An Oncogene-Regulated Epigenetic Switch in T Cell Acute Lymphoblastic Leukemia

Although the cure rate for acute lymphoblastic leukemia (ALL), a frequent pediatric leukemia, has improved dramatically, the overall prognosis remains dismal due to frequent disease relapse and the absence of non-cytotoxic targeted therapy options. Up to 25% of children fail frontline therapy and in these cases prognosis is dismal and the cure rate is approximate 20%. Main current therapies are based on intensive induction chemotherapy that is most frequently coupled to intrathecal chemotherapy alone or with cranial irradiation for central nervous system prophylaxis, which has severe short and long-term side effects. Thus, the ultimate and most critical aim for developing new treatments in different types of leukemia is to block the effects of specific cancer-inducing oncogenes. Others and we have previously shown that T cell ALL (T-ALL) is characterized by activating mutations in the NOTCH signaling pathway.

It is currently unclear how key transcription factors in T-ALL such as NOTCH1 recruit the epigenetic machinery and bring together different chromosomal domain, in order to carry out the oncogenic transformation program. We generated evidence that NOTCH1 oncogenic action leads to important epigenetic changes through antagonizing the polycomb repressive complex 2 (PRC2) and leads to loss of the repressive mark histone 3 lysine 27 di/tri-methylation (H3K27me2/3). Moreover, we identified inactivating mutations of the polycomb repressive complex 2 (PRC2), the “writer” of Histone 3 lysine 27 methylation, in primary samples from human patients revealing a tumor suppressor role for the complex in T-ALL. Further extending our work on the H3K27me3 mark, we showed the oncogenic role for the Jumonji d3 (JMJD3) demethylase. Functionally, genomic ablation of the JMJD3 modulator as well as targeting with a specific chemical inhibitor, GSKJ4, generated by GlaxoSmithKline, leads to apoptosis and cell cycle arrest of T-ALL lines and primary cells. Genetic ablation of JMJD3 leads to slower initiation of the disease with significantly improved survival rates of the mice. Surprisingly, UTX acts as a tumor suppressor in the context of the same disease, as part of different transcriptional complexes, and we found that it is genetically inactivated in T-ALL patients. In light of recent developments on novel epigenetic inhibitors against JMJD3, these findings pave the way to specific pharmacological targeting of T cell leukemia.

Based on this activity of Notch1 oncogene on epigenetic marks we further hypothesized that the switch from physiological to oncogenic activity might be mediated by changes in enhancer-promoter interaction networks forming chromosomal domains. A substantial percentage of these interactions are likely to be specific for the malignant state, and their disruption with epigenetic pharmacological inhibitors would not potentially affect healthy tissues. Studies in our laboratory show for the first time in leukemia that NOTCH1 chromatin binding sites are associated with enhancer-promoter interactions at oncogenic loci, using up-to-date chromosome conformation capture technology. We hereby show the importance of these interactions for oncogenic gene expression and pharmacological targeting of leukemic cells. These findings lend further rationale to the use of epigenetic drugs for targeted treatment of T cell leukemia.