USP7 inhibits FOXO1 activity downstream of the BCR in chronic lymphocytic leukaemia cells to enhance cell survival

Signals emanating from the B cell receptor (BCR) are vital for driving cell survival and proliferation in chronic lymphocytic leukaemia (CLL). Our previous studies demonstrated that BCR-mediated signals promote nuclear exclusion and inactivation of the transcription factor forkhead box protein O1 (FOXO1), and transient upregulation of FOXO1 expression. To investigate the mechanisms that regulate FOXO1 inactivation, we focused on the role of deubiquitinase (DUB) family proteins in CLL cells. Stratifying the expression of individual DUB proteins, we established that ubiquitin specific protease 7 (USP7), USP8, USP9x, USP10 and USP14 are significantly upregulated compared to healthy donor B cells. USP7 interacts with FOXO1 in primary CLL cells suggesting that modulation of USP7 activity may impact on FOXO1 function. Using selective USP7 inhibitors, P5091 and HBX19818, or USP7 knockdown, combined with ibrutinib treatment we demonstrated enhanced FOXO1 nuclear localisation over ibrutinib alone, and increased FOXO1 activity, leading to modulation of selected FOXO target genes. This resulted in reduced CLL cell viability and enhanced cell cycle arrest in MEC1 cells. These findings suggest that dual targeting of BCR signalling and USP7 activity can harness FOXO1 anti-tumour activity and may represent a novel therapeutic approach for the treatment of CLL.