Plasmacytoma induction in radiochimeras

Duplication of Subcytoband 11E2 of Chromosome 11 Is Regularly Associated with Accelerated Tumor Development in v-abl/myc-Induced Mouse Plasmacytomas

Chromosome 11 aberrations constitute the second most frequent chromosomal aberration in mouse plasmacytomas (PCTs) in which both the myc and abl oncogenes are constitutively expressed. In these tumors, previous G-banding studies had revealed numerical aberrations including duplication of the entire chromosome 11 or segments of telomeric bands D and E. The trisomy of chromosome 11 was always associated with accelerated pristane + v-abl/myc-induced PCT development. In the present study, PCT development was studied in a unique BALB/c congenic mouse strain, (T38HxBALB/c) F1, carrying a reciprocal translocation between chromosomes X and 11. After v-abl/myc induction, PCTs in this strain had acquired a nonrandom duplication of subcytoband 11E2. This duplication was always associated with accelerated PCT development. Corresponding synteny regions in the human and rat are changed in many tumors and involved in duplication, amplification, or translocation events. Thus, together with these synteny data, our findings strongly suggest a causal involvement of 11E2 in the acceleration of v-abl/myc-induced PCTs.

Location ofMyc,Igh, andIgk on Robertsonian fusion chromosomes is inconsequential forMyc translocations and plasmacytoma development in mice, but Rb(6.15)-carrying tumors preferIgk-Myc inversions over translocations

The location of the Myc and immunoglobulin (Ig) loci on metacentric Robertsonian (Rb) fusion chromosomes may affect the development of mouse plasmacytomas (Pcts) by changing the probability with which chromosomal Myc-Ig translocations occur. To test this hypothesis, we induced Pcts in BALB/c (C) mice that carried Rb(4.12) and/or Rb(6.15) chromosomes. The Rb mice developed Pcts (n = 198) with similar onset and incidence to that in the inbred C mice. Karyotyping of 70 Rb-carrying Pcts demonstrated that in these tumors, just as in their counterparts in inbred C mice, the Igh heavy-chain locus was translocated with Myc more often than was the Igk light-chain locus. Pcts harboring Igh or Igk on normal and Rb chromosomes showed no bias toward either in generating Myc translocations. These findings indicated that the location of Myc, Igh, and Igk on normal or Rb chromosomes is inconsequential for Myc translocation and Pct development. In contrast, in Rb(6.15) mice, in which chromosomal inversions competed with chromosomal translocations for Igk-Myc juxtapositions, the former occurred more frequently than the latter in the resulting Pcts. This suggested that spatial proximity of Igk and Myc on the same chromosome facilitates the rearrangement of these loci. Myc translocation-dependent mouse Pct may provide a good model system for furthering our understanding of the relationship of higher-order genome organization in the interphase nucleus, origin of chromosomal translocations, and development of cancer.

Ig/myc translocations of the plasmacytoma-prone BALB/c strain occur independently of the genetic and parental origin of the affected chromosomes 6, 12, and 15

Virtually all murine plasmacytomas (MPCs) carry chromosomal translocations that juxtapose myc on chromosome 15 (chr 15) to one of the three immunoglobulin loci carrying chr 6, 12, or 16. Only some mouse strains are susceptible to MPC induction, however, with BALB/c as the outstanding example. Most other strains are resistant. Our earlier studies with reciprocal BALB/c<-->DBA/2 chimeras suggested that part of this susceptibility is determined at the level of the target cell itself (DBA/2 is MPC resistant). The probability of the Ig/myc translocation is one of the possibly relevant variables. Because MPC resistance is dominant over susceptibility, it is conceivable that the translocations prevail due to some deficiency of the Ig rearrangement or Ig-associated repair mechanisms in BALB/c cells. This could be determined at the level of the chromosomes that participate in the translocation or by genes on other chromosomes. Here, we show that the substitution of the BALB/c-derived chr 12, 6, and 15, which carry IgH, kappa, and myc, respectively, with their homologs derived from MPC-resistant mice, did not affect MPC susceptibility. The use of Robertsonian 4.12 and 6.15 chromosomes in this study has also provided us with the opportunity to assess the parental derivation of the chromosomes participating in the translocation. In contrast to the human chronic myeloid leukemia (CML)-associated BCR/ABL fusion transcript, where a strong bias was claimed that was attributed to imprinting, we have found that the parental chromosomes were randomly involved in the translocation. We have also shown that the translocations could be of uniparental or biparental origin.

Differential susceptibility of BALB/c and DBA/2 cells to plasmacytoma induction in reciprocal chimeras

Reciprocal chimeras were generated between BALB/c and DBA/2 mice by inoculating newborn recipients of either strain with bone-marrow (BM) cells of the other through the periorbital vein. DBA/2 mice inoculated with the BALB/c with proven chimerism will be referred to as C----D, the reciprocal as D----C. The BALB/c cells carried a Robertsonian 6;15 (Rb6;15) chromosome marker to facilitate identification. The chimeric mice contained between 5% and 70% of donor cells when examined at 4 to 5 weeks of age. Six of 10 C----D developed plasmacytomas (MPC) after 3 x 0.5 ml monthly pristane treatment (incidence 60%) and 8 of 25 (incidence 32%) after 2 to 3 x 0.5 ml pristane followed by Abelson virus (A-MuLV) infection. Seven of 15 D----C developed MPC after pristane treatment (incidence 47%) and 4 of 17 after pristane + A-MuLV (incidence 24%). All tumors that have arisen in both reciprocal chimeras originated from BALB/c cells independently of the degree of chimerism. All tumors contained an Ig/myc translocation. Among the C----D chimeras, 5 carried t(12;15) and I t(6;15) in the pristane-treated group, while 4 carried t(12;15), I t(6;15) and 3 t(15;16) in the pristane + A-MuLV. Among the D----C chimeras 6 carried t(12;15) and I t(6;15) in the pristane-treated group, while 3 t(12;15) and I t(6;15) in the pristane + A-MuLV. No tumors developed in 18 pristane- and 22 pristane + A-MuLV-treated DBA/2 mice nor in 15 pristane- and 17 pristane + A-MuLV-treated (BALB/c x DBA/2)F1 mice. The data indicate that BALB/c and DBA/2 cells differ in their propensity to transform into plasmacytoma in identical host environments after both pristane and pristane + A-MuLV treatment. They also show that the oil granuloma can support MPC development in either type of chimeric host.

v-abl does not abolish IL-6 requirement by murine plasmacytoma cells

Activation of c-myc by juxtaposition to an immunoglobulin locus and introduction of the v-abl oncogene act synergistically in generating a mouse plasmacytoma (PC). The question arose whether the effect of v-abl could be attributed to a deregulation of interleukin-6 (IL-6) production or responsiveness, in view of the fact that IL-6 exerts potent growth-stimulatory activity on PC cells. We studied the effect of IL-6 on the in vitro growth of primary PCs induced by pristane alone (TEPCs) or by pristane + A-MuLV (ABPCs). Five of 13 TEPCs and 3 of 7 ABPCs responded to IL-6. Macrophage supernatants prepared from both TEPCs and ABPCs had similar stimulatory effects on PC cells. From 30 primary PCs (including both TEPCs and ABPCs), we established 9 in vitro lines, 2 of which expressed v-abl. All were able to grow on macrophage feeder layers. Three types of behavior could be distinguished on the basis of growth in feeder-free cultures in the presence and absence of IL-6. Group I contained 4 IL-6-dependent lines. Group II contained 2 IL-6-independent lines (one v-abl expressor) that grew faster in the presence of IL-6. Group III consisted of 3 feeder-dependent lines (one v-abl expressor) that were not significantly stimulated by IL-6. These findings indicate that v-abl expression does not influence IL-6 dependence or responsiveness by itself. The supernatant of one line in group II was able to stimulate PC cells. All 6 lines of Groups I and II carried a typical (12;15) translocation, while all 3 lines in group III had a variant (6;15) or (15;16) translocation.

The "missing" mouse plasmacytoma (MPC) associated translocation T(15;16) occurs repeatedly in new MPC induction systems

A reciprocal translocation between chromosomes 15 and 16 has been detected in seven murine plasmacytomas induced by a combination of pristane and Abelson virus. Six of the tumors were induced in a new, unconventional experimental system based on transfer of uninfected or Abelson virus infected bone marrow and spleen cells, respectively, into pristane treated mice. All six tumors were of donor type. The seventh tumor appeared in a conventional pristane + Abelson virus treated mouse. This tumor was unusual in carrying both the 15;16 variant translocation and the typical 12;15 translocation, in the same tumor cells. In the new 15;16 variant, the breakpoint of chromosome 15 was at the interphase of the D2/3 sub-bands, as in mouse plasmacytomas with the previously well-known typical 12;15 and variant 6;15 translocations. The breakpoint on chromosome 16 was mapped to band 16B1, corresponding to the presumed cytogenetic site of the Ig-lambda gene. In three of the seven tumors with the 15;16 translocation, the derivative chromosome 15 had undergone a duplication, a feature that has not been previously encountered in the MPC-associated 12;15 and 6;15 translocation carriers. The reciprocal derivative chromosome 16 was lost from one of the seven tumors. We postulate that the 15;16 translocation results in juxtaposition of the myc gene to lambda sequences, probably in a similar orientation as previously described for the variant 6;15 translocation.