The Somatic Hypermutation Activity of a Follicular Lymphoma Links to Large Insertions and Deletions of Immunoglobulin Genes

Abstract

A biopsy specimen from a patient with follicular lymphoma was divided into two fragments. DNA was extracted from one fragment and a 1.2 kb region of the functional heavy chain (IgH) gene was amplified, cloned and sequenced (eight clones). From the other fragment a cell line (HF-1) was started. The IgH gene region was amplified from the cell line, and sequenced without cloning. The nine sequences obtained could be arranged into a genealogical tree where the individual sequences differed from the deduced ancestor by 16-29 single nucleotide changes, some also by an insertion and/or a deletion. It is apparent that the sequence alterations were caused by somatic mutations during the growth of the lymphoma. The comparison of the sequences with two published (allelic) germline sequences of the human JH region showed approximately 20% non-homology. The differences included five additional multinucleotide insertion/deletion changes, the longest of them a 101-nucleotide insertion. Two long insertions were homologous to the adjacent germline sequences. We propose that most of the changes observed, including long deletions and insertions, represent or are linked to somatic hypermutation events of the Ig gene type. Although in a few cases large deletions and insertions (> 2 bp) have been found in mutated immunoglobulin genes, our results, for the first time, firmly link these deletions/insertions to somatic hypermutations; their frequency was found to be 2.2% of the observed mutational events in the non-translated gene regions. HF-1 is the first follicular lymphoma line successfully established from a lymphoma known to have hypermutated its Ig genes during the malignant growth. It is a candidate cell line to be studied for its ability to generate mutations of B cell type in cell cultures.

The genetic variability of the VH genes in follicular lymphoma: the impact of the hypermutation mechanism

“…This study is to our knowledge the ®rst report in literature of insertions and deletions at the coding region of the V H genes expressed by the follicular lymphoma cells. A case report of a FL displaying ins/del at the non-coding introns has been described (Wu & Kaartinen, 1995). 17% of the FLs in our study harboured a total of ®ve deletions, three duplications and two insertions as compared to their GL counterparts.…”

AID-GFP chimeric protein increases hypermutation of Ig genes with no evidence of nuclear localization

“…For instance, one could imagine that as the ratelimiting activity of AID is overcome, the enzymes involved in normal hypermutation become limiting and other repair processes are put in operation. This interpretation could help to explain the variations in the reported frequency of deletions in cell lines and in vivo (23)(24)(25).…”

In vivoandin vitrostudies of immunoglobulin gene somatic hypermutation

“…Ramos constitutively diversi¢es its immunoglobulin V regions by hypermutation. This hypermutation generates immunoglobulin-negative variants through the introduction of nonsense mutations as well as deletions and duplications, as described in vivo (Goossens et al 1998;Wilson et al 1998;Wu & Kaartinen 1995). When transfected with terminal deoxynucleotidyl transferase (TdT) this repertoire of immunoglobulin-negative cells is increased by the presence of mutants that carry short non-templated insertions in V H (but not C m ).…”