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Helicobacter pylori: genotyping - PCR and sequencing

Information 16/02/23.

Helicobacter pylori can cause gastritis symptoms of acute or chronic duodenal ulcer, gastric ulcer, gastric adenocarcinoma and lymphoma of mucosa (MALT: Mucosal Associated Lymphoid Tissue).

This bacterium, however is widely distributed in the human population. In underdeveloped countries can get to meet up to 80% of the population, while in developed countries is between 20 to 50% of the population. These differences in prevalence were mainly attributed to the different existing social and health conditions between developing and developed countries, which would facilitate the dissemination among the population in the former.

Despite this broad distribution in the population infection rates do not correspond to similar rates of morbidity attributed to this bacteria. This is because there are differences in pathogenicity between the different strains of this bacterium, as some pathogenicity factors not found in all strains.

The main factors of pathogenicity of this species that allow invasion, colonization and cell degeneration are: 1) mobility polar flagella that facilitates contact with the mucosa; 2) the urease enzyme produced, on the one hand allows it to survive in the acid environment which, thanks to the decomposition of urea secreted by the mucous membrane with the production of ammonia and carbon, of which the first allows you to create situated one alkaline environment in their environment, and moreover is toxic to cells of the mucosa; 3) the vacuolating cytotoxin (VacA: vacuolating cytotoxin A); and 4) the cytotoxin associated gene A (CagA: cytotoxin associated gene A).

Among the most important factors of virulence are vacuolating cytotoxin A (VacA) and the cytotoxin associated gene A (CagA). The differences in the genes encoding both toxins differentiate genotypes within the same species which have been correlated to the pathology caused by different strains.

The vacuolating cytotoxin (VacA), is a 88kDa protein formed from a precursor protein of 140 kDa. This cytotoxin can be found in all strains of H. pylori, and would be responsible for the appearance of cytoplasmic vacuoles and increased permeability leading to damage to the epithelial cells of the mucosa. Additionally, the cytotoxin VacA is considered responsible for the induction of inflammation, stimulation of apoptosis, inhibition of cell multiplication and damage of intercellular junctions. This cytotoxin is present in almost all strains of H. pylori, but is polymorphic, and is constituted by two variable regions, the signal region (s: signal region) located in the N-terminal end of the protein, and the region medium (m: middle region). There are two main types of "s": s1 and s2, and three minor subtypes S1 (S1A, S1B and s1c). The "m" region can also be differentiated into two main types (m1 and m2) and two smaller subtypes (M1A and M1B). Furthermore, an intermediate region has distinct "i" with two different alleles (i1 and i2). Recently it described another region related to pathogenicity, located between regions "i" and "m", and designated as region "d" differentiating "d1" (no deletion) and "d2" (with a deletion of 81 bp). The "d1" region would be a risk factor for gastric cancer and peptic ulcer disease strains in the West and the Far East. Depending on the combination of the different genotypes and subtypes cow region, different genotypes with different pathogenicity can be established: s1am1 and s1bm1, producing larger amounts of cytotoxin VacA, are considered the most virulent, causing greater epithelial damage, which would be related with pictures of acute gastritis, peptic ulcer and gastric cancer. By contrast, genotype s1m2 vacuolizante produces little amount of toxin. The S2M1 and s2m2 genotypes not considered toxigenic, because of a hydrophilic-aminácidos extension 12 in the form s2, and unable to generate intracellular vacuoles. These two genotypes commonly found in gastric ulcer.

Cytotoxin associated gene A (CagA), is a protein of 125-145 kDa, and n is 60 to 90% of H. pylori strains. CagA protein is highly immunogenic and induce metaplasia, dysplasia and gastric cancer. It is accepted that the presence of CagA cytotoxin in a colonizing strain carries an increased risk of gastritis and gastric cancer. Moreover, the presence or absence of this cytotoxin, also influences the structure of the protein. In this protein they have been described several reasons EPIYA as EPIYA-ABC and ABD-EPIYA. In Western countries, predominantly EPIYA-ABC motif, which correspond to lower incidence of gastritis, gastric ulcer, duodenal ulcer and gastric cancer, while in the Far East dominated the EPIYA-ABD reason that relates to more incidence of such processes.

They described other genes related to pathogenicity, such as iceA gene with two alleles iceA1 and iceA2. The iceA gene would be expressed upon contact with the epithelium during attachment of H. pylori to gastric mucosa. IceA1 expression would be associated with greater increase in IL-8 and increased mucosal inflammation.

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