Translocations and Jumping Rearrangements at 8q24 Result in over-Expression of MYC and are Key Drivers of Disease Progression

Introduction

Rearrangements at 8q24 are seen in up to 47% of multiple myeloma (MM) patients by a combination of fluorescence in situ hybridization (FISH), spectral karyotyping and classical cytogenetics. The gene of interest in this region is MYC, a known oncogene which acts a transcription factor and is involved in various pathways including cell cycle progression, apoptosis and cellular transformation. In myeloma, activation of MYC may be mediated through copy number changes or translocations where B-cell super-enhancers are juxtaposed to MYC resulting in its over-expression. In 30% of samples the partner super-enhancer comes from an Ig loci, and in the remainder the super-enhancer comes from genes commonly associated with myeloma such as FAM46C, FOXO3, and XBP1. The nature and extent of MYC abnormalities in MM still remains debatable, with a range of abnormalities including deletions, trisomies, jumping MYC rearrangements and translocations. Here we identify the range of abnormalities, the clonality and partner chromosomes involved.

Methods

We conducted a systematic investigation using interphase FISH (iFISH), gene expression profiling (GEP), karyotyping, and targeted sequencing to determine the prevalence and genetic basis for MYC aberrations in groups of plasma cell disorders incorportating monoclonal gammopathy of undetermined significance (MGUS, n=97), smoldering MM (SMM, n=64), and newly diagnosed symptomatic MM (NDMM, n=307). Two FISH probes were used that are 1 Mb telomeric and centromeric of MYC, as well as a third probe targeting MYC itself. These were co-hybridized to cells from bone marrow aspirates. Translocations were defined where the MYC probe and an adjacent probe hybridized to another chromosome, whereas jumping translocations were defined where only the MYC probe hybridized to another chromosome. Cells were also analyzed for common IGH translocations, gain of 1q and del(17p) by FISH as well as common myeloma mutations and copy number abnormalities using a targeted sequencing panel.

Results

By iFISH, copy number variations of either loss or gain of the 8q24 region were present in 18.2% of NDMM; and structural aberrations including translocations and jumping MYC rearrangements were present in 28.5%. By karyotyping, the partner chromosomes of the MYC translocations included 1p, 1q, 2p, 4q, 5q, 6q, 14q, or 22q. The most frequent genomic rearrangement was translocations of MYC to chromosome 1 [t(1;8)]. Altogether, MYC alterations were detected in 54.8% of NDMM patients.

As the disease stage progressed more MYC abnormalities were detected from MGUS to SMM to NDMM, especially translocations (0%, 0%, 13.4%) and jumping rearrangements (4.1%, 4.7%, 15.1%). Some MYC abnormalities were detected in MGUS and SMM samples, but they consisted of many different FISH signal patterns which were present in fewer than 20% of cells, indicating a heterogeneous population of subclonal cells at the early stages of myeloma development. These subclonal populations were present in 17% of MGUS samples and 23% of SMM samples.

Using gene expression arrays, samples with translocations and jumping rearrangements had higher levels of MYC expression compared to samples without these variants. Those with monosomy of MYC had lower expression of MYC and those with a heterogeneous population of abnormal MYC signals showed no difference in expression compared to samples with no abnormal signal. These data indicate that jumping MYC rearrangements result in over-expression of MYC in the same way as translocations of 8q24. However, current commercial probe designs do not incorporate the MYC locus and instead label the surrounding region. These assays do not identify jumping MYC rearrangements and as a result under-estimate the frequency of MYC abnormalities in myeloma.

Conclusion

MYC abnormalities are present in 54.8% of NDMM samples and occur by a variety of mechanisms from copy number changes to translocations. MYC translocations and jumping rearrangements are associated with over-expression of MYC, driving pathogenesis of the disease. MYC translocations are not present in MGUS or SMM samples, indicating this is a key lesion mediating the progression of disease to a symptomatic state.