Tyrosine phosphorylation of CARM1 promotes its enzymatic activity and alters its target specificity.

An important epigenetic component of tyrosine kinase signaling is the phosphorylation of histones, and epigenetic readers, writers, and erasers. Phosphorylation of protein arginine methyltransferases (PRMTs), have been shown to enhance and impair their enzymatic activity. In this study, we show that the hyperactivation of Janus kinase 2 (JAK2) by the V617F mutation phosphorylates tyrosine residues (Y149 and Y334) in coactivator-associated arginine methyltransferase 1 (CARM1), an important target in hematologic malignancies, increasing its methyltransferase activity and altering its target specificity. While non-phosphorylatable CARM1 methylates some established substrates (e.g. BAF155 and PABP1), only phospho-CARM1 methylates the RUNX1 transcription factor, on R223 and R319. Furthermore, cells expressing non-phosphorylatable CARM1 have impaired cell-cycle progression and increased apoptosis, compared to cells expressing phosphorylatable, wild-type CARM1, with reduced expression of genes associated with G2/M cell cycle progression and anti-apoptosis. The presence of the JAK2-V617F mutant kinase renders acute myeloid leukemia (AML) cells less sensitive to CARM1 inhibition, and we show that the dual targeting of JAK2 and CARM1 is more effective than monotherapy in AML cells expressing phospho-CARM1. Thus, the phosphorylation of CARM1 by hyperactivated JAK2 regulates its methyltransferase activity, helps select its substrates, and is required for the maximal proliferation of malignant myeloid cells.

CARM1 Is Essential for Myeloid Leukemogenesis but Dispensable for Normal Hematopoiesis.

Chromatin-modifying enzymes, and specifically the protein arginine methyltransferases (PRMTs), have emerged as important targets in cancer. Here, we investigated the role of CARM1 in normal and malignant hematopoiesis. Using conditional knockout mice, we show that loss of CARM1 has little effect on normal hematopoiesis. Strikingly, knockout of Carm1 abrogates both the initiation and maintenance of acute myeloid leukemia (AML) driven by oncogenic transcription factors. We show that CARM1 knockdown impairs cell-cycle progression, promotes myeloid differentiation, and ultimately induces apoptosis. Finally, we utilize a selective, small-molecule inhibitor of CARM1 to validate the efficacy of CARM1 inhibition in leukemia cells in vitro and in vivo. Collectively, this work suggests that targeting CARM1 may be an effective therapeutic strategy for AML.

Tet2 Regulates Osteoclast Differentiation by Interacting with Runx1 and Maintaining Genomic 5-Hydroxymethylcytosine (5hmC).

As a dioxygenase, Ten-Eleven Translocation 2 (TET2) catalyzes subsequent steps of 5-methylcytosine (5mC) oxidation. TET2 plays a critical role in the self-renewal, proliferation, and differentiation of hematopoietic stem cells, but its impact on mature hematopoietic cells is not well-characterized. Here we show that Tet2 plays an essential role in osteoclastogenesis. Deletion of Tet2 impairs the differentiation of osteoclast precursor cells (macrophages) and their maturation into bone-resorbing osteoclasts in vitro. Furthermore, Tet2-/- mice exhibit mild osteopetrosis, accompanied by decreased number of osteoclasts in vivo. Tet2 loss in macrophages results in the altered expression of a set of genes implicated in osteoclast differentiation, such as Cebpa, Mafb, and Nfkbiz. Tet2 deletion also leads to a genome-wide alteration in the level of 5-hydroxymethylcytosine (5hmC) and altered expression of a specific subset of macrophage genes associated with osteoclast differentiation. Furthermore, Tet2 interacts with Runx1 and negatively modulates its transcriptional activity. Our studies demonstrate a novel molecular mechanism controlling osteoclast differentiation and function by Tet2, that is, through interactions with Runx1 and the maintenance of genomic 5hmC. Targeting Tet2 and its pathway could be a potential therapeutic strategy for the prevention and treatment of abnormal bone mass caused by the deregulation of osteoclast activities.

Histone-binding of DPF2 mediates its repressive role in myeloid differentiation.

Double plant homeodomain finger 2 (DPF2) is a highly evolutionarily conserved member of the d4 protein family that is ubiquitously expressed in human tissues and was recently shown to inhibit the myeloid differentiation of hematopoietic stem/progenitor and acute myelogenous leukemia cells. Here, we present the crystal structure of the tandem plant homeodomain finger domain of human DPF2 at 1.6-Å resolution. We show that DPF2 interacts with the acetylated tails of both histones 3 and 4 via bipartite binding pockets on the DPF2 surface. Blocking these interactions through targeted mutagenesis of DPF2 abolishes its recruitment to target chromatin regions as well as its ability to prevent myeloid differentiation in vivo. Our findings suggest that the histone binding of DPF2 plays an important regulatory role in the transcriptional program that drives myeloid differentiation.

Caspase-3 controls AML1-ETO-driven leukemogenesis via autophagy modulation in a ULK1-dependent manner.

AML1-ETO (AE), a fusion oncoprotein generated by t(8;21), can trigger acute myeloid leukemia (AML) in collaboration with mutations including c-Kit, ASXL1/2, FLT3, N-RAS, and K-RAS. Caspase-3, a key executor among its family, plays multiple roles in cellular processes, including hematopoietic development and leukemia progression. Caspase-3 was revealed to directly cleave AE in vitro, suggesting that AE may accumulate in a Caspase-3-compromised background and thereby accelerate leukemogenesis. Therefore, we developed a Caspase-3 knockout genetic mouse model of AML and found that loss of Caspase-3 actually delayed AML1-ETO9a (AE9a)-driven leukemogenesis, indicating that Caspase-3 may play distinct roles in the initiation and/or progression of AML. We report here that loss of Caspase-3 triggers a conserved, adaptive mechanism, namely autophagy (or macroautophagy), which acts to limit AE9a-driven leukemia. Furthermore, we identify ULK1 as a novel substrate of Caspase-3 and show that upregulation of ULK1 drives autophagy initiation in leukemia cells and that inhibition of ULK1 can rescue the phenotype induced by Caspase-3 deletion in vitro and in vivo. Collectively, these data highlight Caspase-3 as an important regulator of autophagy in AML and demonstrate that the balance and selectivity between its substrates can dictate the pace of disease.

All-trans retinoic acid synergizes with FLT3 inhibition to eliminate FLT3/ITD+ leukemia stem cells in vitro and in vivo.

FMS-like tyrosine kinase 3 (FLT3)-mutant acute myeloid leukemia (AML) portends a poor prognosis, and ineffective targeting of the leukemic stem cell (LSC) population remains one of several obstacles in treating this disease. All-trans retinoic acid (ATRA) has been used in several clinical trials for the treatment of nonpromyelocytic AML with limited clinical activity observed. FLT3 tyrosine kinase inhibitors (TKIs) used as monotherapy also achieve limited clinical responses and are thus far unable to affect cure rates in AML patients. We explored the efficacy of combining ATRA and FLT3 TKIs to eliminate FLT3/internal tandem duplication (ITD)(+) LSCs. Our studies reveal highly synergistic drug activity, preferentially inducing apoptosis in FLT3/ITD(+) cell lines and patient samples. Colony-forming unit assays further demonstrate decreased clonogenicity of FLT3/ITD(+) cells upon treatment with ATRA and TKI. Most importantly, the drug combination depletes FLT3/ITD(+) LSCs in a genetic mouse model of AML, and prolongs survival of leukemic mice. Furthermore, engraftment of primary FLT3/ITD(+) patient samples is reduced in mice following treatment with FLT3 TKI and ATRA in combination, with evidence of cellular differentiation occurring in vivo. Mechanistically, we provide evidence that the synergism of ATRA and FLT3 TKIs is at least in part due to the observation that FLT3 TKI treatment upregulates the antiapoptotic protein Bcl6, limiting the drug's apoptotic effect. However, cotreatment with ATRA reduces Bcl6 expression to baseline levels through suppression of interleukin-6 receptor signaling. These studies provide evidence of the potential of this drug combination to eliminate FLT3/ITD(+) LSCs and reduce the rate of relapse in AML patients with FLT3 mutations.

Arginine methyltransferases in normal and malignant hematopoiesis.

Arginine methylation is an abundant covalent modification that regulates diverse cellular processes, including transcription, translation, DNA repair, and RNA processing. The enzymes that catalyze these marks are known as the protein arginine methyltransferases (PRMTs), and they can generate asymmetric dimethyl arginine (type I arginine methyltransferases), symmetric dimethylarginine (type II arginine methyltransferases), or monomethyarginine (type III arginine methyltransferases). The PRMTs are capable of modifying diverse substrates, from histone components to specific nuclear and cytoplasmic proteins. Additionally, the PRMTs can orchestrate chromatin remodeling by blocking the docking of other epigenetic modifying enzymes or by recruiting them to specific gene loci. In the hematopoietic system, PRMTs can regulate cell behavior, including the critical balance between stem cell self-renewal and differentiation, in at least two critical ways, via (i) the covalent modification of transcription factors and (ii) the regulation of histone modifications at promoters critical to cell fate determination. Given these important functions, it is not surprising that these processes are altered in hematopoietic malignancies, such as acute myeloid leukemia, where they promote increased self-renewal and impair hematopoietic stem and progenitor cell differentiation.

Effect of FLT3 ligand on survival and disease phenotype in murine models harboring a FLT3 internal tandem duplication mutation.

Many of the mutations contributing to leukemogenesis in acute myeloid leukemia have been identified. A common activating mutation is an internal tandem duplication (ITD) mutation in the FLT3 gene that is found in approximately 25% of patients and confers a poor prognosis. FLT3 inhibitors have been developed and have some efficacy, but patients often relapse. Levels of FLT3 ligand (FL) are significantly elevated in patients during chemotherapy and may be an important component contributing to relapse. We used a mouse model to investigate the possible effect of FL expression on leukemogenesis involving FLT3-ITD mutations in an in vivo system. FLT3(ITD/ITD) FL(-/-) (knockout) mice had a statistically significant increase in survival compared with FLT3(ITD/ITD) FL(+/+) (wildtype) mice, most of which developed a fatal myeloproliferative neoplasm. These findings suggest that FL levels may have prognostic significance in human patients. We also studied the effect of FL expression on survival in a FLT3-ITD NUP98-HOX13 (NHD13) fusion mouse model. These mice develop an aggressive leukemia with short latency. We asked whether FL expression played a similar role in this context. The NUP98-HOX13 FLT3(ITD/wt) FL(-/-) mice did not have a survival advantage, compared with NUP98-HOX13 FLT3(ITD/wt) FL(+/+) mice (normal FL levels). The loss of the survival advantage of the FL knockout group in the NUP98-HOX13 model suggests that adding a second mutation changes the effect of FL expression in the context of more aggressive disease.