Heterogeneous breakpoints on the immunoglobulin genes are involved in fusion with the 5′ region of BCL2 in B-cell tumors

Abstract

The 5′ flanking region of the BCL2 gene (5′-BCL2) is a breakpoint cluster of rearrangements with immunoglobulin genes (IGs). In contrast to t(14;18)(q32;q21) affecting the 3′ region of BCL2, 5′-BCL2 can fuse to not only the heavy chain gene (IGH), but also two light chain gene (IGL) loci. We report here cloning and sequencing of a total of eleven 5′-BCL2/IGs junctional areas of B-cell tumors, which were amplified by long-distance polymerase chain reaction-based assays. The breakpoints on 5′-BCL2 were distributed from 378 to 2312 bp upstream of the translational initiation site and, reflecting the alteration of regulatory sequences of BCL2, 5′-BCL2/IGs-positive cells showed markedly higher levels of BCL2 expression than those of t(14;18)-positive cells. In contrast, the breakpoints on the IGs were variable. Two 5′-BCL2/IGH and two 5′-BCL2/IGLκ junctions occurred 5′ of the joining (J) segments, suggesting operation of an erroneous variable (V)/diversity (D)/J and V/J rearrangement mechanism. However, two other 5′-BCL2/IGH junctions affected switch regions, and the κ-deleting element, which is located 24 kb downstream of the constant region of IGLκ, followed the 5′-BCL2 in another case. One 5′-BCL2/IGLκ and two 5′-BCL2/IGLλ junctions involved intronic regions where the normal recombination process does not occur. In the remaining one case, the 5′-BCL2 fused 3′ of a Vλ gene that was upstream of another Vλ/Jλ complex carrying a non-producing configuration, indicating that the receptor editing mechanism was likely involved in this rearrangement. Our study revealed heterogeneous anatomy of the 5′-BCL2/IGs fusion gene leading to transcriptional activation of BCL2, and suggested that the mechanisms underlying the formation of this particular oncogene/IGs recombination are not identical to those of t(14;18).