A Druggable TCF4 and BRD4 Dependent Transcriptional Network Sustains Malignancy in Blastic Plasmacytoid Dendritic Cell Neoplasm
Authors: Ceribelli Michele , Kelly Priscilla N , Pittaluga Stefania , Thomas Craig J , Reizis Boris , Staudt Louis M. 
Published: October 2019

Abstract
Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive malignancy originating from neoplastic transformation of plasmacytoid dendritic cells (pDCs). Although an initial response to chemotherapy is common, BPDCN prognosis is extremely poor and most patients relapse with drug-resistant disease leading to a median overall survival of ~1 year after diagnosis. In an effort to uncover novel molecular vulnerabilities in BPDCN, we combined RNA interference screening with high-throughput drug screening to identify pathways essential for the proliferation and survival of these cancer cells. This functional/chemical genomic approach allowed us to define a master regulatory network in BPDCN that is amenable to therapeutic attack.
First, the E-box transcription factor TCF4, an important regulator of normal pDCs development, emerged from our RNAi screen as a essential for BPDCN viability. Consistently, expression of TCF4 shRNAs induced a robust and time-dependent apoptotic response, specific for BPDCN cells. We defined the TCF4 regulatory network in BPDCN, by profiling gene expression changes following induction of TCF4 shRNAs and by performing TCF4 ChIP-Seq in BPDCN lines. Notably, several TCF4 targets genes were expressed at higher levels in BPDCN than in normal pDCs, including proto-oncogenes such as BCL2, TCL1A/B and MYC. Conversely, TCF4 targets encoding for important regulators of normal pDC function (BCL11A, SPIB, IL3RA and CLEC4C) were selectively expressed at low level in BPDCN cells, indicating that the pDC-specifying function of TCF4 is attenuated in BPDCN to favor oncogenic gene expression programs. Importantly, TCF4 also served as a faithful diagnostic marker of BPDCN, and TCF4 IHC could readily distinguish BPDCN from BPDCN-mimics such as cutaneous AML.
Second, 3 structurally different bromodomain and extra-terminal domain inhibitors (BETi's) emerged as highly toxic to BPDCN cells from our high-throughput drug screening .This was notable since BET inhibitors block the recognition of acetylated histones by BET proteins, one of which, BRD4, was among the hits from our shRNA toxicity screening. Consistently, expression BRD4 shRNAs or treatment with the BETi JQ1 resulted in time and dose dependent induction of apoptosis in BPDCN cells. Notably, TCF4 itself was strongly downregulated by BETi treatment and ectopic TCF4 expression was sufficient to rescue BPDCN cells from JQ1 toxicity, underscoring the functionally dominant, master regulatory role played by TCF4 in BPDCN. Mechanistically, we showed that the effect of BETi on TCF4 expression is mediated by the presence of a BRD4-dependent super-enhancer (SE) that controls TCF4 expression. Surprisingly, TCF4 itself was bound to its own SE and to the majority of the BRD4-dependent SE we mapped in BPDCN, enforcing a widespread regulatory network that is required for the viability of this cancer.
Altogether, we uncovered novel molecular dependencies and identified TCF4 as a lineage-survival oncogene in BPDCN. More importantly, the TCF4 network in BPDCN can be targeted by BETi. Considering that the BETi CPI 203 was well tolerated in vivo andeffective as a single agent in reducing tumor growth in two BPDCN xenograft models, our findings support the clinical evaluation of BETi in this lethal cancer.

Disclosures
No relevant conflicts of interest to declare.