Inhibition Of H3K27-Methylome As a Novel Therapeutic Strategy In Multiple Myeloma

Multiple myeloma (MM) remains incurable, despite recent therapeutic advances; newer insights into the pathogenic mechanisms that cause this disease and additional therapies are urgently needed. Recent studies of the epigenome and in particular the methylome, have shown that myeloma is characterized by widespread epigenetic changes. Epigenetic changes may precede genetic mutations and genomic instability; Ubiquitously Transcribed Tetratricopeptide Repeat Protein X (UTX), a histone H3K27 demethylase may represent such an example. Previous studies have shown inactivating somatic mutations in UTX (KDM6A) in 10% of MM cases. Aberrant methylation of core histone tails and deregulation of the corresponding enzymes such as UTX and JMJD3 have been implicated in leukemia as well as other types of cancers, but their role in MM remains unknown. We evaluated the activity of the selective Jumonji H3K27 demethylase (UTX/JMJD3) inhibitor, GSK-J4, in MM.

In a panel of 15 human MM cell lines (HMCLs) including cell lines resistant to bortezomib and dexamethasone, GSK-J4 induced significant survival and proliferation inhibition, as measured by luminescence-based viability assay (CTG), MTT (inhibition>68% in 48 hours) and ³H thymidine uptake, with the exception of OPM2 that was resistant up to 5 uM concentration. GSK-J4 induced apoptosis as measured by flow cytometry upon staining with Annexin-V/Propidium Iodide. The compound did not induce cytotoxicity in PBMCs from healthy donors and normal human skin fibroblasts. The inhibitory effect of GSK-J4 was observed also in CD138+ primary plasma cells from newly diagnosed MM patients (n=5) compared to PBMCs from healthy donors (n=9) (p<0.001). The compound also resulted in significant inhibition of MM cell colony formation in soft agar after 2 weeks in compared to controls. Moreover, interaction between MM cells and bone marrow stromal cells from MM patients or HS-5 stromal cell line did not overcome the inhibitory effect of GSK-J4 in all the HMCLs (KMS-12BM, LP-1, MM1S, OPM2, RPMI-8226, H929 and INA6) tested. To further investigate the mechanism of GSK-J4-induced apoptosis, we evaluate the activation of caspases 3/7, 8 and 9, and observed significant activation of caspase 3/7 and 9, indicating the involvement of intrinsic apoptotic pathway into the GSK-J4 function, as also confirmed by western blot analysis of Bcl-xl, p53, and Bax.

We evaluated the enzymatic activity of UTX/JMJD3 in HMCLs by using a fluometric-based assay. The sensitivity of the HMCLs to demethylase inhibitor was directly related with UTX/JMJD3 activity (p<0.05). As the methylation of H3K27 mark plays a major role in the maintenance of active and silent states of gene expression in important cellular processes, we next mapped H3K27me3 and H3K27me2 chromatin modifications by genome-wide chromatin immunoprecipitation followed by DNA sequencing (ChIP-seq) analysis in RPMI8226 and KMS-12BM cells before and after GSK-J4 treatment and in UTX knockdown cells. Transcription factors of OSKM complex: Oct4, Sox2, and Nanog were found to be targets of aberrant demethylation after treatment with GSK-J4, indicating possible involvement of H3K27 demethylases in pathogenesis of MM through the regulation of “stemness” genes. Western blot analysis showed that the inhibitor reduced the expression of these genes.

In conclusion, we demonstrate a significant beneficial impact of inhibition of H3K27 demethylation in MM. These results provide new insights into the mechanism of altering methylome as a potent epigenetic intervention in treatment of MM.