Microsatellite instability in gastric MALT lymphoma

Microsatellite Instability and Promoter Hypermethylation of DNA repair genes in Hematologic Malignancies: a forthcoming direction toward diagnostics

OBJECTIVE

The objective of our review is to highlight the significance of microsatellite hypervariation in diagnostics of hematologic malignancies.

METHODS

For the past few decades, extensive experiments in cancer research have explored all the possible pathways and a number of deleterious mutations that either make the tumor suppressor genes (TSGs) dysfunctional or cause the proto-oncogenes to behave abnormally by changing the cellular phenotype hence rendering disease. To prevent the deleterious effects of mutations and to protect the genomic integrity, our system possesses multiple repair mechanisms. DNA Mismatch Repair (MMR) and Direct Reversal of Damage (DRD) are two repair mechanisms which help in removal of faulty base pairs and alkyl adduct formation respectively to avoid long term effects of toxicity, tumorigenesis and mutagenesis. There are nine major MMR genes - MutS homolog (MSH2, MSH3, MSH4, MSH5, MSH6), MutL homolog (MLH1, MLH3), human post-meiotic segregation genes (PMS1, PMS2), and three major damage reversal genes - O⁶-methylguanine-DNA-methyltransferase (MGMT), ABH2 and DEPC1.

RESULTS

Any malfunction in DNA repair machinery can cause microsatellite instability (MSI), a form of genomic abnormality with hyper mutable repeats that is directly associated with cancer. Microsatellites are short, repetitive sequences, non-randomly distributed and localized in 3'-UTR (Untranslated Region), introns, coding regions and promoters. Besides MSI, evidence on promoter hypermethylation of selected repair genes also points toward a prominent reason for cancer initiation and progression.

CONCLUSION

The presence of specific microsatellite marker hyper-mutability and consistent promoter hypermethylation in leukemia or lymphoma can be considered as a part of routine diagnostic test in clinical laboratories.

Analysis of microsatellite instability in gastric mucosa-associated lymphoid tissue lymphoma

In Helicobacter pylori gastritis, constant antigenic stimulation triggers a sustained B-cell proliferation. Errors made during this continuous DNA replication are supposed to be corrected by the DNA mismatch repair mechanism. Failure of this mismatch repair mechanism has been described in hereditary non-polyposis colorectal cancer (HNPCC) and results in a replication error phenotype. Inherent to their instability during replication, microsatellites are the best markers of this replication error phenotype. We aimed to evaluate the role of defects in the DNA mismatch repair (MMR) mechanism and microsatellite instability (MSI) in relation to the most frequent genetic anomaly, translocation t(11;18)(q21;q21), in gastric mucosa-associated lymphoid tissue (MALT) lymphoma. Therefore, we examined 10 microsatellite loci (BAT25, BAT26, D5S346, D17S250, D2S123, TGFB, BAT40, D18S58, D17S787 and D18S69) for instability in 28 patients with MALT lymphomas. In addition, these tumors were also immunostained for MLH1, MSH2, MSH6 and PMS2, as well as screened for the presence of t(11;18)(q21;q21) by real-time polymerase chain reaction (RT-PCR). We found MSI in 5/28 (18%) lymphomas, with MSI occurring in both t(11;18)(q21;q21)-positive and -negative tumors. One tumor displayed high levels of instability, and, remarkably, this was the only case displaying features of a diffuse large B-cell lymphoma. All microsatellite unstable lymphomas showed a loss of MSH6 expression. In conclusion, our data suggest that a MMR-defect may be involved in the development of gastric MALT lymphomas, and that a defect of MSH6 might be associated with those MSI-driven gastric lymphomas.

A quantitative trait locus responsible for inducing B-cell lymphoblastic lymphoma is a hotspot for microsatellite instability

While the molecular mechanisms underlying microsatellite instability (MSI) have been exhaustively investigated, identifying the patterns of MSI distribution within diverse cancer genomes has remained an elusive issue. In the present study, we conducted genome-wide MSI screening in B-cell lymphoblastic lymphomas (B-LBL) which spontaneously develop in the SL/Kh strain of mice. Tumor samples harvested from 16 mice were investigated using a framework map consisting of 150 microsatellite markers spaced at increments of roughly 0.5-3.0 centimorgans, spanning the entirety of mouse chromosomes (mus musculus chromosomes [MMU]) 3-6. MMU3 contains a quantitative trait locus (QTL), Bomb1 (bone marrow pre-B1), known to induce an aberrant expansion of pre-B cells in bone marrow prior to the onset of B-LBL in SL/Kh mice. The remaining chromosomes were selected on the basis of those most closely resembling MMU3 in terms of total estimated length (maximum variance 10 Mb). MSI was confirmed at 2 Collapse

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MESH Headings

• Animals
• Lymphoma, B-Cell/genetics
• Mice
• Microsatellite Instability
• Quantitative Trait Loci

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Affiliation(s)

Richard H Kaszynski Department of Forensic Medicine and Molecular Pathology, Graduate School of Medicine, Kyoto University, Kyoto, Japan
Shinya Akatsuka
Takuya Hiratsuka
Guang Jin
Munetaka Ozeki
Tomoko Okuno
Takuro Nakamura
Toshiaki Manabe
Tetsuya Takakuwa
Hiroshi Hiai
Shinya Toyokuni
Keiji Tamaki
Tatsuaki Tsuruyama

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Helicobacter pylori and Gastric Malignancy

Last year there were a few important clinical data on long-term prognosis of mucosa-associated lymphoid tissue (MALT) lymphoma after eradication of Helicobacter pylori. Similarly, there were clinical data to support the role of eradication of H. pylori in the primary prevention of gastric cancer. We understand more about the host susceptibility as well as the pathogenetic mechanisms of the bacteria in the gastric mucosa. This article summarizes the important breakthroughs in H. pylori and gastric malignancies last year.