The “Trans-Amplicon” in Myeloma; a Mechanism of Clonal Selection Favoring Retention of Derivative Chromosomes Involved in Translocations.

Background: Multiple mechanisms exist for the positive selection of genes involved in oncogenesis such as gene amplification and transcriptional deregulation associated with chromosome translocations. Here we describe the case of a myeloma cell line that over time acquired extra copies of a derivative chromosome 4 involved in the t(4;14). These changes occurred over time both in vitro (in the generated JMW myeloma cell lines) as well as in vivo from extramedullary myeloma samples collected from the patient at various time points. In the case of the t(4;14)(p16;q32) 30% can be unbalanced translocations, always with loss of the der(14) and consequent loss of the FGFR3 gene expression, suggesting an indispensable role for MMSET located in the der(4).

Results: We performed serial analysis on patient samples obtained at 9, 10 and 12 mos from diagnosis, along with a time course analysis of the cell line. At diagnosis the patient tumor cells displayed a complex clinical karyotype subsequently characterized by hypodiploidy, a t(4;14)(p16;q32), 13 monosomy and 17p13.1-. The remaining chromosome 13 and 17 were involved in an unbalanced reciprocal translocation. Notably, over time the cell line duplicated the total chromosome number becoming near-tetraploid. At the time of diagnosis, FISH analysis was performed on the patient sample revealing the translocation t(4;14)(p16.3:q32) in nearly all cells. The t(4;14) was also present in the stable cell line. It was noted that the patient cells exhibited more than one fusion signal per cell for the t(4;14) by FISH. As the disease progresses both in vitro and in vivo, the number of t(4;14)(p16.3:q32) fusion signals detected per cell increased with time suggesting a positive selection of the derivative chromosomes of the translocation. We further analyzed the t(4;14) in the JMW cells by FISH using MMSET/VH, and FGFR3/CH probe combinations. The full-length (MMSET/FGFR3+VH/CH) probe was used to detect the total number of fusions in the JMW cells. At the time of analysis (cell line passage #23), the majority of the cells (69%) had 6 fusion signals per cell while 23% had 5 fusions and 8% of the cells had 4 fusions. We found that most of the cells (88%) displayed 4 fusion signals detected using the MMSET/VH probes. In parallel studies using the FGFR3/CH probes, 61% of the cells had 2 fusion signals and 39% with 1 fusion signal. It is evident that the tumor cells display positive selection of the derivative chromosome der(4) of the t(4;14)(p16.3;q32), accompanied by an increased dosage of the target gene (MMSET) upregulated by the translocation.

Conclusion: We present evidence for the amplification of the translocated chromosome in multiple myeloma, which we have termed “trans-amplicons”. Trans-amplicons may provide a selective growth and survival advantage to the myeloma tumor cells in an analogous manner to amplicons of DNA segments, but with much more powerful transcriptional consequences. Importantly, the evolution of genomic instability exemplified by the “trans-amplicons” in the patient cells was paralleled in vitro in the JMW cells.