在急性早幼粒细胞白血病（APL）中，早幼粒细胞白血病-视黄酸受体α（PML/RARα）融合蛋白破坏PML核体（NB），导致微斑点的形成。然而，我们的理解主要来自形态学观察，缺乏对 PML/RARα 介导的微斑点形成背后的机制及其在 APL 白血病发生中的作用的深入了解。这项研究提供了证据，揭示了液-液相分离 (LLPS) 作为 PML/RARα 介导的微斑点形成的关键机制。包含大部分 PML 和一小部分 RARα 的本质无序区域促进了这一过程。我们证明了 PML/RARα 介导的缩合物中含有溴结构域的蛋白 4 (BRD4) 的共组装，这与野生型 PML 形成的 NB 不同。在不存在 PML/RARα 的情况下，PML NB 和 BRD4 斑点作为两个独立的相存在，但 PML/RARα 的存在会破坏 PML NB 并将 PML 和 BRD4 重新分配到不同的相中，形成 PML/RARα 组装的微斑点。全基因组分析揭示了 PML/RARα 诱导的 BRD4 在整个基因组中的重新分布，并优先结合超级增强子和广泛启动子 (SEBP)。从机制上讲，BRD4 被 PML/RARα 招募到核凝聚物中，促进 BRD4 染色质结合，发挥 APL 存活所必需的转录激活作用。通过化学抑制（1, 6-己二醇）扰乱 LLPS 可显着降低 PML/RARα 和 BRD4 的染色质共占用，从而减弱其靶基因激活。最后，在原发性 APL 患者样本中进行的一系列实验验证证实，PML/RARα 通过凝聚物形成微斑点，招募 BRD4 来共同组装凝聚物，并共同占据 SEBP 区域。我们的研究结果阐明了 PML/RARα 组装微斑点的生物物理、病理和转录动力学，强调了 BRD4 在介导转录激活（使 PML/RARα 启动 APL）中的重要性。

In acute promyelocytic leukemia (APL), the promyelocytic leukemia-retinoic acid receptor alpha (PML/RARα) fusion protein destroys PML nuclear bodies (NBs), leading to the formation of microspeckles. However, our understanding, largely learned from morphological observations, lacks insight into the mechanisms behind PML/RARα-mediated microspeckle formation and its role in APL leukemogenesis. This study presents evidence uncovering liquid-liquid phase separation (LLPS) as a key mechanism in the formation of PML/RARα-mediated microspeckles. This process is facilitated by the intrinsically disordered region containing a large portion of PML and a smaller segment of RARα. We demonstrate the coassembly of bromodomain-containing protein 4 (BRD4) within PML/RARα-mediated condensates, differing from wild-type PML-formed NBs. In the absence of PML/RARα, PML NBs and BRD4 puncta exist as two independent phases, but the presence of PML/RARα disrupts PML NBs and redistributes PML and BRD4 into a distinct phase, forming PML/RARα-assembled microspeckles. Genome-wide profiling reveals a PML/RARα-induced BRD4 redistribution across the genome, with preferential binding to super-enhancers and broad-promoters (SEBPs). Mechanistically, BRD4 is recruited by PML/RARα into nuclear condensates, facilitating BRD4 chromatin binding to exert transcriptional activation essential for APL survival. Perturbing LLPS through chemical inhibition (1, 6-hexanediol) significantly reduces chromatin co-occupancy of PML/RARα and BRD4, attenuating their target gene activation. Finally, a series of experimental validations in primary APL patient samples confirm that PML/RARα forms microspeckles through condensates, recruits BRD4 to coassemble condensates, and co-occupies SEBP regions. Our findings elucidate the biophysical, pathological, and transcriptional dynamics of PML/RARα-assembled microspeckles, underscoring the importance of BRD4 in mediating transcriptional activation that enables PML/RARα to initiate APL.