Identification of WNK1 as a therapeutic target to suppress IgH/MYC expression in multiple myeloma.

Multiple myeloma (MM) remains an incurable hematological malignancy demanding innovative therapeutic strategies. Targeting MYC, the notorious yet traditionally undruggable oncogene, presents an appealing avenue. Here, using a genome-scale CRISPR-Cas9 screen, we identify the WNK lysine-deficient protein kinase 1 (WNK1) as a regulator of MYC expression in MM cells. Genetic and pharmacological inhibition of WNK1 reduces MYC expression and, further, disrupts the MYC-dependent transcriptional program. Mechanistically, WNK1 inhibition attenuates the activity of the immunoglobulin heavy chain (IgH) enhancer, thus reducing MYC transcription when this locus is translocated near the MYC locus. WNK1 inhibition profoundly impacts MM cell behaviors, leading to growth inhibition, cell-cycle arrest, senescence, and apoptosis. Importantly, the WNK inhibitor WNK463 inhibits MM growth in primary patient samples as well as xenograft mouse models and exhibits synergistic effects with various anti-MM compounds. Collectively, our study uncovers WNK1 as a potential therapeutic target in MM.

Prosurvival kinase PIM2 is a therapeutic target for eradication of chronic myeloid leukemia stem cells.

A major obstacle to curing chronic myeloid leukemia (CML) is the intrinsic resistance of CML stem cells (CMLSCs) to the drug imatinib mesylate (IM). Prosurvival genes that are preferentially expressed in CMLSCs compared with normal hematopoietic stem cells (HSCs) represent potential therapeutic targets for selectively eradicating CMLSCs. However, the discovery of such preferentially expressed genes has been hampered by the inability to completely separate CMLSCs from HSCs, which display a very similar set of surface markers. To overcome this challenge, and to minimize confounding effects of individual differences in gene expression profiles, we performed single-cell RNA-seq on CMLSCs and HSCs that were isolated from the same patient and distinguished based on the presence or absence of BCR-ABL. Among genes preferentially expressed in CMLSCs is PIM2, which encodes a prosurvival serine-threonine kinase that phosphorylates and inhibits the proapoptotic protein BAD. We show that IM resistance of CMLSCs is due, at least in part, to maintenance of BAD phosphorylation by PIM2. We find that in CMLSCs, PIM2 expression is promoted by both a BCR-ABL-dependent (IM-sensitive) STAT5-mediated pathway and a BCR-ABL-independent (IM-resistant) STAT4-mediated pathway. Combined treatment with IM and a PIM inhibitor synergistically increases apoptosis of CMLSCs, suppresses colony formation, and significantly prolongs survival in a mouse CML model, with a negligible effect on HSCs. Our results reveal a therapeutically targetable mechanism of IM resistance in CMLSCs. The experimental approach that we describe can be generally applied to other malignancies that harbor oncogenic fusion proteins or other characteristic genetic markers.

PRMT2 interacts with splicing factors and regulates the alternative splicing of BCL-X.

Protein arginine N-methyltransferase 2 (PRMT2) functions in JAK-STAT and Wnt/ß-catenin signalling pathways, serves as a nuclear receptor-dependent transcriptional co-activator, and represses NF-κB and E2F1 transcription factor activities to promote apoptosis. We have previously demonstrated that PRMT2 interacts with PRMT1 and increases its activity. Here, we reveal associations using proteomics between the PRMT2 SH3 domain and splicing factors including Src-associated in mitosis 68 kDa protein (SAM68), a PRMT1 substrate and trans-acting factor that mediates BCL-X alternative splicing. We determined that PRMT2 interacts with SAM68 in cells and regulates its subcellular localization via the SH3 domain of PRMT2, prompting us to investigate the potential role of PRMT2 in BCL-X alternative splicing. We found that the expression of the full-length, wildtype form of PRMT2 promotes an increase in the BCL-X(L)/BCL-X(s) ratio in TNF-α or LPS stimulated cells. These results indicate that active PRMT2 may play a role during inflammation in alternative splicing regulation.

Arginine methylation in yeast proteins during stationary-phase growth and heat shock.

Arginine methyltransferases (RMTs) catalyze the methylation of arginine residues on proteins. We examined the effects of log-phase growth, stationary-phase growth, and heat shock on the formation of methylarginines on yeast proteins to determine if the conditions favor a particular type of methylation. Utilizing linear ion trap mass spectrometry, we identify methylarginines in wild-type and RMT deletion yeast strains using secondary product ion scans (MS(3)), and quantify the methylarginines using multiple reaction monitoring (MRM). Employing MS(3) and isotopic incorporation, we demonstrate for the first time that Nη1, Nη2-dimethylarginine (sDMA) is present on yeast proteins, and make a detailed structural determination of the fragment ions from the spectra. Nη-monomethylarginine (ηMMA), Nδ-monomethylarginine (δMMA), Nη1, Nη1-dimethylarginine (aDMA), and sDMA were detected in RMT deletion yeast using MS(3) and MRM with and without isotopic incorporation, suggesting that additional RMT enzymes remain to be discovered in yeast. The concentrations of ηMMA and δMMA decreased by half during heat shock and stationary phase compared to log-phase growth of wild-type yeast, whereas sDMA increased by as much as sevenfold and aDMA decreased by 11-fold. Therefore, upon entering stressful conditions like heat shock or stationary-phase growth, there is a net increase in sDMA and decreases in aDMA, ηMMA, and δMMA on yeast proteins.

The CREB Coactivator CRTC2 Is a Lymphoma Tumor Suppressor that Preserves Genome Integrity through Transcription of DNA Mismatch Repair Genes.

The CREB-regulated transcription coactivator CRTC2 stimulates CREB target gene expression and has a well-established role in modulating glucose and lipid metabolism. Here, we find, unexpectedly, that loss of CRTC2, as well as CREB1 and its coactivator CREB-binding protein (CBP), results in a deficiency in DNA mismatch repair (MMR) and a resultant increased mutation frequency. We show that CRTC2, CREB1, and CBP are transcriptional activators of well-established MMR genes, including EXO1, MSH6, PMS1, and POLD2. Mining of expression profiling databases and analysis of patient samples reveal that CRTC2 and its target MMR genes are downregulated in specific T cell lymphoma subtypes, which are microsatellite unstable. The levels of acetylated histone H3 on the CRTC2 promoter are significantly reduced in lymphoma in comparison to normal tissue, explaining the decreased CRTC2 expression. Our results establish a role for CRTC2 as a lymphoma tumor suppressor gene that preserves genome integrity by stimulating transcription of MMR genes.

A large-scale RNAi-based mouse tumorigenesis screen identifies new lung cancer tumor suppressors that repress FGFR signaling.

UNLABELLED: To discover new tumor-suppressor genes (TSG), we developed a functional genomics approach in which immortalized but nontumorigenic cells were stably transduced with large-scale shRNA pools and tested for tumor formation in mice. Identification of shRNAs in resulting tumors revealed candidate TSGs, which were validated experimentally and by analyzing expression in human tumor samples. Using this approach, we identified 24 TSGs that were significantly downregulated in human lung squamous cell carcinomas (hLSCC). Amplification of fibroblast growth factor receptor 1 (FGFR1), which aberrantly increases FGFR signaling, is a common genetic alteration in hLSCCs. Remarkably, we found that 17 of the TSGs encode repressors of FGFR signaling. Knockdown of 14 of these TSGs transformed immortalized human bronchial epithelial cells and, in most cases, rendered them sensitive to FGFR inhibitors. Our results indicate that increased FGFR signaling promotes tumorigenesis in many hLSCCs that lack FGFR1 amplification or activating mutations. SIGNIFICANCE: A functional genomics approach identifies new lung TSGs whose loss aberrantly increases FGFR signaling to promote tumorigenesis. These TSGs are frequently downregulated in hLSCCs, indicating that increased FGFR signaling promotes tumorigenesis in many hLSCCs lacking FGFR1 amplification or activating mutations.

MS³ fragmentation patterns of monomethylarginine species and the quantification of all methylarginine species in yeast using MRM³.

Protein arginine methylation is one of the epigenetic modifications to proteins that is studied in yeast and is known to be involved in a number of human diseases. All eukaryotes produce Nη-monomethylarginine (ηMMA), asymmetric Nη1, Nη1-dimethylarginine (aDMA), and most produce symmetric Nη1, Nη2-dimethylarginine (sDMA) on proteins, but only yeast produce Nδ-monomethylarginine (δMMA). It has proven difficult to differentiate among all of these methylarginines using mass spectrometry. Accordingly, we demonstrated that the two forms of MMA have indistinguishable primary product ion spectra. However, the secondary product ion spectra of δMMA and ηMMA exhibited distinct patterns of ions. Using incorporation of deuterated methyl-groups in yeast, we determined which secondary product ions were methylated and their structures. Utilizing distinct secondary product ions, a triple quadrupole multiple reaction monitoring cubed (MRM(3)) assay was developed to measure δMMA, ηMMA, sDMA and aDMA derived from hydrolyzed protein. As a proof-of-concept, δMMA and ηMMA were measured using the MRM(3) method in wild type and mutant strains of Saccharomyces cerevisiae and compared to the total MMA measured using an existing assay. The MRM(3) assay represents the only method to directly quantify δMMA and the only method to simultaneously quantify all yeast methylarginines.

A protein arginine N-methyltransferase 1 (PRMT1) and 2 heteromeric interaction increases PRMT1 enzymatic activity.

Protein arginine N-methyltransferases (PRMTs) act in signaling pathways and gene expression by methylating arginine residues within target proteins. PRMT1 is responsible for most cellular arginine methylation activity and can work independently or in collaboration with other PRMTs. In this study, we demonstrate a direct interaction between PRMT1 and PRMT2 using co-immunoprecipitation, bimolecular fluorescence complementation, and enzymatic assays. As a result of this interaction, PRMT2 stimulated PRMT1 activity, affecting its apparent V(max) and K(M) values in vitro and increasing the production of methylarginines in cells. Active site mutations and regional deletions from PRMT1 and -2 were also investigated, which demonstrated that complex formation required full-length, active PRMT1. Although the inhibition of methylation by adenosine dialdehyde prevented the interaction between PRMT1 and -2, it did not prevent the interaction between PRMT1 and a truncation mutant of PRMT2 lacking its Src homology 3 (SH3) domain. This result suggests that the SH3 domain may mediate an interaction between PRMT1 and -2 in a methylation-dependent fashion. On the basis of our findings, we propose that PRMT1 serves as the major methyltransferase in cells by forming higher-order oligomers with itself, PRMT2, and possibly other PRMTs.

Förster resonance energy transfer measurements of cofactor-dependent effects on protein arginine N-methyltransferase homodimerization.

Protein arginine N-methyltransferase (PRMT) dimerization is required for methyl group transfer from the cofactor S-adenosyl-L-methionine (AdoMet) to arginine residues in protein substrates, forming S-adenosyl-L-homocysteine (AdoHcy) and methylarginine residues. In this study, we use Förster resonance energy transfer (FRET) to determine dissociation constant (K(D)) values for dimerization of PRMT1 and PRMT6. By attaching monomeric Cerulean and Citrine fluorescent proteins to their N-termini, fluorescent PRMTs are formed that exhibit similar enzyme kinetics to unconjugated PRMTs. These fluorescent proteins are used in FRET-based binding studies in a multi-well format. In the presence of AdoMet, fluorescent PRMT1 and PRMT6 exhibit 4- and 6-fold lower dimerization K(D) values, respectively, than in the presence of AdoHcy, suggesting that AdoMet promotes PRMT homodimerization in contrast to AdoHcy. We also find that the dimerization K(D) values for PRMT1 in the presence of AdoMet or AdoHcy are, respectively, 6- and 10-fold lower than the corresponding values for PRMT6. Considering that the affinity of PRMT6 for AdoHcy is 10-fold higher than for AdoMet, PRMT6 function may be subject to cofactor-dependent regulation in cells where the methylation potential (i.e., ratio of AdoMet to AdoHcy) is low. Since PRMT1 affinity for AdoMet and AdoHcy is similar, however, a low methylation potential may not affect PRMT1 function.