Burkitt lymphoma cell line carrying a variant translocation creates new DNA at the breakpoint and violates the hierarchy of immunoglobulin gene rearrangement.

The Burkitt lymphoma cell line KK124, which contains a reciprocal t(8;22) translocation, was shown to have rearranged in a region 3' to the c-myc proto-oncogene on chromosome 8 and 5' to the lambda constant region on chromosome 22. The breakpoint was cloned and sequenced, revealing that c-myc and a portion of its 3' region abutted a complete lambda variable gene that had undergone V-J recombination. Since this cell line expresses kappa light chain, this lambda rearrangement violates the previously proposed hierarchy of immunoglobulin gene rearrangement. A novel duplication of normal chromosome 8 sequences was also found at the breakpoint. The first exon of c-myc and its flanking sequence from the translocated allele was sequenced and compared with a normal counterpart. Extensive mutation was found within the first exon in contrast to its 3' and 5' flanking regions. S1 nuclease analysis revealed that it was the translocated c-myc being expressed and that there was a promoter shift from P2 to P1. The detailed structural analysis of this cell line provides clues concerning mechanisms of chromosomal translocation and c-myc deregulation in Burkitt lymphomas.     

Variant (6;15) translocations in murine plasmacytomas involve a chromosome 15 locus at least 72 kb from the c-myc oncogene.

The variant (6;15) translocations in murine plasmacytomas join the myc oncogene-bearing band of chromosome 15 and the immunoglobulin kappa band of chromosome 6. We recently cloned a region from chromosome 15 linked to C kappa and have now used probes from that region to define the major locus of plasmacytoma variant translocations, which we denote pvt-1. In five of nine plasmacytomas we analysed, the 6;15 translocation resulted from reciprocal recombination between the C kappa locus and a 4.5-kb region of pvt-1. Moreover, nearby we located the region shown by others to have undergone a complex (15;12;6) translocation in plasmacytoma PC7183. All the chromosome 6 breakpoints fell between 1 and 3 kb 5' to C kappa but only two were near J kappa genes. Thus the J kappa -C kappa region appears to be a recombination 'hot spot' in lymphocytes, but the breaks are unlikely to be mediated via V/J recombination enzymes. Comparison of a cloned 108-kb region across pvt-1 and another of 52 kb across c-myc established that the pvt-1 breakpoints lie at least 72 kb from the c-myc promoters. Since c-myc is expressed at a substantial level, the 6;15 translocation apparently activates c-myc. Activation may occur directly, at a remarkable distance along the chromosome, or indirectly, via a putative pvt-1 gene product.     

Breakage on chromosome 2 brings the Ck gene to a region 3' of c-myc in a Burkitt's lymphoma line carrying a (2;8) translocation

We have shown using in situ hybridization that the constant region of the kappa light chain immunoglobulin gene (Ck) is translocated from chromosome 2 to chromosome 8 in Burkitt's lymphoma cells with a (2;8) translocation. The Ck gene then ends up adjacent to and on the 3' side of c-myc. The breakpoint probably falls between the gene for the variable region of the kappa light chain (Vk) and the Ck gene.     

c-myc and immunoglobulin kappa light chain constant genes are on the 8q+ chromosome of three Burkitt lymphoma lines with t(2;8) translocations.

We have determined the localization of c-myc and the immunoglobulin kappa light chain genes on the 8q+/2p- chromosomes of the three Burkitt lymphoma lines BL21, LY66 and LY91 with t(2;8) translocation by in situ hybridization. BL21 is characterized by a complex translocation in which a piece of chromosome 9 appears to be located between the fragments of chromosome 8 and 2 on the 8q+ chromosome. Our data indicate that in all three cell lines the c-myc gene is located on the 8q+ chromosome proximal to the breakpoint in band 8q24. In all cell lines examined the cluster of kappa variable genes has remained on the 2p- chromosome. In LY91 cells the major part of the joining region remained on 2p-, while the joining region has moved to 8q+ in the cell lines BL21 and LY66. In all three cell lines the constant kappa light chain gene was found on the 8q+ chromosome. The fact that an essentially identical pattern was found in the cell line BL21, with the complex translocation, suggests that the insertion of the piece of chromosome 9 into the 8q+ chromosome might be a secondary event. Our present data fit into the concept that in all Burkitt lymphoma lines investigated so far, including cases with t(8;14) and the variant translocations t(2;8) and t(8;22), the c-myc gene becomes situated at the 5' side of an immunoglobulin constant gene. This may have implications for the generation of somatic mutations in the coding and non-coding part of the c-myc gene.     

Mapping of the c-myc, pvt-1 and immunoglobulin kappa genes in relation to the mouse plasmacytoma-associated variant (6;15) translocation breakpoint.

A variant mouse plasmacytoma (MPC)-associated translocation chromosome has arisen by pericentric inversion and exchange of the distal segments of a Robertsonian 6;15 fusion chromosome in the CAK TEPC 1198 mouse plasmacytoma, as described earlier. In situ hybridization was performed on the normal and the inverted Rb chromosomes, using myc and kappa probes. On the normal Rb chromosome, myc was in the 15 D2/3 region, whereas kappa hybridized in the 6 C2 area, as expected. On the inverted Rb chromosome, myc remains on the centrometric side of the translocation breakpoint on the chromosome 15-derived portion, whereas kappa has moved to the chromosome 6-derived segment that joined the same breakpoint on the telomeric side. Taken together with our recent demonstration that the murine c-myc locus is oriented 'head up' on chromosome 15, and with the results of Cory and co-workers concerning the relationship between the kappa gene and the associated pvt-1 region in the CAK TEPC 1198 tumor, the following conclusions can be drawn: (i) in the variant translocation of the CAK TEPC 1198 MPC, the breakage occurs 3' of the c-myc gene, as in the human Burkitt lymphoma-associated variant translocations; (ii) the pvt-1 gene on chromosome 15 is distal to the myc gene; (iii) the kappa light chain locus is oriented 'head up' on mouse chromosome 6 and faces pvt-1 and, beyond it, c-myc, in a head-to-tail configuration.     

Clustered somatic mutations in and around first exon of non-rearranged c-myc in Burkitt lymphoma with t(8;22) translocation.

We have examined the restriction map of the c-myc gene in 15 BL cell lines carrying the variant t(8;22) translocation in which c-myc is known to remain on chromosome 8. Using 3 restriction enzymes cutting outside the c-myc domain (EcoRI, BamHI, HindIII), we found no evidence for a c-myc/Ig lambda rearrangement in 14 BL cell lines. In the last one, BL 37, the 3' flanking region was rearranged corresponding to the already identified breakpoint located 400 pb downstream from the c-myc gene (9). Using 4 restriction enzymes cutting inside the c-myc gene (PvuII, PstI, SacI, HincII) we looked for discrete abnormalities within the gene limits, and we found in 9 BL cell lines several abolished and created sites, compatible with multiple independent somatic mutations. They are significantly clustered in the 5' non coding region, with a striking prevalence at the end of exon 1. The role of mutations in the non-coding first exon region for the deregulation of c-myc expression is discussed.     

c-myc Gene rearrangements involving gamma immunoglobulin heavy chain gene switch regions in murine plasmacytomas

In murine plasmacytomas, the c-myc gene has frequently been found to undergo rearrangement by virtue of a T(12;15) chromosome translocation. The immunoglobulin heavy chain gene switch region (S alpha) constitutes the target for most of these recombinations particularly in IgA producing plasmacytomas. We sought to identify non-S alpha myc target sites in several IgG producing tumors. The c-myc target in MPC-11 (a BALB/c IgG2b producing plasmacytoma) has been cloned, localized to the Igh-C locus and identified as the gamma 2a heavy chain gene switch region (S gamma 2a). Furthermore, by Southern blot hybridization, we have determined that the S gamma 2b region is the c-myc target in two NZB IgG2b producing plasmacytomas. The potential relation between Ig class expressed and c-myc translocation target is discussed.     

Structural analysis of both products of a reciprocal translocation between c-myc and immunoglobulin loci in Burkitt lymphoma.

The balanced translocations that occur between the c-myc and immunoglobulin loci in Burkitt lymphoma provide an unusual opportunity to analyze both products of a reciprocal recombination. Accordingly, we have determined the structure of the two reciprocal products of a translocation that joins the 5' portion of the c-myc gene on chromosome 8 to the immunoglobulin mu switch recombination signal on chromosome 14. By determining the nucleotide sequences at the translocation crossover points of both product chromosomes, we precisely locate these points with respect to nearby genes. This determination allows us to conclude that translocation involves nonhomologous recombination, is highly conservative of c-myc sequences (deleting only 16 bp at the crossover point), but deletes over 2 Kb of immunoglobulin sequences from the mu switch signal. The mu constant and c-myc genes are joined head-to-head about 3 Kb apart, while the IgH enhancer and an aberrantly rearranged D/J region are linked to sequences 5' of c-myc on the reciprocal product.