Down-regulation Of C-myb Research Articles

Effect of RGT-61159 on inhibition of oncogene c-MYB synthesis and tumor growth inhibition in a broad range of ACC PDX models, at well tolerated doses in rodents and non-human primates.

6107 Background: RGT-61159 is a first in-class oral inhibitor of the oncogenic transcription factor c-MYB via selective alteration of its RNA splicing machinery. RGT-61159 selectively induces the inclusion of the cryptic “poison” exon into the c-MYB RNA transcripts, resulting in the robust elimination of c-MYB canonical mRNA transcript and consequently of its protein in cells. Overactivation of the c-MYB oncogene, primarily due to chromosome rearrangements, is a hallmark of adenoid cystic carcinoma. (ACC). ACC is a rare and aggressive type of cancer for which effective treatment does not exist. By inhibiting oncogenic MYB protein production, RGT-61159 has the potential to efficiently and selectively inhibit proliferation or induce cell death of cancer cells overexpressing MYB protein. Methods: RGT-61159 cell killing activity was measured against a broad panel of cancer cell lines in a CellTiter-Glo assay. MYB RNA and MYB protein levels were assessed after drug treatment by RT-qPCR and JESS assay respectively. Anti-tumor activity of RGT-61159 as single agent or in combination was evaluated in a panel of ACC PDX mouse models harboring different molecular alterations in c-MYB, including ACCX6, ACCX11, ACCx5M1 and ST105B2. Results: Here, we showed that RGT-61159 has potent cancer cell killing activity (EC50 – 100nM -200nM) against a large panel of cancer cell lines overexpressing c-MYB, while sparing normal cells. Robust c-MYB RNA and c-MYB protein depletion (>80%) was observed in treated cells confirming RGT-61159 on-target cell killing effect. In addition, RGT-61159 single agent showed a remarkable anti-tumor activity (up to 90% TGI) at tolerated doses in the four ACC PDX models evaluated: ACCX6, ACCx11, ACCx5M1 and STB105B2. Importantly, we confirmed that RGT-61159 anti-tumor activity correlated with c-MYB target modulation. RGT-61159 efficacious doses reduced c-MYB transcript and c-MYB protein levels (by >80%) in all ACC PDx models evaluated. In addition, RNAseq analysis of ACC tumor after treatment showed that RGT-61159 drug treatment reversed the published ACC overexpression gene signature. Notably, the optimal RGT-61159 regimen was well tolerated, as assessed by changes in animal body weight and clinical signs in rodents and non-human primate. Finally, we showed that the combination of RGT-M001, an analog of RGT-61159, with the NOTCH Inhibitor AL-101 resulted in significant tumor regression at well tolerated doses. Conclusions: Altogether, these data demonstrated that RGT-61159 efficiently and safely targets the undruggable oncogenic transcription factor c-MYB. Down-regulation of c-MYB by the RGT-61159 RNA-targeting small molecule is a very promising therapeutic strategy to treat ACC and other cancers driven by c-MYB dysregulation.

Abstract 3306: RGT-M001, a first-in-class small molecule mRNA degrader of the oncogenic transcription factor c-Myb, demonstrated remarkable single agent anti-tumor efficacy in cancer patient-derived xenograft model

Abstract Dysregulated transcription factors (TFs) represent a unique class of drug targets that drive aberrant gene expression programs involved in the major hallmarks of cancers. However, developing drug-like small molecules targeting TF proteins for clinical purposes has proven to be extremely challenging. Here, we present a novel and innovative approach to degrade the RNA transcript of the master oncogenic transcription factor C-MYB, thereby preventing its undesirable expression, using potent and orally available small molecules. C-MYB is a transcription factor that regulates differentiation and proliferation programs in normal cells, including hematopoietic, stem and epithelial cells. In numerous cancer types, such as adenoid cystic carcinoma (ACC), leukemia, colorectal cancer, and breast cancer, C-MYB is a well-established oncogenic transcription factor, overactivated via different mechanisms, including chromosomal translocations and gene amplification. Here, we leveraged our integrative RNA-targeting platform to identify and validate actionable cryptic exon target sites in the human C-MYB gene. We identified RGT-M001, a potent small molecule that can selectively induce the inclusion of the cryptic exon into the final C-MYB transcripts, resulting in a robust decrease of C-MYB canonical transcripts and C-MYB protein in cells. In a functional assay, we demonstrated that RGT-M001 has potent cell-killing activity against a large panel of cancer cell lines overexpressing C-MYB (EC50 ~ 20 - 300nM), while sparing normal cells. To confirm RGT-M001's on-target effect, we demonstrated a robust correlation between RGT-M001 cell-killing activity and the knockdown of C-MYB RNA and protein. We further investigated the anti-tumor activity of RGT-M001 in an ACC patient-derived xenograft (PDx) mouse model harboring C-MYB-NFBI fusion. Recurrent or metastatic ACC is a malignant neoplasm of predominantly salivary gland origin for which effective approved therapies are lacking; the best reported ORR being 15.6% for lenvatinib. As a single agent, RGT-M001 reduced in vivo C-MYB transcript levels by >80% at peak drug exposure and induced a remarkable tumor growth inhibition response (~70% TGI) in the ACCX11 PDx mouse model, surpassing the therapeutic benchmark Lenvatinib (40% TGI). Importantly, the RGT-M001 regimen was well tolerated. Finally, we showed that the combination of RGT-M001 with the Notch Inhibitor AL-101 resulted in complete inhibition of tumor growth. In conclusion, these data demonstrate that small molecules targeting RNA are a safe and effective approach to address previously undruggable protein targets. Down-regulation of C-MYB by our RNA-targeting small molecules is an attractive therapeutic strategy to treat ACC and other cancers driven by C-MYB dysregulations. Citation Format: Norman Lu, Patricia Soulard, Kay Li, Heather Sadlish, Chris Yates, Xiubin Gu, Ibrahim Kays, Jae Lee, Sam Hasson, Zhiping Weng, Simon Xi, Travis Wager. RGT-M001, a first-in-class small molecule mRNA degrader of the oncogenic transcription factor c-Myb, demonstrated remarkable single agent anti-tumor efficacy in cancer patient-derived xenograft model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3306.

Estrogens revert neutrophil hyperplasia by inhibiting Hif1α-cMyb pathway in zebrafish myelodysplastic syndromes models

Myelodysplastic syndromes (MDS) are characterized by daunting genetic heterogeneity and a high risk of leukemic transformation, which presents great challenges for clinical treatment. To identify new chemicals for MDS, we screened a panel of FDA-approved drugs and verified the neutrophil hyperplasia inhibiting role of 17β-estradiol (E2, a natural estrogen) in several zebrafish MDS models (pu.1G242D/G242D, irf8Δ57Δ/57 and c-mybhyper). However, the protective mechanism of estrogen in the development of hematological malignancies remains to be explored. Here, analyzing the role of E2 in the development of each hematopoietic lineage, we found that E2 exhibited a specific neutrophil inhibiting function. This neutrophil inhibitory function of E2 is attributed to its down-regulation of c-myb, which leads to accelerated apoptosis and decreased proliferation of neutrophils. We further showed that knockdown of hif1α could mimic the neutrophil inhibiting role of E2, and hif1α overexpression could reverse the protective function of E2. Collectively, our findings highlight the protective role of E2 on MDS by inhibiting hif1α-c-myb pathway, suggesting that E2 is a promising and effective drug for hematopoietic tumors associated with abnormal neutrophil hyperplasia.

Osteogenic Potential of the Transcription Factor c-MYB.

The transcription factor c-MYB is a well-known marker of undifferentiated cells such as haematopoietic cell precursors, but recently it has also been observed in differentiated cells that produce hard tissues. Our previous findings showed the presence of c-MYB in intramembranous bones and its involvement in the chondrogenic steps of endochondral ossification, where the up-regulation of early chondrogenic markers after c-myb overexpression was observed. Since we previously detected c-MYB in osteoblasts, we aimed to analyse the localisation of c-MYB during later stages of endochondral bone formation and address its function during bone matrix production. c-MYB-positive cells were found in the chondro-osseous junction zone in osteoblasts of trabecular bone as well as deeper in the zone of ossification in cells of spongy bone. To experimentally evaluate the osteogenic potential of c-MYB during endochondral bone formation, micromasses derived from embryonic mouse limb buds were established. Nuclear c-MYB protein expression was observed in long-term micromasses, especially in the areas around nodules. c-myb overexpression induced the expression of osteogenic-related genes such as Bmp2, Comp, Csf2 and Itgb1. Moreover, alizarin red staining and osteocalcin labelling promoted mineralised matrix production in c-myb-overexpressing cultures, whereas downregulation of c-myb by siRNA reduced mineralised matrix production. In conclusion, c-Myb plays a role in the osteogenesis of long bones by inducing osteogenic genes and causing the enhancement of mineral matrix production. This action of the transcription factor c-Myb might be of interest in the future for the establishment of novel approaches to tissue regeneration.

MiR34a Contributes to Defective Erythropoiesis in RPS19 Insufficient Hematopoietic Stem Cells

Abstract 2404Diamond-Blackfan Anemia (DBA) is characterized by defective erythropoiesis, congenital abnormalities, and predisposition to cancer. Approximately 25% of DBA patients have a defect in the RPS19 gene, which encodes a ribosomal protein on the 40S ribosomal subunit. Previous studies in zebrafish and mice have shown that knockdown of RPS19 is correlated with the upregulation of p53, a tumor suppressor gene that regulates cell cycle and apoptosis. However, the molecular pathways that link ribosome dysfunction and defective erythropoiesis remain unknown.Among the targets of p53 is microRNA34a (miR34a). Overexpression of miR34a in human myeloid leukemia K562 cells inhibits cell proliferation, leads to cell cycle arrest at the G1 stage, and promotes differentiation toward the megakaryocytic lineage in a p53-independent manner. MiR34a downregulates numerous proteins, including c-Myb, which is essential for normal erythropoiesis and has been found to be expressed at lower levels in bone marrow cells from DBA patients. We hypothesize that miR34a induction downstream of p53 activation in RPS19 insufficient normal hematopoietic progenitor cells contributes to defective erythropoiesis observed in DBA patients.To study the effects of RPS19 insufficiency and downstream signaling pathways, we transduced primary CD34+ fetal liver cells with two different shRNA constructs against RPS19. Knockdown was confirmed by Western blot analysis and by qRT-PCR (73%, p< 0.0001 for shRNA1, and 87%, p< 0.0001 for shRNA2, respectively), compared with a scrambled control shRNA. Cells were sorted for a GFP marker 72 hours post-transduction and either collected for downstream analysis or propagated further in methylcellulose.Our results demonstrated that knockdown of RPS19 in primary human CD34+ fetal liver cells leads to an increase in p53 protein by Western blot analysis, and upregulation of miR34a mRNA (3-fold, p<0.01). This upregulation correlates with downregulation of miR34a target genes including c-Myb (p<0.01) and c-Myc (p<0.01). Induction of miR34a and downregulation of c-Myb and c-Myc, occurs through a p53-dependent pathway since the expression of those genes was not altered in p53-null K562 cells transduced with RPS19 shRNA lentivirus. To correlate molecular pathways with the differentiation potential of RPS19 insufficient CD34+ fetal liver cells, methylcellulose colony assays were performed. Total colony formation was reduced by 87% (p<0.01) in cells treated with RPS19 shRNA compared to scrambled control shRNA. Erythroid colony formation in methylcellulose containing IL-3, SCF, GM-CSF, and EPO, decreased by 90% (p<0.01) in shRNA transduced cells compared to control shRNA. FACS analysis for erythroid markers CD71 and Glycophorin A showed a similar pattern, with the CD71+GlyA+ population representing 2.6% of RPS19 shRNA transduced cells, compared to 9.9% in cells transduced with the scrambled control. Preliminary data with lymphoblastoid cell lines (LCLs) from DBA patients with RPS19 insufficiency demonstrate 2-fold upregulation of miR34a. Therefore, we conclude that RPS19 insufficiency in human CD34+ fetal liver cells leads to p53-dependent upregulation of miR34a, down-regulation of miR34a target genes c-Myc and c-Myb, and defects in erythroid differentiation. Disclosures:No relevant conflicts of interest to declare.

Role of miR‐150‐targeting c‐Myb in colonic epithelial disruption during dextran sulphate sodium‐induced murine experimental colitis and human ulcerative colitis

Chronic inflammatory bowel diseases (IBDs) are associated with differential expression of genes involved in inflammation and tissue remodelling. We surveyed the expression profile of apoptosis-related microRNAs by real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) in a dextran sulphate sodium (DSS) murine model of colitis. We found that miR-150 was strongly elevated, whereas c-Myb, a transcription factor and a target gene of miR-150, was significantly reduced in colon tissue after DSS treatment. Interestingly, elevation of miR-150 and down-regulation of c-Myb were also observed in human colon with active ulcerative colitis compared to the normal colon. Supporting the observation of DSS treatment inducing colonic cell apoptosis, Bcl-2, an anti-apoptotic protein known to be regulated by c-Myb, was reduced in colon tissue of DSS-treated mice. Furthermore, forced expression of pre-miR-150 in colonic epithelial HT29 cells strongly elevated miR-150 levels and decreased c-Myb and Bcl-2 levels, thus enhancing cell apoptosis induced by serum deprivation. Together, the present study presents the first evidence that miR-150 and its targeting of c-Myb may serve as a new mechanism underlying the colonic epithelial disruption in DSS-induced murine experimental colitis and in active human IBD.

Specific PAF antagonist WEB-2086 induces terminal differentiation of murine and human leukemia cells.

A pharmacological approach to neoplasia by differentiation therapy relies on the availability of cytodifferentiating agents whose antitumor efficacy is usually assayed first on malignant cells in vitro. Using murine erythroleukemia cells (MELCs) as the model, we found that WEB-2086, a triazolobenzodiazepine-derived PAF antagonist originally developed as an anti-inflammatory drug, induces a dose-dependent inhibition of MELC growth and hemoglobin accumulation as a result of a true commitment to differentiation. MELCs treated for 5 days with 1 mM WEB-2086 show greater than or equal to 85% benzidine-positive cells, increased expression of alpha- and beta-globin genes, and down-regulation of c-Myb. This differentiation pattern, which does not involve histone H4 acetylation and is abrogated by the action of phorbol 12-myristate 13-acetate, recalls the pattern induced by hexamethylene bisacetamide (HMBA). In addition to MELCs, human erythroleukemia K562 and HEL and myeloid HL60 cells are massively committed to maturation by WEB-2086 and, with some differences, by its analog, WEB-2170. This suggests that WEB-2086, structurally distant from other known inducers, might be a member of a new class of cytodifferentiation agents active on a broad range of transformed cells in vitro and useful, prospectively, for anticancer therapy due to their high tolerability in vivo.

C-Jun is Induced to High Continuous Expression During Differentiation of Hematopoietic Cells and is Regulated Independently from C-Fos.

The c-jun proto-oncogene is usually expressed at a low level in most cell types but is induced rapidly and transiently in response to stimuli. The present results show that stimulation of U-937 cells to macrophage differentiation by a phorbol ester or vitamin D3 induced a continuous high level expression of c-jun mRNA and protein. High level expression of c-jun mRNA was also observed in various other human differentiated hematopoietic cells, represented by normal macrophages, HL-60 promyelocytic cells differentiated towards macrophages or granulocytes and in myeloma, but not in immature cell lines. These results might suggest a role of c-jun in the differentiation of several hematopoietic lineages. In U-937, a differential regulation of c-jun and c-fos was demonstrated both with respect to kinetics and to the level of regulation. Furthermore, in subclones of U-937, where maturation and growth arrest could be separated, c-jun was induced in the absence of c-fos and vice versa. Expression of c-jun and c-fos mRNA correlated with maturation and growth arrest, respectively, in these clones. cAMP generating agents, which induced c-jun and c-fos, did, however, not induce terminal differentiation, indicating that other signals, e.g. those associated with down-regulation of c-myb and c-myc, are also required.

Expression of c-fos, c-myb, and c-myc in human monocytes: correlation with monocytic differentiation

Terminal differentiation of human monocytic leukemia cells (THP-1 cells) was associated with the induction of c-fos, the down regulation of c-myb, and no significant change in the level of c-myc expression. Gamma interferon, which resulted in a slight decrease in c-myb but no change in c-fos or c-myc expression, had a transient antiproliferative effect without a morphological or functional differentiation of THP-1 cells. Resting human peripheral blood monocytes have a high c-fos, a low c-myc, and no detectable c-myb expression. These findings suggest that a switch in c-fos/c-myb expression is associated with the terminal differentiation of cells of the monocytic lineage.

Expression of c-fos, c-myb, and c-myc in Human Monocytes: Correlation With Monocytic Differentiation

Terminal differentiation of human monocytic leukemia cells (THP-1 cells) was associated with the induction of c-fos, the down regulation of c-myb, and no significant change in the level of c-myc expression. Gamma interferon, which resulted in a slight decrease in c-myb but no change in c-fos or c-myc expression, had a transient antiproliferative effect without a morphological or functional differentiation of THP-1 cells. Resting human peripheral blood monocytes have a high c-fos, a low c-myc, and no detectable c-myb expression. These findings suggest that a switch in c-fos/c-myb expression is associated with the terminal differentiation of cells of the monocytic lineage.