Evaluation of Microsatellite Instability in Gastric MALT Lymphomas.

BACKGROUND: Microsatellite instability (MSI) is a major mechanism of carcinogenesis in hereditary nonpolyposis colorectal cancer, sporadic colon cancer, as well as other solid tumors. It is also encountered in gastritis and other chronic inflammatory conditions. Gastric MALT lymphomas are associated with chronic antigenic stimulation due to Helicobacter pylori (H.P) infection, leading to chronic gastritis, before converting to overt lymphoma.

AIM: The evaluation of the presence of MSI in gastric MALT lymphoma lesions at diagnosis and during follow-up, after H.P eradication and/or chemotherapy.

METHODS: Genomic DNA was extracted from formalin-fixed paraffin-embedded MALT lymphoma gastric biopsies. Blood genomic DNA was used as control. A pentaplex PCR was performed using primers for 5 different mononucleotide loci (NR21, NR24, NR27, BAT25 and BAT26). Sense oligonucleotides were labeled with WellRead fluorophores and products were run on a CEQ8000 apparatus (Beckman Coulter). Allele scoring was performed using the specific software. For the definition of MSI, 2/5 loci had to show banding shifts or gains outside the quasimonomorphic area for each marker.

RESULTS: 10 MALT lymphoma patients at diagnosis were studied and peaks were compared between stomach and blood samples. All 5 markers studied did not reveal any additional peaks when blood and gastric DNA were compared. In 4 patients follow-up gastric biopsies were available, after H.P eradication, as well as after chemotherapy treatment. There was no evidence of MSI in the serial gastric biopsies of these patients.

CONCLUSIONS: MSI was not detected in any gastric biopsies of this small group of MALT patients, either at diagnosis, or after treatment. The role of MSI in the lymphomagenesis of this distinct type of lymphoma needs to be further evaluated in serial specimens in a larger series of patients.