and several clinical outcomes.

Literature search strategy
At the first, a systematic search was performed using global databases such as Scopus, Web of Science, and PubMed to collect all the studies relevant to our purpose. All selective studies were related to the association between homB and clinical outcomes such as PUD, duodenal ulcer, gastric ulcer, and gastric cancer (GC). In this study, articles published up to December 2020 were retrieved separately by two authors (MK1 and MK2).
Search terms were selected based on MeSH thesaurus including "Helicobacter pylori", "H. pylori", and "homB"; articles were search regardless of publication date and language.

Inclusion criteria
In this step, we used the full-text of all published casecontrol studies examining the relationship between the homB and severe clinical outcomes (patients with gastritis and NUD were considered as control group, while patients with PUD, GU, DU, and GC were assumed as case group). The inclusion criteria in adult populations included; (1) published researches on the presence of homB confirmed by polymerase chain reaction (PCR) or immunoblotting, (2) exploratory studies on the association between H. pylori homB gene and severe clinical outcomes, (3) articles published in English, and (4) articles about human adults. All relevant documents were independently reviewed by two authors (MK1 and MK2).

Statistical analysis
All statistical analysis were performed using Comprehensive Meta-Analysis (CMA) software version 2.2 (Biostat, Englewood, NJ, USA). The relationship between homB and clinical outcomes was estimated according to the summary odds ratio with 95% CIs. Heterogeneity between studies was assessed through parameters such as I 2 index and Cochrane Q test, so that in cases of high heterogeneity (I > 25% and Cochrane Q test p value > 0.05) and non-significant heterogeneity, we used from random-effects model, and fixed-effects model, respectively. Finally, the publication bias of selected studies was measured using funnel plot asymmetry, Egger's p value, and Begg's p value test [33].

Characteristics of selected studies
Following the initial systematic search, 138 related articles were collected and 126 articles were deleted according to inclusion criteria. The details of comprehensive search processing and study selection are summarized in Fig. 1. In the screening phase, we removed the irrelevant articles such as articles with unclear results and articles that did not meet our criteria. In total, out of 12 studies that met our inclusion criteria, 2930 patients and 2016 strains of H. pylori were evaluated. Of these, two studies compared the relationship between homB and clinical outcomes in both Western and Asian countries [24,26], Six studies have been conducted on Western countries [21,22,25,[29][30][31], as well as four studies on the population of Asia [23,27,28,32] (Table 1). Final results of some of eligible studies were contradictory and varied [22,24,26,27].

Association between homB and PUD
With the exception of the study by Casarotto  According to the information, infection with homB-expressing strains appears to increase the risk of PUD (Fig. 2). Due to the high heterogeneity between studies, we used subgroup analysis to determine the role of homB in the development of primary infection to PUD in Western and Asian countries. Interestingly

Publication bias analysis
Publication bias was estimated based on both Begg's p value and Egger's p value tests, although no significant publication bias was observed. However, funnel plot asymmetry indicated a slight publication bias in the current meta-analysis.

Discussion
H. pylori is one of the most successful pathogens that colonizes the stomach of half the world's population. This bacterium can cause serious clinical consequences such as chronic gastritis, PUD, gastric atrophy and GC [34,35]. According to documents, approximately 63% of GC cases worldwide are caused by H. pylori infection, and the bacterium is also responsible for 75% of gastric ulcers and 90% of duodenal ulcers [27]. The strains of this bacterium are genetically diverse and harbor different virulence genes [36,37]. Studies in recent decades have shown that these genes are strain-specific (e.g. vacA, cagA, and omp) and play an important role in the immunopathogenesis of H. pylori and in the development of serious clinical outcomes [13,17,22,27,32,38]. In several studies, the role of the homB gene in the pathogenesis of this pathogen was controversial; difference in results are related to differences in diet, environmental condition, hygiene status, age, socioeconomic level, and low sample size [29,39]. Nevertheless, in the present study, we conducted a comprehensive literature review to assess the role of homB in the progression of primary infection to PUD and GC in Western and Asian countries. Oleastro et al. in their study showed that the presence of the homB gene is significantly higher than the homA gene in Portuguese children with PUD; "on" genotypes consistent cagA/vacAs1/ hopQI/oipA/homB strongly were associated with PU disease in children under four years of age [22]. In contrast, in studies on populations of Iraq, Turkey, and South Korea, none of homA and homB genes were correlated with PU disease [26,40]. Interestingly, all studies in Western children have shown that the homB gene is associated with PUD, while the homA gene is more prevalent in the NUD [21,22,25]. In present study, frequency of homB gene in patients with affected to PUD and GC, severe clinical outcomes significantly was more prevalent than gastritis/NUD cases (54.4% and 39.7%, respectively). In addition, the summary OR showed that there was a significant relationship between homB-positive genotype and progression to PUD, especially in Western countries (OR: 1.16; 1.20-2.14 with 95% CIs; p value: 0.01), while in Asian countries there was no such relationship (OR: 0.89; 0.57-1.40 with 95% CIs; p value: 0.01). Therefore, our findings confirmed the results of previous studies. Also, strains isolated from Western countries contained two copies of the homB gene, but most infectious strains in Asian countries had only one copy of each of the homA and homB genes [19,22,31]. Related articles showed that the number of OMP copies also affects the status of bacterial compatibility and plays a role in the formation of clinical outcomes [14,41,42]. Recently, the role of homB as a cofactor in the increase of gastric adenocarcinoma in Asian countries has attracted much attention. The homB gene enhances the attachment of H. pylori to gastric epithelium, leading to dysregulation of normal signaling pathways and genetic instability [27,40]. In addition, this gene increases the risk of GC through interferences such as inducing the inflammatory response, persistent infection, and gastric atrophy [23]. Abadi et al. showed that 78% of the strains isolated from GC patients contained the homB gene [27]. Jung et al. found that cagA-independent homB was associated with GC in Western countries [23]. However, in a study on the Chinese population, despite the presence of the homB gene in all isolated strains in patients with PUD and GC, no significant correlation was observed [39]. According to our results, a strong correlation was observed between the homB gene and the risk of GC in the Asian population (OR: 3.71; 1.85-7.45 with 95% CIs; p value: 0.01), whereas this correlation did not exist in Western countries (OR: 1.42; 0.79-2.54 with 95% CIs; p value: 0.66). Thus, depending on the geographical area, the homB gene appears to lead to PUD and GC in Western and Asian countries respectively. In several studies, correlation between homB and other virulence factors, especially vacA, cagA, oipA, hopQI, and babA in patients with PUD and GC was investigated. Sterbenc et al. in their study observed that there was no significant difference in the histopathological characteristics of PUD in both groups of children with and without the genotype profiles vacAs1m1/cagA/babA2/hompB [29]. Similar to this study, Oleastro et al. found that homB, independent of the cagA + /vacAs1 genotype profile, increases PUD risk in Western countries [24]. However, in other studies, it was shown that there is a significant correlation between homB and cagA, and homB also acts as a cofactor in complications such as PUD [23,27,40]. Due to the lack of raw data and uncertain results, we could not evaluate the relationship between homB and cagA in patients with PUD, but in GC cases, a weak correlation was observed (OR: 1.47; 0.95-2.28 with 95% CIs; p value: 0.79; I2: 80.15; Q-Value: 15.1; p value: 0.02; Egger's p value: 0.07; Begg's p value: 0.08). In the end it must be said, our study had several limitations such as low sample size, low number of included studies, inaccessibility to raw data, high heterogeneity in some studies, and also slight publication bias based on asymmetry of funnel plot. Hence, we need further studies to confirm the present findings.

Conclusions
Our results suggest that the presence of the homB gene in H. pylori strains contributes to the development of primary infection to severe clinical outcomes. We found that the homB gene could increase the risk of PUD in Western countries as well as GC in Asian countries.