Modular Domains Within Super Enhancer Drive T-ALL Drug Resistance

ORLANDO ­– The change in drug sensitivity by tumor cells resistant to Notch or BET inhibitors results from selective use of individual enhancers that are located within the broad Notch regulated region, a study presented at the American Society of Hematology (ASH) 57^th Annual Meeting & Exposition has  shown.¹

Myc is a critical target of Notch, a common oncogenic driver in T-cell acute lymphoblastic leukemia (ALL) that plays roles in T cell development, in normal and malignant pre-T cells. Researchers had identified a distal enhanced located >1 Mb 3′ of human and murine Myc that binds Notch transcription complexes and physically interacts with the Myc proximal promoter.

It was found that deletion of a 1 kb region surrounding this enhancer ultimately resulted in an inhibition of T cell development and blocked Notch induced T-ALL induction and maintenance. This region contains binding sites for various transcription factors, so researchers sought to identify the exact sequences required for function of this distal enhancer by using CRISPR/CAS9 to specifically alter transcription factor binding sites in the Notch-dependent Myc enhancer (NDME).

“We aimed to precisely disrupt Notch1 binding site in T-ALL with CRISPR/CAS9,” Yumi Ohtani, PhD, of the University of Pennsylvania in Philadelphia, PA, said during her presentation. “The problem is that T-ALL cells die without Myc expression, so we needed a system with exogenous Myc expression.”

Results showed that specifically mutating the RBPj binding site, which blocked Notch/RBPj binding and inhibited Myc expression and T-ALL cell development and survival, eliminated H3K27 acetylation peaks. This finding suggests that Notch binding at a single site in the NDME is necessary for enhancer function and H3K27 acetylation peaks across a very broad region of the enhancer.

“The Notch1 binding site is required for acetylation of H3K27 on Myc and the Myc enhancer,” Dr Ohtani noted.

The study also demonstrated that the Notch-regulated distal enhanced is made up of individual enhancers that are sensitive to Notch or BET domain inhibitors.

“Individual enhancers within a broad super enhancer can select for drug resistance,” Dr Ohtani concluded.

Reference

1. Ohtani Y, Xu S, Petrovic J, et al. Modular domains within a super enhancer determine drug resistance in leukemia. Oral presentation at: 57^th American Society of Hematology (ASH) Annual Meeting & Exposition; December 5-8, 2015; Orlando, FL.