Aberrant Immunoglobulin Class Switch Recombination and Switch Translocations in Activated B Cell-Like Diffuse Large B-Cell Lymphoma.

To elucidate the mechanisms underlying chromosomal translocations in diffuse large B-cell lymphoma (DLBCL), we investigated the nature and extent of immunoglobulin class switch recombination (CSR) in these tumors. We used a Southern blot assay to detect legitimate and illegitimate CSR events in 92 primary tumor samples of either the activated B-cell-like (ABC; n = 50), germinal center B cell-like (GCB; n =31), and primary mediastinal B cell lymphoma (PMBL; n = 11) subgroups of DLBCL. We could clearly distinguish ABC DLBCL from GCB and PMBL by the phenotype shown in these experiments. The frequency of legitimate CSR was significantly lower in ABC DLBCL compared to GCB DLBCL and PMBL (30% vs. 58% vs. 64%; p = 0.012 and p = 0.035 respectively). In contrast, illegitimate switch recombination events were more frequent in ABC than in GCB (56% vs. 32%; p = 0.037). Similarly, ABC DLBCL had a higher frequency of internal deletions within the switch μ region compared to GCB DLBCL and PMBL (48% vs. 13% vs. 0%; p = 0.0012 and p = 0.003, respectively). ABC DLBCLs also had frequent deletions within Sγ (66%) and Sα (26%). Sequence analysis revealed ongoing Sμ deletions within ABC DLBCL tumor clones, with 4–19 separate deletions occurring per case. In addition, Sμ deletions were accompanied by ongoing segmental duplications and apparent AID-dependent somatic mutations in Sμ, consistent with failed attempts at AID-mediated CSR. Unexpectedly, short fragments derived from multiple chromosomes were interspersed within the deleted Sμ regions. Furthermore, aberrant switch recombination was responsible for translocations in ABC DLBCLs involving the known oncogenes BCL6 and MYC. In addition, we were able to identify a novel translocation involving the switch γ3 region and the gene encoding the B-cell transcription factor SPIB in the ABC cell line OCI-Ly3. These findings suggest that ABC DLBCLs have abnormalities in the regulation of CSR that predispose to chromosomal translocations.