Search strategies:
All the available papers published before 2022 were collected by searching in PubMed and Scopus. The keywords included in the research were “H.pylori”, “gastric cancer”, “virulence factors”, “tumor suppressor genes” “ gene mutations” “cagA+” used by Boolean operators to obtain the articles with the keywords in their titles or abstracts.
All records were entered into the computer program Zotero to remove the duplicated articles or merge them. After that, the Rayyan website was used for the title and abstract screening. The included articles were downloaded for full-text screening and eligibility. The Statistical Package for Social Sciences (SPSS v26) was used for analysis.
Inclusion and exclusion criteria:
All the case-control, cross-sectional, cohort studies, and the letter to the editors regarding the H. pylori infection, gastric cancer, and tumor suppressor genes were considered eligible and included in the study.
The eligibility of studies was determined after reviewing and evaluating the titles, abstracts, and full text of the studies. The included studies needed to be focused on the main idea, using standard methods including culture, urea breath test, immunohistochemistry PCR, Sequencing, and PCR-RFLP. However, the studies published in a non-English language, review articles, case reports, studies based on nonclinical samples, laboratory animals, and other autoimmune diseases were excluded.
Background:
H. pylori can trigger a strong immune response that results in inflammation of the gastric mucosa. Gastric cancer is largely caused by the failure to eradicate the organism from the stomach, where it causes a protracted infection (21). However, it is a protracted process governed by the digestive environment, the host, and bacterial virulence factors, leading to gastric issues like peptic ulcer disease and gastric cancer (22). H. pylori possesses numerous virulence factors, including Vacuolating cytotoxin A (VacA) (23), the cytotoxin-associated gene pathogenicity island (cagPAI), an oncoprotein (i.e. cytotoxin-associated gene A (CagA)), and adhesion proteins, all of which are associated with the pathogenicity and development of gastric cancer (24). The purpose of this review was to compile the most recent findings regarding changes in gastric tumor suppressor genes that are associated with CagA and contribute to the pathogenesis and development of gastric cancer.
Cytotoxin-associated Gene Pathogenicity Island (cagPAI):
cagPAI contains about 40 kb of chromosomal DNA that contains about 32 open reading frames (ORFs), such as cag1 to cag26, cagA to cagZ (25). cagPAI encodes the effector protein cagA and the syringe-like structure the bacterial type four (IV) secretion system (T4SS) which delivers the cagA into the epithelial cells of the stomach (26, 27). Although cagPAI integrity is required to encode intact T4SS for H. pylori contact with host cells, cagPAI is not present in all strains and is defective in some strains (28, 29). Infection with strains expressing the entire cagPAI has been linked to severe gastrointestinal problems such as chronic gastritis, peptic ulcer disease, and gastric cancer (30, 31).
Cytotoxin-associated Gene A (CagA):
H.pylori bacteria that can express this protein are thought to be very virulent, whereas strains that cannot express this protein are thought to be less virulent. This protein is approximately 125–145 kDa in length. T4SS assists in its translocation into gastric epithelial cells (29, 32). CagA is made up of a well-known five-amino-acid motif called EPIYA (glutamic acid-proline-isoleucine-tyrosine-alanine), which forms a sequence at the C-terminal region and, along with the nearby sequence, is critical for CagA's biological activity (24).
Depending on geographic variation, H. pylori strains can have four different EPIYA-sequences, namely EPIYA-A, -B, -C, and -D. CagA with the EPIYA-D sequence has a greater ability to de-regulate cellular activities than CagA with the EPIYA-C sequence. CagA containing the first two EPIYA-sequences (EPIYA-A and EPIYA-B) combined with the EPIYA-D sequence is thus considered more virulent than CagA containing the EPIYA-A, EPIYA-B, and EPIYA-C sequences (33).
CagA Translocation
CagA is translocated into gastric epithelial cells after H. pylori strains produce it, and at least 15 cagPAI-encoded proteins are involved in the formation of T4SS (34, 35). CagA is exposed on the bacterial surface via T4SS and interacts with PS patches on the plasma membrane of host cells that have been abnormally externalized due to H. pylori infection. CagA's N-terminal region interacts with the PS patches, causing the bound CagA to be flipped inside and internalized (36).
Tyrosine Phosphorylation:
When CagA is delivered into gastric epithelial cells, its tyrosine (Y) residue in the EPIYA motifs is phosphorylated by cellular kinases such as Csk, Src family kinases (SFKs), and c-Abl (37, 38, 39). CagA has the ability to bind promiscuously to the SH2 domain, which contains host proteins like the pro-oncogene Src homology 2 phosphatase (SHP2), PI3K, Crk, and the adaptor protein Grb2 (40–43).
Pylori cagA+ and Tumor Suppressor Genes:
Apoptosis-stimulating Protein of p53-2 (ASPP2) Tumor Suppressor:
ASPP2 was initially discovered as a p53 binding protein, but it has now been shown to be a self-sufficient tumor suppressor that collaborates with p53 (and its family members p63 and p73) to inhibit tumor growth in vivo (44, 45). Furthermore, mounting evidence suggests that ASPP2's cellular roles include tight junction development and epithelial cell polarity maintenance. ASPP2 is downregulated in many aggressive tumors, including gastric cancers. Following Hp infection and CagA delivery, the level of ASPP2 increases. Furthermore, after Hp infection, CagA coimmunoprecipitates with ASPP2, altering its proapoptotic activity (46–54).
CagA injection enhances the interaction of p53 and ASPP2. Doxorubicin (Dox), a DNA-damaging chemical that activates p53, induced ASPP2 association with p53 as well as cell apoptosis. The link between CagA and ASPP2 was discovered 90 minutes after infection and grew stronger over time. In contrast, the relationship between ASPP2 and cytoplasmic p53 was discovered at 3 h and peaked at 7 h. This implies that when CagA interacts with ASPP2, the cytoplasmic pool of p53 is recruited.
P53 is a transcription factor that regulates gene expression. Although it is significantly stabilized following DNA damage or cellular stress, the proteasome degrades it quickly. Because ASPP2 binds cytoplasmic p53 during H. pylori infection, the transcriptional activity of p53 may be altered after CagA translocation. According to several lines of research, CagA-mediated suppression of p53 expression is ASPP2 dependent, whereas CagA-induced p53 degradation is mediated by the proteasome. Under normal conditions, the tumor suppressor activity of the ASPP2-p53 pathway is primarily mediated by stimulation of the apoptotic response. CagA, on the other hand, increases the association between p53 and ASPP2, resulting in increased p53 degradation and, as a result, transcriptional suppression in H. pylori-infected cells. CagA inhibits apoptosis by binding the tumor suppressor ASPP2, causing p53 to be damaged and its apoptotic activity to be suppressed (56).
Phosphatase And Tensin Homolog (PTEN):
PTEN (phosphatase and tensin homolog) is a tumor suppressor gene (810) found on chromosome 10q23. It has been discovered to regulate the protein kinase B (AKT) and mechanistic target of rapamycin signaling pathways, both of which are involved in apoptosis, cell cycle progression, and cell proliferation. PTEN deficiency has been linked to oncogenesis and somatic mutations in a number of cancers. Tet methylcytosine dioxygenase (Tet)1 has been found to interact with the p53enhancer of zeste 2 polycomb repressive complex 2 subunits (EZH2) signaling pathway to decrease tumors in gastric cancer. Tet1 inhibits cancer formation by activating p53 and inhibiting the carcinogenic protein EZH2, perhaps through DNA demethylation (57, 58). Zhang et al (59) recently reported that In human gastric cancer, the expression of PTEN was found to be dramatically reduced by CagA.
Cyclin-dependent Kinase Inhibitor 2A-CDKN2A:
E-cadherin and CDKN2A are two tumor suppressor genes whose promoter hypermethylation has been associated with H. pylori infection (59, 60). The tumor suppressor p16INK4A deletion has been implicated in the carcinogenic process in a number of malignancies (61–63). It is common for p16INK4A to lose expression in GC, and hypermethylation of its promoter regions is thought to be the main factor in this gene's inactivation (64–66). To the contrary, numerous earlier studies (67, 68) claimed that promoter methylation and p16INK4A showed a strong correlation. The mechanism behind p16INK4A inactivation is unknown, despite the fact that it is acknowledged as a contributing factor to GC carcinogenesis. It is believed that H. pylori plays a significant role in this process (69, 70). The H. pylori genotype has an impact on whether methylation or non-methylation mechanisms are used to inactivate p16INK4A, according to research by Zhang et al. (59). Additionally, they noted that depending on where the tumor is located, different histological subtypes of GC exhibit different patterns of p16INK4A inactivation. Only in Nocardia tumors does methylation of the CDKN2A promoter render p16INK4A inactive in diffuse subtype cancers. In contrast, both cardia and non-cardia tumors exhibit promoter methylation, which is a crucial pathway for deactivating p16INK4A in intestinal cancers. Additionally, the methylation of the CDKN2A promoter is influenced by the H. pylori genotype.
p53 And p27
The p53 gene, which is located on the short arm of chromosome 17 (17p13.1), produces a protein that acts as a transcription factor and controls a number of physiological processes, including cell division, DNA damage response, apoptosis, and angiogenesis. The main transcriptional target of p53 is WAF1 (also known as CIP1, SDI1, mda-6, or CDKN1A). A phosphorylated 21-kDa protein with tumor-suppressing properties is encoded by the p2WAF1/CIP1 gene. P53 mutations are found in between 38% and 71% of gastric cancer tumors, making them relatively common (70, 71).
Another CIP/KIP tumor suppressor protein is encoded by the p27KIP1 gene, which is located on chromosome 12p13. Because p27Kip1 Protein (p27) and p21 share a 42 percent structural similarity, this explains how their ability to inhibit the cyclin D/CDK4, cyclin E/CDK2, and cyclin A/CDK2 complexes to stop the progression of the cell cycle is similar (72, 73). Reduced or absent p27 protein expression is associated with more aggressive characteristics and tumor growth in people with gastric carcinomas (74–76). Lower expression of p27 has been reported to be a predictor of aggressive behavior and a poor prognosis in a variety of malignant tumors, including breast, colon, liver, stomach, lung, brain, prostate, and malignant melanoma (77–80).
According to reports, H. pylori can result in a mutation in the p53 tumor suppressor gene, which in turn can lead to stomach cancer (81). Although p53 alterations have been examined in several studies of gastric cancer associated with H. pylori infection (81–85), there is still some controversy. Furthermore, H. pylori in gastric cancer has been linked to decreased p27 expression (86, 87). However, there is no scientific agreement among studies, and there aren't any studies linking these two suppressor genes to H. pylori (89).
Fragile Histidine Triad (FHIT):
Is a tumor suppressor gene that can be found on chromosome 3p14.2. Early studies on a large number of gastric cancer samples discovered that the tumor suppressor protein fragile histidine triad (FHIT) was lost in the majority of cases (more than 70%), and that it was more common in GC with the diffuse and mixed histotypes than the intestinal histotype. Changes in FHIT gene expression have been found in primary tumors and cancer cells from the lung, breast, head and neck, esophagus, colon and rectum, pancreas, kidney, cervix, and hepatocellular carcinoma. The FHIT protein is involved in a variety of biological functions, including cell cycle regulation, DNA damage sensitivity, and pro-apoptotic signaling (90–95).
CDH1:
CDH1 is a tumor suppressor gene that produces the E-cadherin protein, which is required for cell-cell interactions. Inactivating this gene increases the likelihood of metastasis. The methylation of the CDH1 promoter during the early stages of gastric carcinogenesis remains a mystery. It is found in epithelial cells and participates in cellular processes like adhesion, morphology, migration, and development. It plays a role in cellular processes such as adhesion, morphology, migration, and development, as well as in cell architecture and tissue integrity (96, 97).
It has been found in a variety of cancers, including gastric cancer, and its inactivation has been linked to tumor growth via invasion and metastasis. CDH1 mutations were found in approximately 50% of diffuse histological type gastric carcinomas, and CDH1 hypermethylation was discovered to be the second source of gene expression inactivation in two families with familial stomach cancer and CDH1 mutations (98).