Variable heavy chain gene analysis of follicular lymphomas: correlation between heavy chain isotype expression and somatic mutation load

Abstract

The expansion of follicular lymphomas (FLs) resembles, both morphologically and functionally, normal germinal center B-cell growth. The tumor cells proliferate in networks of follicular dendritic cells and are believed to be capable of somatic hypermutation and isotype switching. To investigate the relation between somatic mutation and heavy chain isotype expression, we analyzed the variable heavy (VH) chain genes of 30 FL samples of different isotypes. The VH genes of the FLs were heavily mutated (29.3 mutations on average). In addition, isotype-switched lymphomas contained more somatic mutations than immunoglobulin M–positive lymphomas (33.8 mutations per VH gene versus 23.0, respectively). In all but one of the FLs, the ratios of replacement versus silent mutations in the framework regions were low, independent of the absolute number of somatic mutations and the level of intraclonal variation. Analysis of relapse samples of 4 FLs showed no obvious increase in somatic mutation load in most FLs and a decrease in intraclonal variation in time. In 3 of 4 cases, we obtained evidence for selection of certain subclones, rather than clonal evolution. Our findings question if intraclonal variation is always a reflection of ongoing somatic hypermutation. This may have implications for the concept of antigen-driven lymphomagenesis.

Among B cell non-Hodgkin's lymphomas, MALT lymphomas express a unique antibody repertoire with frequent rheumatoid factor reactivity

“…Thus, in general FL, DLBCL, BL, and myelomas, all carrying significantly hypermutated IgV genes, seem to recognize unique epitopes, suggesting that they arise randomly out of the pool of B cells selected for nonself-antigens, most likely during the germinal center reaction. This is in accordance with previous observations that the germline IgV H gene usage of these B-NHLs is similar to that of normal peripheral B cells (18,60). In contrast, B-CLL and MCL cases showed a high degree of overall CDR3 homology (31 and 44%, respectively) (Table V).…”

“…As additional controls, IgM LIDAs were generated of four follicular lymphomas (FL1, FL6Ј94, FL8Ј92, and FL13), one B-CLL, B-CLL26, and two anti-Rhesus(D) Ab producing B cell lines (8D8 and LOS3; reference 57). The IgV H and IgV sequences of these FLs and B-CLL have been previously reported by us (18,58,59). The lymphomas originally expressed IgM, with exception of M14, M22 (both IgA ϩ ), and M23 FL6Ј94, FL8Ј92 (both IgG ϩ ) (Tables I and II).…”

“…In 17 of the 24 lymphomas (71%), the replacement versus silent (R/S) mutation ratios in the framework regions (FRs) of the IgV H genes, were significantly Ͻ 1.5, implying that, in spite of the high mutation frequencies, selective forces had preserved the BCR in these lymphomas (unpublished data). Intraclonal variation (ICV) was determined for 16 IgV H genes and for 8 IgV genes (18). In 10 out of the 16 (63%) MALT lympho-…”

“…Analysis of the Ig variable ( IgV ) genes supported the concept of Ag-driven lymphomagenesis in FL and MALT lymphoma. The IgV heavy ( IgV H ) and IgV light ( IgV L ) chain genes of these malignancies are heavily mutated, compatible with a GC or post-GC derivation (18)(19)(20)(21)(22). The mutation patterns unequivocally indicate that Ag-based selection occurs at some stage of their development; despite high mutation loads, the overall structure of the Ig is generally being preserved in these lymphomas often during years of disease.…”

Analysis of the intraclonal diversity of HLA‐A0201‐restricted T lymphocyte epitopes in follicular lymphoma idiotype

“…The intraclonal variants displayed additive mutations attesting Id diversification. As previously observed, no correlation was found between the number of consensus Id mutations and intraclonal diversity (Aarts et al, 2000). Intraclonal mutations occurred anywhere on the Id sequence with almost similar frequencies.…”