DNA Fiber Fluorescence In Situ Hybridization Analysis of Immunoglobulin Class Switching in B-Cell Neoplasia: Aberrant CH Gene Rearrangements in Follicle Center-Cell Lymphoma

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.

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

Using DNA fiber fluorescence in-situ hybridization (FISH) and 3-color interphase FISH, 2 cases of follicular lymphoma were identified in which the BCL2 gene was excised from 18q21 and inserted into the immunoglobulin heavy chain (IGH) locus at 14q32. Both the insertion breakpoint at 14q32 and the deletion breakpoint at 18q21 were cloned using inverse polymerase chain reaction. Sequence analysis showed that the JH sequences were juxtaposed to the 5′-side of BCL2, and the DH sequences were juxtaposed to the 3′-side of BCL2. There were breakpoints at both the JH and DH recombination signal sequences, and N-nucleotides were present at all breakpoint junctions. At theBCL2 locus, the 3′-breakpoints in both cases were localized at exactly the same nucleotide position, 6.2 kilobase downstream of the major breakpoint region, directly adjacent to a complete cryptic recombination signal sequence (RSS) consisting of a heptamer, a nonamer, and a 23–base pair (bp) spacer. The BCL25′-breakpoints were approximately 600 bp upstream of the gene, within the CA repeats. Although less evident than for the BCL23′-breakpoints, cryptic RSSs were also identified at these breakpoints, with a 12-bp spacer. On the basis of structural characteristics of these rearrangements, a model is proposed in which the BCL2 gene is deleted from its locus by recombination activation gene-1/-2 (RAG-1/-2)–mediated excision. The gene is subsequently inserted into the recombiningIGH locus, a process involving the formation of hybrid joints between the IGH coding ends and theBCL2 signal ends.