Recurrent immunoglobulin gene translocations identify distinct molecular subtypes of myeloma

Abstract

Background: Chromosome translocations involving the immunoglobulin heavy chain gene (IgH) on 14q32 are a seminal event in the pathogenesis of many B-cell malignancies. Since myeloma is a post-germinal center tumor of mature, isotype switched plasma cells, we hypothesized that 14q32 translocations would usually involve IgH switch regions.

Materials and methods: We analyzed a panel of 21 human myeloma cell lines using a Southern blot assay to detect illegitimate rearrangements involving the switch regions. We then cloned the breakpoints, developed probes for FISH analysis, and characterized the oncogenes dysregulated by the translocations.

Results: Only half of the cell lines demonstrated a 14q32 abnormality by conventional karyotypic analysis, but we were able to identify translocations involving IgH switch regions in 15 of 21 lines, including all of the lines in which a 14q32 translocations was not identified by conventional karyotypic analysis. Six cell lines have an Ig translocation involving 11q13 with overexpression of cyclin D1. Six cell lines have an Ig translocation involving 16q23 with overexpression of c-maf. Five lines have an Ig translocations involving 4p16 with overexpression of FGFR3 and a novel gene, MMSET. The 4p16 breakpoints occur within the 5' introns of MMSET, and are associated with IgH-MMSET hybrid mRNA transcripts. The remaining five cell lines have translocations involving other loci, including: 6p25 (MUM1), 8q24 (c-myc), and 21q22 (?AML1).

Conclusions: Recurrent Ig translocations identify at least three distinct molecular subtypes of myeloma. Our long-term goal is to determine if there are phenotypic, prognostic and therapeutic differences associated with these molecular subtypes.