Immunomodulatory drugs and their therapeutic effect in hematological malignancies through cereblon

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ISSN: 2398-8495
Hematol Med Oncol, 2017 doi: 10.15761/HMO.1000129
Review Article
Hematology & Medical Oncology
Volume 2(3): 1-8
Immunomodulatory drugs and their therapeutic eect in
hematological malignancies through cereblon
Ota Fuchs1*
1Institute of Hematology and Blood Transfusion, Prague, Czech Republic
Abstract
Immunomodulatory drugs (IMiDs), today also known as cereblon (CRBN) binding drugs, are therapeutically important anti-cancer and anti-inammatory drugs.
IMiDs are analogs of their prototype compound thalidomide. IMiDs have immune-modulation, anti-angiogenic, anti-inammatory and anti-proliferative eects.
CRBN is a component and substrate receptor of the Cullin 4 Ring E3 Ubiquitin Protein Ligase complex (CRL4). CRL4 consists of Cullin 4, RING nger protein
(Roc1), and DNA damage binding protein 1 (DDB1). CRBN binds to its substrate proteins and it leads to ubiquitination of these substrates by the CRL4. CRBN
is also involved in IMiDs-mediated T-cell co-stimulation and cytokine production. CRBN is a primary target of thalidomide teratogenicity. e binding of IMiDs
to CRBN is associated with cytotoxicity of IMiDs and is used to treat multiple myeloma (MM), myelodysplastic syndromes (MDS), lymphomas and chronic
lymphocytic leukemia. CRBN is composed of an N-terminal ATP-dependent serine protease Lon-like domain, which links to the E3 ubiquitin protein ligase
complex CRL4, and a C-terminal domain, which binds IMiDs. CRBN binding is mediated by a glutarimide ring in thalidomide, lenalidomide, pomalidomide,
CC-122, CC-220, CC-885 and CC-90009. Development of eector molecules mediating targeted ubiquitination of disease related proteins through cereblon is a
new important way in pharmacology.
Correspondence to: Ota Fuchs, Institute of Hematology and Blood Transfusion,
Prague, Czech Republic, Tel: +420 221 977 313; E-mail: Ota.Fuchs@uhkt.cz
Key words: immunomodulatory drugs, cereblon, cullin 4 E3 ubiquitin protein
ligase, Ikaros transcription factors famil, casein kinase 1A1, translation termination
factor GSPT1, ubiquitination, proteasome
Received: May 13, 2017; Accepted: May 29, 2017; Published: May 31, 2017
Introduction
IMiDs include thalidomide, lenalidomide, pomalidomide
(Figure 1), CC-122, CC-220, CC-885 and CC-90009 till now.
Both, lenalidomide (Revlimid®, initially known as CC-5013), and
pomalidomide (Pomalyst®, initially known as CC-4047, Actimid) are a
synthetic derivative of thalidomide (alomid®, Inmunoprin, Talidex,
Talizer).
Thalidomide [(RS)-2-(2,6-dioxopiperidin-3-yl)-1H-isoindole-
1,3(2H)-dione] was synthesized in Germany, in 1954, from
α-phtaloylisoglutamine, to be used as a non-barbiturate sedative and
antimetic drug (Contergan). In 1957, aer a short period of preclinical
studies, thalidomide was approved for rst trimester gestational sickness
in humans. e appearance of malformations such as phocomelia in the
newborn banned its use three years later. alidomide was responsible
for birth defects in more than 10,000 children [1,2]. e US Food and
Drug Administration (FDA) approved thalidomide in 1998 for the
treatment of erythema nodosum leprosum [3]. alidomide exhibits
potent antiangiogenic and immunomodulatory eects, and is currently
used around the world to treat a range of conditions, mainly multiple
myeloma. However, long-term use of thalidomide has detrimental side
eects, such as peripheral neuropathy [4,5].
Lenalidomide was developed in order to avoid thalidomide side
eects (sedation and neuropathy), and to increase ecacy [6,7].
Lenalidomide shares a number of structural and biological properties
with thalidomide but is safer and more potent than thalidomide
Lenalidomide [3-(4-amino-1-oxo1,3-dihydro-2H-isoindol-2-yl)
piperidine-2,6-dione] is 4-amino-glutarimide analog of thalidomide
with potent immunomodulatory, antiangiogenic and direct neoplastic
cell inhibitory activity [6-26].
Pomalidomide [4-Amino-2-(2,6-dioxopiperidin-3-yl) isoindole-
1,3-dione] is a potent second-generation IMiD [27-34]. Pomalidomide
has direct antiproliferative, pro-apoptotic, and antiangiogenic eects,
as well as modulatory eects on bone resorption and on the immune
system. Chemical structure of the IMiD drugs is shown in Figure 1.
CC-122 hydrochloride is a novel immunomodulatory agent-like
thalidomide analog which directly binds to CRBN and promotes
ubiquitination and degradation of zinc nger transcription factors
Figure 1. Chemical structures of immunomodulatory drugs (IMiDs) including thalidomide,
lenalidomide and pomalidomide. Lenalidomide and pomalidomide are synthetic compounds
derived by modifying the chemical structure of thalidomide.

Fuchs O (2017) Immunomodulatory drugs and their therapeutic eect in hematological malignancies through cereblon
Volume 2(3): 2-8
Hematol Med Oncol, 2017 doi: 10.15761/HMO.1000129
Aiolos and Ikaros in difuse large B cell-lymphoma (DLBCL) [35-39].
CC-220 is a further IMiD that binds to CRBN and is currently in
phase 1b/2aclinical trials for the treatment of relapsed and refractory
multiple myeloma (MM) and in phase 2 clinical trials for systemic
lupus erythematosus (SLE) [40]. CC-220 binds tighter to the CRBN
in the complex of CRL4 E3 ubiquitin protein ligase than lenalidomide
and causes more potent degradation of transcription factors Aiolos
and Ikaros. CC-885 binds also to CRL4CRBN but mediates not only
degradation of Aiolos but also specically binds to the eukaryotic
translation termination factor 3a (eRF3a), also known as GSPT1 (G1 to
S phase transition 1) and mediates degradation of this protein [41,42].
GSPT1 together with the eukaryotic translation termination factor 1
(eRF1) functions in stop codon recognition and nascent protein release
from ribosome in the process of mRNA translation. Lenalidomide
and pomalidomide do not mediate the degradation of GSPT1. CC-
90009 is a further IMiD that binds the E3 protein ubiquitin ligase
CRL4CRBN and mediates degradation of certain lymphoid transcription
factors, including Ikaros (IKZF1) and Aiolos (IKZF3), which are
transcriptional repressors in T-cells. is reduces the levels of these
transcription factors, and modulates the activity of the immune system,
which may include the activation of T-lymphocytes. In addition, this
downregulates the expression of other proteins, including interferon
regulatory factor 4 (IRF4) and c-myc, which plays a key role in the
proliferation of certain cancer cell types.CC-90009 is now in phase 1
clinical trials (NCT 02848001) in subjects with relapsed or refractory
acute myeloid leukemia (AML) in Canada and USA.
Mechanism of immunomodulatory drugs action in the
treatment of multiple myeloma
IMiDs target both MM cells and their microenvironment, while also
modulating the immune system. e exact molecular mechanisms of
the antitumor eects of IMiDs remain uncertain. IMiDs aect various
molecular and cellular elements within the tumor microenvironment.
IMiDS change the concentration of various cytokines that support
tumor cell growth [8-10,27,43,44]. IMiDs disrupt bone marrow stromal
support for malignant MM cells, although the exact mechanisms of
these actions remain unclear. IMiDs decrease the expression of adhesion
molecules that facilitate the interaction between MM cells and bone
marrow stromal cells (BMSCs) [45]. Importantly, the downregulation
of MM cells adherence to BMSCs can overcome the cellular adhesion-
mediated drug resistence by malignant MM cells. is eect of IMiDs
is further increased by their ability to downregulate TNFα [9,43,46,47].
Surprisingly, in contrast to the inhibitory eect of lenalidomide in
BMSCs, lenalidomide was totally ineective in inhibiting TNFα mRNA
expression in MM cells [44].
Immunomodulatory activities of IMiDs
IMiDs are a potent co-stimulator of primary human T cells,
synergizing with stimulation via T-cell receptor complex to increase
IL-2-mediated T-cell proliferation and interferon gamma (IFN-γ)
production [12-14,48-50]. Secretion of IL-2 and IFN-γ increases
the number of natural killer (NK) cells, improves their function and
mediates lysis of MM cells (Figure 2). NKT cells are a heterogeneous
Figure 2. Schematic diagram of ubiquitination and degradation of zinc nger lymphoid transcription factors Ikaros and Aiolos by CRL4cereblonE3 ubiquitin ligase and proteasomes. Binding
of cereblon (CRBN) by lenalidomide induces ubiquitination (marking IKZF1 /Ikaros/ and IKZF3 /Aiolos/) and degradation of both, Ikaros and Aiolos transcription factors. CRBN functions
as a substrate recognition component (substrate receptor) of this E3 ubiquitin ligase enzyme complex. CRL4cerebloncomplex consists of cullin 4A, RING nger protein regulator of cullins
(Roc1), and DNA damage binding protein 1 (DDB1).

Fuchs O (2017) Immunomodulatory drugs and their therapeutic eect in hematological malignancies through cereblon
Volume 2(3): 3-8
Hematol Med Oncol, 2017 doi: 10.15761/HMO.1000129
group of T-cells that recognize lipids and glycolipids presented by
CD1d molecules. NKT cells activated in presence of lenalidomide,
have greater ability to secrete IFN-γ. Lenalidomide enhances antigen-
specic expansion of NKT cells [51]. Regulatory T-cells (Tregs) are a
component of the immune system that suppresses immune response of
other cells. Tregs were elevated in MM patients. IMiDs strongly inhibits
Tregs proliferation via decreased FOXP3 mRNA expression [52].
Anti-angiogenic properties of IMiDs
All IMiDs have anti-angiogenic activity. alidomide has
predominant anti-angiogenic activity while lenalidomide and
pomalidomide have far greater immune enhancing eects [43].
It appears that anti-angiogenesis occurs via the modulation of
chemotactic factors involved in endothelial cells migration including
TNFα, vascular endothelial growth factor (VEGF) and basic broblast
growth factor (bFGF) from BMSCs rather than a direct inhibition
of endothelial cells proliferation [53,54]. Inhibition of VEGF and
bFGF production by IMiDs is likely to have multiple other biological
eects beyond that of anti-angiogenesis, including inhibition of IL-6
production by BMSC. VEGF and bFGF up-regulate IL-6 and other
pro-inammatory cytokines [55].
Inhibition of cell cycle and induction of apoptosis by
IMiDs
IMiDs induce cell cycle arrest via upregulation of tumor suppressor
genes (cyclin dependent kinase inhibitors p15INK4b, p16INK4a, p21Cip1,Waf1,
p27Kip1; early response transcription factors / Erg1, Erg2 and Erg3/) and
apoptosis by caspase activation [56,57]. e activity of the transcription
factor NF-κB inhibition by IMiDs resulted in reduced expression of
anti-apoptotic proteins including cellular inhibitor of apoptosis protein
2 (cIAP2) [58] and FLIP [Fas-associated protein with death protein
(FADD)-like interleukin-1β-converting enzyme (FLICE) inhibitor
protein] [59]. ese anti-apoptotic proteins inhibit caspase-8, that is
on the other hand stimulated by IMiDs [57].
Disruption of bone marrow stromal support for malig-
nant MM cells
IMiDs down-regulate the expression of adhesion molecules
[leukocyte function-associated antigen 1 (LFA-1, CD11a), intercellular
adhesion molecule 1 (ICAM-1, CD54), vascular cell adhesion molecule
1 (VCAM-1, CD106) and very late antigen 4 (VLA-4)] that facilitate
the interaction between MM cells and BMSC. As we described, IMiDs
inhibit NF-κB, a transription factor that has important growth and
anti-apoptotic roles and which is connected with the upregulation of
intracellular adhesion molecules and many cytokines [60].
IMiDs eect on myeloma cell proliferation
e direct anti-MM eect of IMiDs was shown to occur through
the induction of a G1 phase of cell cycle growth arrest of MM cells
[54] and was associated with a decrease in interferon regulatory factor
4 (IRF4), a transcription factor that is critical for MM cell growth and
survival, (Figure 2), [61,62].
Raje, et al. [63] showed strong synergism of anti-MM activity of
rapamycin (Rapamune), a specic mTOR inhibitor, combined with
CC-5013. Importantly, this combination was able to overcome drug
resistance when tested against MM cell lines resistant to conventional
chemotherapy. Moreover, the combination, but not rapamycin alone,
was able to overcome the growth advantage conferred on MM cells by
interleukin-6 (IL-6), insulin-like growth factor-1 (IGF-1), or adherence
to bone marrow stromal cells (BMSCs). Combining rapamycin
and CC-5013 induced apoptosis of MM cells. Dierential signaling
cascades, including the mitogen-activated protein kinase (MAPK) and
the phosphatidylinositol 3’-kinase / Akt kinase (PI3K /Akt) pathways
[64-67], were targeted by these drugs individually and in combination,
suggesting the molecular mechanism by which they interfere with MM
growth and survival. ese studies, therefore, provide the framework
for clinical evaluation of mTOR inhibitors combined with IMiDs to
improve patient outcome in MM.
IMiDs down-regulate CCAAT/enhancer-binding protein-β (C/
EBPβ) resulting in abrogation of cell proliferation [68]. Overexpression
of C/EBPβ rescued MM cells from IMiD-induced inhibition
of proliferation, indicating that C/EBPβ is critical in mediating
antiproliferative eects. IMiD-induced decrease of C/EBPβ protein led
to impaired transcription of interferon regulatory factor 4 (IRF4).
Down-regulation of IRF4 by lenalidomide was conrmed by
longitudinal studies of bone marrow samples from 23 patients obtained
before and during lenalidomide treatment using CD138⁺/IRF4⁺ double
labeling. In contrast to down-regulation of C/EBPβ protein, IMiD
compounds did not alter C/EBPβ mRNA levels or protein stability,
suggesting translational regulation of C/EBPβ. We could demonstrate
that C/EBPβ protein expression is under eIF4E-translational control
in MM. Furthermore, inhibition of the eIF4E-C/EBPβ axis by IMiD
compounds was not observed in IMiD-resistant MM cells.
However, targeting translation at a dierent level by inhibiting
eukaryotic translation initiation factor 4E-binding protein 1
phosphorylation overcame resistance, suggesting that this pathway is
critical and might be a target to overcome drug resistance. MM cell lines
and primary MM cells strongly expressed C/EBPβ, whereas normal B
cells and plasma cells had little or no detectable levels of C/EBPβ [69].
Silencing of C/EBPβ led to down-regulation of transcription factors
such as IRF4, XBP1, and BLIMP1 accompanied by a strong inhibition
of proliferation. Further, silencing of C/EBPβ led to a complete down-
regulation of antiapoptotic B-cell lymphoma 2 (BCL2) expression. In
chromatin immunoprecipitation assays, C/EBPβ directly bound to
the promoter region of IRF4, BLIMP1, and BCL2. C/EBPβ is involved
in the regulatory network of transcription factors that are critical for
plasma cell dierentiation and survival. Targeting C/EBPβ may provide
a novel therapeutic strategy in the treatment of multiple myeloma.
Exposure plasma cells to lenalidomide activated the Wnt/β-catenin
pathway and its downstream targets such as cyclin D1 and MYC [70].
e accumulation of β-catenin during treatment with lenalidomide
might be cause of drug resistance [70]. N-cadherin-based interaction
between MM cells and osteoblasts block MM cell growth. erefore,
the high levels of N-cadherin expression in osteoblasts confers strong
proliferation block on MM cells. Since β-catenin associates with
N-cadherin at the cell membrane, N-cadherin adhesion is disrupted,
β-catenin is released and will translocate to the nucleus leading to
the transcription of target genes, and ultimately, cell proliferation
[71]. Resistance of myeloma to lenalidomide is an emerging clinical
problem, and though it has been associated in part with activation of
Wnt/β-catenin signaling, the mediators of this phenotype remained
undened. Lenalidomide-resistant models were found to overexpress
the hyaluronan (HA)-binding protein CD44, a downstream Wnt/β-
catenin transcriptional target [72]. Consistent with a role of CD44 in
cell adhesion-mediated drug resistance (CAM-DR), lenalidomide-
resistant myeloma cells were more adhesive to bone marrow stroma
and HA-coated plates. Blockade of CD44 with monoclonal antibodies,
free HA or CD44 knockdown reduced adhesion and sensitized to

Fuchs O (2017) Immunomodulatory drugs and their therapeutic eect in hematological malignancies through cereblon
Volume 2(3): 4-8
Hematol Med Oncol, 2017 doi: 10.15761/HMO.1000129
lenalidomide. Wnt/β-catenin inhibition by FH535, a compound that
suppresses both Wnt/β-catenin and peroxisome proliferator-activated
receptor (PPAR) signaling, enhanced the activity of lenalidomide, as
did interleukin-6 neutralization with siltuximab.
Cereblon as the direct target protein of IMiDs
Ito, et al. [73-76] developed a new anity bead technology
for isolating ligand-binding proteins. Polymer-coated beads were
constructed that allow single-step purication of ligand target
molecules. ese beads include styrene-glycidyl-methacrylate (SG)
beads and ferrite-glycidyl-methacrylate (FG) beads. FG beads were
used for the purication of thalidomide-binding proteins from various
cell extracts. alidomide-modied beads were incubated with cell
extracts and then washed with buer. Bound proteins were eluted with
free thalidomide and analyzed by gel electrophoresis. Only two specic
protein bands were detected (55 kDa and 127 kDa). ese proteins
were identied as CRBN and DDB1. As we described in Abstract and
Introduction, CRBN, like DDB1, is a component of the cullin 4 ring E3
ubiquitin ligase complex (CRL4). Auto-ubiquitination of CRBN was
inhibited by thalidomide in vitro, suggesting that thalidomide is an
inhibitor of E3 ubiquitin ligase [77].
Cereblon as a primary target for thalidomide teratoge-
nicity
e zebrash was adopted as a model animal for in vivo study of
thalidomide teratogenicity [73-76]. Zebrash have a protein zCrbn,
which is 70% homologous to human CRBN and possesses thalidomide
binding activity. alidomide exposure induces n and ear (otic
vesicle) defects in zebrash. A reduction in in the size of otic vesicle was
observed in zebrash following thalidomide exposure. Zebrash ns
and tetrapod limbs are homologous with respect to early patterning
and gene expression, although skeletal structures in adult sh and
tetrapods are quite dierent. Shortening of the pectoral ns along
the proximo-distal axis and the inhibition of broblast growth factor
8 (Fgf8) gene expression were found in thalidomide-treated zebrash
embryos. Finally, chicks were employed for the investigation of the
conserved role of CRBN. Down-regulation of Fgf8 and Fgf10 cause
multiple birth defects and limb deformities.
e role of Ikaros family proteins in IMiDs and CRBN
mechanism
Using distinct but complementary proteomic techniques and
systems, three groups have recently simultaneously reported that
IMiDs induced the CRBN-dependent proteasomal degradation of
IKZF1 (Ikaros) and IKZF3 (Aiolos) [50,78,79]. Schema is shown in
Figure 2. IKZF1, a zinc nger transcription factor initially discovered
as a regulator of the T cell receptor, is required for hematopoiesis,
particularly lymphocyte development and plasma cell maturation. Loss
of function mutations of IKZF1 and IKZF3 are associated with acute
lymphoblastic leukemia, consistent with a tumor suppressor function.
On the other hand, IKZF1 and IKZF3 are required for the viability of
many MM cell lines. IKZF1 and IKZF3 are also involved in the complex
process of chromatin remodeling, and the nature of their interactions
is poorly understood.
IKZF1 bound and activated the IRF4 gene promoter and loss
of IKZF1 led to decreased IRF4 and MYC expression. However,
lenalidomide could also inhibit MM cell lines with high basal levels
of IRF4 unchanged by drug treatment, suggesting that other IKZF1/3
targets can play a role in the therapeutic response to IMiDs. IKZF1/3 are
known repressors of IL-2 gene promoter. e degradation of IKZF1/3
in response to IMiDs explains enhanced T cell IL-2 production. Hence,
many of the eects of IMiDs can be explained by a unied mechanism:
IMiDs re-target the cullin 4 ring E3 ubiquitin ligase activity toward
IKZF1/3 in a change-of-function eect (Figure 2).
An alternative CRL4CRBN substrate in the lenalidomide-
responsive myelodysplastic syndrome with del(5q)
Krӧnke, et al [18] identied a novel target casein kinase1A1
(CSNK1A1) by quantitative proteomics in the myeloid cell line KG-1.
CSNK1A1 is encoded in the del(5q) commonly deleted region and the
gene is haploinsucient. Lenalidomide treatment leads to increased
ubiquitination of CSNK1A1 and decreased protein abundance (Figure
3). CSNK1A1 was shown as a therapeutic target in a murine model of
AML [80,81] and in MDS with del(5q) [82,83]. CSNK1A1 negatively
regulates β-catenin which drives stem cell self-renewal and CSNK1A1
haploinsuciency causes the initial clonal expansion in patients with
the del(5q) MDS and contributes to the pathogenesis of del(5q) MDS.
Further inhibition of CSNK1A1 in del(5q) MDS is associated with
del(5q) cells apoptosis and p53 activation. e inhibition of CSNK1A1
reduced RPS6 phosphorylation, induced p53 expression, and triggered
myeloid dierentiation program. TP53-null leukemia did not respond
to CSNK1A1 inhibition, strongly supporting the importance of the p53
expression for the yield of CSNK1A1 inhibition. CSNK1A1 mutations
have been recently found in 5-18% of MDS patients with del(5q)
[82,84-87]. ese mutations are associated similarly to the eect of
TP53 mutations with rise to a poor prognosis in del(5q) MDS [87].
While CSNK1A1 is CRL4CRBN target in del(5q) MDS,
CRL4CRBN targets in lower risk non-del(5q) remain to be determined.
e mechanism of action of lenalidomide is still unclear in non-del(5q)
MDS cells.
Importance of valosin-containing protein/p97 for the
degradation of all known CRL4CRBN targets
Valosin-containing protein (VCP)/p97 is ATPase which delivers
ubiquitinated proteins for degradation in proteasomes. VCP is
required for degradation of all known CRL4CRBN targets (IKZF1,
IKZF3, casein kinase 1α, and the translation termination factor
GSPT1) [88]. VCP promotes also degradation of glutamine synthetase
(GS). GS is important for de novo synthesis of glutamine and functions
tn detoxification of glutamate and ammonia. GS is associated with
diseases including cancer, Alzheimer´s disease and Huntington
disease [89].
Immunomodulatory drugs disrupt the cereblon-
CD147-MCT1 axis to exert anti-cancer activity and ter-
atogenicity in a ubiquitin-independent way
CRBN promotes the activation of the CD147 (basigin; BSG) -
MCT1 (solute carrier family 16 members 1; SLC16A1) transmembrane
complex. is complex activatesvarious biological functions, including
angiogenesis, proliferation, invasion and lactate export. Binding of
IMiDs to CRBN leads to destabilization of the CD147-MCT1 complex
and to the inhibition of tumor growth [90].
Measurement of cereblon mRNA and cereblon protein
levels as biomarkers for IMiDs response
e measurement of CRBN mRNA by quantitative RT-PCR
typically uses commercial assays, such as predesigned TaqMan assays,

Fuchs O (2017) Immunomodulatory drugs and their therapeutic eect in hematological malignancies through cereblon
Volume 2(3): 5-8
Hematol Med Oncol, 2017 doi: 10.15761/HMO.1000129
where primers and probe are optimized with respect to known gene
splicing information. For CRBN mRNA, the current “best coverage
assay” detects the exon 8-exon 9 junction and exon 10 as a measure of
gene expression [91]. is assay (Hs00372271_m1; Applied Biosystems,
Life Technologies Corp.) measures all CRBN mRNA variants that
are translated to functional protein with exception of the variants
with removed exon 10 (part of IMIDs binding region). e presence
of multiple CRBN mRNA splice variants complicates the transcript
measurement by Aymetrix array.
We have found that MDS patients with isolated del(5q) (the so-
called 5q minus syndrome) have higher levels of full-length CRBN
mRNA than other patients with lower risk MDS, linking higher levels
of a known lenalidomide target CRBN and an MDS subgroup known
to be especially sensitive to lenalidomide [92].
Currently available commercial antibodies are neither sensitive
nor specic for reliable detection of CRBN protein levels. Gandhi et
al. [91] characterized a monoclonal antibody CRBN65 and compared
its properties with the commonly used, currently available commercial
antibodies against CRBN. is antibody is the most sensitive and
specic and can detect as little as 200 pg of CRBN protein via Western
blot [91].
Appropriate antibodies and validated assays for cereblon protein
detection and CRBN gene expression that account for the known gene
splicing information are needed for CRBN measurements in the clinic.
While it has been shown that CRBN and IRF4 levels correlate
with lenalidomide responsiveness in MM patients, previous in vitro
investigations using cytogenetically discrete human myeloma cell
lines have not been able to replicate this phenomenon [93,94]. e
implications of this apparent discordance are two-fold. First, these
results show that it is important to investigate CRBN and IRF4 gene
expression in both CD138- and CD138+ myeloma cells. Second,
because there are multiple isoforms of CRBN, it may be important to
Figure 3. Schematic diagram of ubiquitination and degradation of casein kinase 1α by CRL4cereblon E3 ubiquitin ligase and proteasomes. Binding of cereblon (CRBN) by lenalidomide induces
ubiquitination (marking CK1α) and degradation of casein kinase 1α. CRBN functions as a substrate recognition component (substrate receptor) of this E3 ubiquitin ligase enzyme complex.
CRL4cereblon complex consists of cullin 4A, RING nger protein regulator of cullins (Roc1), and DNA damage binding protein 1 (DDB1).
study expression levels of each one in the context of both in vivo and
in vitro settings [91].
Conclusion and perspectives
e studies showing IMiDs induced the CRBN-dependent
proteasomal degradation of IKZF1, IKZF3, and caein kinase 1α
[18,50,78,79] have greatly advanced our understanding of the
mechanism of action of IMiDs in MM and MDS with del(5q). e
small-molecule drug lenalidomide modulates the activity of the CRBN-
CRL4 E3 ubiquitin ligase complex to increase ubiquitination of two
transcription factors, IKZF1 and IKZF3, or casein kinase 1α. It does
so by specic binding to one component of the system, cereblon.
Crystal structures of these complexes were studied in detail [83,94,95].
All these studies may have wider implications for the targeting of E3
ubiquitin ligases in drug discovery for other diseases. ere are very
few approved and experimental drugs that modulate the ubiquitin
system like lenalidomide in this case. Other small molecules could be
developed to alter the specicity of ubiquitination complexes with the
aim of inducing the specic degradation of previously “undruggable”
oncoproteins such as Ras and Myc in many dierent types of cancer
[96-99]. Nevertheless, there are still gaps in our understanding of
the mechanism of action of IMiDs in MM and other hematological
malignancies. For example, the proteasomal inhibitor bortezomib
(VelcadeTM, PS-341) is used for the treatment of MM. Bortezomib
is used eciently also in combination with lenalidomide. ere is an
apparent paradox, because inhibition of proteasomal destruction of
IKZF1 and IKZF3 by bortezomib contradicts the preposed mechanism.
Lu et al. [80] hypothesize that since the proteasomal inhibition by
bortezomib is incomplete with therapeutic dosing, this might allow
sucient destruction of IKZF1 and IKZF3 while retaining other
therapeutic eect of bortezomib. Alternatively, they hypothesize that
IKZF1 and IKZF3 once polyubiquitylated, may be inactive or act as
dominant-negatives.

Fuchs O (2017) Immunomodulatory drugs and their therapeutic eect in hematological malignancies through cereblon
Volume 2(3): 6-8
Hematol Med Oncol, 2017 doi: 10.15761/HMO.1000129
Future dissection of CRBN direct down-stream substrates and
CRBN indirect down-stream factors will help to identify mechanisms
of IMiD action and nd new biomarkers for prediction of IMiD
response and IMiD resistence as well as developing a new therapy to
treat the patients with MM.
It is also possible to speculate whether or not anti-myeloma
activity of IMiDs is associated with the interaction between CRBN
and AMPK (AMP-activated protein kinase) or KCNT1 (Potassium
Sodium-Activated Channel Subfamily T Member 1). Both AMPK and
KCNT1 are potential substrates of the CRBN-CRL4 E3 ubiquitin ligase
complex, but they may not be associated with downstream signaling
that leads to anti-myeloma activity of IMiDs.
Acknowledgements
is work was supported by the project for conceptual development
of research organisation No 00023736 (Institute of Hematology and
Blood Transfusion) from the Ministry of Health of the Czech Republic.
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