DIRECT SEQUENCE-ANALYSIS OF 14Q+ AND 18Q- CHROMOSOME JUNCTIONS AT THE MBR AND MCR REVEALING CLUSTERING WITHIN THE MBR IN FOLLICULAR LYMPHOMA

The t(14; 18) translocation is a highly consistent feature of follicular lymphoma although the underlying mechanism generating this fusion remains uncertain. The breakpoints on chromosome 18 are at one of two sites, designated mbr and mcr, in the bcl-2 gene. A polymerase chain reaction strategy has been developed for amplification and direct sequencing of the resultant 14q+ and 18q- reciprocal junctions. Sequence analysis of the amplified 14q+ junction established that 21 tumours contained a bcl-2 (mbr) sequence to an immunoglobulin J(H) region, the majority being J5 or J6. A nonrandom pattern of breakpoints within the mbr region was found. Clustering of the breakpoint occurred with over 60% of the translocations clustering within 10 bases. There was a second cluster within the mbr 50 bases 3' of the first cluster. One of these junctions had an unusual configuration with the bcl-2 and J(H) sequences separated by a recognisable D(H) region. This suggests that at least some of the junctional sequences, previously thought of as N insertions, may be fragments of unrecognised D(H) regions. In one of these tumours it was possible to sequence the reciprocal 18q- junction, showing it to consist of a D(H)/bcl-2 (mbr) fusion. Analysis of both reciprocal junctions for a translocation in the mcr region of bcl-2, showed that this 18q- junction also consisted of D(H) fused to a bcl-2 sequence. In contrast to previous analyses, which demonstrated either loss or duplication of bcl-2 sequences at the breakpoints, the bcl-2 sequence was conserved during the mbr and mcr translocations in this study. Although the precise mechanism of the t(14; 18) translocation remains unclear, these data demonstrate that breakpoints are nonrandomly distributed within the mbr.