BCOR loss promotes both retinoblastoma growth and susceptibility to IGF1R inhibition

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BACKGROUND: BCL-6 corepressor (BCOR) loss-of-function alterations are common in clinically aggressive retinoblastoma. The study aim was to determine if BCOR loss promotes the growth and dissemination of retinoblastoma cells, and identify the pathways it regulates in these retinal tumors of childhood. METHODS: Gain- and loss-of-function strategies were used to modulate BCOR levels in a panel of retinoblastoma cell lines, and the effects on proliferation, clonogenicity, apoptosis, and migration were assessed in vitro and in murine xenograft models. RESULTS: BCOR knockdown or knockout in retinoblastoma lines with high protein levels increased tumor growth, invasion, clonogenicity, and chemoresistance in vitro, while increased expression in low BCOR lines slowed growth. Growth of retinoblastoma xenografts was similarly sensitive to BCOR gain or loss. BCOR reduction resulted in upregulation of IGF1 and activation of IGF1 receptor (IGF1R) signaling, and the effects of IGF1R inhibition were dependent on BCOR level. In vitro, reduction of retinoblastoma growth and induction of apoptosis by the IGF1R inhibitors linsitinib and AEW541 were also significantly stronger in cells with low BCOR as compared to controls. Both linsitinib and AEW541 suppressed colony formation in a dose-dependent manner in BCOR knockout or knockdown cells. Finally, high BCOR levels rendered retinoblastoma xenografts insensitive to linsitinib. CONCLUSIONS: Loss of BCOR function is associated with more aggressive retinoblastoma cell line growth and chemoresistance, at least in part due to increased IGF1R signaling. Inhibiting IGF1R pharmacologically had a marked anti-tumor effect in aggressive retinoblastoma lacking BCOR, suggesting it as a new therapeutic target, although this still needs to be confirmed in clinical samples with BCOR mutations.

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