Our findings that inhabitants of the Andean region of Colombia were all colonised by hpEurope strains and that individuals living on the Pacific coast were infected with strains of European or African ancestry, with the latter predominating, suggest that Mestizos from the HR mountain region have lost their ancestral Amerindian strains, and that the Mulattos of the LR coastal region [18
], still carry H. pylori
strains from their African ancestors brought to the New World during the slave trade. Amerindians are hypothesised to have been infected originally with hspAmerind strains that are part of the hpEAsia group [34
], but the Mestizo subjects in the Andean region in our study were exclusively infected with hpEurope strains, suggesting that hpEurope strains possess a competitive advantage compared to indigenous hspAmerind strains. In a study of 22 Mestizos from Colombia and Venezuela, subjects harboured only hpEurope and hpAfrica1 strains [38
], but the relationship between MLST classification of strains and cancer risk parameters was not assessed. It has been reported that humans can be colonised with more than one H. pylori
]. However, when we tested for bacterial genetic diversity using PCR with enterobacterial repetitive intergenic consensus sequence primers, we found that in all cases randomly selected for analysis (n=28), colonies isolated from antrum and corpus for each subject possessed the same DNA fingerprint. In addition, in all of the 12 cases that were further studied, 4 colonies (2 from antrum and 2 from corpus) were all identical.
We have shown that as expected, persons from the HR region have more histologic and DNA damage than those from the LR region. The data presented herein suggest that these findings are attributable to differences in the phylogeographic origin of the strains harboured by these subjects. In accordance with this concept, we have studied 10 strains from each of the risk regions cocultured with gastric epithelial cells, and found a significant increase in oxidative DNA damage (measured by flow cytometry for 8-oxoguanosine [37
]) with the HR versus
LR strains; similarly, we also detected increased DNA damage in cells infected by hpEurope strains compared to hpAfrica1 strains (data not shown). These data support our hypothesis that H. pylori
strains from the two different risk regions exhibit substantial differences in pathogenicity that relate to their phylogeographic origin.
Our findings are in distinction to previous considerations that in addition to ethnic differences, the high altitude and starch-based diet in HR regions compared to the low altitude and seafood-based diet in LR regions may explain differences in gastric cancer rates in Latin America [18
]. Our group has recently reported that there are currently no significant difference in household condition index, crowding, vitamin supplement consumption, salt intake, and smoking status between the LR and HR regions [20
], indicating that these are not confounding variables in our study. Furthermore, we have analysed our data based on smoking history [20
], and have found that histopathology score and 8-OHdG staining were not different between smokers and non-smokers. In the LR region, where inhabitants are infected with either hpAfrica1 strains or hpEurope strains, we have shown that the divergence in H. pylori
genetic features, as reflected by strain phylogeographic origin, is a very important factor that could explain the distinct patterns of pre-malignant histologic injury and oxidative DNA damage that we observed. Because the subjects in the LR region are ethnographically similar, but their gastric tissues exhibited a significant increase in the frequency of atrophy, intestinal metaplasia, and dysplasia if they were infected with hpEurope strains rather than hpAfrica1 strains, this suggests the importance of phylogeographic strain origin. However, host genetics may also have an effect, and to this end we intend to investigate the mitochondrial DNA of Colombian subjects from the LR and HR regions in future studies.
It has been reported that there is a strong association between the presence of the cag
PAI or the vacA
s1m1 genotype and an increased likelihood of developing gastric cancer in Latin American populations [41
]. Consistent with this, we found that patients in this study who were infected with cagA−
strains (tables S1
) had very low histopathology scores with non-atrophic gastritis only (data not shown). Interestingly, all of the cagA−
strains in this study were classified as hpEurope (table S2
). The histologic comparisons in the current study focused on subjects who were infected with cagA+ vacA
s1m1 strains, because including the cagA−
strains would be a confounding factor. Importantly, we have demonstrated that among subjects all harbouring cagA+
s1m1 strains, there are large differences in histologic disease progression and epithelial DNA damage that correlate with the MLST type of the strains. This provides evidence that other genetic characteristics of the strains (besides the cagA
status) are strongly associated with disease risk. Our phylogenetic data provide a new strategy for stratifying risk, and should serve as a foundation for studies searching for specific H. pylori
genetic factors, that vary between strains of European versus
African origin in order to gain insights into gastric carcinogenesis. We expect that next generation sequencing technology used for whole genome sequencing of H. pylori
could be a useful future approach to identify specific determinants of cancer risk.
For Summary Box
What is already known about this subject?
- Helicobacter pylori is the main causative agent for peptic ulceration and gastric cancer.
- High prevalence rates of H. pylori generally correlate with a high risk of gastric cancer, but there are areas of the world with low gastric cancer rates despite high H. pylori prevalence.
- H. pylori strains with cagA and vacA s1m1 virulence factors are the most commonly associated with gastric cancer risk.
- In Colombia, people living in the Andean mountain region have a 25-fold greater risk of developing gastric cancer than people living in the coastal region.
What are the new findings?
- H. pylori strains infecting human subjects from the Colombian mountain region are exclusively of European ancestry.
- H. pylori strains of European or African phylogeographic origin are found in the coastal region, with the latter predominating.
- European phylogeographic origin of the H. pylori strains is strongly associated with more advanced histologic lesions and increased DNA damage in gastric epithelial cells, regardless of whether the subjects were from the high risk mountain region or the low risk coastal region.
How might it impact on clinical practice in the foreseeable future?
- We have found that the ancestral origin of H. pylori strains is a strong predictor of gastric cancer risk.
- Therefore, determination of phylogeographic origin of H. pylori-infecting strains may be a useful strategy for risk stratification and clinical management, including intensification of eradication programs and surveillance endoscopy.