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Capicua ( CIC )-rearranged sarcomas are an aggressive subset of undifferentiated round cell sarcomas. CIC::DUX4, the proto-typical CIC fusion oncoprotein is associated with rapid clinical progression and chemotherapy resistance leading to poor clinical outcomes. Recent studies have identified additional CIC fusions (CIC::NUTM1, CIC::FOXO4, and CIC::LEUTX) that largely retain CIC-binding specificity but leverage C-terminal binding partners (NUTM1, FOXO4, and LEUTX) to potentially activate transcriptional programs that drive oncogenesis. Moreover, the recent development of preclinical models to study CIC::DUX4 sarcoma have advanced our understanding of the underlying biological mechanisms and uncovered key dependencies that can be translated into rational therapies. In this review, we will highlight these recent advancements in CIC -rearranged sarcoma biology with a vision for clinical translation to improve patient outcomes.

CIC-DUX4 rearrangements define an aggressive and chemotherapy-insensitive subset of undifferentiated sarcomas. The CIC-DUX4 fusion drives oncogenesis through direct transcriptional upregulation of cell cycle and DNA replication genes. Notably, CIC-DUX4-mediated CCNE1 upregulation compromises the G1/S transition to confer a dependence on the G2/M cell cycle checkpoint. Through an integrative transcriptional and kinase activity screen using patient-derived specimens, we now show that CIC-DUX4 sarcomas depend on the G2/M checkpoint regulator WEE1 as part of an adaptive survival mechanism. Specifically, CIC-DUX4 sarcomas depended on WEE1 activity to limit DNA damage and unscheduled mitotic entry. Consequently, genetic or pharmacologic WEE1 inhibition in vitro and in vivo led to rapid DNA damage-associated apoptotic induction of patient-derived CIC-DUX4 sarcomas. Thus, we identified WEE1 as a vulnerability targetable by therapeutic intervention in CIC-DUX4 sarcomas.

Transcription factor fusions (TFFs) are present in ∼30% of softtissue sarcomas. TFFs are not readily “druggable” in a direct pharmacologic manner and thus have proven difficult to target in the clinic. A prime example is the CIC-DUX4 oncoprotein, which fuses Capicua (CIC) to the double homeobox 4 gene, DUX4. CIC-DUX4 sarcoma is a highly aggressive and lethal subtype of small round cell sarcoma found predominantly in adolescents and young adults. To identify new therapeutic targets in CIC-DUX4 sarcoma, we performed chromatin immunoprecipitation sequencing analysis using patient-derived CIC-DUX4 cells. We uncovered multiple CIC-DUX4 targets that negatively regulate MAPK-ERK signaling. Mechanistically, CIC-DUX4 transcriptionally up-regulates these negative regulators of MAPK to dampen ERK activity, leading to sustained CIC-DUX4 expression. Genetic and pharmacologic MAPKERK activation through DUSP6 inhibition leads to CIC-DUX4 degradation and apoptotic induction. Collectively, we reveal a mechanism-based approach to therapeutically degrade the CIC-DUX4 oncoprotein and provide a precision-based strategy to combat this lethal cancer.

Transcription factor fusion genes create oncoproteins that drive oncogenesis and represent challenging therapeutic targets. Understanding the molecular targets by which such fusion oncoproteins promote malignancy offers an approach to develop rational treatment strategies to improve clinical outcomes. Capicua-double homeobox 4 (CIC-DUX4) is a transcription factor fusion oncoprotein that defines certain undifferentiated round cell sarcomas with high metastatic propensity and poor clinical outcomes. The molecular targets regulated by the CIC-DUX4 oncoprotein that promote this aggressive malignancy remain largely unknown. We demonstrated that increased expression of ETS variant 4 (ETV4) and cyclin E1 (CCNE1) occurs via neomorphic, direct effects of CIC-DUX4 and drives tumor metastasis and survival, respectively. We uncovered a molecular dependence on the CCNE-CDK2 cell cycle complex that renders CIC-DUX4-expressing tumors sensitive to inhibition of the CCNE-CDK2 complex, suggesting a therapeutic strategy for CIC-DUX4-expressing tumors. Our findings highlight a paradigm of functional diversification of transcriptional repertoires controlled by a genetically aberrant transcriptional regulator, with therapeutic implications.

Clinical divergence between patients harboring -rearrangements is frequently observed. For example, the prototypical fusion associates with soft tissue tumors while fusions typically localize to the CNS (brain/spinal cord). The basis for these differences is poorly understood due to a lack of molecular tools. To address this need, we generated patient-informed, synthetic coding sequences for , , and and validated them in structure-function studies. We found that CIC::NUTM1 drives a transcriptional program distinct from that of CIC::DUX4 due to a C-terminal NUTM1 functional domain, CIC::LEUTX weakly activates CIC target genes through LEUTX transactivation sequences, and ATXN1::DUX4 upregulates CIC target genes via the ATXN1 AXH domain. Our findings indicate that the fusion binding partner may alter overall fusion oncoprotein activity. Thus, these first generation synthetic tools provide an unprecedented resource to study -family fusions beyond and allow for the dissection of this rare subgroup of cancers.

CIC-DUX4 is a rare and understudied transcription factor fusion oncoprotein. CIC-DUX4 co-opts native gene targets to drive a lethal form of human sarcoma. The molecular underpinnings that lead to oncogenic reprograming and CIC-DUX4 sarcomagenesis remain largely undefined. Through an integrative ChIP and RNA-Seq analysis using patient-derived CIC-DUX4 cells, we define CIC-DUX4 mediated chromatin states and function. We show that CIC-DUX4 primarily localizes to proximal and distal cis-regulatory elements where it associates with active histone marks. Our findings nominate key signaling pathways and molecular targets that enable CIC-DUX4 to mediate tumor cell survival. Collectively, our data demonstrate how the CIC-DUX4 fusion oncoprotein impacts chromatin state and transcriptional responses to drive an oncogenic program in undifferentiated sarcoma. CIC-DUX4 sarcoma is a rare and lethal sarcoma that affects children, adolescent young adults, and adults. CIC-DUX4 sarcoma is associated with rapid metastatic dissemination and relative insensitivity to chemotherapy. There are no current standard-of-care therapies for CIC-DUX4 sarcoma leading to universally poor outcomes for patients. Through a deep mechanistic understanding of how the CIC-DUX4 fusion oncoprotein reprograms chromatin state and function, we aim to improve outcomes for CIC-DUX4 patients.

CIC-DUX4 rearrangements define an aggressive and chemotherapy-insensitive subset of undifferentiated sarcomas. The CIC-DUX4 fusion drives oncogenesis through direct transcriptional upregulation of cell cycle and DNA replication genes. Notably, CIC-DUX4- mediated CCNE1 upregulation compromises the G1/S transition, conferring a potential survival dependence on the G2/M cell cycle checkpoint. Through an integrative transcriptional and kinase activity screen using patient-derived specimens, we now show that CIC-DUX4 sarcomas depend on the G2/M checkpoint regulator, WEE1, as an adaptive survival mechanism. Specifically, CIC-DUX4 sarcomas depend on WEE1 activity to limit DNA damage and unscheduled mitotic entry. Consequently, genetic or pharmacologic WEE1 inhibition in vitro and in vivo leads to rapid DNA damage-associated apoptotic induction of patient-derived CIC-DUX4 sarcomas. Thus, we identify WEE1 as an actionable therapeutic vulnerability in CIC-DUX4 sarcomas.

The dysregulation of ETS family transcription factors drives human prostate cancer. The majority of prostate cancer is the result of chromosomal rearrangements that lead to aberrant ETS gene expression. The mechanisms that lead to fusion independent ETS factor upregulation and prostate oncogenesis remain unknown. Here, we show that two neighboring transcription factors, Capicua (CIC) and ETS2 repressor factor (ERF), which are co-deleted in human prostate tumors can drive prostate oncogenesis. Concurrent CIC and ERF loss commonly occurs through focal genomic deletions at chromosome 19q13.2. Mechanistically, CIC and ERF co-bind the proximal regulatory element and mutually repress the ETS transcription factor, ETV1. Targeting ETV1 in CIC and ERF deficient prostate cancer limits tumor growth. Thus, we have uncovered a fusion independent mode of ETS transcriptional activation defined by concurrent loss of CIC and ERF. Competing Interest Statement The authors have declared no competing interest.