A dual role for PSIP1/LEDGF in T-cell acute lymphoblastic leukemia [CUT&RUN]

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy. Although intensified therapeutic protocols have improved the outcome of T-ALL patients, they coincide with severe short- and long-term side effects. In addition, no salvage therapeutic strategies are available for primary therapy-resistant or relapsed T-ALL, resulting in a dismal outcome for these patients. It highlights the need to identify new targets in T-ALL biology that allow the development of less toxic targeted therapies. PSIP1, a histone mark reader, is a dependency factor in KMT2A-rearranged myeloid leukemia, but is dispensable for normal hematopoiesis, making it an attractive therapeutic target. Nonetheless, rare recurrent inactivating mutations and deletions of PSIP1, suggest that PSIP1 could act as a tumor suppressor in T-ALL. Here, we demonstrate that the loss of Psip1 accelerates T-ALL initiation in mice and we identified a correlation with reduced H3K27me3 binding. Contrastingly, loss of PSIP1 impaired cell proliferation in several human and murine T-ALL cell lines. In these cell lines, PSIP1 loss leads to a significant downregulation of COX20, an assembly factor of the cytochrome c oxidase in the mitochondria, and is associated with a reduction in mitochondrial respiration. Similarly to what was observed for PSIP1, loss of COX20 expression also leads to an impairment of proliferation in these T-ALL cell lines. These data corroborate that PSIP1 can exert a dual role in the context of T-ALL, either as a tumor suppressor gene during tumor initiation or as a dependency factor in tumor maintenance. Overall design: For this study, quantitave spike-in controlled CUT&RUN was performed to evaluate alterations in H3K27me3 levels in thymi of healthy mice (42 days old) with and without Psip1 expression. Aditionally to investigate where in the genome PSIP1 binds, CUT&RUN was performed to onthe human T-ALL cell line Jurkat for PSIP1 and H3K36me3.

T细胞急性淋巴细胞白血病（T-cell acute lymphoblastic leukemia, T-ALL）是一种侵袭性血液系统恶性肿瘤。尽管强化治疗方案已改善了T-ALL患者的预后，但同时伴随严重的短期与长期不良反应。此外，针对原发耐药或复发的T-ALL患者，目前尚无挽救性治疗策略，导致此类患者预后极差。这凸显了挖掘T-ALL生物学新靶点的迫切需求，以开发毒性更低的靶向治疗手段。

PSIP1是一种组蛋白标记阅读器（histone mark reader），在KMT2A重排髓系白血病中属于细胞依赖因子，且对正常造血功能非必需，因此是极具潜力的治疗靶点。然而，罕见的复发性失活突变与缺失事件提示，PSIP1在T-ALL中可能发挥肿瘤抑制因子的作用。

本研究证实，Psip1缺失会加速小鼠T-ALL的发生，并发现该现象与H3K27me3结合水平降低存在相关性。与之相反，PSIP1缺失会损害多株人源及鼠源T-ALL细胞系的增殖能力。在上述细胞系中，PSIP1缺失会显著下调COX20的表达——该基因是线粒体细胞色素c氧化酶的组装因子，且与线粒体呼吸作用减弱相关。与PSIP1的表型类似，COX20表达缺失同样会损伤此类T-ALL细胞系的增殖能力。上述数据证实，PSIP1在T-ALL的病理进程中可发挥双重作用：在肿瘤发生阶段作为肿瘤抑制因子，而在肿瘤维持阶段则作为细胞依赖因子。

本研究的实验设计如下：为评估Psip1表达与否对健康小鼠（42日龄）胸腺中H3K27me3水平的影响，我们采用了基于定量spike-in对照的CUT&RUN技术。此外，为探究PSIP1在基因组中的结合位点，我们对人源T-ALL细胞系Jurkat开展了针对PSIP1与H3K36me3的CUT&RUN实验。