Stomach cancer incidence is known to increase with age with the peak incidence occurring at 60-80 years. Cases in patients younger than 30 years are very rare[4,5]. In India, the age range for stomach cancer is 35-55 years in the South and 45-55 years in the North. The disease shows a male preponderance in almost all countries, with rates two to four times higher among males than females[3,6].

Gastric cancer can develop both in the proximal and the distal region. Distal gastric cancers predominate in developing countries, among blacks, and in the lower socio-economic groups. Dietary factors and Helicobacter pylori (H. pylori) infection are major risk factors for the development of distal tumors. Proximal tumors are more common in developed countries, among whites, and in higher socio-economic classes. The major risk factors for proximal cancers are gastroesophageal reflux disease and obesity. Distal tumors continue to predominate in Japan in contrast to the increasing prevalence of proximal tumors in the rest of the world[7].

The steady decline in the incidence and mortality of stomach cancer in most affluent countries has been attributed to changes in dietary pattern, food storage, and control of H. pylori infection. The incidence of gastric cancer varies in different parts of the world with highest incidence rates documented in Eastern Asia, Eastern Europe, and South America, while North America and Africa show the lowest recorded rates[3,8-10]. Stomach cancer is the fifth most common cancer in Europe with 159 900 new cases and 118 200 deaths reported in 2006[11]. The population of Linxian, China is known to have one of the highest rates of oesophageal/gastric cardia cancer in the world[12]. In India, the incidence of gastric carcinoma is higher in the southern and north-eastern states with Mizoram recording an age-adjusted rate of 50.6 and 23.3 for men and women respectively[13,14]. A recent assessment of 556 400 deaths due to cancer in India in 2010 based on a nationally representative survey found that stomach cancer with a mortality rate of 12.6% is the second most common fatal cancer[15].

Significant variations in the incidence of gastric cancer have been observed between different ethnic groups living in the same region; African-Americans, Hispanics and Native Americans are affected more than Caucasians in the United States. High frequency of gastric cancer has been documented in Maoris of New Zealand[16]. However, the geographical distribution of gastric cancer cannot be ascribed to racial differences alone. For example, natives of Japan and China living in Singapore have higher rates than their counterparts in Hawaii. Furthermore, people who migrate from high incidence areas such as Japan to low-incidence regions such as the United States were found to have reduced gastric cancer risk[9,16].

H. pylori, a Gram-negative microaerophilic, spiral bacterium found in the gastric mucosa in patients with severe gastritis and chronic atrophic gastritis, has been recognized as an important risk factor for gastric cancer[2,21]. The results of several meta-analyses concluded that H. pylori infection is associated with an approximately two-fold increased risk of developing gastric cancer[22]. In a prospective study involving 1526 Japanese patients who had duodenal ulcers, gastric ulcers, gastric polyps or non-ulcer dyspepsia, 2.9% of H. pylori infected patients subsequently developed gastric cancer while none of the uninfected patients developed tumors[23]. In 1994, the International Agency for Research on Cancer categorized H. pylori as a “Group 1 human carcinogen” based on a plethora of studies[24].

Currently, approximately 50 per cent of the world’s population is infected by H. pylori. The prevalence of H. pylori infection varies markedly in different countries in Asia with seroprevalence rates higher in developing countries than in industrialized, developed nations[25].

The identification of H. pylori as a risk factor for gastric carcinogenesis has stimulated extensive research on the mechanisms by which H. pylori induces carcinogenesis. A combination of a virulent organism, a permissive environment, and a genetically susceptible host is considered essential for H. pylori-induced gastric cancer. H. pylori has been suggested to trigger a cascade of events that promote the sequential progression of normal gastric epithelium through atrophic gastritis, intestinal metaplasia, and dysplasia to carcinoma[26-28]. The bacterium secretes several products that cause gastric mucosal damage such as urease, protease, phospholipase, ammonia, and acetaldehyde. H. pylori disrupts gastric barrier function via urease-mediated myosin II activation[29].

Generation of oxidative stress is recognized as a virulence factor in H. pylori-infected hosts. H. pylori infection induces the production of reactive oxygen and nitrogen species and suppresses the host antioxidant defense mechanisms, leading to oxidative DNA damage. However, H. pylori, which is endowed with a variety of antioxidant enzymes is spared from oxidative stress and the damage is solely restricted to the gastric mucosa of the susceptible host[30]. H. pylori although not directly mutagenic, has been suggested to favor the formation of mutagenic substances through inflammatory mediators or by impairing the mismatch repair pathway[26,31]. Kim et al[26] demonstrated that H. pylori infection promotes gastric carcinogenesis by increasing endogenous DNA damage whilst decreasing repair activities and by inducing mutations in the mitochondrial and nuclear DNA. Aberrant DNA methylation induced by H. pylori infection has been found to be a significant risk factor for gastric cancer[32].

Epidemiological evidence suggests that H. pylori strains containing the cag pathogenicity island (cagPAI) are more virulent. The cagPAI is a 40-kb genome segment that encodes approximately 30 genes including the cytotoxin-associated gene A (cagA). The virulent cagA positive strains increase the risk of non-cardia gastric cancer of both intestinal and diffuse types, but not the risk of cardia cancer. The CagA protein is delivered into gastric epithelial cells where it undergoes tyrosine phosphorylation by SRC family kinases. Phosphorylated CagA specifically binds to and activates SHP2, a phosphatase that transmits positive signals for cell growth and motility. Thus H. pylori acting via cagA activates growth factor receptors, increases proliferation, inhibits apoptosis, and promotes invasion and angiogenesis[33].

Gene expression profiling of gastric antral mucosa samples from H. pylori infected patients by microarray analysis followed by quantitative real-time PCR assays have revealed differential expression of 38 genes, indicating that H. pylori infection leads to evasion of host defense, enhanced inflammatory and immune responses, activation of NF-κB and Wnt/β-catenin signaling pathways, perturbation of metal ion homeostasis, and induction of carcinogenesis[34].

A survey of literature on the role of diet in the pathogenesis of gastric cancer using PubMed as a search platform has revealed over 2000 epidemiological and experimental studies. Populations at high risk for stomach cancer have been shown to consume diets rich in starch and poor in protein quality, and are not inclined to eat fresh fruits and vegetables. Both high starch and low protein diet may favor acid-catalyzed nitrosation in the stomach and cause mechanical damage to the gastric mucosa[35-37]. Using an ecological approach, Park et al[38] found a negative association between refrigerator use, fruit intake, and gastric cancer mortality and positive associations between salt/sodium intake and gastric cancer mortality and incidence in Korea.

Both epidemiological and experimental studies strongly support the role of excessive salt intake in gastric carcinogenesis. D’Elia et al[39] reported a direct correlation between dietary salt intake and risk of gastric cancer with progressively increasing risk across consumption levels based on a meta-analysis of prospective studies. Consumption of large amount of salted fish, soy sauce, pickled vegetables, cured meat and other salt-preserved foods enhances H. pylori colonization, and increases the risk of gastric cancer through direct damage to the gastric mucosa resulting in gastritis. Salt is also known to induce hypergastrinemia and endogenous mutations, promoting epithelial cell proliferation which eventually leads to parietal cell loss and gastric cancer progression[40,41]. Reports from this laboratory as well as by other workers have demonstrated that saturated sodium chloride (S-NaCl) promotes the development of N-methyl-N’-nitro-N-nitrosoguanidine (MNNG)-induced rat gastric carcinomas[42,43].

Dietary nitrates are found either naturally in foods such as cabbage, cauliflower, carrot, celery, radish, beets, and spinach or added during preservation. In addition, the nitrate content of fertilizers, soil, and water also contribute to dietary nitrate. Nitrite, nitrate, and nitrosating agents can be synthesized endogenously by reactions mediated by bacteria and/or activated macrophages. Nitrosation of a number of naturally occurring guanidines and L-arginine-containing polypeptides produces mutagenic compounds. Dietary nitrate is converted to carcinogenic N-nitroso compounds (NNC) by gastric acid thereby increasing gastric cancer risk. Small quantities of preformed NNC and nitrosamines may also be present in some foods including cured meats, dried milk, instant soups, and coffee dried on direct flame[44-46].

In addition to specific components of the diet, certain cooking practices are also associated with increased risk of gastric cancer. These include broiling of meats, roasting, grilling, baking, and deep frying in open furnaces, sun drying, salting, curing, and pickling, all of which increase the formation of NNC. Polycyclic aromatic hydrocarbons such as benzo[a]pyrene formed in smoked food have been incriminated in many areas of the world with high stomach cancer rates[47,48].

Alcohol, a gastric irritant is an important risk factor for gastric cancer. Zaridze et al[49] have reported an increased risk of stomach cancer in men and women who regularly consume strong alcoholic beverages. A direct correlation was observed between consumption of alcohol and tobacco and the risk of gastric cancer in a population-based prospective cohort study[50]. A study from this laboratory demonstrated a positive correlation between alcohol consumption and cigarette smoking with the blood lipid profile in gastric cancer patients[51]. The European Prospective Investigation into Cancer and Nutrition (EPIC) project found a significant association between the intensity and duration of cigarette smoking and gastric cancer risk[52]. Smoking history was found to be a significant independent risk factor for death from gastric cancer in patients who had undergone curative surgical resection[53]. Smoking is known to decrease prostaglandins that maintain gastric mucosal integrity[54]. Tobacco smoke has been reported to induce the development of precursor gastric lesions such as gastritis, ulceration, and intestinal metaplasia. Smokers tend to have a higher incidence of H. pylori infection and gastroduodenal inflammation than non-smokers[55].

Gastric cancer is a known manifestation of inherited cancer predisposition syndromes similar to hereditary nonpolyposis colon cancer and Li-Fraumeni syndrome. According to the OMIM database, 90 per cent of gastric cancers are sporadic, whereas 10 per cent are hereditary. The first documented report of familial predisposition to gastric cancer was described for Napoleon Bonaparte’s family (OMIM_192090) with Napoleon, his father, grandfather, brother, and three sisters, all dying of stomach cancer at a relatively early age[56]. The Scandinavian twin study in the Swedish, Danish, and Finnish twin registries found an increased risk of stomach cancer in the twin of an affected person[57]. Family members usually share the same environment and have similar socioeconomic status. These risk factors act independently or in conjunction with genetic factors thereby increasing the risk of stomach cancer.

A positive correlation has been recognized between increased stomach cancer risk and a number of occupations including mining, farming, refining, and fishing as well as in workers processing rubber, timber, and asbestos[58,59]. Occupational exposure to dusty and high temperature environments such as in cooks, wood processing plant operators, food and related products machine operators was associated with a significant increased risk of gastric cancer of the diffuse subtype[60]. A German uranium miner cohort study however found a positive statistically non-significant relationship between stomach cancer mortality and occupational exposure to arsenic dust, fine dust, and absorbed dose from α and low-linear energy transfer radiation[61].

CIN, recognized as the most common instability occurring in sporadic gastric tumors, may manifest as gain or loss of whole chromosomes (aneuploidy) or parts of chromosomes [loss of heterozygosity (LOH), translocations, and amplifications][65]. Comparative genomic hybridization analysis has revealed numerous DNA copy number variations with gains in chromosomal regions 6p21, 9p34, 11q23, 17p13, 19p13, and 22q13, especially in younger patients[66]. Using laser microdissection, Tsukamoto et al[67] demonstrated DNA copy number variations in gastric cancer patients with a high frequency of 20q13 chromosome gain as well as upregulation of 114 candidate genes in the regions of amplification, and downregulation of 11 genes in the regions of deletion. LOH at chromosomes 1p, 2q, 3p, 4p, 5q, 6p, 7p, 7q, 8p, 9p, 11q, 12q, 13q, 14q, 17p, 18q, 21q, and 22q that are possible sites of tumor suppressor genes is believed to play a crucial role in gastric carcinogenesis. A high frequency of LOH was found in the adenomatous polyposis coli (APC), p53, nm23 and Rb loci[65,68]. Several factors have been suggested to contribute to CIN in gastric cancer patients including aberrations in chromosome segregation, DNA damage response, cell cycle regulation, H. pylori infection, tobacco, and dietary nitrates.

MSI, resulting from errors in DNA replication is seen in 15-20 per cent of gastric cancers with a higher frequency in familial cases. The high frequency of MSI associated with advanced, invasive, intestinal-type gastric cancer has been suggested to be due to epigenetic inactivation of the mismatch repair gene hMLH1, whereas mutations in transforming growth factor-β (TGF-β) RII, insulin-like growth factor II (IGFII) R, and BAX genes in sporadic gastric tumors with MSI display a decreased tendency for invasion and nodal metastasis[65,69,70]. Cytosine-adenine repeat instability, LOH of the APC, and deleted in colon cancer genes have been documented in well-differentiated tumors[71].

Mutational activation and/or amplification of several oncogenes has been documented in gastric cancer. The K-ras oncogene was found to be mutated (codon-12) in intestinal-type cancer and its precursor lesions, intestinal metaplasia, and adenoma, but not in diffuse-type cancer[72]. Overexpression of c-erbB2 a cell surface receptor of the tyrosine kinase family is more common in intestinal-type gastric cancer, whereas in diffuse-type gastric cancers, amplification of c-met, a transmembrane tyrosine kinase receptor, and aberrations in the FGFR2/ErbB3/PI3 kinase pathway have been frequently documented[63,73]. A high correlation was observed between EZH2 the human homolog of the Drosophila protein ‘‘Enhancer of Zeste’’, with intestinal-type cancer and the risk of distant metastasis[74].

Alterations in a number of TSGs have been documented in the pathogenesis of stomach cancer. The p53 gene is frequently inactivated in gastric carcinomas as well as in precursor lesions by LOH, missense mutations, or frameshift deletions[63,65,72]. GC-AT transitions of the p53 gene are common in diffuse-type gastric cancer induced by carcinogenic N-nitrosamines produced from dietary amines and nitrates[72,75]. LOH and mutations of PTEN on chromosome 10q23.31 were observed in gastric cancers as well as in precancerous lesions[76]. The RUNX3 gene, a tumor suppressor, is also involved in the complex process of gastric oncogenesis[77]. Hypermethylation of RUNX3 promoter in chronic gastritis, intestinal metaplasia, and gastric adenomas, suggests that this gene is a target for epigenetic gene silencing in stomach cancer[78]. Hypermethylation of nuclear retinoic acid receptor β has been documented in intestinal-type gastric cancers but not in the diffuse-type[79].

Gene abnormalities and aberrant expression of cell cycle regulators play a pivotal role in the pathogenesis of gastric cancer. Overexpression of cyclin E and CDK together with aberrant p53 expression and downregulation of p27, a common event in gastric cancer, is associated with increased aggressiveness and poor prognosis[80,81]. A meta-analysis of cell proliferation-related genetic polymorphisms revealed a significantly higher risk of diffuse-type of stomach cancer in individuals harboring TP5372Pro polymorphisms[82]. Immunohistochemistry and TUNEL staining performed on tissue array slides containing 293 gastric carcinoma specimens showed a positive correlation between the expression of cyclin D1, p21, or p27 with early pTNM stages, tumor cell proliferation and good prognosis, but an inverse correlation with lymph node metastasis. However, p27 expression inversely correlated with the apoptosis index indicating that these cell cycle regulators may serve as candidate molecular markers for early gastric carcinoma[83]. High levels of circulating cell-free human telomerase reverse transcriptase mRNA in gastric cancer patients suggests that this molecule may be useful as a noninvasive diagnostic and prognostic marker[84].

Several growth factors and cytokines produced by the gastric tumor microenvironment regulate differentiation, activation, and survival of multiple cell types. Extensive changes in the expression profiles of the components of the TGF-β signaling pathway and its downstream targets occur during the sequential progression of the normal epithelium through chronic atrophic gastritis and dysplasia to carcinoma. These changes include a progressive increase in the expression of TGFB1/2, TGFBR1, MYC and TP53, enhanced expression of SMAD4, CDKN1A, SMAD1/2/3, SMAD2/3 and CDKN1B in dysplasia that decreased in carcinoma, and enhanced expressed of TGFBR2, SMAD7, RELA, and CDC25A both in dysplasia and carcinoma[85]. A systematic review and meta-analysis of interleukin (IL)-1B cluster gene polymorphisms at positions -511, -31, and +3954 and the receptor IL-1RN variable number tandem repeat (VNTR) polymorphisms revealed that IL-1B -511 T allele and IL-1 RN*2 VNTR are significantly associated with an increased risk of developing gastric carcinoma especially the non-cardia or intestinal-type and among Caucasians[86].

Mutational inactivation and downregulation of genes encoding cell-adhesion molecules that function as tumor suppressors have been documented in gastric cancer. Inactivation of E-cadherin, a product of the CDH1 gene has been suggested to play an important role in cell motility, growth, and invasion of gastric cancer[87]. Rare genetic alterations of IQ motif-containing GTPase-activating protein 1 gene, also called p195 (locus 15q26), a negative regulator of cell-cell adhesion at adherens junctions were found to occur in diffuse gastric cancers[88].

Expression of the proangiogenic vascular endothelial growth factor (VEGF) was demonstrated to correlate with poor survival in gastric cancer patients[89]. VEGF-A was found to be a significant marker for the presence of tumor cells in the bone marrow, whereas VEGF-D is a useful predictor of the lymphatic spread of tumor cells in gastric cancer patients indicating that the metastatic spread of gastric cancer could be determined, in part, by the profile of VEGF family members expressed in the primary tumour of gastric cancer patients[90]. Using human gastric cancer specimens, in vitro cell experiments, and in vivo animal experiments, Lee et al[91] demonstrated that hypoxia-independent promotion of the AKT-HIF-1α-VEGF pathway contributes to gastric cancer tumorigenesis and angiogenesis.

miRNAs located within regions of LOH, amplification, fragile sites, and in other cancer-associated genomic regions regulate a number of important biological processes relevant to carcinogenesis including proliferation, apoptosis, differentiation, angiogenesis, metastasis, and immune response and they function as both oncogenes and tumor suppressor genes. miR dysregulation plays a key role in the pathogenesis of gastric cancer. Studies have shown that miRs that function as oncogenes, such as miR-21, miR-106a and miR-17, were upregulated, whereas miRs that function as tumor suppressors, including miR-101, miR-181, miR-449, miR-486, let-7a, were downregulated in gastric cancer[92]. In addition, genetic polymorphism of miR-196a-2 that interfers with its normal binding with target mRNA such as homeobox gene cluster and annexin A1 was associated with a significantly increased risk of gastric cancer[93]. H. pylori infection was demonstrated to induce dysregulation of cancer-associated miRNAs including oncogenic (miR-106b) and tumor suppressor (let-7) miRNAs with hypermethylation of the tumor suppressor miRNAs miR-124a-1, miR-124a-2 and miR-124a-3[94,95].

The advent of genomics, proteomics, and transcriptomics has enabled successful detection of the comprehensive molecular alterations that occur during neoplastic transformation of the gastric mucosa. In Japan, a genome-wide linkage study identified chromosome 2q33-35 as a potential susceptibility locus for proximal gastric cancer[96]. Analysis of the microarray gene-expression data of 54 paired gastric cancer and adjacent noncancerous gastric tissues identified gene signatures of different grades and different stages of gastric cancer. While a 19-gene signature distinguished between high- and low-grade gastric cancers, an expanded 198-gene panel allowed the stratification of cancers into four grades plus control and a 10- and 9-gene signature enabled classification of early- and advanced-stage cancer respectively[97]. Sun et al[98] analysed multiple gene expression patterns and their exact roles in gastric carcinogenesis using high-throughput tissue microarray technique. The results showed that while p53 was useful for distinguishing low-grade dysplasia from high-grade dysplasia, high-level expression of cyclin E might be an indicator for malignant transformation of dysplasia. Gene expression profiles by Affymetrix technology and quantitative polymerase chain reaction and in situ hybridization on tissue microarrays revealed that the majority of alterations associated with early gastric cancer are retained in advanced gastric cancer, with additional gene expression changes in AGC compatible with a progression model of gastric carcinogenesis. Molecular characterization of 8 primary gastric carcinomas, corresponding xenografts, and 2 novel gastric carcinoma cell lines revealed comparable histological features and expression of several markers as revealed by immunohistochemistry, copy number, and hypermethylation of up to 38 genes[99].

Comprehensive protein profiling of paired surgical specimens of primary gastric adenocarcinomas and non-tumor mucosae from Japanese patients by 2-D gel electrophoresis and liquid chromatography-electrospray ionic tandem mass spectrometry revealed increases in manganese dismutase and nonhistone chromosomal protein HMG-1 with decreases in carbonic anhydrases I and II, glutathione-S-transferase and foveolin precursor (gastrokine-1) (FOV), an 18-kDa stomach-specific protein with putative tumor suppressor activity. RT-PCR analysis also revealed significant downregulation of FOV mRNA expression in tumor tissues, underscoring its potential use as an effective biomarker for diagnosis and molecular target for chemotherapy[100].

Although the role of genetic alterations in gastric cancer has long been recognized, global changes in the epigenetic landscape with reference to DNA methylation, histone methylation and histone acetylation have only been recently documented. While global hypomethylation leads to activation of oncogenes and genomic instability, promoter hypermethylation is associated with transcriptional silencing of TSGs and DNA mismatch repair genes[101]. Diverse CpG island methylator phenotypes have been identified in gastric cancer that serve as good prognostic indicators[102]. Meta-analysis indicated aberrant methylation of 77 genes in gastric cancer, suggesting the potential clinical value of DNA methylation as a marker for risk prediction and prognosis[103]. Hypermethylation of promoters of genes involved in cell cycle control, metabolism of essential nutrients, and production of inflammatory mediators, has been described in H. pylori infection as well as in gastric cancer[104].

E-cadherin, a member of the APC pathway, and CDH4 (encoding R-cadherin), are hypermethylated in gastric tumors. In particular CDH4 methylation is an early diagnostic marker for gastrointestinal tumorigenesis[105]. Epigenetic inactivation by hypermethylation of the RAS-related gene, RASSF1A isoform, a negative effector of K-ras, and activation of the R-RAS oncogene by hypomethylation has been reported in gastric carcinomas[106].

Histone acetylation and deacetylation catalyzed by histone acetyltransferases and histone deacetylases (HDACs) play an important role in chromatin remodeling. Histone H4 acetylation in both the promoter and coding regions of the p21WAF1/CIP1 gene in cells expressing dominant-negative p53 was significantly reduced in gastric cancer cells expressing wild-type p53[107]. Epigenetic modifications also play an important role in miRNA deregulation in gastric cancer[95].

Thus, genetic and epigenetic alterations can lead to perturbations in normal cellular homeostasis eventually culminating in neoplastic transformation of the gastric mucosa. In particular, disruption in a number of regulatory pathways, evasion of apoptosis and increased progression through the cell cycle could create a permissive environment for genomic instability, invasiveness, and metastasis.