Helicobacter Pylori
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HELICOBACTER PYLORI Helicobacter pylori was considered by a previous IARC Working Group in 1994 (IARC, 1994). Since that time, new data have become available, these have been incorporated into the Monograph, and taken into consideration in the present evaluation. 1. Exposure Data and several strains are awaiting official classifica- tion (Table 1.1). Helicobacter pylori is a highly heterogenous According to the usual site of colonization, bacterium with a large genomic diversity. In addi- Helicobacter species can be divided into gastric tion, humans may sometimes harbour multiple and enteric or enterohepatic Helicobacter types. strains, and H. pylori can change genotypically Some gastric Helicobacter species from and phenotypically during colonization in a animals can infect humans: H. bizzozeroni, single host (Suerbaum & Josenhans, 2007). H. salomonis, H. felis, H. candidates, H. suis. Bacausee they are extremely difficult to grow in cultures, the exact speciation is usually not 1.1 Taxonomy, structure, and biology done, and they are known as “Gastrospirullum 1 .1 .1 Taxonomy hominis” or “H. heilmannii” (De Groote et al., 2005). The presence of spiral-shaped bacteria on human gastric mucosa was first recognized 1 1. .2 Structure of the bacterium nearly one hundred years ago (Pel, 1913). These bacteria were originally named Campylobacter H. pylori is a spiral or slightly curved Gram- pylori (C. pylori) (Warren, 1983). negative rod with 2–6 characteristic unipolar In 1989, a new genus, Helicobacter, was flagella. The bacterium has bluntly rounded proposed, and C. pylori was renamed Helicobacter ends and measures 2.5–4.0 µm in length and pylori (Goodwin et al., 1989). Recently (Garrity 0.5–1.0 µm in width. The cell wall is smooth et al., 2005), the genus Helicobacter has been and may be coated with a prominent glycocalyx included with the genus Wolinella in the with a thickness of up to 40 nm (Goodwin et al., family Helicobacteraceae which, with the 1989); it is covered with ring-like subunits with family Campylobacteraceae, constitutes the a diameter of 12–15 nm. Occasionally, the bacte- Epsilonproteobacteria. rium may contain bacteriophages. The flagella Over the past 20 years, 23 Helicobacter measure 2.5 µm in length and around 30 nm in species have been validated, and two candidates thickness, and have a distinctive terminal bulb (Goodwin & Worsley, 1993). The bacterium 385 IARC MONOGRAPHS – 100B chronic atrophic gastritis (Tomb et al., 1997; Alm Table 1 1. Validated species of the genus et al., 1999; Oh et al., 2006). Helicobacter Strains 26695, J99 and HPAG1 have a Species circular chromosome of 1667867 base pairs (bp), Helicobacter pylori 1643831 bp and 1596366 bp, respectively. HPAG1 Helicobacter acinonychis also has a single 9369 bp plasmid, pHPAG1. Helicobacter aurati The percentage of genome-coding sequences Helicobacter bilis of strains 26695, J99 and HPAG1 is around Helicobacter bizzozeronii 92%, and they contain 1552 (Alm et al., 1999; Helicobacter canadensis Boneca et al., 2003), 1495 (Tomb et al., 1997; Alm Helicobacter canis Helicobacter chrolecystus et al., 1999), and 1536 (Oh et al., 2006) predicted Helicobacter cinaedi protein-coding genes, respectively. In these three Helicobacter felis small genomes, 1379 open reading frames (ORFs) Helicobacter fennelliae are common to all three strains and about 10% Helicobacter ganmani of the genes are strain-specific Alm( et al., 1999); Helicobacter hepaticus 117 and 89 genes present in strains 26695 and J99, Helicobacter mesocricetorum respectively, are absent in the other strain (Alm Helicobacter muridarum Helicobacter mustelae et al., 1999); in contrast, 43 of the HPAG1 genes Helicobacter nemestrinae are either not detectable at all or incompletely Helicobacter pametensis represented in the 26695 and J99 genomes (Oh Helicobacter pullorum et al., 2006). Helicobacter rodentium A comparison of the three genomic sequences Helicobacter salomonis revealed that the genetic organization was similar Helicobacter trogontum in all three strains. However, it confirmed the Helicobacter typhonius Candidatus Helicobacter bovis panmictic structure of H. pylori, which is the Candidatus Helicobacter suis result of a high mutation rate (microdiversity, i.e. high polymorphism among orthologous genes), displays remarkable motility in viscous solutions, and free recombinations (Falush et al., 2003). A and the flagella play a central role in this motility significant macrodiversity (presence or absence (Hazell et al., 1986; Suerbaum et al., 1993). of the genes) was also observed (Raymond et al., In certain circumstances, H. pylori can evolve 2004). A comparative genomic analysis of isolates from this typical helical form to a coccoidal from 15 Caucasians (Salama et al., 2000) allowed form. Some studies suggested that they are live to extend the pool of strain-specific genes from organisms (Sisto et al., 2000; Willén et al., 2000), 6–7% (as determined from the comparison of the but others concluded that they are degenerating first two sequenced genomes) to 18–22%. More organisms (Kusters et al., 1997). recently, a large study was conducted on 56 H. pylori strains and four H. acinonychis strains, 1 1. .3 Structure of the genome with whole genome microarrays. They concluded that the core genome present in all H. pylori The genome of three H. pylori strains has now isolates contains 1111 genes, with a weighted been fully sequenced: strain 26695 from a patient average of 27% of the genome variably present in with gastritis, strain J99 from a duodenal ulcer different isolates Gressmann( et al., 2005). patient, and strain HPAG1 from a patient with Besides the cag pathogenicity island, which is known to be a variable region, half of the 386 Helicobacter pylori strain-specific genes are clustered in a hypervari- H. pylori can be present transiently in the able region, known as the ‘plasticity zone’ (Salama mouth when regurgitated, and may also be et al., 2000). The group of genes containing the found in the faeces, but it cannot survive with most variability are those that comprise genes of competing organisms (Parsonnet et al., 1999). unknown function (44%), genes associated with DNA metabolism (most of them are restriction- 1 1. 6. Function of gene products modification systems 54%), outer-membrane (a) Colonizing factors proteins (22%), cellular processes/cagPAI (40%) and others (100%, including transposases) Colonization by H. pylori involves an inter- (Gressmann et al., 2005). action between a large family of Helicobacter The genomic analyses suggest that H. pylori outer membrane proteins (Hop) and the gastric strains have essentially identical metabolic epithelial cells of the host. Several genes involved potential (Table 1.2). in determining the composition of the outer membrane are differentially regulated by a 1 1. 4. Host range phase variation called slipped-strand repair. This phenomenon is possible due to the presence of H. pylori is the Helicobacter species of repeated intragenic sequences, allowing replica- humans. H. pylori isolation from several other tive shifts and mismatchs, leading to changes animal species (monkey, pig, cat, dog) has been in the status of a gene (“on/off”) Salaün( et al., reported, but these reports were anecdotal, and 2004). Such proteins are the blood group antigen these bacteria were most likely acquired from binding adhesion (BabA), sialic acid binding humans. adhesion (SabA), adherence-associated lipopro- tein (AlpA and AlpB), and HopZ. 1 1. .5 Target cells and tissues Lipopolysaccharides play an important The target cell of H. pylori is the gastric role in the interaction between Gram-negative mucus-secreting cells. A low acid output leads bacteria and their host. They are potential stimu- H. pylori to also infect the corpus (Louw et al., lators of the immune system (Moran et al., 1996). 1993). H. pylori lives mainly in the surface mucus TheH. pylori lipopolysaccharides, however, have layer and within the pits, and can adhere to remarkably low activity, and their synthesis may mucus-secreting cells especially close to inter- involve over 20 genes, scattered throughout the cellular junctions (Hazell et al., 1986). It is not genome, unlike other bacteria in which they are found on intestinal-type cells in the case of intes- grouped into a single cluster. tinal metaplasia. In contrast, it has the ability to The expression of fucosyltransferase, an colonize metaplastic gastric cells present in the enzyme essential for the lipopolysaccharide duodenum and elsewhere, for example, in the biosynthesis pathway, is also subject to phase oesophagus, in Meckel’s diverticulum, and in the change, and is a key enzyme allowing H. pylori rectum (Hill & Rode, 1998). to mimic human Lewis antigens, which allows it The main cell receptor for this adherence is the to escape the host immune response (Lozniewski blood group antigen A, and the corresponding et al., 2003). adhesin is named BabA. In a low proportion of It has been suggested that this differen- the cells, H. pylori may be intracellular, a situa- tial regulation and the strain-specific outer- tion which contributes to its persistence (Dubois membrane-related genes may play a role in the & Borén, 2007). severity of H. pylori-related disease, and the 387 IARC MONOGRAPHS – 100B Table 1 .2 Classification of the genes of two Helicobacter pylori strains sequenced Annotation category No. of genes in: H. pylori J99 H. pylori 26695 Functionally classified 877 898 Aminoacid biosynthesis 44 44 Biosynthesis of cofactors etc. 60 59 Cell envelope 160 164 Cellular processes 96 113 DNA replication 23 23 DNA restriction-modification, etc. 66 68 Energy metabolism 104 104 Fatty acid and phospholipid metabolism 28 29 Purine and pyrimidine biosynthesis 34 34 Regulatory functions 32 32 Transcription 13 13 Translation 128 128 Transport and binding proteins 88 87 Conserved with no known function 275 290 Helicobacter pylori specific 343 364 Total 1495 1552 From Doig et al. (1999) ability of H. pylori to persist chronically in its H. pylori strains, and the babB gene is present host (Mahdavi et al., 2002). in almost them all (Colbeck et al., 2006; Hennig et al., 2006).