To the Editor:
The amplification of cellular proto-oncogenes, a common feature of malignant tumors, results in distinct cytogenetic alterations in carcinomas and hematopoietic neoplasms. In carcinomas, it mainly produces elongation of chromosomes by homogeneously staining regions or extrachromosomal elements referred to as double minutes. In hematopoietic malignancies, in which homogeneously staining regions and double minutes are rare, it gives rise to jumping translocations of chromosomal segments containing oncogenes, ie, segmental jumping translocations (SJTs). SJTs were detected recently in treatment-related leukemias in humans.1 They have been shown to relocate chromosomal regions harboring the ABL, MLL, INT-2, and MYC oncogenes to one or more recipient chromosomes, creating, thereby, structurally abnormal chromosomes, unidentifiable marker chromosomes, and partial polysomy of the amplified chromosomal segment. Thus far, SJTs have been demonstrated to lead only to the amplification of unrearranged oncogenes in normal genomic configuration. We describe here a new type of SJT that appears to lead to the multiplication of rearranged and activated (ie, constitutively transcribed) oncogenes. The new SJT was observed in the BALB/c mouse plasmacytoma, MOPC 315, in which it effected the transposition onto two marker chromosomes of a chromosomal segment that contained the same clonotypic MOPC 315-typical T(12;15) translocation breakpoint region. This region is known to harbor a recombined and transcriptionally deregulated Myc gene.
Spectral karyotyping (SKY)2,3 was used to analyze the chromosome complement of a subline of the inflammation-induced mouse plasmacytoma, MOPC 315. It readily identified the Chr T(12;15), the hallmark chromosome of BALB/c plasmacytomas, which is thought to be essential for these tumors, because it contains the transcriptionally deregulated Myc.4 In addition, SKY showed the presence of two chimeric marker chromosomes that contained small hybrid segments of Chr 12- and Chr 15-derived material, the Chrs T(12;15;16;12;15;16) and T(17;15;16;12;15;16). The marker chromosomes were of particular interest, because they could be considered as tripartite chromosomes that were produced by joining Chrs 12, 15, and 16, and Chrs 17, 15, and 16, respectively, and then inserting a chromosomal segment that harbored a T(12;15) breakpoint region (Fig1A). To test this hypothesis, the Chr T(12;15) and the two marker chromosomes were separated by flow sorting.5 The genomic DNA obtained from flow-sorted chromosomes was amplified by degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR)6 to prepare fluorescence in situ hybridization (FISH) probes for the reverse painting of normal mouse chromosomes. Reverse painting was used to determine the purity of the flow-sorted chromosomes and to confirm and map their composite nature as seen by SKY (Fig 1B). To demonstrate that all three flow-sorted tumor chromosomes contained the MOPC 315-typical translocation breakpoint region, a clonotypic junction fragment between Myc and the switch region of the Ig heavy-chain α locus was generated. This was accomplished by PCR using DNA obtained from the flow-sorted chromosomes as template. The DNA sequence analysis of the PCR fragments showed the presence on all three chromosomes of the same T(12;15) translocation breakpoint region previously determined to be unique for MOPC 3157 (Fig 1C). These findings were interpreted to mean that in MOPC 315 the identicalMyc rearrangement was relocated by SJTs to two marker chromosomes.
The mechanism of the SJT that amplified the T(12;15) breakpoint region in MOPC 315 is not known. Possibilities include, first, the involvement of extrachromosomal precursors, such as episomes or double minutes containing T(12;15) translocation segments, before a postulated chromosomal reintegration event; second, the occurrence of illegitimate, nonreciprocal, trans-chromosomal recombinations between hyperreplicative or fragile sites; and third, the involvement of recombinogenic repetitive sequences at the breaksites. The latter explanation is supported by findings that breakpoints of jumping translocations, the next close relatives of SJT, are usually found at sites of repetitive DNA, eg, in centromeres or pericentromeric heterochromatin,8 telomeres, subtelomeric regions, variant telomeric repeats or interstitial telomeric sequences,9-11or constitutive heterochromatin12,13; however, this has not been shown for MOPC 315. Furthermore, it is conceivable that theMyc gene facilitates its own amplification via SJTs as a consequence of a Myc-induced mutator phenotype. This hypothesis is based on the proposal that Myc acts as a mutator gene in plasmacytomas14 and the finding that chromosomal translocations were induced by another oncogene, the SV40 large T-antigen.15
In conclusion, it is suggested that SJTs may be not only a mechanism for increasing the copy number of unrearranged oncogenes, but also a tumor progression mechanism that leads to the amplification of rearranged, transcriptionally active oncogenes. Additional studies are warranted to determine the prevalence of SJTs in mouse plasmacytomas and to explore if recombined oncogenes can jump in human leukemias and lymphomas, too.
The authors thank Dr J.F. Mushinski for reading the letter and making helpful editorial suggestions.