Genetic pathways of two types of gastric cancer.

Multiple genetic and epigenetic alterations in oncogenes, tumour-suppressor genes, cell-cycle regulators, cell adhesion molecules, DNA repair genes and genetic instability as well as telomerase activation are implicated in the multistep process of human stomach carcinogenesis. However, particular combinations of these alterations differ in the two histological types of gastric cancer, indicating that well-differentiated or intestinal-type and poorly differentiated or diffuse-type carcinomas have distinct carcinogenetic pathways. In the multistep process of well-differentiated-type carcinogenesis, the genetic pathway can be divided into three subpathways: an intestinal metaplasia-->adenoma-->carcinoma sequence, an intestinal metaplasia-->carcinoma sequence and de novo. In the multistep process of well-differentiated-type or intestinal-type gastric carcinogenesis, infection with Helicobacter pylori may be a strong trigger for hyperplasia of hTERT-positive 'stem cells' in intestinal metaplasia. Genetic instability and hyperplasia of hTERT-positive stem cells precede replication error at the D1S191 locus, DNA hypermethylation at the D17S5 locus, pS2 loss, RARbeta loss, CD44 abnormal transcripts and p53 mutation, all of which accumulate in at least 30% of incomplete intestinal metaplasias. All of these epigenetic and genetic alterations are common events in intestinal-type gastric cancer. An adenoma-->carcinoma sequence is found in about 20% of gastric adenomas with APC mutations. In addition to these events, p53 mutation and loss of heterozygosity (LOH), reduced p27 expression, cyclin E expression and the presence of c-met 6.0-kb transcripts allow malignant transformation from the above precancerous lesions to intestinal-type gastric cancer. DCC loss, APC mutations, 1q LOH, p27 loss, reduced tumour growth factor (TGF)-beta type I receptor expression, reduced nm23 expression and c-erbB gene amplification are frequently associated with an advanced stage of intestinal-type gastric cancer. The de-novo pathway for carcinogenesis of well-differentiated gastric cancer involves LOH and abnormal expression of the p73 gene that is responsible for the development of foveolar-type gastric cancers with pS2 expression. On the other hand, LOH at chromosome 17p, mutation or LOH of p53 and mutation or loss of E-cadherin are preferentially involved in the development of poorly differentiated gastric cancers. In addition to these changes, gene amplification of K-sam, and c-met and p27 loss as well as reduced nm23 obviously confer progression, metastasis and diffusely productive fibrosis. Mixed gastric carcinomas composed of well-differentiated and poorly differentiated components exhibit some but not all of the molecular events described so far for each of the two types of gastric cancer. Besides these genetic and epigenetic events, well-differentiated and poorly differentiated gastric cancers also organize different patterns of interplay between cancer cells and stromal cells through the growth factor/cytokine receptor system, which plays an important role in cell growth, apoptosis, morphogenesis, angiogenesis, progression and metastasis. Meta-analysis of epidemiological studies and animal models show that both intestinal and diffuse types of gastric cancer are equally associated with H. pylori infection. However, H. pylori infection may play a role only in the initial steps of gastric carcinogenesis. Differences in H. pylori strain, patient age, exogenous or endogenous carcinogens and genetic factors such as DNA polymorphism and genetic instability may be implicated in two distinct major genetic pathways for gastric carcinogenesis.