The Ten-eleven translocation (Tet) proteins (including Tet1/2/3) have been recently identified as critical regulators in epigenetic regulation through converting 5-methylcytosine (5mC) to 5- hydroxymethylcytosine (5hmC), leading to active or passive DNA demethylation. Although TET1, the founding member of the TET gene family, was first identified as a fusion partner of the mixed lineage leukemia (MLL) gene in acute myeloid leukemia (AML), its pathological role in leukemia is unknown. In contrast to the down-regulation and potential tumor suppressor roles of all three TET genes reported in various solid tumors and the frequent loss-of-function mutations of TET2 in hematopoietic malignancies, we recently found that TET1 (but not TET2 /3) was significantly up-regulated in MLL-rearranged AML;MLL fusions bind to the promoter region of TET1 and promote its expression directly, which is associated with an increased level of 5hmC. Depletion of Tet1 expression by small hairpin RNAs (shRNAs) or genetic knockout significantly inhibits MLL-AF9-mediated cell transformation in vitro and leukemogenesis in vivo. In addition, knockdown of TET1 expression by small interfering RNA (siRNA) oligos significantly decreases viability/growth and increases apoptosis of human MLL- AF9 leukemic cells. Furthermore, we showed that several essential downstream direct targets of MLL fusions, such as HOXA9, MEIS1, and PBX3 that have been shown to be critical for the induction/maintenance and leukemia stem cells (LSCs) self-renewal of MLL-rearranged AML, are also direct target genes of TET1. Hypothesis: TET1 plays an essential oncogenic role in the pathogenesis of MLL-rearranged leukemia through promoting expression of a group of oncogenic target genes and repressing expression of a group of tumor- suppressor target genes, and thus TET1 is a viable therapeutic target in treating MLL-rearranged leukemia.
Specific Aims : 1) To determine the essential role of Tet1 in both development and maintenance of MLL- rearranged leukemia as well as in the self-renewal of relevant leukemia stem cells (LSCs);2) To identify critical target genes of Tet1 in MLL-rearranged leukemic cells. Study Design: 1) First, we will perform in vitro binding and transcriptional studies to validate that TET1 is a direct target of all five major MLL-fusions (i.e, MLL-AF9, -AF6, -AF10, -ELL and -ENL). Next, we will use Tet1 knockout/knockdown models coupled with MLL-fusion bone marrow transplantation models to determine the essential role of Tet1 in both development and maintenance of all five major subtypes of MLL-rearranged AML and in the self-renewal of relevant LSCs. We will also use xeno-transplantation models to determine whether targeting TET1 is a potential therapeutic approach to treat MLL-rearranged AML (Aim 1). 2) We will perform Chip-Seq, single-base resolution 5hmC/5mC-Seq, and RNA-Seq to identify critical potential targets of Tet1, followed by the validation and functional studis of a group of selected targets of Tet1 in MLL-rearranged leukemia in order to elucidate the underlying molecular mechanisms (Aim 2).
Around 10% of acute leukemias, including ~80% of all infant acute leukemia, carrying a chromosomal rearrangement involving the MLL (mixed lineage leukemia) gene located on human chromosome 11, are usually associated with poor survival and the majority of patients cannot be cured with contemporary treatment;thus, better understanding of the molecular mechanisms underlying the pathogenesis of MLL- rearranged leukemia, and development of effective novel therapeutic strategies based on such understanding, are urgently needed. The major goal of this proposal is to determine whether expression of TET1, a gene involving genomic DNA modification and relevant epigenetic gene regulation, is required for both development and maintenance of MLL-rearranged leukemia and for the self-renewal of relevant leukemia stem cells (LSCs), whether targeting TET1 is a potential therapeutic approach to treat MLL-rearranged leukemia, and what genes (including both protein-coding genes and microRNAs) are critical downstream direct targets of TET1 in MLL- rearranged leukemia. The success of this project will not only shed new light on our understanding of the pathological role of TET1 (an epigenetic regulator) and of the molecular mechanisms underlying the development, maintenance, and LSC self-renewal of MLL-rearranged leukemia, but also may lead to the ultimate development of specific and effective novel therapeutic approaches to treat this presently therapy- resistant disease through targeting of TET1 and/or its critical downstream target genes.