Alternative promoters and exons, somatic mutation and deregulation of the Bcl-2-Ig fusion gene in lymphoma.

Abstract

The most common translocation in human lymphoma, the t(14;18)(q32;q21), generates heterogeneous 4.2‐7.2 kb Bcl‐2‐immunoglobulin (Ig) chimeric mRNAs resulting from alternative Bcl‐2 5′ exons and varied Ig 3′ untranslated regions (UT). The normal human Bcl‐2 gene has a three exon structure with an untranslated first exon, a facultative 220 bp intron I, but an enormous 370 kb intron II. S1 protection and primer extension analysis defined initiation sites in exon II associated with classic promoter elements and a decanucleotide (ATG‐CAAAGCA) homologous with Ig variable region enhancers. Multiple initiation sites were also found in a GC‐rich region with Sp1 binding motifs in exon I. Most t(14;18) breakpoints cluster within the 3′ UT of Bcl‐2 implicating that event in gene deregulation. The Bcl‐2 gene introduced into the Ig constant (C gamma) locus of SU‐DHL‐6 displayed somatic mutation. While Bcl‐2–Ig mRNAs demonstrated an unaltered 2.5 h half‐life, the Bcl‐2–Ig gene revealed an inappropriately high rate of transcription for a mature B‐cell. This indicates the translocated Bcl‐2 allele has escaped normal control mechanisms.

Bcl-xS and Bax induce different apoptotic pathways in PC12 cells

“…pCMV ± SEAP was obtained from Dr Pradier (Rhone-Poulenc Rorer). The vectors pcDNA3Bcl-2, pcDNA3Bcl-2 mI-3 (G145A), pcDNA3Bcl-2 mI-4 (G145E) and pcDNA3Bcl-2DC22 (with deletion of 22 amino acids from the C-terminal, which contains the signal anchor sequence) were generated by inserting cDNAs of human wild-type Bcl-2 (Seto et al, 1998) and its mutant cDNAs (Oltvai et al, 1993;Yin et al, 1994) into the EcoRI site of pcDNA3. The Bax expression vector pcDNA3HA muBax contains mouse Bax cDNA (Oltvai et al, 1993) and was generated in Dr Korsmeyer's laboratory (Washington University, St. Louis, MO, USA).…”

t(14;18), a journey to eternity

“…Exons 2 and 3 are separated by a large intron of about 225 kb. The MBR is located in the untranslated region of exon 3, 40 and therefore, the protein coding domain is maintained during the t(14;18). Dependent on splicing of intron 2, bcl-2 encodes for two different mRNAs, Bcl-2␣ and Bcl-2␤ 41 of which only Bcl-2␣ seems to have biological relevance.…”

Molecular features of a new human lymphoma cell line carrying both BCL2 and BCL6 gene rearrangements

“…The BCL2 gene is composed of three exons and two potential promoter regions have been identi®ed (Seto et al, 1988). The P1 promoter is associated with exon 1 and is GC-rich with multiple SP1 sites.…”

“…In established B-cell lines, the former promoter is used predominantly, while use of the latter is minimal (Seto et al, 1988). It is of special interest that reported breakpoints of the 5'-BCL2 are clustered within a limited region immediately 5' to the P1 promoter.…”

“…The latter PCR products were cloned into the plasmid and the restriction map encompassing the junction was constructed. Comparison with the map of the germline 5'-BCL2 and IGk as well as sequence analysis of the junctional area revealed that breakage occurred in BCL2 *560 bp upstream of the transcription initiation site of exon 1, as de®ned by Seto et al (1988) (Figure 2b). The BCL2 sequences were followed by the Jk4 segment (Hieter et al, 1982) of the IGk in the divergent orientation, and a sequence of 19 bp of unknown origin was inserted at the junction ( Figure 2c).…”