CpG island hypermethylation and global genomic hypomethylation are common epigenetic features of cancer cells. Less attention has been focused on histone modifications in cancer cells. We characterized post-translational modifications to histone H4 in a comprehensive panel of normal tissues, cancer cell lines and primary tumors. Using immunodetection, high-performance capillary electrophoresis and mass spectrometry, we found that cancer cells had a loss of monoacetylated and trimethylated forms of histone H4. These changes appeared early and accumulated during the tumorigenic process, as we showed in a mouse model of multistage skin carcinogenesis. The losses occurred predominantly at the acetylated Lys16 and trimethylated Lys20 residues of histone H4 and were associated with the hypomethylation of DNA repetitive sequences, a well-known characteristic of cancer cells. Our data suggest that the global loss of monoacetylation and trimethylation of histone H4 is a common hallmark of human tumor cells.

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.

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.

Here, we examine the reasons why the exquisite sensitivity of APL to ATRA-based differentiation therapy has not been extended to other of AML subtypes. Current differentiation-based combinatorial approaches to target AML will also be analyzed. Finally, we will evaluate the potential of novel strategies, high-throughput screening, and functional genomics to uncover new differentiation-based therapies for AML.