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Blood, 15 March 2008, Vol. 111, No. 6, pp. 3145-3154. Prepublished online as a Blood First Edition Paper on December 21, 2007; DOI 10.1182/blood-2007-06-092122.
NEOPLASIA The MMSET protein is a histone methyltransferase with characteristics of a transcriptional corepressor1 Division of Hematology/Oncology and 3 Department of Pediatrics, Mount Sinai School of Medicine, New York, NY; 2 Division of Hematology/Oncology, Kobe University Graduate School of Medicine, Kobe, Japan; 4 Division of Hematology/Oncology, Northwestern University Feinberg School of Medicine, Robert H. Lurie Comprehensive Cancer Center, Chicago, IL; 5 Division of Hematological Malignancy, Dana-Farber Cancer Institute, Boston, MA; 6 Mayo Clinic, Scottsdale, AZ; and 7 Structural Biology Program, Mount Sinai School of Medicine, New York, NY MMSET, identified by its fusion to the IgH locus in t(4;14)-associated multiple myeloma, possesses domains found within chromatin regulators, including the SET domain. MMSET protein is overexpressed and highly associated with chromatin in myeloma cell lines carrying t(4;14). MMSET possesses methyltransferase activity for core histone H3 lysine 4 and histone 4 lysine 20, whereas MMSET made in cells only modified H4. Segments of MMSET fused to the Gal4 DNA binding domain repressed transcription of a chromatin-embedded Gal4 reporter gene. MMSET-mediated repression was associated with increased H4K20 methylation gene and loss of histone acetylation. Consistent with this repressive activity, MMSET could form a complex with HDAC1 and HDAC2, mSin3a, and the histone demethylase LSD1, suggesting that it is a component of corepressor complexes. Furthermore, MMSET coexpression enhances HDAC1- and HDAC2-mediated repression in transcriptional reporter assays. Finally, shRNA-mediated knockdown of MMSET compromised viability of a myeloma cell line, suggesting a biologic role for the protein in malignant cell growth. Collectively, these data suggest that, by acting directly as a modifier of chromatin as well as through binding of other chromatin-modifying enzymes, MMSET influences gene expression and potentially acts as a pathogenic agent in multiple myeloma.
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