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.

Appropriate hematopoietic stem cell (HSC) self-renewal reflects the tight regulation of cell cycle entry and lineage commitment. Here, we show that Id1, a dominant-negative regulator of E protein transcription factors, maintains HSC self-renewal by preserving the undifferentiated state. Id1-deficient HSCs show increased cell cycling, by BrdU incorporation in vivo, but fail to efficiently selfrenew, leading to low steady-state HSC numbers and premature exhaustion in serial bone marrow transplant assays. The increased cycling reflects the perturbed differentiation process, because Id1 null HSCs more readily commit to myeloid differentiation, with inappropriate expression of myeloerythroid-specific genes. Thus, Id1 appears to regulate the fate of HSCs by acting as a true inhibitor of differentiation.differentiation ͉ transcriptional regulation ͉ cell fate determination H ematopoietic stem cells (HSCs) have the ability to both maintain their own pool (self-renewal) and to constantly generate all mature peripheral blood cell lineages. The selfrenewal capacity of HSCs is clearly sufficient to reconstitute the hematopoietic system upon stress, such as bone marrow transplantation or myeloablative therapy. The functional reserve of HSCs depends on the precise control of cell proliferation and cell fate determination, because either excessive HSC commitment to differentiate or impaired self-renewal divisions can result in HSC exhaustion in experimental models.Decisions regarding self-renewal vs. commitment are based on microenvironmental cues, which predominantly use the Notch, Wnt, and Shh signaling pathways (1-7). However, the critical, cell-intrinsic regulators of HSC commitment are unknown.Here, we demonstrate a unique function of the inhibitor of DNA binding 1 (Id1) dominant-negative basic helix-loop-helix transcription factor in restricting myeloid lineage commitment in HSCs and preserving their self-renewal capacity. Id1 is a member of the family of four proteins (Id1-4) known to inhibit the activity of the E protein bHLH transcription factors by restraining their ability to bind DNA. By modulating the function of transcription factors such as the E2A and Ets1, the Id proteins have been shown to regulate cell proliferation and cell fate determination both in vitro and in vivo (8)(9)(10)(11).We have examined the hematopoietic compartment of Id1 Ϫ/Ϫ mice and found a decreased frequency and diminished selfrenewal capacity of HSCs in Id1-deficient bone marrow. These changes were associated with an increased turnover and accelerated myeloid differentiation of HSCs. Furthermore, Id1 null HSCs show increased expression of key myeloerythroid transcription factors, indicating a critical role of Id1 in the transcriptional repression of myeloid lineage commitment. Thus, Id1-dependent genetic programs are critical in preventing the premature differentiation and exhaustion of HSCs.

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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 t(15;17) translocation in acute promyelocytic leukemia (APL) yields a PML/ RAR-␣ fusion messenger RNA species that can be detected by reverse transcription-polymerase chain reaction (RT-PCR) amplification. Breakpoints within intron 3 of PML produce a short PML/RAR-␣ isoform, whereas breakpoints within intron 6 result in a longer form. Using RT-PCR, serial evaluations were performed on the bone marrow of 82 patients with APL (median follow-up, > 63 months) who received retinoic acid (RA) induction followed by postremission treatment with chemotherapy, RA, and biologic agents.Sixty-four patients attained a clinical complete remission and had at least 2 RT-PCR assays performed after completing therapy. Forty of 47 patients (85%) with newly diagnosed APL who were induced using RA had residual disease detectable by RT-PCR before additional therapy. After 3 cycles of consolidation therapy, residual disease was found in only 4 of 40 evaluable patients (10%). Among newly diagnosed patients who had 2 or more negative RT-PCR assays, only 3 of 41 (7%) had a relapse, whereas all 4 patients (100%) who had 2 or more positive results had a relapse. Among 63 newly diagnosed patients, those who expressed the short isoform appeared to have shorter disease-free and overall survival durations than patients who expressed the long isoform. These data indicate that 2 or more negative RT-PCR assays on bone marrow, performed at least 1 month apart after completing therapy, are strongly associated with long-term remissions.