Immunogenetic Profiling of Follicular Lymphoma Reveals Universal N-Glycosylation Sites Introduced into the B Cell Receptor by Somatic Mutation and Suggests Relevance to Lymphoma Pathogenesis.

Abstract

Immunogenetic analysis of B-cell malignancies can provide important information that relates to the cellular origin and clonal history of these lymphomas and give clues as to possible pathogenic mechanisms. In follicular lymphoma (FL), immunoglobulin variable region (V) genes are commonly somatically mutated and display intraclonal heterogeneity consistent with location in the germinal centre (GC). In this analysis of 44 cases of FL we find that, with minor exceptions, both the VH and VL gene usage reflects that of the normal B cell repertoire, indicative of a common antigenic drive and in support of a final transforming event in the GC. We have previously reported a high incidence of potential N-glycosylation sites in the VH genes of FL, which have been introduced by the process of somatic mutation. Here we have assessed both the VH and VL genes and find that sites are universally present and further demonstrate that they are available for functional glycosylation. The majority of sites are found in VH (81%) and are located predominantly within CDR2 and CDR3, with few sites present in the FRs. Sites are also evident in VL (45%) where they are focused mainly in CDR3 and CDR1. A minor subset (10%) has sites in VL only. In total, 26 different N-glycosylation motifs were observed, with NIS being the most commonly used. The natural motif in the V4–34 germline gene appears unimportant, and can be lost. Scrutiny of the somatic mutations giving rise to these motifs reveals that the acquisition of sites was predominantly (73%) achieved by a single amino acid (aa) replacement to Asn at position 1 of the motif, either with or without an additional, non-essential aa replacement at another position. Common ‘hotspots' were observed within the CDR2 for the VH gene segments V3–23, V3–48, V3–07 and V3–15. It appears likely that the acquisition of N-glycosylation sites in the antigen-binding site during somatic mutation in the GC and the subsequent addition of oligosaccharides is important to the lifestyle of FL and may provide a critical second tumorigenic event. In turn, it may be possible to exploit this seemingly essential feature to develop novel therapeutic approaches.