Relapsed MALT Lymphomas Are Associated with a Restricted VH3-30.3 Immunoglobulin VH Gene Repertoire and Are Targeted by Aberrant Somatic Hypermutation

Mucosa associated lymphoid tissue (MALT) type lymphomas are B-cell neoplasms that develop out of a reactive infiltrate; Sjoegren syndrome, Hashimoto thyroiditis, and Helicobacter pylori gastritis provide the pathogenetic background. Direct antigen stimulation through surface immunoglobulin (Ig) molecules may be playing an important role in the development of MALT lymphomas. In rare cases MALT lymphomas may undergo transformation into extranodal diffuse large B-cell lymphomas (eDLBCL). Previous reports suggested salivary gland MALT lymphomas expressed a restricted Ig VH gene repertoire with over use of VH1-69 gene segments. Because knowledge about the VH gene used by MALT lymphoma of gastric and extragastric origin and by eDLBCL is limited, we sequenced the VH genes from 11 MALT lymphomas [5 of gastric and 6 of extragastric origin (3 salivary gland, 3 thyroid)] and from 10 eDLBCL, all arisen from MALT lymphoma and still exhibiting a low grade component [= so called “transformed MALT lymphoma”: 5 of gastric and 5 of extragastric origin (3 salivary gland and 2 thyroid)]. MALT lymphomas used gene segments of the VH1 (1–69: 2 salivary gland and 1 thyroid), VH3 (3–30.3: 1 thyroid), VH4 (4–34: 1 thyroid, 1 salivary gland and 2 gastric), VH5 (5–51: 2 gastric) and VH6 (6-1: 1 gastric) families. Extranodal DLBCLs used segments derived from VH1 (1–69: 1 gastric and 1 salivary gland), VH2 (2–70: 1 thyroid), VH3 (3–23: 1 gastric; 3–30: 1 gastric; 3–30.3: 1 gastric and 2 salivary glands) and VH4 (4–34: 1 gastric) families as shown in table 1. The VH1-69 gene segment was found in 3 of 6 extragastric MALT lymphomas (2 salivary glands and 1 thyroid), in one gastric and one salivary gland eDLBCL but in no gastric MALT lymphoma. Further, 4 of the 21 lymphomas relapsed, 3 eDLBCL and one MALT lymphoma – and remarkably, all of them used the VH3-30.3 gene segment. Comparing the frequency of somatic hypermutation (SHM) of the immunoglobuline locus and aberrant somatic hypermutation (ASHM) of the four proto-oncogenes PAX-5, PIM-1, Rho/TTF and c-MYC between the relapsed (n=4) and non relapsed lymphomas (n=17) a ~2.9 fold (5.8 vs 2.0 × 10⁻²/bp, p=0.017) and 2.1 fold (0.067 vs 0.032 × 10⁻²/bp, p=0.027) higher mutation rate for SHM and ASHM in relapsed lymphomas was observed. Also, the AID mRNA relative expression number was 1.7 fold higher in the group of relapsed lymphomas (4.66 vs 2.71, p=0.049). Performing immunohistochemical analysis for AID a significant positive correlation (p=0.01; correlation coefficient (Spearman rho) = 0.794) was observed when comparing protein expression with mRNA levels. These results are consistent with the model in which only certain B cells displaying specific patterns of V_H immunoglobuline molecules binding to an as yet undefined antigen together with a highly aberrant somatic hypermutation process are preferentially selected for malignant transformation and determine the natural course of disease.

Table1: V_H gene sement used by MALT lymphomas and eDLBCL