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Gastric cancer is constituted by two histomorphological entities ‘intestinal’ and ‘diffuse’, however lesions with similar morphologies may differ in biological aggressiveness and response to therapy. Two distinct molecular pathways have been identified in gastric carcinogenesis: the microsatellite mutator phenotype and a phenotype associated with chromosomal and intrachromosomal instability.
Mounting evidence suggests that microsatellite mutator phenotype alterations and expression of the products of cancer-related genes are early markers of cell transformation, and may serve to identify the gastric carcinoma histotypes. The lack of a clear genetic basis, lends weight to the notion that gastric cancer is not a monomorphic entity but may be affected by environmental factors. Helicobacter pylori is the most important environmental risk factor associated with sporadic gastric cancer. Exposure of gastric epithelial cells to bacterium results in the generation of reactive oxygen species and inducible nitric oxide synthase that in turn may cause genetic alterations leading to cancer in a subset of subjects. Thus, gastric cancer may be considered the result of an interplay between host genetic profile and environmental toxic agents. The new technologies of molecular analysis will help to establish an individual's risk of developing gastric cancer and will lead to novel biological therapeutic strategies.
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In the last 50 years the incidence of gastric cancer has declined, probably due to the changing influence of environmental factors, however it continues to be the second most common cancer and the second leading cause of cancer death.1 Today, the estimated crude rates of gastric cancer incidence accounts for approximately 10% of cancers worldwide.2
Gastric cancers are largely resistant to radiotherapy and chemotherapy, and surgery represents the only treatment with curative potential. Two-thirds of the Western gastric cancer patients, however, are diagnosed in advanced stages, when surgery can only be palliative. The American Cancer Society estimates that the 5-year relative survival rate ranges from 10 to 20% and nearly 22 000 new cases/year are expected.3 Gastric cancer therefore remains a major clinical challenge, due to its poor prognosis and limited treatment options.
Phenotypically, gastric cancer is divided into two histomorphological entities denominated ‘intestinal’ and ‘diffuse’, which differ in epidemiology, pathogenesis, clinical outcome and genetic profile.4 Intestinal-type gastric carcinoma is the final result of a multistep process that starts from chronic gastritis and progresses to atrophy, intestinal metaplasia and dysplasia; it is more common in elderly men. The diffuse type of gastric cancer has no identifiable histologic precursor lesions, occurs prevalently in women under the age of 50 years and has a less favourable prognosis.
Thus far, the histological findings and the classification based on primary tumour size, lymph node involvement and presence of metastasis (TNM) have been the parameters used in clinical management, however lesions with similar morphologies may differ in biological aggressiveness, prognostic associations and response to therapy. All this raises the need to develop new classifications, based on biological parameters. Understanding the molecular changes underlying the pathogenesis of gastric cancer is a prerequisite for the identification of early markers of cell transformation, which might, in turn, indicate the individual risk of cancer, and might also lead to the novel treatment modalities.
At the genetic level, the hallmark of cancer is genome destabilization.1, 5 At least two molecular phenotypes, associated with distinct pathways of genome destabilization, have been identified in gastrointestinal cancer (Figure 1): the phenotype with high-level microsatellite instability and the phenotype associated with chromosomal and intrachromosomal instability.6–9
The high-level microsatellite instability phenotype is caused by altered repair of DNA replication errors, associated with mutational inactivation of one of at least five mismatch repair genes.7–10In vitro studies indicate that the mismatch repair system may be implicated not only in the repair of spontaneous DNA replication errors but also in the repair of toxic DNA damage.5–7 In high-level microsatellite instability gastric cancers, the presence of multiple hits, inactivating genes controlling cell growth, apoptosis and DNA repair, clearly demonstrates the complexity of the mutational interactions contributing to malignancy.10, 11 Correlations among mismatch repair, DNA methylation, environmental carcinogens and inflammatory processes in the gastric mucosa implicate mismatch repair deficiency in the early phase of gastric carcinogenesis.11–13 Increased DNA methylation might inactivate mismatch repair genes and be responsible for the high-level microsatellite instability profile.14, 15 The high-level microsatellite instability phenotype is relatively frequent in gastric cancer, accounting for more than 15% of diffuse and familial gastric cancer cases.10, 13, 14 Thus, the high-level microsatellite instability phenotype appears to be associated with a distinct molecular and histogenetic pathway of gastric carcinogenesis.13, 14 We have recently studied microsatellite instability status, evaluated at the BAT 25, BAT 26 and SMAD2 mononucleotide repeats, in a familial gastric cancer kindred. Neither gastric cancer cases (three patients) nor unaffected first-degree relatives (three subjects), however, had microsatellite instability alterations.15
Chromosomal instability, due to mutations in genes controlling the segregation of genetic material during mitosis, is characterized by chromosomal rearrangements and loss or gain of chromosomes, which in turn can induce oncogene activation and/or tumour-suppressor-gene inactivation.16, 17 Mutations and altered expression of genes implicated in cell proliferation and death (i.e. p53, bcl-2, SC-1), chromosomal segregation (i.e. APC), cell adhesion (i.e. E-cadherin, β-catenin), signal transduction (i.e. K-ras) and neoangiogenesis (i.e. vascular endothelial growth factor) may occur during chromosomal and intrachromosomal instability-related gastric carcinogenesis (Table 1).9, 17, 18 We recently have analysed some proteins (CDC25 phosphatase-related proteins, p16INK4 and the fragile histidine triad [Fhit] oncosuppressor protein) implicated in controlling the cell proliferation and death equilibrium. In fact, from a biological viewpoint, the chronic imbalance between cell proliferation and apoptosis is the first step of the gastric carcinogenesis as in all tumours.1 CDC25 phosphatases are a novel class of CDK cell cycle activators, p16INK4 is a cyclin-dependent kinase inhibitor up-regulated in response to such physiological events as cellular senescence and differentiation processes, and Fhit is a new oncosuppressor gene that induces apoptosis and inhibits cell proliferation.20–22 We studied the expression of these proteins in 137 patients with gastric cancer in relation to histotype, grade and stage of the tumour. Our results have shown that the overexpression of CDC25 phosphatases and p16INK4 is an early and frequent event of gastric cancer regardless of tumour histotype.23 In contrast, the expression of the Fhit oncoprotein (immunohistochemistry and mRNA RT-PCR analyses) seems to be related to the intestinal histotype and well-differentiated gastric cancer (study in progress).
Table 1. Genetic alterations in gastric cancer histotypes (Modified from Tahara 199530and Moss 1998.18)
|DNA-repair error||Genetic instability||16--39||Early||0||--|
|K-ras mutation||Signal transduction|| 0||--||10||Late|
|p53 LOH/mutation ||Transcription factor||50||Late||50||Early|
|APC LOH/mutation||Signal transduction||30*||Late||60||Early|
|p16||Cell cycle inhibitor||31||Early||12||Early|
|Cyclin E amplification||Cell cycle regulator||10||Early||20||Early|
|bcl-2 LOH||Apoptosis inhibitor|| 0||--||43||--|
|DCC LOH||Cell adhesion|| 0||--||50||Late|
|E-cadherin mutation||Cell adhesion||50||Late||10||Late|
|β-catenin mutation||Cell adhesion|| 0||--||27||Late|
Apoptosis plays a fundamental role in multicellular organism ensuring, in contrast to necrosis, a rapid and complete removal of cells that are no longer required or dangerous for the organism.24 Loss of heterozygosity at the bcl-2 locus, an apoptosis inhibitor, is associated with intestinal type tumour, whereas the expression of the SC-1 antigen, an apoptosis receptor, is associated with diffuse type tumours.25–27 A fairly new treatment for gastric cancer is the administration of the monoclonal antibody SC-1, which induces apoptosis and inhibits proliferation of gastric neoplastic cells.28
Mutation of p53 has been described in around 50% of histologically advanced gastric cancers.17–19, 29 Although the p53 mutation occurs in a high per cent (30–40%) of early intestinal-type gastric cancer, it is found in fewer than 5% of early diffuse-type gastric cancers. Inactivation of p53, which would induce apoptosis, arrest growth and inhibit neoangiogenesis, represents a promising anticancer therapy.
Up to 60% of intestinal-type gastric cancers have mutation or loss of heterozygosity of the APC gene.30 These alterations are rare in diffuse-type gastric cancer but may be associated with signet-ring cell carcinoma.17, 31 An association between K-ras mutations and preneoplastic lesions has been described in patients with chronic atrophic gastritis.19, 32 A consensus appears to be emerging that K-ras is associated with intestinal metaplasia and represents intestinal phenotypes in gastric carcinoma.33
Microsatellite instability, chromosomal and intrachromosomal instability, and the expression of the products of cancer-related genes are early markers of cell transformation and therefore are useful for an early diagnosis of tumour. It is noteworthy that the combination of molecular changes differ between intestinal- and diffuse-type gastric cancers, suggesting that the two forms may have unique genetic pathways, which could represent targets for novel, specific treatments.9, 17–28
Decreased cell–cell or cell–matrix interactions, which occur later in solid tumorigenesis, are common in gastric cancers and may relate more to the tendency to produce metastasis than to initial transformation processes.17 Beta-catenin mutations have very recently been detected in intestinal-type tumours, but are absent from the diffuse type.34, 35 Germline mutations of the E-cadherin gene have been detected in 50–70% of diffuse-type gastric cancers and are responsible for a small subset of familial gastric cancers.36–38 The International Gastric Cancer Consortium recommends testing for E-cadherin mutations in patients with diffuse type gastric cancer.39 Thus, E-cadherin is an excellent marker for diagnosis and an attractive target for novel therapeutic interventions.
Gastric cancer cells also express a wide variety of growth factors, hormones and cytokines that act in an autocrine or paracrine mechanism to modulate the complex interactions between tumour cells and stromal cells (Table 2).17, 19, 30 In this regard, c-met (a protoncogene encoding hepatocyte growth factor) and K-sam (a fibroblast growth factor) are preferentially amplified in diffuse type gastric cancer, whereas c-erbB-2 is selectively overexpressed in intestinal tumours and may serve as a prognostic marker of tumour invasion and lymph node metastasis.40, 41 Tahara proposed an interesting explanation of the mechanisms underlying the diffuse and intestinal types of gastric cancer.8 The C-met gene encodes the c-met protein, a receptor for hepatocyte growth factor. In the presence of cadherins, differentiation leads to tumours that have gland-like structures: i.e. to intestinal type cancers, whereas the absence of cadherin molecules results in the diffuse type of growth of cancer cells. Tumours showing vascular endothelial growth factor have a worse prognosis than tumours negative for vascular endothelial growth factor.30
Table 2. Growth factors alterations in gastric cancer histotypes (Modified from Werner, 2001.17)
|Genetic alteration||Function||Diffuse %||Intestinal %|
|c-met amplification||Growth factor receptor||39||19|
|K-sam amplification||Growth factor receptor||33||0|
|c-erbB-2 amplification||Growth factor receptor||0||20|
|EGFR overexpression||Growth factor receptor||25||50|
|EGF overexpression||Growth factor||20||40|
|TGFα overexpression||Growth factor||55||60|
|VEGF overexpression||Growth factor||12||46|
Despite a large body of data, the genetic basis of gastric cancer remains unknown. The prevalence and type of gene errors seem to vary between intestinal- and diffuse-type cancers, and also within the same subtype. None of the gene errors identified thus far in gastric cancer are totally specific or unique, which indicates that gastric cancer is not a monomorphic entity.
The lack of an unequivocal genetic basis lends weight to the theory that gastric cancer may be affected by environmental factors. Helicobacter pylori is the most important environmental risk factor associated with sporadic gastric cancer.42–45 Intestinal type gastric cancer, the most frequent histotype of sporadic gastric cancer, can be considered the result of a multistep process starting with H. pylori infection and progressing from gastritis to gastric atrophy, intestinal metaplasia and dysplasia.42H. pylori is rarely found in the mucosa of patients with gastric cancer, due to the modified gastric microenvironment, but antibodies against H. pylori are detected in nearly 70% of patients affected by the intestinal or diffuse histotype of gastric cancer.44 The link between H. pylori infection and cancer has been demonstrated by epidemiological data and in experimental animal models.46–50 Exposure of gastric epithelial cells to H. pylori results in the generation of reactive oxygen species and an increased level of inducible nitric oxide synthase that in turn is responsible for the endogenous formation of nitrite and nitroso compounds.51, 52 Nitric oxide synthase can also deaminate DNA and cause mutations of tumour suppressor genes, and possibly other oncogenes such as c-met, and initiate genetic alterations of gastric cells leading to gastric malignancy.53 We found chromosomal and intrachromosomal instability alterations in gastric bioptic samples in relatives of a familial gastric cancer cluster and in a subset of patients with chronic atrophic gastritis only in the presence of H. pylori infection. These alterations disappeared 12 months after H. pylori eradication.54
Most infected individuals, however, have asymptomatic chronic gastritis and only a small minority develop gastric cancer.44, 45 In a prospective study of 1526 Japanese subjects, 2.9% of infected patients developed gastric cancer within 8 years versus none of the uninfected patients.55 In this regard, host genetic factors may be important in determining the response to H. pylori infection, thereby suggesting an interplay between genetic and environmental factors. In fact, a DNA polymorphism in the IL-1 gene cluster results in atrophy of the gastric corpus and increased risk for H. pylori-gastric carcinoma.56 The absence of the DQA10102 allele is associated with a greater incidence of glandular atrophy and gastric cancer in H. pylori-infected subjects.53, 57 Thus, H. pylori plays a key role in both types of gastric cancer by giving rein to the genetic predisposition in the case of the diffuse type, and inducing molecular alterations in the case of the intestinal type (Figure 2).