Many diffuse large B-cell lymphomas (DLBCLs) of the stomach are believed to represent high-grade transformation of low-grade marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue type, which is of memory B-cell origin, displaying evidence for positive antigen selection and a low level of ongoing somatic mutation of the rearranged immunoglobulin heavy-chain variable region (V(H)) genes. The pattern of somatic mutation has been studied little in intestinal DLBCLs.
To assess evidence for antigen selection and the levels of ongoing mutation, we analyzed the ratio of replacement to silent mutations, as well as the frequency of intraclonal sequence variation in gastric and small intestinal DLBCLs that showed no concomitant low-grade component.
Genomic DNA was extracted from formalin-fixed paraffin blocks of gastric (n = 6) and small intestinal (n = 6) DLBCLs. The complementarity-determining region 2 and framework region 3 sequences (<200 base pairs) of the rearranged immunoglobulin V(H) gene were obtained from polymerase chain reaction-amplified product, and the ratio of replacement-to-silent mutations and the frequency of intraclonal sequence variation were determined.
Clustering of replacement mutations in complementarity-determining region 2 with a high (>2.9) ratio of replacement-to-silent mutations was observed in 5 gastric DLBCLs, whereas it was recognized in only 1 intestinal DLBCL. Intraclonal sequence variation was observed in 6 intestinal and 5 gastric DLBCLs. The frequency of ongoing mutation was much higher in the intestinal (median, 0.33%) than in the gastric DLBCLs (median, 0.13%), but the difference was not statistically significant (P =.09).
The mutation pattern was consistent with positive antigen selection in gastric DLBCLs, but not in the intestinal tumors. Ongoing mutation was much more frequent in the intestinal than in the gastric DLBCLs. These findings suggest that positive antigen selection plays a major role in a significant proportion of gastric tumors, whereas germinal center reaction with aberrant mutation is important in small intestinal DLBCLs.