Systematic review and meta-analysis: the relationship between the Helicobacter pylori dupA gene and clinical outcomes

Helicobacter pylori (H. pylori) infection is now accepted as the major cause of chronic gastritis. Several epidemiological studies have shown that H. pylori infection is also linked to the severe gastritis-associated diseases, including peptic ulcer and gastric cancer (GC) [1]. In 1994, the International Agency for Research on Cancer categorized H. pylori infection as a definite group I carcinogen [2]. The infection remains latent in the majority of infected patients, with only approximately 20% of infected individuals developing severe diseases. One possible reason for the varying outcomes of H. pylori infection relates to differences in the virulence of H. pylori strains in addition to host, environmental, and dietary factors.

Several H. pylori virulence factors associated with peptic ulcer and GC have been reported, including cagA, vacA, babA and oipA[1, 3–6]. Lu et al. [7] described a novel virulence factor, duodenal ulcer promoting gene (dupA), which encompassed both jhp0917 and jhp0918 located in the plasticity region of the H. pylori genome. Interestingly, the dupA gene is homologous to virB4, a gene encoding a component protein of the type IV secretion system (TFSS) in Agrobacterium tumefaciens. They reported that infections with dupA-positive strains increased the risk for duodenal ulcer (DU) but were protective against gastric atrophy, intestinal metaplasia and GC in the Japanese, Korean and Columbian subjects. Intriguingly, dupA is the first genetic factor of H. pylori to be associated with differential susceptibility to DU and GC, and thus it could be considered as a disease specific virulence marker. The pathogenic mechanism of dupA appears to involve the induction of interleukin (IL)-8 production in the antrum, leading to antrum-predominant gastritis, a well-recognized characteristic of DU [7].

However, the role of dupA was controversial subsequently since several studies were unable to reproduce the observation in other population including Japan [8–12]. Although Hussein recently reported the systematic review which proved the high prevalence of dupA-positive H. pylor i in patients with DU [13], he determined the conclusion based on simple combined data but not meta-analysis (e.g., he did not show the meta-analysis model, heterogeneity, publication bias). In this study, we aimed to examine the relationship between the dupA gene and clinical outcomes based on the method of meta-analysis.

A literature search was performed using the PubMed databases for articles published through August 2010, using the following text words: 1) dupA, and 2) pylori or Helicobacter. We did not include abstract alone or unpublished articles.

Our present meta-analysis shows that the presence of the dupA gene was significantly associated with DU. Although several studies failed to show the positive association between the dupA status and clinical outcomes, the meta-analysis confirmed the original report in 2005 [7]. Especially, the presence of the dupA gene was associated with DU in Asian countries; however it was not in Western countries. This difference may be due to the different prevalence of the dupA gene between Asian and Western countries. Furthermore, the study by Gomes et al. was the resource of heterogeneity due to larger difference compared with other studies. This mean the simple combined calculation such as the report by Hussein [13] is not strictly accurate to concluded although the trend was not changed. In addition, we found that the several miscount exist in the report by Hussein (e.g., he counted the mean age but not the number of subjects from the study Zhang et al. [24] and mistook the calculation from the study by Pacheco et al. [23]).

When Argent et al. combined Belgian and South African populations, the presence of the dupA gene was significantly associated with the presence of GC. There was also a non-significant trend towards an association between the dupA gene and DU in the combined Belgian and South African population. However, we did not include the South Africa in Western countries. It is not relevant to combine random populations since it is unclear whether the South African strains were taken from patients of European descent and recent studies confirmed that the genomic structures of some South African strains (i.e., HpAfrica2 type) were relatively different from those from the European population (i.e., HpEurope type) [25, 26].

In a study examining strains from Brazilian children and adults, the prevalence of the dupA gene was extremely high (92%; 445/482) irrespective of clinical outcomes [10]. Interestingly, the frequency of the dupA gene was significantly higher in strains from children than in those from adults. H. pylori infection is typically acquired in childhood and persists throughout life unless treated with a combination of anti-acid and antimicrobial therapy, so it is speculated that the dupA gene might be lost during long-term infections in which the gastric mucosa gradually develop chronic atrophic gastritis and GC. Therefore, their results might partially support the original hypothesis that the dupA gene is a marker associated with gastric damage that leads to the development of gastric cancer. Therefore, we need to pay attention to the age of each disease in case-control study. For example, mean age of DU was younger than that of gastritis in the study by Zhang et al. (41 years old in DU vs 59 years old in gastritis) [24]. Case-control study matched age- and sex- should be performed in the future.

Overall, there are distinct geographical variations in the prevalence of the dupA gene, and there appears to be an association between dupA and DU in some populations but not in others. As Argent et al. [8] reported, the association of dupA with DU in only some populations could reflect differences in the definition or diagnosis of ulcers or in the use of drugs that either cause or heal ulcers in these populations. In addition, the discrepancy could be related to the limitation of PCR techniques for detecting the intact dupA gene. In some studies, only one set of primer pairs for jhp0917 and jhp0918 was used [7, 9, 23, 24]: use of multiple primer pairs is recommended for detection of the dupA gene in future studies. None of the previous reports considered the frameshift mutation after position 1385 as a criterion for the presence of the dupA gene [7, 10]. More importantly, Gomes et al. reported frameshift mutations in 14/86 (16%) dupA-positive sequenced samples [10]; a single adenine insertion after position 1426 of dupA or at position 2998 of the jhp0917-jhp0918 gene of the J99 strain that created a premature stop codon and may have considerable effects on protein expression or function. In their study, they counted the truncated samples as dupA-positive; however, it is clear that these mutated sequences would not produce intact DupA protein. For example, the expression of the blood group antigen binding adhesin (BabA) protein does not always correlated with the babA gene expression [27]. It will be better to detect intact dupA by measuring intact DupA protein using immunoblotting techniques, which has not been reported previously. In addition, the vir genes exist before and after the region of the dupA locus [16]. In the strain Shi470, for example, virB2, virB3, virB4 (dupA), virB8, virB9, virB10, virB11, virD4, and virD2 were detected. These are structurally similar to the type IV secretion system (T4SS) called cag PAI and ComB and thought to be the third T4SS. Recently, T4SS containing dupA was named as tfs3a, and T4SS having virB4 sequence, but not dupA was named as tfs3b[28]. These observations suggest that only strains that are intact dupA-positive and form a novel type IV secretion system might be involved in gastroduodenal diseases. If this is true, examining the presence of DupA/dupA alone might not be sufficient. Study on DupA is still in their early stages, and great progress is expected in the near future.

Infection with the dupA-positive H. pylori increased the risk for DU overall and this evidence was significant in Asian countries. In contrast, the relationship between dupA and GC and GU was not clear from the meta-analysis. Case-control study matched age- and sex- should be performed in the future.

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