**2. The risk factors of GC**

GC is defined as a tumor with multifactorial etiologies. Environmental alteration and genetic factors play major roles in GC, particularly, virus infection, dietary habits, and lifestyles are recognized to be critical factors.

*H. pylori* is a common human pathogen, contributing to both malignant and benign diseases. Among all the risk factors of GC, *H. pylori* is considered to be the most predominant factor, up to 80% of GC cases are led by *H. pylori* infection. At least 660,000 new diagnoses are making annually [5, 6]. Data in western countries indicated that *H. pylori* is a major risk factor for only non-cardia GC instead of cardia GC [5]. It has been noted that the death number of GC is decreasing in many developed regions, partially related to eradication therapy and improved living condition in certain populations and regions [7]. Given the worldwide aging problem and the migration trend of people from high to low prevalence regions, the mortality is very likely to increase in the future [8]. *H. pylori* infection alone is not sufficient to trigger GC. Chiba et al. suggested *H. pylori* infection contributed to GC development via two potential mechanisms, either caused chronic inflammation or directly acted on epithelial cells [9]. In general, *H. pylori* colonizes the gastric mucosal epithelium and induces chronic inflammation at the beginning of infection, while persistent inflammation resulted in GC [10, 11]. Cytokines and chemokines produced by the tumor microenvironment facilitate cell proliferation and migration, and keep cells from apoptosis and immune detection [12]. Cytokines released by different cell sources have been described as suggestive indicators in the progression of GC. The serum level of Tumor necrosis factor-α (TNF--α) in GC was found greatly reduced, however, levels of TNF-α in stage III or IV GC patients showed a significant elevation when compared to levels in earlier stage patients [13]. Besides, the secretion of Interleukin-1 (IL-1) contributes to tumor cell proliferation and progression. In addition, IL-6, IL-10, and Transforming growth factor-β (TGF-β) improve survival of GC cells by promoting cell invasion and suppression of antitumor immunity. IL-11, IL-17, IL-18, IL-22, and chemokines secreted by a specific cell type also improve the progression of GC [14]. Major types of virulence factors of *H. pylori* include cytotoxin VacA, the type IV secretion system (T4SS), and the CagA effector protein. These factors are associated with multiple cellular responses, such as induction of oxidative stress and epithelial barrier disruption, in various model systems [15].

Asia, Eastern Europe, and certain areas of Latin America [3]. Particularly, adenocarcinomas, which developed from the glands of the most superficial layer or the mucosa, take up 90% of GC. The rest types of GC include mucosa-associated lymphoid tissue (MALT) lymphomas,

The risk factors include environmental factors, such as *Helicobacter pylori* infection, cigarettes smoking and high-salt diet, and host genetic alterations. Although the incidence of GC has shown a dramatic decrease in recent years due to *H. pylori* eradication, the overall survival is still quite poor due to its silent nature, late clinical presentation, and genetic heterogeneity. Thus, comprehensive understanding of the detailed molecular mechanisms and accurate pathogenesis of GC will improve patient outcome. Recently, several kinds of molecular classification of GC have been provided to reveal the genomic landscape of GC and decipher the crucial molecular changes. Among them, The Cancer Genome Atlas (TCGA) classification is a milestone for the molecular characterization of GC. Clinical translation of these molecular findings will provide novel strategies for early GC detection and promote precision therapies

GC is defined as a tumor with multifactorial etiologies. Environmental alteration and genetic factors play major roles in GC, particularly, virus infection, dietary habits, and lifestyles are

*H. pylori* is a common human pathogen, contributing to both malignant and benign diseases. Among all the risk factors of GC, *H. pylori* is considered to be the most predominant factor, up to 80% of GC cases are led by *H. pylori* infection. At least 660,000 new diagnoses are making annually [5, 6]. Data in western countries indicated that *H. pylori* is a major risk factor for only non-cardia GC instead of cardia GC [5]. It has been noted that the death number of GC is decreasing in many developed regions, partially related to eradication therapy and improved living condition in certain populations and regions [7]. Given the worldwide aging problem and the migration trend of people from high to low prevalence regions, the mortality is very likely to increase in the future [8]. *H. pylori* infection alone is not sufficient to trigger GC. Chiba et al. suggested *H. pylori* infection contributed to GC development via two potential mechanisms, either caused chronic inflammation or directly acted on epithelial cells [9]. In general, *H. pylori* colonizes the gastric mucosal epithelium and induces chronic inflammation at the beginning of infection, while persistent inflammation resulted in GC [10, 11]. Cytokines and chemokines produced by the tumor microenvironment facilitate cell proliferation and migration, and keep cells from apoptosis and immune detection [12]. Cytokines released by different cell sources have been described as suggestive indicators in the progression of GC. The serum level of Tumor necrosis factor-α (TNF--α) in GC was found greatly reduced, however, levels of TNF-α in stage III or IV GC patients showed a significant elevation when compared to levels in earlier stage patients [13]. Besides, the secretion of Interleukin-1 (IL-1) contributes to tumor cell proliferation and progression. In addition, IL-6, IL-10, and Transforming

which originate from the muscles surrounding mucosa areas of the stomach [4].

for GC patients.

8 Stomach Disorders

**2. The risk factors of GC**

recognized to be critical factors.

*Epstein-Barr virus* (EBV), also termed as human herpesvirus 4 (HHV-4), is one of the most prevalent viruses in humans. EBV was the first virus identified in human carcinoma in 1964 [16]. Approximately 95% of adults in the world are infected by EBV due to the positive detection of serological EBV markers. EBV infection may not cause severe symptoms and disease. After primary infection, EBV establishes a carrier state called latent stage. However, latent EBV infection could subsequently become high-risk oncogenic factors associated with human malignancies. EBV has been known as another significant pathogen exists in GC cells. According to worldwide data, EBV-associated GC (EBVaGC) accounts for 10%, in average, of total GC cases. Most of the EBVaGC cases were epithelial tumors, while lymphoepithelioma-like carcinomas take up 90% of the rest rare EBVaGC cases. EBVaGC presents distinct clinic properties, such as predominance among male and younger individuals and predominant proximal stomach location [17]. EBV infection can be achieved by two different entry mechanisms, either via B cell entry or fusion with epithelial cells directly [18, 19]. EBVaGC occurs in the upper and middle stomach in the majority. Fukayama et al. indicated the tumor distribution in the stomach, with the proportion that 58% in the cardia, 33% in the body, and 9% in the antrum [20, 21]. They also depicted the appearance of EBVaGC as ulcerated or saucer-like, with obvious thickened gastric wall. Moreover, the lymph node was less frequently involved during early stage within the submucosa. These characteristics are proposed to be favorable prognosis indicators. Histological studies provide evidence that immune cell infiltration was a feature of EBVaGC. For instance, infiltrating lymphocytes, containing EBV-specific cytotoxic T cells, communicate with carcinoma cells directly, in the opposite, cytokine IL-1β was upregulated to recruit noninfiltrating lymphocytes against this cell-cell contact [22, 23]. Therefore, in EBVaGC, immune responses in tumor microenvironment also accompany with the progression of EBVaGC [24].

Dietary factors: dietary risks, including salty and spicy food intake, cigarettes smoking, frequent coffee, and high-temperature drinking habits are positively associated with GC. Intriguingly, excess salt intake showed susceptibility to EBVaGC and *H. pylori*-induced GC. Habitual excess salt intake was suggested to progressively increase the risk across consumption levels of GC via a meta-analysis [25]. The association between salty food intake and *H. pylori* infection was also evaluated in a cross-sectional study of 634-middle age male cohort. The result supported habitual salt-rich Japanese food intake was prevalent in *H. pylori*-induced GC cases [26]. It is probably due to the increase of *H. pylori* colonization and persistent infection [27]. Besides salty food intake, Camargo et al. indicated that smoking was strongly associated with EBVaGC by a case-case comparison between EBV-positive tumors and EBV-negative tumors [28]. Although alcohol drinking was a suggestive risk factor of GC, heavy alcohol drinking, rather than moderate alcoholic drinks, was significantly correlated with GC development [29, 30].
