Therapeutic Targeting of the Histone Ubiquitination-Methylation Axis in T Cell Leukemia

Acute lymphoblastic leukemia (ALL) is a highly aggressive blood cancer affecting children and adults. Certain high-risk disease subsets have poor outcomes and often debilitating therapy-related toxicities stemming from direct inhibition of the oncogenes. We hypothesize that the process of oncogenic transformation is driven by aberrant activity of oncogene-associated chromatin modifying partners. These changes create a chromatin environment unique to the malignant state and, therefore, disruption of critical oncogenic chromatin signatures would not likely affect healthy tissues. We have generated strong evidence for the intertwined roles between the NOTCH1 oncogenic pathway and deubiquitinase enzymes in T cell leukemia, members of the ubiquitin-specific proteases (USP) family in particular. Members of the USP family physically interact with NOTCH1 and the lysine 27 on histone H3 (H3K27) demethylase JMJD3 and this methylation-ubiquitination biology-related axis coordinates regulation of transcriptional initiation and elongation, vital for the survival of leukemia cells. Interestingly transcription of USP genes is positively controlled by NOTCH1 creating a feedback loop in leukemia.

We have further characterized this oncogenic axis using a combination of small molecule inhibitors and genetic engineering of USPs in ALL cell lines, primary patient samples and primagraft models of disease. We are able to show that a) USP activity is important for certain oncogenic pathways (such as NOTCH1) in leukemia; b) Oncogenes and USP enzymes co-bind certain areas in the leukemia genome; c) Ubiquitination of histone H2B acts in a combinatorial fashion with H3K27me, is a major epigenetic change affected by the USP activity in leukemia and controls d) transcriptional elongation. Finally we demonstrate that f) chemical inhibition or down-regulation of USPs affect leukemia growth in vitro and in vivo. Ongoing and future studies include manipulation of USP levels in mouse models of leukemia as well as combinatorial use of USP inhibitors with chemotherapeutic regiments in vitro using matched diagnosis-relapsed primary samples and in xenograft studies.

Information gained from these studies will lend rationale towards the use of small molecule inhibitors against USP proteins in clinical trials for the treatment of aggressive and relapsed ALL.