Enhancers act to regulate cell type specific gene expression by facilitating the transcription of target genes. In mammalian cells active or primed enhancers are commonly marked by monomethylation of Histone H3 at lysine 4 (H3K4me1) in a cell-type specific manner. Whether and how this histone modification regulates enhancer-dependent transcription programs in mammals is unclear. In this study, we conducted SILAC Mass-spec experiments with mono-nucleosomes and identified multiple H3K4me1 associated proteins, including many involved in chromatin remodeling. We demonstrate that H3K4me1 augments the association of the chromatin remodeling complex BAF to enhancers in vivo and that in vitro, H3K4me1 nucleosomes are more efficiently remodeled by the BAF complex. Crystal structures of BAF component BAF45c reveal that monomethylation, but not trimethylation, is accommodated by BAF45c’s H3K4 binding site. Our results suggest that H3K4me1 plays an active role at enhancers by facilitating the binding of the BAF complex and possibly other chromatin regulators.

We apologize for an error that we just found in the paper published online on January 9th, 2018. The 2nd column/6th row of Figure 5B (Hoxd13-DKO) was inadvertently duplicated from 2nd column/5th row of Figure 5B (Hoxd13-WT). A corrected version of Figure 5B is provided below. No conclusion was affected by this error, but we apologize for not detecting it before publication.We also regret for the failure to acknowledge the technical support by Ms Rong Hu towards the generation of ChIP-seq of Rad21 in wild type and dCD MLL3/4 mutant mouse ES cells. We would like to add the following statement in the section of Acknowledgments: "We also thank Ms Rong Hu for her technical assistance in ChIPseq experiments."

APTO-253 is a phase I clinical stage small molecule that selectively induces CDKN1A (p21), promotes G-G cell-cycle arrest, and triggers apoptosis in acute myeloid leukemia (AML) cells without producing myelosuppression in various animal species and humans. Differential gene expression analysis identified a pharmacodynamic effect on MYC expression, as well as induction of DNA repair and stress response pathways. APTO-253 was found to elicit a concentration- and time-dependent reduction in MYC mRNA expression and protein levels. Gene ontogeny and structural informatic analyses suggested a mechanism involving G-quadruplex (G4) stabilization. Intracellular pharmacokinetic studies in AML cells revealed that APTO-253 is converted intracellularly from a monomer to a ferrous complex [Fe(253)]. FRET assays demonstrated that both monomeric APTO-253 and Fe(253) stabilize G4 structures from telomeres, MYC, and KIT promoters but do not bind to non-G4 double-stranded DNA. Although APTO-253 exerts a host of mechanistic sequelae, the effect of APTO-253 on MYC expression and its downstream target genes, on cell-cycle arrest, DNA damage, and stress responses can be explained by the action of Fe(253) and APTO-253 on G-quadruplex DNA motifs. .

No abstract

We apologize for an error that we just found in the paper published online on January 9th, 2018. The 2nd column/6th row of Figure 5B (Hoxd13-DKO) was inadvertently duplicated from 2nd column/5th row of Figure 5B (Hoxd13-WT). A corrected version of Figure 5B is provided below. No conclusion was affected by this error, but we apologize for not detecting it before publication.We also regret for the failure to acknowledge the technical support by Ms Rong Hu towards the generation of ChIP-seq of Rad21 in wild type and dCD MLL3/4 mutant mouse ES cells. We would like to add the following statement in the section of Acknowledgments: "We also thank Ms Rong Hu for her technical assistance in ChIPseq experiments."

Four carbonic anhydrase (CA) cDNA clones were isolated from a 48 h dark-grown cotton (Gossypium hirsutum L.) seedling cDNA library. Nucleotide sequence analysis revealed two different CA isoforms designated GhCA1 and GhCA2. The encoded polypeptides possess N-terminal serine/threonine-rich regions indicative of plastid transit peptides, and approximately 80% sequence identity to other plant plastidial beta-CAs. The GhCA1 cDNA encodes a nearly complete preprotein of 323 amino acids with a molecular mass of 34.9 kDa and a predicted mature protein of 224 amino acids with a molecular mass of 24.3 kDa. Eleven nucleotide differences within ORFs of GhCA1 and GhCA2 result in 5 conservative amino acid substitutions. The 3' GhCA2 untranslated region contains five additional substitutions and one single nucleotide addition. GhCA1 clones, nearly full-length or with 70% of the transit peptide deleted, were expressed as LacZ alpha fusion proteins in E. coli. Lysates of these strains contained 9-fold higher levels of CA activity as compared to untransformed controls and this activity was inhibited by CA-specific inhibitors. Sulfanilamide, acetazolamide, ethoxyzolamide, each at 10 mM, inhibited recombinant CA activity approximately 50%, 65%, and 75%, respectively. In plant tissue homogenates these inhibitors reduced CA activity by 50%, 70%, and 95%, respectively. Although CA activity was bighest in extracts of mature cotton leaves, probing total RNA with GhCA1 revealed CA transcript levels to be highest in the cotyledons of dark-grown cotton seedlings. Collectively, our data indicate the presence of a plastid-localized CA in cotyledons of germinated seeds, suggesting a role for CA in postgerminative growth.

The c-Myc multifunctional transcription factor protein, a product on the c-myc proto-oncogene, contributes to the pathogenesis of many types of human cancers through mechanisms of proliferation, apoptosis, cell cycle progression and cellular senescence. c-Myc is frequently overexpressed in acute myeloid leukemia, yet strategies to effectively modulate c-Myc function do not exist. We evaluated inhibition of c-myc gene expression by APTO-253, a small molecule anticancer agent that is being developed clinically for the treatment of acute myelogenous (myeloid) leukemia (AML) and high risk myelodysplastic syndromes (MDS). We first confirmed that c-Myc mRNA level were significantly higher in AML cell lines as compared to peripheral blood mononuclear cells (PBMCs) isolated from healthy human donors. However, the c-Myc expression in AML cells was inhibited by APTO-253 in dose-dependent and time-dependent manners at both the mRNA and protein levels. Likewise, APTO-253 was found to induce AML cell apoptosis in dose-dependent and time-dependent manners as demonstrated by positive Annexin-V staining and increases in cleaved poly (ADP-ribose) polymerase (c-PARP). APTO-253 induced AML cells arrest at G1/G0 phase of cell cycle by increasing p21 expression and decreasing expression of cyclin D3 and cyclin-dependent kinases 4/6 (CDK4/6). For the p53 positive cell lines MV4-11 and EOL-1, p53 was also increased by APTO-253 at early time points (less than 6-hour treatment), suggesting that p53-dependent cell cycle arrest and apoptosis is mechanistically operative as a consequence of treatment with APTO-253. Importantly, we demonstrated that APTO-253 selectively targeted tumor cells but not normal healthy cells, with MV4-11 AML cells and normal PBMCs having IC50s of 0.25±0.03µM and more than 100µM, respectively. Our previous studies (56th ASH abstract #4813) showed that APTO-253 induces the Krüppel-like Factor 4 (KLF4) transcription factor and was effective and well tolerated as a single agent in multiple AML xenograft models without causing bone marrow suppression. Taken together, our results suggested that APTO-253 may serve as an effective and safe agent for AML chemotherapy, and that APTO-253 mechanistically inhibits c-Myc expression in AML cells and subsequently induces cell cycle arrest and apoptosis. Disclosures No relevant conflicts of interest to declare.

APTO-253 is a small molecule with antiproliferative activity against cell lines derived from a wide range of human malignancies. We sought to determine the mechanisms of action and basis for resistance to APTO-253 so as to identify synthetic lethal interactions that can guide combination studies. The cellular pharmacology of APTO-253 was analyzed in Raji lymphoma cells and a subline selected for resistance (Raji/253R). Using LC/MS/ESI analysis, APTO-253 was found to convert intracellularly to a complex containing one molecule of iron and three molecules of APTO-253 [Fe(253)]. The intracellular content of Fe(253) exceeded that of the native drug by approximately 18-fold, and Fe(253) appears to be the most active form. Treatment of cells with APTO-253 caused DNA damage, which led us to ask whether cells deficient in homologous recombination (i.e., loss of BRCA1/2 function) were hypersensitive to this drug. It was found that loss of either BRCA1 or BRCA2 function in multiple isogenic paired cell lines resulted in hypersensitivity to APTO-253 of a magnitude similar to the effects of PARP inhibitors, olaparib. Raji cells selected for 16-fold acquired resistance had 16-fold reduced accumulation of Fe(253) RNA-seq analysis revealed that overexpression of the ABCG2 drug efflux pump is a key mechanism of resistance. ABCG2-overexpressed HEK-293 cells were resistant to APTO-253, and inhibition of ABCG2 reversed resistance to APTO-253 in Raji/253R. APTO-253 joins the limited repertoire of drugs that can exploit defects in homologous recombination and is of particular interest because it does not produce myelosuppression. .