Frequent allelic imbalance but infrequent microsatellite instability in gastric lymphoma

Specific defects in DNA repair pathways are reflected by DNA microsatellite instability (MSI) and play an important role in carcinogenesis. Reported frequencies in gastric non-Hodgkin's lymphomas (NHL) vary from 14% to as high as 90%. Another form of genetic instability in tumours is allelic imbalance (AI) due to loss or gain of genetic material at a specific chromosomal region. This might point to the presence of a tumour suppressor gene or oncogene. We examined both MSI and AI in 26 gastric lymphomas (10 low-grade and 13 high-grade MALT lymphomas and three cases lacking MALT features and categorised as diffuse large B cell lymphoma (DLCL)). Tumour components and normal cells (epithelium, muscle) were microdissected from paraffin-embedded resection samples. Contrary to other studies we did not observe frequent MSI when investigating 18 different loci distributed over 12 chromosomes. Microsatellite instability of a single locus was found in 1/10 (10%) low-grade MALT lymphomas and 2/13 (15%) high-grade MALT lymphomas. These data indicate that DNA mismatch repair genes do not play a role in the pathogenesis of these lymphomas. Allelic imbalance was detected in 60% (6/10) of low-grade MALT lymphomas, in 62% (8/13) of high-grade MALT lymphoma and in 67% (2/3) of DLCL. In high-grade lymphomas more loci showed AI (one to seven loci, with a mean of 2.5 loci per case) than in the low-grade lymphomas (one to two loci, with a mean of 1.3 loci per case), possibly reflecting an increased genomic instability.