The Multiple Myeloma SET Domain (MMSET) Protein Is a Histone H3 and H4 Methyltransferase with Properties of a Transcriptional Co-Repressor.

Over 40% of cases of multiple myeloma (MM) are associated with translocations of the immunoglobulin heavy (IgH) chain gene gene. The t(4;14) translocation, present in ca. 20% of myeloma cases, results in the overexpression of two potential oncogenes, MMSET and FGFR3, via juxtaposition of their endogenous promoters to regulatory elements of the IgH locus. The presence of t(4;14), and MMSET overexpression, is an adverse prognostic factor in MM irrespective of FGFR3 expression. MMSET contains several conserved motifs found in proteins involved in chromatin function (PWWP, HMG, PHD domains) and in the epigenetic control of transcription (SET domain). Accordingly, we found that the two main isoforms of the MMSET protein exhibit exclusive nuclear localization in both transfected fibroblasts and myeloma cells carrying t(4;14). Towards our goal of defining the ability of MMSET to affect gene regulation and contribute to the disease pathogenesis, we found that the SET domain of MMSET possesses in vitro methyltransferase activity specific for core histones H3 and H4. Using a computational approach and theoretical extrapolation from the solved NMR structure of vSET, we identified residues in the active site of MMSET essential for catalysis, whose mutation drastically reduces enzymatic activity.

Reporter assays using Gal4 fusion constructs showed that both the amino terminus of MMSET, containing the PWWP and HMG domains, as well as the SET-containing carboxy terminus act as transcriptional repressors. MMSET interacts physically and functionally with a number of known co-repressor molecules, such as HDAC1, HDAC2, Sin3a, and SIRT1, but not HDAC4 or HDAC6. As such, MMSET co-expression enhances HDAC1 and HDAC2-mediated repression in transcriptional reporter assays, and MMSET repression is partially relieved by the addition of an HDAC inhibitor. A yeast two hybrid screen identified a number of other functional partners of MMSET, including ZNF331/RITA (Rearranged in Thyroid Adenoma), a KRAB domain/zinc finger protein previously implicated in malignancy. MMSET and ZNF331 co-localize in the nuclei of transfected fibroblasts, co-immunoprecipitate, and display cooperative repression in reporter assays. Collectively, these data support the idea that MMSET is a biologically active, bifunctional transcriptional mediator acting as a HMT enzyme in chromatin remodeling and as a complex adaptor in the recruitment of repressor species.

Presently we are modeling the biological effects of MMSET through a conditional overexpression system in a B cell line. While low levels of MMSET are ubiquitiously expressed, induction of high levels of MMSET expression in the B cell line is associated with growth suppression and G1 arrest. While paradoxical for a presumed oncoprotein, such actions have been observed for other disease-associated proteins such as Runx1/MTG8. In contrast, a myeloma cell line harboring t(4;14) proliferates in the presence of high level MMSET expression. RNAi-mediated knockdown of MMSET in these cells induces apoptotic cell death. This suggests that MMSET may be critical for growth and survival of myeloma cells. Profiling of gene expression changes in these systems should link the transcriptional and biological activities of MMSET.