Targeting the HBZ Viral Oncoprotein Transcriptional Network in Adult T-cell leukemia/lymphoma

Adult T-cell Leukemia/Lymphoma (ATLL) is a frequently incurable disease associated with the human lymphotropic virus type I (HTLV-I). The transcription factor HBZ is the only virally encoded gene that is expressed in all ATLL cases, but it is unclear why it may be essential in ATLL and how it might be targeted therapeutically. Here we performed RNA interference screening of ATLL lines and discovered that their proliferation depends on the transcription factors BATF3 and IRF4, which regulate the identity of normal immune cells. These factors, which are highly expressed in ATLL, cooperatively bind and transactivate genes that distinguish ATLL from other T cell malignancies, including BATF3 itself. HBZ binds to an ATLL-specific BATF3 super-enhancer and thereby regulates the expression of BATF3 and its downstream targets, including the oncogene MYC. The BET protein inhibitor JQ1 collapsed the transcriptional network directed by HBZ and BATF3, and was consequently toxic for ATLL lines and patient samples in vitro, and blocked growth of ATLL xenografts. Our study demonstrates that the HTLV-I virus exploits a regulatory module that can potentially be attacked therapeutically with BET protein inhibitors. For shRNA gene expression profiling, KK1 and ST1 cells were infected with either Ctrl, IRF4, BAFT3_A2, or BAFT3_bp360 shRNAs. Following puromycin selection, shRNA expression was induced for 1 day and 2 days in KK1 cells (n=6), and for 1 day and 2 days in ST1 cells (n=6). For JQ1 gene expression profiling, KK1 (n=4) and ST1 (n=4) cells were treated with either DMSO or JQ1 for 12 hours, 24 hours, 36 hours and 48 hours. For sgRNA gene expression profiling, KK1 and ST1 cells were infected with Ctrl, HBZ_1 or HBZ_2 sgRNAs. KK1 was tranduced by sgHBZ_1 for 7, 8 and 9 days (n=3). KK1 was transduced by sgHBZ_2 for 7 day (n=1). ST1 was tranduced by sgHBZ_1 for 7 and 8 days (n=2). SS1 was transduced by sgHBZ_2 for 7 and 8 days (n=2).