Variability in the interpretation of microsatellite patterns with different electrophoretic conditions

High polymorphism in the trisomic portion of a gastric cancer cell line

BACKGROUND

Genetic instability is a hallmark of malignancy, and microsatellite instability is a widely appreciated mechanism of generating genetic changes. We have recently observed four markers clustered on chromosome 20 that showed the effects of microsatellite instability in the gastric adenocarcinoma cell line SNU-1. Each affected marker had alleles of three different sizes. The aim of this study was to investigate the origin for this high-density polymorphism on a single chromosome.

METHODS

The high polymorphism located on chromosome 20 was confirmed using 37 additional markers. To further evaluate this finding, 15 clones of the cell line were generated and then assayed with the triallelic markers.

RESULTS

All told, almost a third of the markers on chromosome 20 had triallelic patterns, but only 0.3% of the markers not on chromosome 20 showed this result. The number of clones showing allelic variation was an average of 50% greater for chromosome 20 markers than for markers elsewhere. A karyotype analysis showed that the progenitor cell line of SNU-1 was trisomic for chromosome 20, and the high polymorphism on that chromosome is almost certainly due to the trisomy.

CONCLUSIONS

Not only are there more chromosome copies and therefore more gene copies subject to mutation in cells containing trisomy, but also more mutations may be passed on to the progeny. This elevated polymorphism increases the repertoire of genetic changes that could affect cellular growth, and may independently increase genomic instability.

Microsatellite instability and gastric non-invasive neoplasia in a high risk population in Cesena, Italy

BACKGROUND/AIMS

In the natural history of gastric cancer, non-invasive neoplasia (NiN) precedes invasive carcinoma. A histological classification of gastric NiN has recently been proposed by a World Health Organisation international panel of experts. Genetic instability is known to be among the molecular pathways involved in gastric oncogenesis. In this retrospective cross sectional study, microsatellite instability (MSI) was analysed in a consecutive series of NiN and NiN related histological alterations from a northern Italian region at high risk for gastric cancer.

PATIENTS/METHODS

Fifty five consecutive cases (indefinite for NiN, 29 cases; low grade NiN, 17 cases; high grade NiN, nine cases) were analysed by radioactive polymerase chain reaction and electrophoresis for microsatellite alterations at six loci (BAT25, BAT26, D2S123, D5S346, D17S250, and D3S1317). MSI was defined according to the Bethesda criteria distinguishing: (1) no instability in the analysed loci; (2) low frequency MSI (MSI-L); and (3) high frequency MSI (MSI-H). Immunohistochemical expression of MLH1 and MSH2 proteins was also analysed in all cases.

RESULTS

Overall, MSI was found in 11 of 55 cases (indefinite for NiN, five of 29 (MSI-L, four; MSI-H, one); low grade NiN, three of 17 (MSI-L, one; MSI-H, two); high grade NiN, three of nine (MSI-L, one; MSI-H, two).

CONCLUSIONS

In an Italian high risk area for gastric cancer, MSI is part of the spectrum of genetic alterations in gastric non-invasive neoplasia. In European populations at high risk of gastric cancer, DNA repair system alterations are thought to be among the early molecular events in gastric carcinogenesis.

Molecular markers in Helicobacter pylori-associated gastric carcinogenesis

Helicobacter pylori infection is a known risk factor of gastric carcino-genesis. This article presents early molecular alterations associated with H. pylori chronic gastritis and advances in the molecular characterization of preneoplastic intestinal metaplasia (IM) and premalignant gastric mucosal lesions. H. pylori infection induces changes in gene expression, genomic instability and accumulation of gene mutations in the stomach epithelium. Mutations, including LOH and microsatellite instability, and gene hypermethylation are seen not only in gastric cancer, but are already detectable in IM and gastric dysplasia/adenoma. Recent reports using microarray expression analysis identified several gastric epithelial genes that are regulated by H. pylori. Among the many genes showing altered epithelial expression in response to H. pylori, some might be useful as markers to assess gastric cancer risk. Profiles of mutagenesis and gene expression in IM and dysplasia/adenoma have been characterized and represent potential markers of preneoplastic and premalignant lesions during gastric carcinogenesis.

Infrequency of microsatellite instability in complete and incomplete gastric intestinal metaplasia

Chronic inflammation may be associated with microsatellite instability (MSI). To test the hypothesis that MSI frequently occurs in gastric intestinal metaplasia, we examined gastric biopsies from 58 subjects from an area of high risk for gastric cancer. These were selected to have 2 types of intestinal metaplasia: complete (31 subjects) and incomplete or mixed-type (27 subjects). None of the subjects had gastric cancer, but 95% had chronic inflammation with Helicobacter pylori. We used laser capture microdissection to retrieve metaplastic glands to compare with lymphocytes microdissected from the adjacent gastric mucosae in the same subjects. We performed microsatellite analysis using 6 microsatellite loci, including BAT26. None of the cases were found to have reproducible MSI, and only 1 case showed loss of heterozygosity at 1 marker, D3S1067. To test the sensitivity of our assay, we mixed templates to produce bands of different mobility and found that we could detect an aberrant microsatellite pattern if only 2% of the DNA showed that pattern. Our results indicate that MSI is a rare event in intestinal metaplasia in subjects who do not have gastric cancer.

Role of Helicobacter pylori in gastric carcinogenesis

Peptic ulcers and gastric malignancies are the two major complication of the course of Helicobacter pylori-associated chronic gastritis. Both gastric adenocarcinomas and MALT lymphomas occur in association with H. pylori infection, and studies support an etiological association. This article discusses the natural history of H. pylori-related gastric carcinogenesis and criteria to identify people susceptible to H. pylori infection-associated gastric cancer. It then reviews the molecular and genetic mechanisms underlying the malignant transformation of the gastric mucosa associated with H. pylori.

Genomic instability in chronic viral hepatitis and hepatocellular carcinoma

Chronic hepatitis may progress to cirrhosis and hepatocellular carcinoma (HCC). Progressive accumulation of mutations and genomic instability in chronic viral hepatitis might flag an increased risk of HCC development. Genomic instability at dinucleotide microsatellite loci in chromosomes 2, 13, and 17 and at 2 mononucleotide repeat loci was examined in liver tissues from 41 patients, including 30 without HCC (18 patients with chronic hepatitis and 12 with cirrhosis) and 11 with HCC. Genomic instability was detected in 51% of the 41 cases. Allelic imbalance at informative dinucleotide loci occurred in 37% of the cases. In 14 cases (34%), allelic imbalance was detected in chronic hepatitis or cirrhosis without HCC. Allelic imbalance at the chromosome 13 locus was detected in 50% of the cases of chronic hepatitis C. Allelic imbalance at the TP53 chromosome locus and/or at the chromosome 13 locus was significantly more frequent than alterations at the chromosome 2 locus (P =.026). Low-level microsatellite instability was found in 20% of all cases examined and high-level microsatellite instability in 3 patients (7.5%), including 2 cases of chronic hepatitis and 1 case of cirrhosis. Our results show that allelic imbalance occurs frequently in hepatitis-related HCC as well as in chronic hepatitis in patients without HCC. Allelic imbalance at the D13S170 chromosome 13 locus (13q31.2) occurs frequently in chronic hepatitis, suggesting that genomic alterations affecting the long arm of chromosome 13 might be used to monitor the natural progression of chronic hepatitis-associated liver carcinogenesis.

Molecular identification of main cellular lineages as a tool for the classification of gastric cancer

Gastric carcinomas (GC) are heterogeneous tumors comprising variable amounts of cells of different lineage phenotype, including gastric mucous cells (surface--SMC or gland--GMC) and intestinal cells (IC). The evaluation of tumor behavior has classically depended on strictly morphological classifications of tumors. Microsatellite instability (MSI) is frequently detected in GC, but whether MSI affects all gastric cellular lineages or exclusively occurs in unique cellular lineages in GC is not known. The aims of this study were to test a combination of anti-mucin antibodies to classify gastric cancer into predominant cell lineage phenotype and to determine whether MSI in GC is associated with particular cellular tumor phenotypes. Fifty-five GC were immunophenotyped with antibodies specific for SMC, GMC, or IC. DNA was extracted from tumor and non-neoplastic gastric tissues and amplified with 5 microsatellite markers. A mixed cellular pattern was the most frequent phenotype of GC (61%) and was seen in both glandular (63%) and diffuse (58%)-type tumors. No significant difference in the rate of MSI was found in tumors with predominant gastric, intestinal or mixed phenotype. However, tumors with null or low-level expression of cellular lineage differentiation markers displayed MSI more frequently than tumors with high-level expression (40% v 20%). In conclusion, different gastric carcinoma cell lineage patterns can be easily identified with the 3 immunohistochemical markers used in this study. The 3 main cellular lineage components of gastric cancer can be similarly affected by microsatellite instability, consistent with the notion that MSI is an early event in gastric carcinogenesis.

Microsatellite instability in gastric intestinal metaplasia in patients with and without gastric cancer

The role and significance of microsatellite instability (MSI) in gastric carcinogenesis remain unknown. This study determined the chronology of MSI in gastric carcinogenesis by examining intestinal metaplasia (IM) from patients with and without gastric cancer. DNA was obtained from gastric specimens of 75 patients with gastric IM (30 cancer, 26 peptic ulcer, and 19 chronic gastritis patients) and was amplified with a set of eight microsatellite markers. Eight (26. 7%) tumors and seven (9.3%) IM samples (three from cancer-free patients) displayed high-level MSI (three or more loci altered). Low-level MSI (one or two loci altered) was detected in 50% of the tumors, in 40% of IM samples coexisting with cancer, and in 38% of IM tissues of cancer-free individuals. Among the 30 cancer patients, microsatellites were more frequently altered in IM coexisting with tumors that showed MSI (P = 0.003). In addition, patients with low-level MSI in the tumor tissues were more likely to have active Helicobacter pylori infection than those with stable tumors (P = 0.02). In conclusion, this study indicates that MSI occurs not only in gastric IM of patients with gastric carcinoma, but also in IM of cancer-free individuals. These data suggest that the progressive accumulation of MSI in areas of IM may contribute to gastric cancer development, representing an important molecular event in the multistep gastric carcinogenesis cascade.

Marked differences in the frequency of microsatellite instability in gastric cancer from different countries

OBJECTIVE

Previous studies have reported variable rates of microsatellite instability (MSI) in gastric cancer. We investigated the frequency of MSI in invasive gastric carcinoma of patients from three geographic regions.

METHODS

Genomic DNA from gastric cancer and nontumor tissue from 22 Korean, 20 Colombian, and 26 U.S. patients was amplified with five microsatellite markers.

RESULTS

MSI was more frequently seen in gastric cancer from Korea, affecting 50% of patients, in contrast with gastric cancers from the U.S. (7%) and Colombia (15%) (p = 0.003 and p = 0.03, respectively). MSI at one locus was significantly more frequent in gastric cancer from individuals >65 yr (p = 0.01). MSI was similarly associated with both diffuse and intestinal types of gastric cancer.

CONCLUSIONS

MSI affects the two major histological types of gastric cancer, and was more frequent in gastric cancer from Korea than in the other countries, suggesting that the relative importance of different pathways of gastric carcinogenesis may vary in diverse regions of the world.