In multiple myeloma, 14q32 translocations are nonrandom chromosomal fusions driving high expression levels of the respective partner genes

Abstract

In studies of patients with multiple myeloma (MM), gene expression profiling (GEP) of myeloma cells demonstrates substantially higher expression of MMSET, FGFR3, CCND3, CCND1, MAF, and MAFB--the partner genes of 14q32 translocations--than GEP of plasma cells from healthy individuals. Interphase fluorescent in situ hybridization (FISH) was used to discriminate between chromosomal translocations involving different regions of the immunoglobulin heavy chain (IGH) genes at 14q32. With special probes designed for the constant region (IGHC) and the variable region (IGHV), IGH translocations were shown to be definite, nonrandom chromosomal fusions of IGHC with the loci of FGFR3, CCND1, CCND3, MAF, and MAFB genes; and IGHV with the locus of MMSET gene. When correlated with GEP results, the IGH translocations were found to drive expression levels of the partner genes to significantly higher levels (spikes) than copy-number variations. Hence, 42% of IGH translocations were identified among newly diagnosed MM patients (448/1,060). As GEP has become essential for assessing cancer risk, this novel approach is highly consistent with the cytogenetic features of the chromosomal translocations to effectively stratify molecular subgroups of MM on the basis of gene expression profiles of the IGH translocation partner genes in myeloma cells. © 2014 Wiley Periodicals, Inc.

Figure 1

Interphase FISH of a bone marrow specimen of a patient with MM demonstrates reciprocal chromosomal translocations of t(4;14) that recombined FGFR3 with IGHC (a) and MMSET with IGHV (e) in myeloma cells with a cytoplasmic immunoglobulin (cIg) light-chain restriction (lambda). Random t(4;14) was not observed (b, d). The GEP values of FGFR3 (40,928), MMSET (8,688), and MAF (3,772) were above the baseline expression levels of healthy plasma cells. FISH-elucidated t(4;14) co-existed with a gain of MAF gene copy numbers (5×) (c, f). White bars = 2 μm (100× magnification).

Figure 2

(A) diagram shows the chromosomal translocations of 4p16 and 14q32 that occur as the IGHV region (red) is juxtaposed with MMSET (black) at 4p16 (der4); the reciprocal exchange, FGFR3 (open circle) recombines with IGHC (green) at 14q32 (der14). The orientations of gene sequences (5′– 3′) are indicated by the arrows. Yellow bands represent chromosomal fusions. (B) FISH with the probe combinations for t(4;14) to examine metaphase chromosomes of a MM patients whose CD138⁺ PCs′ MMSET (7,113.0) and FGFR3 (17,524.2) expressions by GEP were above the threshold of IGH translocations demonstrates reciprocal fusions of IGHV/MMSET(a) and IGHC/FGFR3 (b); and FISH detected unbalanced t(4;14) in a human myeloma cell line (XG7) with IGHV/MMSET fusion (c) induced MMSET expression (3,309.2) above the threshold, but without a fusion of IGHC and FGFR3 that resulted in low FGFR3 expression at 44.7 (d) by GEP.

Figure 3

In the training set of t(4;14) and t(14;16), patient samples were examined with the following FISH probe combinations: IGHC–MMSET, IGHV–MMSET, IGHC–FGFR3, IHGV–FGFR3, IGHC–MAF, and IHGV–MAF. IGHC and IGHV signals are indicated by open circles (○) in the columns to the left and right, respectively, of the columns for MMSET, FGFR3, or MAF signals, which are indicated by filled circles (●). FISH-identified chromosomal translocations between IGHV and MMMSET, IGHC and FGFR3 (^§§), or IGHC/MAF are shown as half-filled circles (◑). All boxed numbers indicate FISH-confirmed IGH translocations with a partner gene whose expression also spiked above the threshold in GEP. The samples that GEP values were below the thresholds of IGH translocations were partially displayed.(→) and the highlighted rows indicate the case of reciprocal t(4;14) with both MMSET and FGFR3 expressions above the thresholds by GEP and copy number increase of MAF determined by the interphase FISH showing in Figure 1.