V(D)J recombinase-mediated transposition of the BCL2gene to the IGH locus in follicular lymphoma

Different classes of genomic inserts contribute to human antibody diversity

The enormous diversity of antibodies is a key element to combat infections. Antibodies containing pathogen receptors were a surprising discovery that contrasted antibody diversification through classic recombination events. However, such insert-containing antibodies were thus far exclusively detected in African individuals exposed to malaria parasites and were identified as screening byproducts or through hypothesis-driven search. The prevalence and complexity of insertion events remained elusive. In this study, we devise an unbiased, systematic approach to identify inserts in the human antibody repertoire. We show that inserts from distant genomic regions occur in the majority of donors and are independent of Plasmodium falciparum preexposure. Our findings suggest that four distinct classes of insertion events contribute diversity to the human antibody repertoire.

Recombination of antibody genes in B cells can involve distant genomic loci and contribute a foreign antigen-binding element to form hybrid antibodies with broad reactivity for Plasmodium falciparum. So far, antibodies containing the extracellular domain of the LAIR1 and LILRB1 receptors represent unique examples of cross-chromosomal antibody diversification. Here, we devise a technique to profile non-VDJ elements from distant genes in antibody transcripts. Independent of the preexposure of donors to malaria parasites, non-VDJ inserts were detected in 80% of individuals at frequencies of 1 in 10⁴ to 10⁵ B cells. We detected insertions in heavy, but not in light chain or T cell receptor transcripts. We classify the insertions into four types depending on the insert origin and destination: 1) mitochondrial and 2) nuclear DNA inserts integrated at VDJ junctions; 3) inserts originating from telomere proximal genes; and 4) fragile sites incorporated between J-to-constant junctions. The latter class of inserts was exclusively found in memory and in in vitro activated B cells, while all other classes were already detected in naïve B cells. More than 10% of inserts preserved the reading frame, including transcripts with signs of antigen-driven affinity maturation. Collectively, our study unravels a mechanism of antibody diversification that is layered on the classical V(D)J and switch recombination.

E mu/S mu transposition into Myc is sometimes a precursor for T(12;15) translocation in mouse B cells

Misguided immunoglobulin (Ig) class switch recombination (CSR) has been implicated in the origin of Myc-activating chromosomal translocations, T(12;15), in BALB/c mouse plasmacytomas (PCTs). CSR has also been involved in the progression of T(12;15); for example, the approximation of Myc to the 3'-C alpha enhancer. This study provides evidence for an additional mechanism by which aberrant CSR may facilitate T(12;15): transposition of Ig heavy-chain (IgH) sequences to Myc. Five IgH transposons containing the intronic heavy-chain enhancer, E mu, and a truncated switch mu region, S mu, were found in the first intron of Myc in lymph node cells of IL-6 transgenic BALB/c mice. In two cases E mu/S mu transposition primed Myc to get involved in apparent trans-chromosomal CSR to C gamma 1, presumably leading to T(12;15). Translocations preceded by E mu/S mu transposition can sometimes be distinguished from de novo translocations by molecular fingerprints in translocation breakpoint regions (Ig switch region [S] inversions and unusual gene orders in composite S regions). The presence of such fingerprints in some PCTs suggests that the tumors sometimes evolve from transposition-bearing precursors. We propose that E mu/S mu transposition to Myc may facilitate plasmacytomagenesis by sensitizing Myc to undergo T(12;15) translocation. T(12;15), in turn, juxtaposes Myc to the 3'-C alpha enhancer, which appears to be required for deregulating Myc in a manner that is conducive to PCT development.

Mechanisms of chromosomal translocations in B cell lymphomas

Reciprocal chromosomal translocations involving the immunoglobulin (Ig) loci are a hallmark of most mature B cell lymphomas and usually result in dysregulated expression of oncogenes brought under the control of the Ig enhancers. Although the precise mechanisms involved in the development of these translocations remains essentially unknown, a clear relationship has been established with the mechanisms that lead to Ig gene remodeling, including V(D)J recombination, isotype switching and somatic hypermutation. The common denominator of these three processes in the formation of Ig-associated translocations is probably represented by the fact that each of these processes intrinsically generates double-strand DNA breaks. Since isotype switching and somatic hypermutation occur in germinal center (GC) B cells, the origin of a large number of B cell lymphomas from GC B cells is likely closely related to aberrant hypermutation and isotype switching activity in these B cells.