Acute promyelocytic leukemia (APL), a cytogenetically distinct subtype of acute myeloid leukemia (AML), characterized by the t(15;17)-associated PML-RARA fusion, has been successfully treated with therapy utilizing all-trans-retinoic acid (ATRA) to differentiate leukemic blasts. However, among patients with non- APL AML, ATRA-based treatment has not been effective. Here we show that, through epigenetic reprogramming, inhibitors of lysine- specific demethylase 1 (LSD1, also called KDM1A), including tranylcypromine (TCP), unlocked the ATRA-driven therapeutic response in non-APL AML. LSD1 inhibition did not lead to a large-scale increase in histone 3 Lys4 dimethylation (H3K4me2) across the genome, but it did increase H3K4me2 and expression of myeloid-differentiation–associated genes. Notably, treatment with ATRA plus TCP markedly diminished the engraftment of primary human AML cells in vivo in nonobese diabetic (NOD)- severe combined immunodeficient (SCID) mice, suggesting that ATRA in combination with TCP may target leukemia-initiating cells. Furthermore, initiation of ATRA plus TCP treatment 15 d after engraftment of human AML cells in NOD-SCID γ (with interleukin-2 (IL-2) receptor γ chain deficiency) mice also revealed the ATRA plus TCP drug combination to have a potent anti-leukemic effect that was superior to treatment with either drug alone. These data identify LSD1 as a therapeutic target and strongly suggest that it may contribute to AML pathogenesis by inhibiting the normal pro-differentiative function of ATRA, paving the way for new combinatorial therapies for AML.

SummaryWe undertook a comprehensive clinical and biological investigation of serial medulloblastoma biopsies obtained at diagnosis and relapse. Combined MYC family amplifications and P53 pathway defects commonly emerged at relapse, and all patients in this group died of rapidly progressive disease postrelapse. To study this interaction, we investigated a transgenic model of MYCN-driven medulloblastoma and found spontaneous development of Trp53 inactivating mutations. Abrogation of p53 function in this model produced aggressive tumors that mimicked characteristics of relapsed human tumors with combined P53-MYC dysfunction. Restoration of p53 activity and genetic and therapeutic suppression of MYCN all reduced tumor growth and prolonged survival. Our findings identify P53-MYC interactions at medulloblastoma relapse as biomarkers of clinically aggressive disease that may be targeted therapeutically.

Infant high grade gliomas appear clinically distinct from their counterparts in older children, indicating that histopathologic grading may not accurately reflect the biology of these tumors. We have collected 241 cases under 4 years of age, and carried out histological review, methylation profiling, custom panel and genome/exome sequencing. After excluding tumors representing other established entities or subgroups, we identified 130 cases to be part of an 'intrinsic' spectrum of disease specific to the infant population. These included those with targetable MAP-kinase alterations, and a large proportion of remaining cases harboring gene fusions targeting ALK (n=31), NTRK1/2/3 (n=21), ROS1 (n=9) and MET (n=4) as their driving alterations, with evidence of efficacy of targeted agents in the clinic. These data strongly supports the concept that infant gliomas require a change in diagnostic practice and management.

Histone deacetylases (HDACs) perform an important function in transcriptional regulation by modifying the core histones of the nucleosome. We have now fully characterized a new member of the Class II HDAC family, HDAC9. The enzyme contains a conserved deacetylase domain, represses reporter activity when recruited to a promoter, and utilizes histones H3 and H4 as substrates in vitro and in vivo. HDAC9 is expressed in a tissue-specific pattern that partially overlaps that of HDAC4. Within the human hematopoietic system, expression of HDAC9 is biased toward cells of monocytic and lymphoid lineages. The HDAC9 gene encodes multiple protein isoforms, some of which display distinct cellular localization patterns. For example, full-length HDAC9 is localized in the nucleus, but the isoform lacking the region encoded by exon 7 is in the cytoplasm. HDAC9 interacts and co-localizes in vivo with a number of transcriptional repressors and co-repressors, including TEL and N-CoR, whose functions have been implicated in the pathogenesis of hematological malignancies. These results suggest that HDAC9 plays a role in hematopoiesis; its deregulated expression may be associated with some human cancers.

Histone acetyltransferase (HAT) activities of proteins such as p300, CBP, and P/CAF play important roles in activation of gene expression. We now show that the HAT activity of p300 can also be required for down-regulation of transcription by a DNA binding repressor protein. Promyelocytic leukemia zinc finger (PLZF), originally identified as a fusion with retinoic acid receptor alpha in rare cases of all-trans-retinoic acid-resistant acute promyelocytic leukemia, is a transcriptional repressor that recruits histone deacetylasecontaining corepressor complexes to specific DNA binding sites. PLZF associates with p300 in vivo, and its ability to repress transcription is specifically dependent on HAT activity of p300 and acetylation of lysines in its C-terminal C 2 -H 2 zinc finger motif. An acetylation site mutant of PLZF does not repress transcription and is functionally deficient in a colony suppression assay despite retaining its abilities to interact with corepressor/ histone deacetylase complexes. This is due to the fact that acetylation of PLZF activates its ability to bind specific DNA sequences both in vitro and in vivo. Taken together, our results indicate that a histone deacetylase-dependent transcriptional repressor can be positively regulated through acetylation and point to an unexpected role of a coactivator protein in transcriptional repression.Alterations of chromatin structure by covalent modification of nucleosomal histones at specific lysine residues in their amino-terminal tails play a major role in regulation of gene expression (26). Over the past few years, a large number of chromatin-modifying factors and complexes have been identified and characterized (see references 1, 15, and 27) and references therein for reviews). Intrinsic histone acetyltransferase (HAT) activities have been found to be associated with a number of transcriptional coactivator proteins, such as p300 (62) and P/CAF (79) (see also references 12 and 44 for reviews).The results of these studies have provided an explanation for the direct relationship between histone acetylation and gene transcription. However, in addition to histones, acetylation of gene-specific (see below) and basal transcription factors (36) by specific HATs has also been shown to play a role in regulating gene expression. For example, p53 is acetylated at specific lysine residues by p300 in vitro and in vivo (32). Acetylation of p53 contributes to its activation by facilitating a transition to a conformation with a higher affinity to its target DNA (32). Key hematopoietic transcription factors such as GATA1 (9) and ELKF (83) were also shown to be acetylated in their DNA binding domains leading to enhanced DNA binding and transcriptional activation. Acetylation has also been shown to antagonize the activities of transcriptional repressors by inhibiting their association with corepressor proteins (8,35,82). In the case of hypoxia-inducible factor 1␣ (HIF-1␣), acetylation represented a prerequisite for recruitment of the ubiquitin mediated degradation system ...