BET degraders reveal BRD4 disruption of 7SK and P-TEFb is critical for effective reactivation of latent HIV in CD4+ T-cells.

Unlabelled: HIV cure strategies that aim to induce viral reactivation for immune clearance leverage latency reversal agents to modulate host pathways which directly or indirectly facilitate viral reactivation. Inhibition of bromo and extra-terminal domain (BET) family member BRD4 reverses HIV latency, but enthusiasm for the use of BET inhibitors in HIV cure studies is tempered by concerns over inhibition of other BET family members and dose-limiting toxicities in oncology trials. Here, we evaluated the potential for bivalent chemical degraders targeted to the BET family as alternative latency reversal agents. We observed that despite highly potent and selective BRD4 degradation in primary CD4+ T-cells from ART-suppressed donors, BRD4 degraders failed to induce latency reversal as compared to BET inhibitors. Furthermore, BRD4 degraders failed to mimic previously observed synergistic HIV reactivation between BET inhibitors and an activator of the non-canonical NF-κB pathway. Mechanistic investigation of this discrepancy revealed that latency reversal by BET inhibitors is not related to the abatement of competition between Tat and BRD4 for P-TEFb, but rather the ability of BRD4 to disrupt 7SK and increase the levels of free P-TEFb. This activity is dependent on the shift of BRD4 from chromatin-bound to soluble and retargeting of P-TEFb to chromatin, which is dependent on intact BRD4 but independent of the bromodomains.

Importance: Multiple factors and pathways contribute to the maintenance of HIV latency, including bromo and extra-terminal domain (BET) family member BRD4. While small molecule inhibitors of the BET family result in latency reversal, enthusiasm for the use of BET inhibitors in HIV cure is limited due to toxicity concerns. We examined BRD4-selective chemical degraders as alternatives to BET inhibitors but found two robust degraders failed to induce latency reversal. We observed key differences in the ability of BET inhibitors versus BET degraders to disrupt P-TEFb, a key cellular activator of transcription and a complex required for HIV reactivation. We present a new model for the role of BRD4 in HIV latency and propose that BRD4 be reconsidered as an activator rather than a repressor of HIV transcription in the context of HIV cure strategies.