BCL6 Interacting Corepressor (BCOR) Functions As Lineage-Specific Tumor Suppressor in B Lymphoid and Myeloid Leukemia
Authors: Shojaee Seyedmehdi , Geng Huimin , Gearhart Micah , Bardwell Vivian , Muschen Markus 
Published: October 2019

Abstract
Abstract 1301

Background:
BCL6-interacting corepressor (BCOR) has been first characterized as a BCL6-binding protein and co-repressor of BCL6 along with NCOR and SMRT. The main function of BCOR is to potentiate the transcriptional repression by BCL6. BCOR is also capable of binding with different histone deacetylases(HDACs), demethylase and H2A ubiquitin ligase which suggests chromatin modification as a mechanism of BCL6-independent transcriptional repression by BCOR. Using next generation sequencing, it has been shown that BCOR is recurrently mutated in myelodysplastic syndrome (MDS; Tiacci et al. Blood 2012) and acute myeloid leukemia (AML; Yoshida et al. Nature 2011). BCOR mutations in AML are typically associated with lack or low expression of a truncated BCOR protein which suggest a tumor-suppressor role for BCOR in MDS and AML. Recently we have shown that BCOR's main ligand, BCL6 promotes drug-resistance and survival in B cell lineage Ph+ALL and CML (Duy et al., Nature 2011). Considering the interactive function of BCL6 and BCOR we hypothesized that BCOR, although a tumor suppressor in MDS and AML, acts in concert with BCL6 to promote survival and drug-resistance in Ph+ ALL and CML.

Results:
To study the role of BCOR in a genetic experiment, we developed a CML and Ph+ ALL leukemia model based on bone marrow progenitor cells from male mice carring a loxP-flanked allele of Bcor (Bcorfl/Y) and mice in which BCOR overexpression can be induced by excision of a loxP-flanked Stop-cassette (BcorLSL). In Bcorfl/Ymice, exon 3 of Bcor is flanked by loxP sites and in BcorLSL mice an extra allele of Bcor has been inserted into Rosa26 locus with a stop codon flanked by loxP sites upstream of the start codon. On the basis of this model, bone marrow hematopoietic stem and progenitor cells from Bcorfl/Y and BcorLSL mice were transformed with BCR-ABL1 under B lymphoid (IL7; Ph+ ALL) and stem/progenitor cell (IL3, IL6, SCF; CML) conditions. Cre-mediated deletion of loxP sequences causes the deletion of Bcor in Bcorfl/Y cells and the over-expression of Bcor in BcorR26cells. Transduction of the ALL and CML cells with tamoxifen (4-OHT)-inducible Cre-ERT2 or ERT2 empty vectors were used to inducibly activate Cre in Ph+ ALL- and CML-like leukemia cells. Inducible deletion of Bcor in B cell lineage (CD19+ B220+) Ph+ ALL results in massive cell death. Surprisingly, deletion of Bcor had no significant effect on CML-like (LSK and myeloid phenotype) cells. Gain-of-function studies with the BcorLSL system revealed that inducible overexpression of BCOR significantly increases the ability of Ph+ ALL-like leukemia cells to form colonies in semi-solid methylcellulose culture, whereas inducible activation of BCOR in BcorLSL CML-like cells had the opposite effect. Comparing the basal level of Bcor between Ph+ ALL and CML shows that there is significantly higher amount of Bcor in Ph+ ALL as compared to CML. In vivo injection and Cre-mediated deletion of BCOR in Bcorfl/YPh+ ALL shows that the Bcor is required to initiate B cell lineage Ph+ ALL in vivo and cause fatal disease (p=0.001). Five out of 7 mice, injected with Bcorfl/-Ph+ ALL survive with no sign of tumor growth until the end of experiment (240 days after injection) while all mice injected with wild type cells died within the first 15 days. In addition, we tested the impact of Bcor deletion and Bcor overexpression on TKI-sensitivity of Ph+ ALL and CML-like leukemia cells. While Bcor-deletion dramatically sensitized Ph+ALL cells to TKI-treatment (e.g. Imatinib), no effects on TKI-sensitivity were observed in CML-like leukemia.

Conclusion:
Our genetic studies demonstrate an unexpected lineage-specific effect of BCOR function in B cell lineage Ph+ ALL and myeloid lineage CML. While both leukemia types are induced by BCR-ABL1, BCOR function was critical for the survival and leukemia-initiation of Ph+ ALL, but not CML. The findings in B cell lineage Ph+ ALL are consistent with our hypothesis that BCOR mediates TKI-resistance and survival in Ph+ ALL in concert with BCL6. Our findings in CML support a role of BCOR as tumor suppressor in myeloid leukemia as was observed for MDS and AML. In myeloid leukemias, BCOR activity may predominantly involve HDACs and, hence, BCL6-independent pathways of transcriptional repression.

Disclosures:
No relevant conflicts of interest to declare.