Abstract PL05-02: MMSET: A pathogenic factor and therapeutic target in multiple myeloma.
Authors: Licht Jonathan D. , Popovic Relja , Min Dong-Joon , Martinez-Garcia Eva , Mrksch Milan , Kelleher Neil , Will Christine , Ezponda Teresa 
Published: June 2012

Multiple Myeloma (MM), a neoplasm of plasma cells accounts for up to 10% of hematological malignancies. The MMSET (Multiple Myeloma SET domain) protein is a histone methyl transferase (HMT) overexpressed as a result of the translocation t(4;14), present in about 15% of multiple myeloma patients. MMSET is a nuclear protein with multiple domains critical for gene regulation including the SET domain, which encodes histone methyl transferase activity, and protein interaction domain including PHD, and PWWP domains. Although studies have shown the involvement of MMSET in heart development and tumor progression, its mode of action in the pathogenesis of MM is largely unknown. We found that overexpression of MMSET induces global increase in H3K36 methylation with concomitant loss of global H3K27 methylation. This is also accompanied by significant changes in over DNA methylation in these cells. These changes cause physical loosening of the chromatin structure, increased micrococcal nuclease accessibility, leading to altered gene expression. Pathways affected by MMSET overexpression include cell cycle, apoptosis and response to DNA damage. The HMT activity of MMSET is important for growth stimulation by MMSET since reexpression of MMSET in a t(4;14) myeloma cell line in which the rearranged MMSET allele was disrupted by homologous recombination (KMS11-KO) rescued growth only when the HMT activity of the protein was intact. Conversely shRNA-mediated deletion of MSMET in t(4;14)+ cell lines leads to growth arrest and cell death, together suggesting that MMSET is a therapeutic target in multiple myeloma.

We investigated the mechanism by which MMSET induces global chromatin changes. KMS-11-KO cells were stably repleted with wild-type MMSET or deletion constructs. The complete H3K36/H3K27 switch mediated by MMSET requires all 4 PHD finger domains of the protein, the second PWWP domain and the functional SET domain. Furthermore, these domains are also required for the increased proliferation as well as the aberrant gene expression caused by overexpression of the wild type MMSET. We also found that the PHD and PWWP domains of MMSET were required for its ability to bind to specific chromatin marks and likely play a role in firmly binding MMSET to chromatin. Furthermore deletion of PHD domain 4 yielded a form of MMSET able to increase H3K36 methylation but unable to reduce H3K37 methylation. Wild-type MMSET but not the PHD4 deletion mutant could bind to the H3K27 histone demethyalse JMJD3. This further implicates a histone demethylase as a possible target in this form of myeloma. Recently, it has been shown that double stranded DNA breaks lead to unwinding of chromatin in a manner regulated by the DNA damage response (DDR). Considering this and the global changes induced by MMSET in chromatin structure, we hypothesized that MMSET could affect the DDR by inducing global changes in chromatin structure. We observed that cells overexpressing MMSET displayed more DNA damage at baseline as measured by alkaline electrophoresis comet assay and had higher levels of phosphorylated H2AX, a common DNA damage marker. To try to explain the observed resistance of t(4;14)+ myelomas to chemotherapy, we incubated cells in the presence of melphalan, an alkylating agent currently used in myeloma treatment. Paradoxically, despite the higher baseline level of phosphorylated H2AX and higher levels of single- and double-strand breaks of DNA upon melphalan treatment, MMSET overexpressing cells show better survival in response to the drug. The cells with high MMSET levels were more resistant to the action of melphalan and displayed less apoptosis after the treatment.

After repleting MMSET expression in the KMS11-KO cells, we observed that the SET domain of the protein is important for these effects as KO cells expressing the HMT inactive mutant grew more slowly and formed fewer colonies in the presence of melphalan than KO cells expressing wild type MMSET. We also profiled gene expression in cells with high and low MMSET levels using Illumina HumanWG-6 v3.0 expression arrays and performed functional annotations using Ingenuity Pathway analysis. There was an enrichment of genes implicated in cell cycle regulation (CCNE2, E2F2, TP53INP1, CDC25A) and DNA damage-repair (DNA-PK, GADD45G). We then studied the cell cycle profile of KMS11-KO treated with high dose melphalan for two hours. We performed cell cycle profile with BrdU staining two days after the release from the drug. We observed that cells with high MMSET levels could cycle normally and overcame the cell cycle arrest but cells with low MMSET were arrested in G0-G1 and G2-M phases. Collectively this information suggests that the normal role of MMSET in DNA repair is dysregulated by its overexpression in myeloma.

An underlying paradox of MM is the continued proliferation and self-renewal of a highly differentiated cell. Plasma cell development requires the BLIMP1 transcription factor, which stimulates terminal B cell differentiation and represses c-MYC, a master regulator of cell growth. By contrast, malignant plasma cells express c-MYC and proliferate while maintaining the specialized machinery required for immunoglobulin production. We found that MMSET overexpression was associated with increased c-MYC protein but not mRNA expression and no change in the half-life of the MYC proteins. miRNA profiling of t(4;14) myeloma cells before and after depletion of MMSET showed that MMSET repressed expression of miR-126*, which was predicted to target c-MYC. miR126* but not a point mutant of mir126* inhibited translation of a c-MYC-UTR-luciferase reporter. Overexpression of miR126* in t(4;14) myeloma cells suppressed c-MYC expression and cell growth, while growth suppression and inhibition of c-MYC expression in myeloma cells after MMSET knock-down was partially blocked by an antagomir directed against mir126*. Growth suppression mediated by MMSET knockdown was reversed by expression of exogenous miRNA-resistant form of c-MYC. MMSET was recruited to the miR126* promoter along with a co-repressor complex and in this case repressed miR126* expression. To model the effects of an MMSET inhibitor in combination with histone deactylase (HDAC) inhibitors, cells were subjected to trichostatin A, MMSET depletion or both. HDAC inhibitor treatment on its own elevated mir126* levels and decreased c-MYC expression while an HDAC inhibitor combined with MMSET depletion led to complete silencing of c-MYC expression and increased cell death. These studies motivate the development of MMSET inhibitors.

We assayed MMSET in vitro using bacterially purified enzyme and recombinant histones. In vitro, MMSET is a promiscuous HMT altering H3K36 and H3K27 residues and methylating itself on C-terminal lysine residues. Previously identified HMT inhibitors including chaetocin and BIX-01294 inhibited MMSET activity in vitro. We developed a screen for additional MMSET inhibitors using a high throughput mass spectroscopic assay in which the HMT activity of MMSET was detected by the presence of a 14Da shift in molecular weight of a small peptide representing the H3K36 methylation site of histone H3. Of 10,000 compounds screened, 7 showed &gt;50% inhibition, none of these structurally similar to known HMT inhibitors. Secondary assays confirmed activity of at least three of these compounds that are undergoing further testing in vitro and in cell culture.

In conclusion MMSET mediates extensive changes in chromatin structure and function that drive malignancy. MMSET inhibition, alone in combination with chemotherapy or other agents that affect chromatin such as HDAC inhibitors may represent future therapies for t(4;14)-associated multiple myeloma

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr PL05-02.