The binding of immunomodulatory drugs (IMiDs), including lenalidomide, to CRBN is associated with cytotoxicity of IMiDs used in the treatment of multiple myeloma, myelodysplastic syndromes (MDS) and lymphomas. Cereblon (CRBN) is named for its putative role in cerebral development, especially in memory and learning. CRBN forms an E3 ubiquitin ligase complex with damaged DNA binding protein 1 (DDB1), cullin-4 (CUL4) and regulator of cullin 1 (ROC1). This complex regulates DNA repair, DNA replication and transcription. CRBN is a primary target of thalidomide teratogenicity (Ito et al., Science 2010; 327:1345-1350). Down-regulation of the CRBN expression is associated with the development of marked IMiDs resistance in human multiple myeloma cells (Zhu et al., Blood 2011; 118: 4771-4779; Lopez Girona et al., Leukemia 2012; 26: 2326-2335; Heintel et al., Br J Haematol 2013; 161: 695-700; Broyl et al., Blood 2013; 121: 624-627; Lodé et al., Br J Haematol 2013; Jul 17. doi: 10.1111/bjh.12478). CRBN expression is thus required for the antimyeloma activity of IMiDs.
To gain insight into the role of CRBN in lower risk myelodysplastic syndromes with or without 5q deletion and into the mechanisms of lenalidomide action, we study CRBN expression in these two groups of lower risk MDS patients and in healthy controls. We further study the expression of DDB1 and IRF4 (interferon regulatory factor 4, one of target genes of CRBN).
Mononuclear cells were isolated both from bone marrow [23 low risk MDS patients with 5q- deletion, 37 low-risk MDS patients with normal chromosome 5 (non5q-) and 24 healthy controls] and from peripheral blood [38 patients with 5q- , 52 non 5q- patients and 25 healthy controls] by Ficoll-Paque PLUS gradient separation, and washed with phosphate-buffered saline. The rest of the red cells were lysed. Total RNA was isolated using RNA isolation solvent and complementary DNA was synthesized from total RNA using SuperScript II reverse transcriptase. Relative levels of the CRBN, DDB1 and IRF4 mRNAs were determined by TaqMan-based quantitative real-time PCR and by calculation to the level of housekeeping glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA. The experiments were performed in duplicate. Informed consent was obtained from all patients and healthy controls. Evaluation of 2-ddCt indicates the fold change in gene expression relative to the control.
The median of CRBN mRNA levels in total RNA isolated from peripheral blood mononuclear cells was the highest (3.6) in lower risk MDS with 5q deletion, lower (1.5) in non5q- MDS and lowest (1.2) in healthy controls. Similar results were obtained in bone marrow mononuclear cells (medians 3.3 in 5q- syndrome, 2.2 in non5q- MDS and 1.3 in healthy controls). The differences between the 3 groups were statistically significant (p˂0.05, Mann-Whitney test) with the exception of non5q- MDS in comparison with healthy controls in mononuclear cells of peripheral blood. Very similar results were obtained in the analysis of DDB1 and IRF4 mRNAs levels, which correlated with CRBN levels. We analyzed CRBN mRNA levels before and in the course of the lenalidomide treatment of 5q- low risk MDS (6 patients). We obtained high CRBN mRNA levels before and during the treatment by lenalidomide in all lenalidomide responders (4 patients). We found a sharp decrease of CRBN mRNA in mononuclear cells of bone marrow in two patients who stopped responding to lenalidomide therapy and subsequently progressed to higher risk MDS. A/G polymorphism located at -29 nt of the 5x UTR of CRBN does not act as a biomarker of response to treatment with lenalidomide in 5q- syndrome.
5q- low risk MDS patients have the highest levels of CRBN mRNA in comparison to lower risk myelodysplastic syndromes with normal chromosome 5 and to healthy controls. DDB1 and IRF4 mRNAs levels correlate with CRBN levels. High levels of CRBN mRNA were detected in all lenalidomide responders during the course of the therapy. Significant decrease of CRBN levels during treatment by lenalidomide is associated with loss of response to treatment and disease progression. Similarly to the treatment of multiple myeloma, these results suggest that high levels of CRBN mRNA are necessary for the efficacy of lenalidomide in low risk 5q- patients.
Supported by the MH CR grant NT/13836-4/2012, PRVOUK-27/LF1/2 and by the MH CR project for development of research institute (UHKT).
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.