Expression of CRBN, IKZF1, and IKZF3 does not predict lenalidomide sensitivity and mutations in the cereblon pathway are infrequent in multiple myeloma.

The immunomodulatory drug thalidomide, and its analogs, lenalidomide, and pomalidomide (IMiDs), have become essential components of the standard treatment for multiple myeloma (MM), and have led to significant improvement of survival in patients with this devastating disease. Cereblon (CRBN), the direct target of IMiDs, has been proposed as a predictive biomarker of response to IMiDs. Using standard immunohistochemistry in formalin-fixed paraffin embedded (FFPE) bone marrow samples of 23 patients treated with a lenalidomide-containing regimen, we found that the malignant plasma cells of all the patients stained positive for CRBN, IKZF1, and IKZF3, regardless of sensitivity to IMiDs. Moreover, we detected no mutations in CRBN, IKZF1, IKZF3, CUL4A, or IRF4, but found expanded TP53-mutated clones in two out of seven sequential samples. Thus, our data argue against the use of CRBN and its downstream targets as predictive biomarkers of IMiD response in MM and confirm clonal evolution patterns during lenalidomide resistance.

Dual inhibition of DNMTs and EZH2 can overcome both intrinsic and acquired resistance of myeloma cells to IMiDs in a cereblon-independent manner.

Thalidomide and its derivatives, lenalidomide and pomalidomide (also known as IMiDs), have significantly changed the treatment landscape of multiple myeloma, and the recent discovery of cereblon (CRBN) as their direct biological target has led to a deeper understanding of their complex mechanism of action. In an effort to comprehend the precise mechanisms behind the development of IMiD resistance and examine whether it is potentially reversible, we established lenalidomide-resistant (-LR) and pomalidomide-resistant (-PR) human myeloma cell lines from two IMiD-sensitive cell lines, OPM2 and NCI-H929, by continuous culture in the presence of lenalidomide or pomalidomide for 4-6 months, until acquirement of stable resistance. By assessing genome-wide DNA methylation and chromatin accessibility in these cell lines, we found that acquired IMiD resistance is associated with an increase in genome-wide DNA methylation and an even greater reduction in chromatin accessibility. Transcriptome analysis confirmed that resistant cell lines are mainly characterized by a reduction in gene expression, identifying SMAD3 as a commonly downregulated gene in IMiD-resistant cell lines. Moreover, we show that these changes are potentially reversible, as combination of 5-azacytidine and EPZ-6438 not only restored the observed accessibility changes and the expression of SMAD3, but also resensitized the resistant cells to both lenalidomide and pomalidomide. Interestingly, the resensitization process was independent of CRBN. Our data suggest that simultaneous inhibition of DNA methyl transferases and EZH2 leads to an extensive epigenetic reprogramming which allows myeloma cells to (re)gain sensitivity to IMiDs.