A novel strategy for transcriptional reprogramming of lymphoid leukemia cells

Abstract
Lymphoid B-ALL and TCLL leukemia consists of leukemic blast cells (LBCs) arrested at early stages of
differentiation which exhibit high proliferative potential and capability for self-renewal. The idea to induce
reprogramming of leukemic and other cancer cells, leading to cell maturation and senescence, gained high
popularity, but its clinical applications are rarely successful and are mainly limited to the therapy of the APL
leukemia with ATRA and arsenic trioxide. This strategy may have been unsuccessful due to a gap in the
knowledge of the mechanisms through which transcriptional reprogramming occurs. Our published data
suggest that normal hematopoietic progenitor cells (HPCs) undergo transient de-condensation of chromatin to
allow lineage-specific transcription factors (TFs) to bind to their gene targets and to activate new transcriptional
programs, leading to cell differentiation. This transient de-condensation occurs through very low accumulation
of H3K27me3, on DNA just after DNA replication. H3K27me3 is a mark of the most condensed arrays of
nucleosomes in the genome and is found at regulatory regions of all repressed genes. Our results suggest that
tested cultured and primary lymphoid B-ALL and TCLL cells have lost this inherent ability to ‘open’ nascent
chromatin, thus creating a barrier for their transcriptional reprogramming. In this proposal, we will test a new
reprogramming strategy, which overcomes these barriers of reprogramming-based therapies and may lead to
elimination of leukemic cells. The key feature of this new strategy is the first step, which includes
pharmacological inhibition of the H3K27me3 histone methyltransferases (HMTs) EZH1/EZH2, thus creating de-
condensed structure of nascent chromatin at regulatory regions of all genes. At the second step, we will use
small molecules to activate endogenous inducible TFs, which can then readily bind to their target genes due to
the de-condensed structure of nascent chromatin. Tumor cells, including leukemic cells, 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 suggest that induction by small molecule inducers leads to transcriptional reprogramming of
cell lines and primary B-ALL and TCLL cells, changes in their immunophenotype and apoptosis. Moreover, this
strategy strongly suppresses lymphoid leukemia burden in mice. The goal of this project is to develop a widely
applicable treatment strategy for transcriptional reprogramming and loss of cell viability for many types of
lymphoid leukemic cells. To this end, we propose to: 1. Extend and generalize the lymphoid leukemic cells
reprogramming approach; 2. Examine the mechanisms and biological outcomes of reprogramming of lymphoid
leukemic cells; 3. Examine the effects of our treatment strategy in vivo.

Grant Number: 5R01CA268899-04
NIH Institute/Center: NIH
Principal Investigator: BRUNO CALABRETTA