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Loss of Capicua alters early T cell development and predisposes mice to T cell lymphoblastic leukemia/lymphoma

Significance Capicua (CIC) is a protein that regulates gene transcription, and its dysfunction leads to several neurological diseases. CIC is frequently mutated in several cancers, but mechanistic studies on its tumor suppressor function have been limited. Here, we showed that deletion of Cic in mice causes T cell acute lymphoblastic leukemia/lymphoma (T-ALL) and disrupts early T cell development. We also found that loss of CIC up-regulates the oncogenic RAS program, both before and after the onset of T-ALL. Moreover, we detected activation of the NOTCH1 and MYC transcriptional programs, which we propose cooperate with the RAS pathway to drive tumor development. Our study demonstrates that CIC is a tumor suppressor for lymphoid malignancies and elucidates the tumorigenic events upon loss of CIC. Capicua (CIC) regulates a transcriptional network downstream of the RAS/MAPK signaling cascade. In Drosophila, CIC is important for many developmental processes, including embryonic patterning and specification of wing veins. In humans, CIC has been implicated in neurological diseases, including spinocerebellar ataxia type 1 (SCA1) and a neurodevelopmental syndrome. Additionally, we and others have reported mutations in CIC in several cancers. However, whether CIC is a tumor suppressor remains to be formally tested. In this study, we found that deletion of Cic in adult mice causes T cell acute lymphoblastic leukemia/lymphoma (T-ALL). Using hematopoietic-specific deletion and bone marrow transplantation studies, we show that loss of Cic from hematopoietic cells is sufficient to drive T-ALL. Cic-null tumors show up-regulation of the KRAS pathway as well as activation of the NOTCH1 and MYC transcriptional programs. In sum, we demonstrate that loss of CIC causes T-ALL, establishing it as a tumor suppressor for lymphoid malignancies. Moreover, we show that mouse models lacking CIC in the hematopoietic system are robust models for studying the role of RAS signaling as well as NOTCH1 and MYC transcriptional programs in T-ALL.