Inducing transcriptional reprogramming of leukemic B-cells by facilitating transcription factors binding to nascent decondensed chromatin

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Project Summary/Abstract B cell acute lymphoblastic leukemia (B-ALL) consist of Leukemic Blast Cells (LBCs) that become arrested at distinct stages of B cell development because of underlying alterations in transcriptional and epigenetic programming. The inability to differentiate confers a capacity for unlimited self-renewal and increased proliferative capability. Based on the successes in the treatment of the APL leukemia with ATRA and arsenic, leading to cell maturation and senescence, the differentiation-based therapies became a popular, but rarely successful, therapeutic strategy. We propose that this strategy has been unsuccessful due to a gap in the knowledge of the underlying mechanism through which transcriptional reprogramming occurs. My new strategy is based on our group’s recent observations that differentiation of normal hematopoietic progenitor cells (HPCs) is dependent on their ability to transiently decondense their post-replicative chromatin upon induction with lineage specifying cytokines. At the early stages of DNA replication, this decondensed state of nascent chromatin offers a window of opportunity for lineage-specific transcription factors (TFs) to overcome the barrier of the condensed structure of nucleosomes at repressed genes and to bind and activate their target sites. Preliminary results indicate that B-ALL LBCs have lost the inherent ability to open nascent chromatin, thus creating a barrier in their transcriptional reprogramming that differentiation-based therapies cannot overcome. In this proposal, to overcome this barrier of condensed nascent chromatin and to reprogram LBCs, I will utilize the following approach. First, I will pharmacologically ablate H3K27me3 to decondense nascent chromatin by inhibiting H3K27me3 histone methyltransferases EZH1/2. Second, I will use small molecules to activate inducible transcription factors, which can then readily bind to their target genes due to the decondensed structure of nascent chromatin. Cancers are commonly known to accumulate mutations in inducible TFs and receptors; thus, screens of small molecule inducers for a variety of TFs/receptors will be performed to determine the best possible inducer for distinct subtypes of B-ALL. Preliminary results provide strong indications that induction by novel small molecule ligands/inducers may lead to transcriptional reprogramming in B-ALL LBCs, which results in an almost complete loss of viability of these cells. This strategy has also shown significant promise in increasing the efficacy of and in decreasing the effective dosages of clinical standard of care drugs, such as steroid hormones. Furthermore, my results indicate that synchronization of B-ALL cells into the entry of the S-phase greatly increase the efficiency of our reprogramming strategy. Overall, our conceptual advance is in that we are not attempting to induce hematopoietic differentiation of LBCs, but rather through decondensing nascent chromatin induce transcriptional reprogramming to aberrant lineages, which may lead to terminal maturation, and thus to senescence, reduced proliferation, and cell death.