Keywords

1 Introduction

Helicobacter pylori (H. pylori) has a genome of 1,600 genes, which are well conserved, but its variants are very diverse and complex in terms of combination and structure of the genes by region depending on the physiological and ecological changes in the strains and hosts, which makes an interesting subject as a model for adaptation and evolution of microorganisms [1, 2].

H. pylori strains have evolved continuously within the human body when mankind left Africa and moved to America and Oceania approximately 58,000 years ago, and it is predicted that the virulence of H. pylori has been changed along [1, 3] (Fig. 6.1). For example, a genetic analysis of a strain isolated from Peruvians in South America demonstrated that while cag pathogenicity island (PAI) gene of Peruvian natives did not contain cytotoxin-associated gene A (cagA) gene, cagA gene of Europeans was newly inserted after the arrival of Europeans. And by a comparison of cagA DNA sequences, it was also confirmed that genes of Indo-Aryans and Neolithic man in the Crescent region coexist in the Indian strain. These genetic diversities of virulence factors including cagA and vacuolating cytotoxin (vacA) depending on the region suggest a possibility that pathogenicity of H. pylori is an acquired characteristic for survival and adaptation through the process of evolution and transfer [3–5].

Fig. 6.1
figure 1

Distribution of H. pylori genotypes before Columbus found the New World and human migration to America and Oceania began. There are seven modern H. pylori population types: hpEurope, hpEastAsia, hpAfrica1, hpAfrica2, hpAsia2, hpNEAfrica, and hpSahul. hpEurope includes almost all H. pylori strains isolated from ethnic Europeans, including people from countries colonized by Europeans. Most H. pylori isolates from East Asia belong to hpEastAsia, which includes hspMaori (Polynesians, Melanesians, and native Taiwanese), hspAmerind (American Indians), and hspEAsia (East Asia) subpopulations. hpAsia2 strains are isolated in South, Southeast, and Central Asia; hpAfrica1 in West Africa, South Africa, and African Americans. hpNEAfrica is predominantly made up of isolates from Northeast Africa. hpAfrica2 is very distinct from any other type and has currently only been isolated in South Africa. hpSahul is a novel group specific to H. pylori strains isolated from Australian Aborigines and Highlanders of New Guinea. H. pylori is predicted to have spread from East Africa over the same time period as anatomically modern humans (~58,000 years ago) and has remained intimately associated with their human hosts ever since. Estimated global patterns of H. pylori migration are indicated by arrows, and the numbers show the estimated time since they migrated (years ago). The broken arrow indicates an unconfirmed migration pattern (Adapted from Yamaoka et al. [9])

Full size image

H. pylori live in the human stomach for a long time, but most of those infected do not have symptoms and only some of those infected are taken with diseases, which are not necessarily same diseases. Thus, the reason why those infected show different results is because bacterial factors, in particular virulence factors, as well as host and environmental factors play an important role. Although genetic studies of virulence factors have been actively conducted based on the already known DNA sequence of H. pylori, crucial virulence factor genes responsible for inducing diseases have not been fully established yet, which can be explained by high degree of variations in the DNA sequences of each H. pylori (genetic polymorphism). Such diverse variations are caused by point mutation, substitution, insertion, or deletion, and occasionally several strains of H. pylori with different genetic backgrounds are observed in a single individual. Through these observations, it is thought that not only endogenous mutations but also chromosomal rearrangements or recombination occur between each strain [1]. So far, there have been many studies to examine the relationship between diseases and known virulence factors, such as CagA, VacA, induced by contact with epithelium (IceA), outer inflammatory protein (OipA), duodenal ulcer promoting gene (dupA) and blood group A antigen-binding adhesion (BabA) [6–14] (Table 6.1). In this chapter, we will discuss the relationship of the genetic polymorphism of these virulence factors and gastroduodenal diseases with geographic differences.

Table 6.1 Association of Helicobacter pylori virulence factors with host responses and disease outcome
Full size table

2 Cytotoxin-Associated Gene A (cagA)

CagA is the most studied virulence factor of H. pylori, which is located at one end of the cag PAI. The cag PAI encodes a type IV secretion system, responsible for the injection of the CagA protein into the host cells [15–17].

CagA-producing strains are reported to be associated with severe clinical outcomes, especially in Western countries. Approximately 60–70 % of isolated H. pylori strains from Western countries are known to be positive for cagA, which cause more severe inflammatory reactions with increased interleukin (IL)-8 production. It has been reported that individuals infected with cagA-positive strains of H. pylori are at a higher risk of peptic ulcer or gastric cancer than those infected with cagA-negative strains [6, 18]. The studies conducted in Western countries showed that the prevalence of CagA antibodies was significantly higher in peptic ulcer patients or duodenal ulcer patients compared to control, that is, 100 % for peptic ulcer disease, 85–100 % for duodenal ulcer, and 30–60 % for control [7, 8].

A study using Western blotting method for serological detection of antibodies against cagA reported that serum antibodies to cagA were detected more frequently in gastric carcinoma patients (91 %) than control (72 %) [19].

A cohort study (mean follow-up period of 11.5 years) for 58 subjects infected with H. pylori showed that infection with cagA-positive H. pylori strains is associated with an increased risk for the eventual development of atrophic gastritis and intestinal metaplasia [20]. However, it is difficult to determine the importance of cagA in clinical outcomes in East-Asian countries including Korea because nearly all H. pylori strains possess cagA [8, 10, 12, 13].

2.1 cagA Type: Western Versus East Asian

There has been an increasing tendency in the last decade to explain the higher incidence of gastric cancer in East Asia using the concept of East-Asian-type cagA and Western-type cagA [9]. There are different numbers of repeat sequences located in the 3′ region of the cagA gene of different H. pylori strains. Each repeat region of the cagA protein contains Glu-Pro-Ile-Tyr-Ala (EPIYA) motifs, including a tyrosine phosphorylation site. It has now become more common to name the first repeat region as EPIYA-A and EPIYA-B segments and to name the second repeat region as EPIYA-C or EPIYA-D segments [21]. Western-type cagA contain EPIYA-A, EPIYA-B, and EPIYA-C segments. By contrast, East-Asian-type cagA contain the EPIYA-A, EPIYA-B, and EPIYA-D segments, but not the EPIYA-C segment (Fig. 6.2). Individuals infected with East-Asian-type cagA strains were reported to have an increased risk of peptic ulcer or gastric cancer compared with those with Western-type cagA strains [9, 11].

Fig. 6.2
figure 2

Structural polymorphism in cagA. Western-type cagA contain EPIYA-A, EPIYA-B, and EPIYA-C segments. By contrast, East-Asian-type cagA contain the EPIYA-A, EPIYA-B, and EPIYA-D segments, but not the EPIYA-C segment. The EPIYA motif in each segment (shown in green) represents the tyrosine phosphorylation sites of cagA. The sequence flanking the tyrosine phosphorylation site of the EPIYA-D segment (EPIYATIDF), but not the EPIYA-C segment (EPIYATIDD), matches perfectly the consensus high-affinity binding sequence for the SH2 domains of SHP2. In Western countries, the incidence of gastric cancer is significantly higher in patients infected with strains containing multiple EPIYA-C segments than in patients infected with strains containing a single EPIYA-C segment (i.e., ABCCC vs. ABC). By contrast, almost all East-Asian strains contain a single EPIYA-D segment. cagA forms dimers in cells in a phosphorylation-independent manner, and the CagA multimerization (CM) sequence (also named the conserved repeat responsible for phosphorylation-independent activity [CRPIA] or MARK2/PAR1b kinase inhibitor [MKI]) in yellow was identified as the site responsible for dimerization, for inhibition of MARK2/PAR1b kinase, and for the interaction of cagA with activated c-Met (Adapted from Yamaoka [9])

Full size image

However, there are limitations to explain the higher incidence of gastric cancer in East Asia using the concept of East-Asian-type cagA and Western-type cagA because the incidence of gastric cancer is also high in some regions where Western-type cagA strains are reported to account for the majority of H. pylori strains (e.g., in Peru and Columbia (age standardized rate [ASR] per 100,000 population 21.2 and 17.4, respectively)) [22]. In addition, in Africa the rate of H. pylori infection is high (e.g., 70–97 % of patients with dyspepsia are infected with H. pylori, as are 80 % of asymptomatic volunteers), but gastric cancer is generally uncommon; this seemingly contradictory situation is known as the “African enigma” [23]. The incidence of gastric cancer is extremely high in Mali, West Africa (ASR per 100,000 population 20.3), and the frequency of gastric cancer among women in this country is higher than it is among women in Japan (ASR per 100,000 population 19.3 vs. 18.2) [9]. Accordingly, these facts cannot be explained by the presence of East-Asian-type cagA versus Western-type cagA alone. In a study comparing the cagA gene repeat sequences found in Columbia (ASR per 100,000 population 17.4) with those found in the USA (ASR per 100,000 population 4.1) to explain the geographic difference in the incidence of gastric cancer, 100 H. pylori isolates from patients with simple gastritis (30 from Columbia and 70 from the USA) were analyzed; 57 % of the isolates from Columbia had two EPIYA-C segments, whereas only 4 % of the isolates from the USA had two EPIYA-C segments (Y. Yamaoka, unpublished data) [9, 11]. Overall, the number of EPIYA-C segments may explain, to some extent, the geographic difference in the incidence of gastric cancer in Western countries [9]. Further research is required to determine whether these subtypes are involved in the pathogenesis of gastric cancer.

In the meta-analysis of case control studies with age- and sex-matched controls, which provided raw data in East-Asian countries including Japan, Korea, and China, in eight studies, the pooled prevalence of CagA seropositivity was 71.6 % (1,019 out of 1,423) in cases and 62.7 % (1,595 out of 2,542) in controls. The estimated overall OR was 1.50 (95 % confidence interval [CI], 1.30–1.72). In meta-analysis in a random effect model, overall OR was 1.81 (95 % CI, 1.30–2.11). This shows that the gastric cancer risk for cagA-positive cases was higher overall than in H. pylori-infected subjects; however, the OR in East-Asian countries was smaller than the result of the meta-analysis that included Western countries (1.81 vs. 2.64). In addition, the presence of anti-CagA antibodies increases the risk of gastric cancer in the H. pylori-negative population. The prevalence of anti-CagA antibodies ranged from 18.2 % to 81.8 % in gastric cancer patients and 9.8–60.2 % in controls [24]. The lower frequency of higher titer IgG antibody in advanced cancer may be due to the increasing extent of intestinal metaplasia associated with transition from the intestinal type of early gastric cancer to advanced cancer, such that the local environment is no longer ideal for the growth of H. pylori [9, 25]. CagA antibodies may be positive in patients who have a negative H. pylori serologic test since CagA antibodies can potentially remain positive for a longer period of time than the anti-H. pylori antibody [9, 24]. This evidence confirms that CagA antibodies can potentially remain positive for a longer period of time than the anti-H. pylori antibody [9]. Accordingly, anti-CagA antibody was related to gastric cancer in both H. pylori-positive and H. pylori-negative populations in East-Asian population [9]. However, it is necessary to evaluate the availability of anti-H. pylori antibody plus anti-CagA antibody for screening for risk of gastric cancer.

3 Vacuolating Cytotoxin (vacA)

All the H. pylori strains have a functional vacA, which encodes a vacuolating cytotoxin. However, there is significant sequence diversity in vacA genes across the many H. pylori isolate strains [26–30] (Fig. 6.3). There is variation in the vacuolating activity of different H. pylori strains, primarily due to differences in the vacA gene structure at the signal (s)-region (s1 and s2) and the middle (m)-region (m1 and m2) [9, 26]. In vitro experiments demonstrated that s1/m1 strains are the most cytotoxic, followed by s1/m2 strains, whereas s2/m2 strains have no cytotoxic activity and s2/m1 strains are rare [26]. vacA s1/m1 is the most common strain in East Asia including Korea [27].

Fig. 6.3
figure 3

vacA allelic diversity and structure. Significant allelic diversity exists in three regions of the vacA gene: the signal region (s1 and s2), the intermediate region (i1, i2, and i3), and the mid-region (m1 and m2) (Adapted from Palframan et al. [30])

Full size image

3.1 Geographic Differences in vacA Genotypes

It has been known that there are geographic differences in the distribution of both the vacA s- and m-region subtypes [26] (Fig. 6.4). Subtype s1a is predominant in Northern Europe and Australia, whereas subtype s1b is prevalent in South America. Subtype s1c is the major subtype in East Asia, but is extremely rare in Western Europe. The most common vacA genotypes in Korea are s1c for the s-region and m1 for the m-region [26–28].

Fig. 6.4
figure 4

Distribution of vacA s- and m-region and cagA genotypes of H. pylori strains from different parts of the world. For each region, the prevalence of each type (s1a, s1b, s1c, s2, m1, m2a, and m2b; cagA positive) is given as a percentage of the total number of strains (shown in parentheses). Only strains containing a single vacA genotype are represented (Adapted from Van Doorn et al. [26])

Full size image

There have been many reports that individuals infected with s1 or m1 H. pylori strains have an increased risk of peptic ulcer or gastric cancer compared with individuals infected with s2 or m2 strains [26–28]. In East Asia including Korea, however, most H. pylori strains have an s1-type s-region; therefore, the pathogenic difference cannot be explained by the type of s region present [9].

The clear geographic differences in the distribution of both the vacA s- and m-region subtypes strongly suggest a geographic structure of H. pylori populations throughout the world [26]. The distribution of both s- and m-region allelic types in Central and South America is similar to that in the Iberian Peninsula. This distribution of H. pylori vacA genotypes may reflect the extensive cultural and socioeconomic relationships between these parts of the world during past centuries. Particular H. pylori strains may have been spread through Central and South America by colonization of these areas by Spanish and Portuguese descendants. Similarly, subtype s1a is more common in the northern European countries and its predominance in Commonwealth countries (Canada and Australia) again may reflect historic relationships. It is unknown whether the evolution of both subtypes occurred in Europe. Within Europe, the distribution gradient of subtypes s1a (northern and eastern Europe) and s1b (Iberian Peninsula) was highly consistent among all tested strains from 12 different countries. If subtypes s1a and s1b already existed for a long time in Europe and were freely movable, a more homogeneous distribution would have been expected. On the other hand, if transmission of H. pylori is highly local, only occurring over very short distances during childhood, this may have prevented a broader geographic distribution of each subtype. An alternative hypothesis would be that the distribution of different subtypes reflects particular adaptation of H. pylori to specific host populations [26–28, 31, 32].

In Korean H. pylori strain, a wide diversity has been observed in vacA s1-region. In the study performed by Kim et al. [12], vacA s1a–s1c was determined as the most common subtype in South Korea and considering the positivity of genotypes, vacA s1c and s1a were identified as the major genotypes. s1a–s1c was significantly frequent in benign gastric ulcer (73.2 %, p = 0.023), gastric cancer (73.6 %, p = 0.018), and dysplasia (71.9 %, p = 0.048) than control (58. 7 %) (Tables 6.2 and 6.3).

Table 6.2 Positivity of vacA, cagA, iceA, oipA, and dupA of H. pylori in 401 colonies from Korea
Full size table
Table 6.3 vacA and iceA subtypes of H. pylori in 401 colonies from Korea
Full size table

On the other hand, a study performed in Korea for peptic ulcer and gastritis patients by Park et al. [31] showed that s1c, s1a, and s1b were found in 66.1 %, 35.6 %, and 0 % of H. pylori isolates, respectively. Another study [8] analyzed H. pylori isolates from Seoul, Korea; s1c and s1a were found in 90 % and 7 %, respectively. In both studies, s1 subtypes were not related to clinical outcome. This explanation may support the reason for the possible infection of multiple strains in Korea and other countries. However, further studies will be required to clarify these diversities.

Regarding the combination of the vacA s and m genotypes, Japanese strain from Okinawa showed that the vacA s1/m1 genotype was significantly higher in strains from gastric ulcer (79.2 %) and gastric cancer (87.5 %) than those from gastritis (59.2 %) (p = 0.002 and 0.006, respectively). The prevalence of the vacA s1/m2 genotype tended to be higher in strains from patients with duodenal ulcer than those from patients with gastritis (27.2 % vs. 17.3 %), although the difference did not reach statistical significance (p = 0.08). The prevalence of the vacA s2/m2 genotype was significantly higher in strains from gastritis patients than in those from gastric ulcer, duodenal ulcer, and gastric cancer patients (22.4 % vs. 11.9 %, 10.5 %, and 4.2 %, p = 0.04, 0.01, and 0.04, respectively) [33–35]. These results suggested that diverse vacA genotypes contribute to the clinical outcomes in Okinawa and low incidence of gastric cancer in Okinawa [9, 11].

With respect to the m-region, there is variation within East Asia. Although m1 strains are common in parts of north East Asia, such as Japan and South Korea, m2 strains are predominant in parts of south East Asia, such as Taiwan and Vietnam [35–37]. As the incidence of gastric cancer is higher in the north than in the south of East Asia, the m-region may play a role in the regional difference in disease pattern [9]. Even within Vietnam, the incidence of gastric cancer is approximately 1.5 times higher in Hanoi in the north than in Ho Chi Minh in the south of the country. Comparison of two geographically distant cities in Vietnam, Hanoi and Ho Chi Minh, showed that the vacA m1 genotype, thought to be more toxic than the vacA m2 type, is more prevalent in Hanoi, where the incidence of gastric cancer is higher than in Ho Chi Minh [36, 37]. These data support the hypothesis that the vacA m1 type is closely associated with gastric carcinogenesis [9].

In 2007, a third disease-related region of vacA was identified between the s-region and the m-region; it was named the intermediate (i)-region [38]. Yamaoka et al. [9] reported that all s1/m1 strains were classified as type i1, and all s2/m2 strains were classified as type i2, but s1/m2 strains were classified as either type i1 or i2, and i1 strains were shown to be more pathogenic. Typing of the i-region was also reported to be more effective for determining the risk of gastric cancer in Iranian patients than typing of the s-region or m-region [39]. However, in a study of patients from East and Southeast Asia, there was no association between the i-region and disease [11, 40].

More recently, a fourth disease-related region – the deletion (d) region – was identified between the i-region and the m- region [9]. The d-region is divided into d1 (no deletion) and d2 (a 69–81 bp deletion). The study of Western strains demonstrated that d1 was a risk factor for gastric mucosal atrophy; however, almost all East-Asian strains are classified as s1/i1/d1. Therefore, further researches are needed to clarify association between d- or i- region and clinical outcome [11, 39, 40].

4 Induced by Contact with Epithelium (iceA)

An initial series of studies showed that iceA has two main allelic variants, iceA1 and iceA2. The expression of iceA1 was upregulated on contact between H. pylori and human epithelial cells, and the iceA1 genotype was linked with enhanced mucosal IL-8 expression and acute antral inflammation [9, 11, 41]. The iceA type 1 allele is reported to be predominant in Japan and Korea, and the iceA type 2 allele in the United States and Colombia [8, 9, 35, 41].

In a meta-analysis [42] including 50 studies with a total of 5,357 patients to confirm the relationship between the iceA allelic type and clinical outcomes, the overall prevalence of iceA1 was significantly higher by 64.6 % (1,791/2,771) in Asian countries than in Western countries (64.6 % vs. 42.1 %), whereas the prevalence of iceA2 was more prevalent in Western countries than in Asian countries (45.1 % vs. 25.8 %). Sensitivity analysis showed that the presence of iceA1 was significantly associated with peptic ulcer (OR 1.25; 95 % CI, 1.08–1.44); however, the presence of iceA2 was inversely associated with peptic ulcer (OR 0.76; 95 % CI, 0.65–0.89). These findings were significant in Western countries. And the presence of iceA was not associated with gastric cancer. Most studies examined the cagA status; however, only 15 studies examined the correlation and only 2 showed a positive correlation between the presence of cagA and iceA1. It is possible that iceA is a discriminating factor for peptic ulcer which is independent of cagA [9, 11]. However, it is a result that has not been confirmed in other countries, such as Japan and Korea [8, 11, 41]. Kim et al. [12] reported that the positivity of iceA1 in Korean H. pylori isolates was more than 95 %, and iceA2 was variable from 35 % to 55 % among clinical disease. In addition, strains expressed with iceA2 alone were only about 5 %, and most strains were detected with iceA1 along with iceA2. This result could support a wide diversity of H. pylori infection in South Korea.

In summary, despite numerous attempts to relate vacA genotypes to outcome or disease pathogenesis, no consistent associations or demonstrable biologic basis for the putative associations has appeared. Further studies are warranted.

5 Outer Membrane Protein

Outer membrane protein has been shown to act as an adhesion that facilitates bacterial attachment to the host epithelium. Approximately 4 % of the H. pylori genome is predicted to encode outer membrane proteins. There have been many studies that investigate the expression status of outer membrane protein such as OipA, BabA, or BabB in different clinical outcomes [9, 43–45].

5.1 Outer Inflammation Protein (oipA)

oipA was initially identified as a proinflammatory response-inducing protein based, in part, on the fact that oipA isogenic mutants reduced the production of IL-8 from gastric epithelial cell lines [9, 43–45]. Transcription of IL-8 genes in both oipA and cag PAI dependent through interactions with different binding sites are involved, such as transcription factors within the IL-8 promoter for nuclear factor-κB (NF-κB), activator protein 1 (AP-1), and interferon-stimulated responsive element (ISRE)-like element [9]. oipA functional status was related to clinical presentation, H. pylori density, and gastric inflammation. cag PAI, babA2, or vacA status appears important only as surrogate markers for a functional oipA gene. It is also important to reconfirm that the presence of cagA, vacA, and oipA are linked such that typically H. pylori either produce all of these proteins or none of them and that clinical outcomes, such as peptic ulcer and gastric cancer, are associated with strains with and without these virulence factors. However, strains with recognized virulence factors tend to produce more severe inflammation and are associated with higher risk of these important clinical outcomes [9, 11].

In a study [45] analyzed H. pylori isolates from the United States and Colombia, an independent univariate analysis, showed that the oipA “on,” cag PAI-positive, vacA s1 genotype, and the babA-positive type were all related to a risk of duodenal ulcer. Importantly, a multiple logistic regression analysis showed that only the oipA “on” status was an independent determinant predictor of duodenal ulcer from gastritis. This finding was confirmed in another study [46] based on a non-overlapping cohort of 200 patients who were examined for four outer membrane proteins, OipA, BabA, BabB, and sialic acid-binding adhesion (SabA), by immunoblot, in which multiple logistic regression analysis showed that only oipA-positive status was an independent determinant predictor of gastric cancer vs. gastritis and duodenal ulcer vs. gastritis.

However, strains in Asia appeared to be different from those in Western countries in the aspect of outer membrane proteins and their actions [9, 11]. Kim et al. [12] reported that oipA was more frequently detected in duodenal ulcer and gastric cancer, but significant effect on gastroduodenal diseases was not found in Korean H. pylori isolates. These results could also be an evidence of the different distribution of virulence factors according to geographic differences.

The H. pylori oipA has been demonstrated to be a potential antigen for a vaccine. Recently, oipA have been tested in mice and vector-based approaches and/or multicomponent vaccines have been investigated [46]. The study showed that H. pylori oipA encoding construct is capable of inducing humoral and cellular responses in immunized mice. The antibody response profiles elicited by the DNA vaccine alone administered intradermally (the gene gun method) showed that it produced a Th2 immune response, while co-delivery of IL-2 and LTB gene encoding constructs promoted a Th1-biased immune response. Further studies warranted for developing vaccination for H. pylori.

5.2 Duodenal Ulcer Promoting Gene A (dupA)

In 2005, the first disease-specific H. pylori virulence factor that induced duodenal ulcer and had a suppressive action on gastric cancer was identified and was named duodenal ulcer promoting gene A (dupA) [47, 48]. The presence of dupA was associated with elevated IL-8 production in the antrum (i.e., antrum-predominant gastritis – a feature of duodenal ulcer disease) and has been reported to induce IL-12 production from monocytes [49].

In an initial study of a total of 500 H. pylori isolates, including 160 from Japan, 175 from South Korea, and 165 from Colombia, the positive rate for the dupA gene was high in patients with duodenal ulcer and low in patients with gastric cancer, regardless of a patients’ nationality (42 % vs. 9 % on average) [47]. In the study analyzed 401 Korean H. pylori isolates by Kim et al. [12], the prevalence of dupA was 48.0 %. Infection by dupA-positive H. pylori showed an increased risk of gastric ulcer (OR 33.06; 95 % CI, 11.91–91.79) and duodenal ulcer (OR 15.60; 95 % CI, 6.49–37.49). More than 75 % of colonies with gastric ulcer and duodenal ulcer expressed dupA, which suggests that dupA may be a fundamental factor for the development of peptic ulcer diseases in South Korea.

However, Brazil, Singapore, Malaysia, and Japan failed to demonstrate a correlation between the presence of the dupA gene and disease [9]. An academic report on Brazilian strains by Queiroz et al. [50] showed that a dupA gene mutation (deletion or insertion) was found in 50 % of patients with gastric cancer, whereas it was found in only approximately 20 % of patients with duodenal ulcer. As a result, the positive rate for the functional dupA gene was considerably higher in patients with duodenal ulcer than in patients with gastric cancer. Further investigation might clarify the effect of functional dupA on various gastroduodenal diseases.

5.3 Blood Group A Antigen-Binding Adhesion (babA)

babA-mediated adherence of H. pylori to the gastric epithelium plays a critical role in the efficient delivery of bacterial virulence factors that damage host tissue [51, 52]. Interactions between BabA and Lewis b (Leb)-related antigens are the best characterized adhesion receptor interactions in H. pylori. The babA gene was initially cloned from strain CCUG17875, which contains a silent babA1 gene and an expressed babA2 gene [9, 51]. A number of studies have suggested a relation between babA2-positive H. pylori and increased cellular mucosal inflammations and an increased risk of developing clinical outcomes [9].

Gerhard et al. [51] reported that the presence of babA2 could be regarded as a good indicator of the ability of strains to express the Lewis b antigen-binding adhesion and that babA2 is significantly associated with duodenal ulcer in H. pylori isolated from a German population. The incidence of the babA2 genotype was about 72 % in their study (duodenal ulcer 100 %, gastric cancer 77.8 %, and gastritis 51.4 %).

However, Kim et al. [53] reported that the incidence of babA was low and was not related to peptic ulcer disease in Korea. The presence of the babA genes in H. pylori isolates from peptic ulcer and gastritis patients was 27.3 % and 26.3 %, respectively (p = 0.578). In addition, the four pathogenicity-related genes, cagA, vacA s1c/m1, iceA1, and babA, did not correlate with other genes. In a Japanese study by Fujimoto et al. [54], all strains from East Asia expressed BabA protein, and 24 (9.8 %) of the Western strains were babA negative. For these strains, the babA-negative status was correlated inversely with cagA or vacA s status (i.e., only 1 [4.2 %] and none [0 %] of these babA-negative strains were cagA- or vacA s1-positive, respectively). Most (91 %) Western strains were classified as either cagA-positive/vacA s1-positive/BabA-H (triple positive, 76 %), cagA-positive/vacA s1-positive/BabA-L (6.1 %), or cagA-negative/vacA s2-positive/babA-negative strains (9.4 %). However, there was no relationship between the triple-positive strains and clinical outcome. babA-negative status is associated with mild gastric injury and lower H. pylori density. babA-negative strains also are associated infrequently with duodenal ulcer or gastric cancer.

However, because babA-negative status is linked closely to cagA-negative/vacA s2 status, potential interactions between these different putative virulence factors cannot be ruled out.

In summary, it remains unclear how BabA expression is regulated or if expressing low levels of BabA has a direct role in the pathogenesis of duodenal ulcer or gastric cancer [11]. Further studies are warranted.

6 Conclusions

The H. pylori virulence factors are important for clarifying the role of H. pylori in the regional differences in the gastric cancer distribution and the pathogenesis of clinically significant diseases such as gastric cancer or peptic ulcer. If all the studies to data are put together, cagA, vacA, and oipA are the factors associated with gastric cancer, and dupA can be considered to be an important virulent factor for duodenal ulcer, but because different studies have showed different results and particularly results were different by geographic region, more research is needed.

The biggest reasons why there is a limit in clarifying the relationship between H. pylori and diseases only by the virulence factors are as follows: H. pylori is composed of nearly 1,600 genes; thus, it is possible that a pathogenic gene which has not yet been identified plays a critical role. Moreover, pathogenesis of gastroduodenal diseases including gastric cancer involves several factors such as diet, environmental changes caused by human movement, host factors, or duration of H. pylori infection, which should be taken into account along with the virulent factors in the pathogenesis of diseases. In addition, it is needed to better understand and interpret the research methods and terminology when the study results associated with H. pylori virulence factors are comprehended and analyzed. Tests for virulence factors are to be made through the use of the strains isolated from the host in vitro or an animal model; however, the actual results caused by these virulence factors in the human body can be different from the results obtained in the laboratory. Therefore, it should be kept in mind that actual working mechanisms of the H. pylori virulence factors in the host can be much more complicated and divers than we imagine.