TY  - GEN
ID  - heidok35218
CY  - Heidelberg
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/35218/
N2  - Sarcomas are a heterogeneous group of mesenchymal tumours arising in bone and soft tissue with a higher incidence in children and young adults. Owing to sarcomas? molecular diversity and lack of available experimental models, progress in treatment has stagnated in the past 5 decades. Many sarcomas arise due to translocations that fuse transcriptional and epigenetic regulators, resulting in characteristic transcriptional landscapes. A commonly affected target is BCL6 co-repressor (BCOR), a member of the non-canonical Polycomb repressive complex 1.1 (ncPRC1.1) which promotes gene silencing by ubiquitinating lysine 119 of histone H2A (H2AK119ub). ncPRC1.1 is crucial in development and differentiation. In sarcomas, BCOR fuses to recurrent partners, leading to remarkably overlapping transcription and DNA methylation patterns, indicative of a convergent pathological mechanism. However, this mechanism is still not understood, and we do not know how these mutations affect the role of BCOR within ncPRC1.1.
I modelled two oncofusions, BCOR::CCNB3 and ZC3H7B::BCOR, in immortalized human mesenchymal stem cells by simultaneous induction of the fusions and silencing of the endogenous BCOR gene. RNA sequencing (RNA-seq) revealed drastic transcriptional remodeling which mimicked a patient-derived BCOR sarcoma transcriptional signature, and upregulation of many PRC targets. Canonically, assembly of ncPRC1.1 is initiated by binding of BCOR and PCGF1 to the targeting subunit KDM2B. Despite drastically remodelling the transcriptional landscape, I found that the BCOR oncofusions are retained at KDM2B-marked ncPRC1.1 sites genome-wide. Using co-immunoprecipitation (co-IP) and an imaging-based in vitro chromatin recruitment assay, I showed that the BCOR fusions interact with PCGF1 and KDM2B and participate in active ncPRC1.1 complexes that deposit H2AK119ub, ruling out a loss-of-function mechanism. To investigate how, then, these oncofusions were contributing to the sarcoma transcriptional signature, I analyzed their interactomes by co-IP coupled to mass spectrometry (co-IP-MS). Thus, I identified novel interactors gained by the BCOR fusions, including many transcriptional activators and cofactors. Shared between the two fusions were the histone acetyltransferases EP300/CREBBP, which were previously identified as fusion partners to BCOR in other cancers. EP300/CREBBP recruitment to the BCOR fusions was mediated by the fusion partners CCNB3 and ZC3H7B. To confirm their role in transcriptional regulation in sarcomas, I developed a sarcoma patient-derived cell line with a BCOR::CCNB3 alteration (BC3). In the BC3 cells, EP300 and H3K27ac co-occupied BCOR::CCNB3 genome loci. EP300 inhibition and CRISPR/Cas9 knock out of BCOR::CCNB3 both led to downregulation of PRC target gene sets. Despite the transcriptional effect, EP300 inhibition in the BC3 cells did not lead to a proliferative defect. 
This work demonstrated that BCOR::CCNB3 and ZC3H7B::BCOR recruit transcriptional activators that lead to transcriptional rewiring by a gain-of-function mechanism.  EP300/CREBBP, which were identified amongst the top interactors, were found to contribute to the characteristic transcriptional profiles. Together, this work reveals key mechanistic insights into the role of fusion proteins in sarcomas. These findings lay the groundwork for future studies to elucidate how these altered transcriptional programs contribute to carcinogenesis in patients and provide mechanistic rationale for novel therapeutic targets.
Y1  - 2025///
A1  - Pejkovska, Anastasija
TI  - Epigenetic Deregulation by BCOR Oncogenic Fusions in Sarcomas
AV  - restricted
ER  -