Secondary genomic rearrangements involving immunoglobulin or MYC loci show similar prevalences in hyperdiploid and nonhyperdiploid myeloma tumors

Abstract

The pathogenesis of multiple myeloma (MM) is thought to involve at least two pathways, which generate hyperdiploid (HRD) or nonhyperdiploid (NHRD) tumors, respectively. Apart from chromosome content, the two pathways are distinguished by five primary immunoglobulin heavy chain (IGH) rearrangements (4p16, FGFR3, and MMSET; 6p21, CCND3; 11q13, CCND1; 16q23, MAF; 20q12, MAFB) that are present mainly in NHRD tumors. To determine the prevalence and structures of IGH, immunoglobulin (IG) light chain, and MYC genomic rearrangements in MM, we have done comprehensive metaphase fluorescent in situ hybridization analyses on 48 advanced MM tumors and 47 MM cell lines. As expected, the prevalence of the five primary IGH rearrangements was nearly 70% in NHRD tumors, but only 12% in HRD tumors. However, IGH rearrangements not involving one of the five primary partners, and IG light chain rearrangements, have a similar prevalence in HRD and NHRD tumors. In addition, MYC rearrangements, which are thought to be late progression events that sometimes do not involve an IG heavy or light chain locus, also have a similar prevalence in HRD and NHRD tumors. In contrast to the primary IGH rearrangements, which usually are simple balanced translocations, these other IG rearrangements usually have complex structures, as previously described for MYC rearrangements in MM. We conclude that IG light chain and MYC rearrangements, as well as secondary IGH rearrangements, make similar contributions to the progression of both HRD and NHRD MM tumors.

Figure 1

Structures of IGH translocations in MM tumors. (A) The 1 Mb IGH locus is depicted with boxed coding regions, enhancer elements, and direction of transcription (arrow). Eα1 and Eα2 enhancers control gene expression bidirectionally (flanking arrowheads), but insulator sequences (thick vertical lines) may block centromeric effects. Eμ is the bidirectional intronic enhancer. The VH cosmid (red line) located 100 kb from the telomere is essentially a 14q tel probe. The CH BAC (green line) detects Eα1 and Eα2 sequences. (B) FISH analysis detected a classical balanced t(8;14) in the ARK HMCL. CH (green) and MYC (red) colocalize on der(14) (purple). The diagram depicts a breakpoint centromeric to MYC (8q24) and telomeric to the Eα1 enhancer (shaded box) that presumably dysregulates MYC. (C) FISH analysis detected a variant unbalanced t(8;14) in tumor #17. CH (green) and VH (red) colocalize on mod der(8) (C1). MYC (red) is juxtaposed to CH (green) on mod der(8) (purple) (C2). The diagram depicts a breakpoint telomeric to MYC and centromeric to Eα2; MYC may be juxtaposed and thus dysregulated either by an inverted Eα2 enhancer or an Eα2 enhancer with deleted insulator sequences (alternative shaded boxes). (D, E) FISH detection of classical t(4;14) and CH insertion at 11q13 in tumor #47. (D) One copy of CH (green) colocalizes with VH (green) on normal chromosome 14, a second copy of CH colocalizes with FGFR3 (red) on der(14), one copy of VH is on der(4) (purple), and one copy of FGFR3 is on chromosome 4 (purple) (E). A third copy of CH (green) is shown to colocalize with CCND1 (red) on der(11) (purple), while a second copy of CCND1 is on normal chromosome 11 (purple).

Figure 2

Complex chromosome rearrangements in MM tumors. Sequential FISH (A) and SKY (B) analysis of the complex IGH translocation t(14;20)t(4;20) in tumor #20. Metaphase chromosomes were hybridized with CH (green), VH (green), FGFR3 (red), and wcp4 (purple). The arrowhead shows a copy of CH at the t(4;14) breakpoint, while the arrow indicates a copy of VH on a small chromosome. The same metaphase chromosomes were stripped and rehybridized with SKY probes to show that the VH is on der(20) (arrow) and that chromosome 20 sequences are inserted at the t(4;14) breakpoint (arrowhead). Examples of chromosomes with rearrangements of the IGH (C), IGL (D), and MYC (E) loci are shown for selected MM tumors. Relative positions of CH, VH, Cλ, and target gene sequences (MAFB, CCND1, MMSET, MYC) are indicated.

Figure 3

Structures of Igλ translocations in MM tumors. (A) The 1 Mb IGL locus is depicted with boxed coding regions and direction of transcription (arrow). The 3′ enhancer (E) is depicted as controlling gene expression bidirectionally, except for putative telomeric insulator sequences (thick vertical line). The Vλ cosmid (red line) detects centromeric sequences, the Cλ-BAC detects JC and 3′E sequences (green line) and Cλ-cos detects only the 3′E. (B) FISH analysis detected a classical balanced t(8;22) in tumor #13. Cλ (green) and telomeric 22 sequences (purple) translocate to der(8) (B1). Cλ (green) colocalizes with MYC (red) on der(8) (purple) (B2). (C) FISH analysis detected a variant unbalanced and complex t(8;22) in tumor #40. Cλ (green) and Vλ (red) colocalize on der(22) (purple) (C1). MYC (red) is juxtaposed to Cλ (green) on der(22) (C2).

Figure 4

Complex chromosome rearrangements in HMCL. Examples of chromosomes with complex rearrangements of the IGH (A), IGL (B), and IGK (C) loci are shown for selected HMCL. Relative positions of CH, VH, Cλ, Cκ, and target gene sequences (CCND1, MAF, MYC, and NMYC) are indicated. Karyotypic abnormalities for the MYC locus that do not involve an IG locus are shown (D).

Figure 5

Primary and secondary genomic rearrangements during MM pathogenesis. There are two proposed pathways of pathogenesis for MM: hyperdiploid (HRD) and nonhyperdiploid (NHRD). Primary translocations that are mediated mainly by errors in IGH switch recombination or somatic hypermutation in germinal center B cells occur mostly in NHRD tumors. Secondary IG genomic rearrangements (TLC) that can occur at all stages of pathogenesis and MYC genomic rearrangements that occur at late stages of pathogenesis have a have a similar prevalence in HRD and NHRD tumors.