The chromosomal translocations found in acute myelogenous leukemia (AML) generate oncogenic fusion transcription factors with aberrant transcriptional regulatory properties. Although therapeutic targeting of most leukemia fusion proteins remains elusive, the posttranslational modifications that control their function could be targetable. We found that AML1-ETO, the fusion protein generated by the t(8;21) translocation, is acetylated by the transcriptional coactivator p300 in leukemia cells isolated from t(8;21) AML patients, and that this acetylation is essential for its self-renewal–promoting effects in human cord blood CD34+ cells and its leukemogenicity in mouse models. Inhibition of p300 abrogates the acetylation of AML1-ETO and impairs its ability to promote leukemic transformation. Thus, lysine acetyltransferases represent a potential therapeutic target in AML.

Exposure to ionizing radiation during childhood markedly increases the risk of developing papillary thyroid cancer. We examined tissues from 26 Ukrainian patients with thyroid cancer who were younger than 10 years of age and living in contaminated areas during the time of the Chernobyl nuclear reactor accident. We identified nonoverlapping somatic driver mutations in all 26 cases through candidate gene assays and next-generation RNA sequencing. We found that 22 tumors harbored fusion oncogenes that arose primarily through intrachromosomal rearrangements. Altogether, 23 of the oncogenic drivers identified in this cohort aberrantly activate MAPK signaling, including the 2 somatic rearrangements resulting in fusion of transcription factor ETS variant 6 (ETV6) with neurotrophic tyrosine kinase receptor, type 3 (NTRK3) and fusion of acylglycerol kinase (AGK) with BRAF. Two other tumors harbored distinct fusions leading to overexpression of the nuclear receptor PPARγ. Fusion oncogenes were less prevalent in tumors from a cohort of children with pediatric thyroid cancers that had not been exposed to radiation but were from the same geographical regions. Radiation-induced thyroid cancers provide a paradigm of tumorigenesis driven by fusion oncogenes that activate MAPK signaling or, less frequently, a PPARγ-driven transcriptional program.

Ch22q LOH is preferentially associated with RAS mutations in papillary and in poorly differentiated thyroid cancer (PDTC). The 22q tumor suppressor NF2, encoding merlin, is implicated in this interaction because of its frequent loss of function in human thyroid cancer cell lines. Nf2 deletion or Hras mutation are insufficient for transformation, whereas their combined disruption leads to murine PDTC with increased MAPK signaling. Merlin loss induces RAS signaling in part through inactivation of Hippo, which activates a YAP-TEAD transcriptional program. We find that the three RAS genes are themselves YAP-TEAD1 transcriptional targets, providing a novel mechanism of promotion of RAS-induced tumorigenesis. Moreover, pharmacological disruption of YAP-TEAD with verteporfin blocks RAS transcription and signaling, and inhibits cell growth. The increased MAPK output generated by NF2 loss in RAS-mutant cancers may inform therapeutic strategies, as it generates greater dependency on the MAPK pathway for viability.

Defining the role of epigenetic regulators in hematopoiesis has become critically important, as recurrent mutations or aberrant expression of these genes has been identified in both myeloid and lymphoid hematological malignancies. We found that PRMT4, a type I arginine methyltransferase, whose function in normal and malignant hematopoiesis is unknown, is overexpressed in AML patient samples. Overexpression of PRMT4 blocks the myeloid differentiation of human stem/progenitor cells (HSPCs) while its knockdown is sufficient to induce myeloid differentiation of HSPCs. We demonstrated that PRMT4 represses the expression of miR-223 in HSPCs via the methylation of RUNX1, which triggers the assembly of a multi-protein repressor complex that includes DPF2. As part of a feedback loop, PRMT4 expression is repressed post-transcriptionally by miR-223. Depletion of PRMT4 results in differentiation of myeloid leukemia cells in vitro and their decrease proliferation in vivo. Thus, targeting PRMT4 holds potential as a novel therapy for acute myelogenous leukemia.

The l3mbtl1 gene is located on the long arm of chromosome 20 (q12), within a region commonly deleted in several myeloid malignancies. L3MBTL1 is a human homolog of the Drosophila polycomb L(3)MBT tumor suppressor protein and thus a candidate tumor suppressor in del(20q12) myeloid disorders. We used the loss-of-function approach to explore the possible tumor suppressive mechanism of L3MBTL1 and found that depletion of L3MBTL1 from human cells causes replicative stress, DNA breaks, activation of the DNA damage response, and genomic instability. L3MBTL1 interacts with Cdc45, MCM2-7 and PCNA, components of the DNA replication machinery, and is required for normal replication fork progression, suggesting that L3MBTL1 causes DNA damage, at least in part, by perturbing DNA replication. An activated DNA damage response and genomic instability are common features in tumorigenesis and a consequence of overexpression of many oncogenes. We propose that the loss of L3MBTL1 contributes to the development of 20q − hematopoietic malignancies by inducing replicative stress, DNA damage, and genomic instability. T he l3mbtl1 gene is located on the long arm of chromosome 20q, within a region on 20q12 commonly deleted in several myeloid malignancies, including myeloproliferative neoplasms, myelodysplastic syndromes, and acute myeloid leukemia (1). It has been proposed that the 20q12 locus contains one or more tumor suppressors, which when lost contribute to the development of these disorders. L3MBTL1 is a human homolog of the Drosophila polycomb group (PcG) protein L(3)MBT. Homozygous mutations of the Drosophila l3mbt gene cause malignant transformation of the adult optic neuroblasts and ganglion mother cells in the larval brain (2). Somatic, focal deletions of other human L3MBTL family members, the l3mbtl2 and l3mbtl3 genes, have recently been found in human medulloblastoma (3). These findings suggest that L3MBTL1 is a candidate tumor suppressor gene in myeloid malignancies associated with 20q12 deletions.The L3MBTL1 protein contains three MBT repeats, which assume a three-bladed propeller-like architecture, as well as a Zn finger and an SPM dimerization domain (4, 5). We previously demonstrated that L3MBTL1 functions as an HDAC-independent transcriptional repressor (6) that binds preferentially to mono-and dimethylated lysines of histones via the second of its three MBT repeats (7,8). The three MBT domains of L3MBTL1 are sufficient to compact nucleosomal arrays. This compaction requires that the nucleosome contain a mono-or dimethylated lysine 26 on histone H1b or lysine 20 on histone H4 (H4K20) (7). L3MBTL1 binds to chromatin most prominently during the S phase of the cell cycle, concomitant with the appearance of the monomethylated H4K20 (H4K20me1) mark (8), suggesting that the biological function of L3MBTL1 may be related to DNA replication.The accurate duplication of DNA during replication is essential for maintaining genomic stability, as uncorrected errors made during this process can lead to DNA breaks, which generate mutat...