NCOR2 Mutations Are Associated with IMiD Resistance in Human Myeloma Cells

The immunomodulatory drug thalidomide, and its analogs, lenalidomide, and pomalidomide (IMiDs) have significantly changed the treatment paradigm of multiple myeloma (MM). Despite this progress, IMiD resistance develops in the majority of patients resulting in the development of refractory disease. Cereblon (CRBN), a direct target, has been implicated in IMiD resistance. However, alternate mechanisms of IMiD resistance independent of CRBN remain largely unknown.

To understand and study the mechanisms responsible for the development of IMiD resistance, we created lenalidomide-resistant (Len-R) and pomalidomide-resistant (Pom-R) human myeloma MM.1s cell lines, by continuous culture in the presence of lenalidomide or pomalidomide for 3 months. Whole genome sequencing of these 2 resistant cell lines compared with parental MM.1s revealed 172 genes with exonic mutations in both Len-R and Pom-R myeloma cells. Furthermore, a protein-protein interaction (PPI) network was constructed based on Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis. The PPI network demonstrated 8 genes that scored a high degree of protein-protein interaction. Among these genes, we identified NCOR2, a corepressor that negatively regulates gene expression, as a downregulated gene in resistant cell lines.

To study this further, we created NCOR2 knock out MM.1s cell lines using CRISPR/cas9 gene modification. Our data demonstrates that depletion of NCOR2 confers IMiD resistance independent to CRBN. Interestingly, Len-R, Pom-R and NCOR2 knock out MM.1s showed increased MYC protein expression, which is essential for myeloma cell survival and proliferation. A BET inhibitor, known to disrupt the binding of BRD4 to chromatin, inhibited the proliferation of Len-R and Pom-R and NCOR2 knock out MM.1s by completely suppressing MYC expression. These results indicate that NCOR2 down regulation in IMiD resistant cells induces MYC upregulation which may in part result in IMiD resistance. Our findings reveal a novel molecular mechanism associated with IMiD resistance, independent of CRBN and suggest that NCOR2-MYC pathway may be a new target for IMiD refractory patients.