Cancer Cell

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Unlocking New Therapeutic TargetsandResistanceMechanisms inMantleCell Lymphoma

Dolors Colomer1 and Elıas Campo1,*1Hospital Clinic, Institut d’Investigacions Biomediques August Pi i Sunyer (IDIBAPS), University of Barcelona, Barcelona 08036, Spain*Correspondence: ecampo@clinic.ub.eshttp://dx.doi.org/10.1016/j.ccr.2013.12.011

Mantle cell lymphoma is an aggressive tumor for which new drugs targeting underlying molecular mecha-nisms are opening promising avenues. A recent study has elucidated the basis for targeting B cell receptorsignaling in these lymphomas and identified somatic mutations in NF-kB regulatory genes that conferresistance to this therapy.

Mantle cell lymphoma (MCL) is an aggres-

sive lymphoid neoplasia derived from

mature B cells and is considered incur-

able with current therapies. The primary

oncogenic driver is the t(11;14)(q13;q32)

translocation, which causes cyclin D1

overexpression. This event is usually

followed by additional chromosomal

alterations that target genes regulating

DNA damage response, cell cycle, and

cell survival pathways (Jares et al.,

2012). The clinical and biological features

of MCL now seem more heterogeneous

than initially recognized. Recent studies

have identified a subset of tumors with

an indolent behavior that tends to pre-

sent with leukemic instead of extensive

nodal disease. The tumor cells have sim-

ple karyotypes, frequent hypermutated

IGHV, and a gene expression signature

that does not include SOX11, a transcrip-

tion factor that promotes oncogenic

growth of conventional MCL (Jares et al.,

2012; Vegliante et al., 2013). These

indolent tumors may acquire additional

oncogenic alterations and eventually

progress to a more aggressive form.

Although the outcome for MCL patients

has improved globally in past years

through the use of new combined

immunochemotherapy and intensive

regimens such as stem cell transplan-

tation, many patients will relapse and

eventually die of the disease.

Increased understanding of the mole-

cular mechanisms driving MCL has

facilitated the development of new

therapies that offer promising avenues

(Perez-Galan et al., 2011). The relevance

of B cell receptor (BCR) signaling to the

pathogenesis of several lymphomas

has propelled investigation of a series of

drugs that interfere with these pathways

(Young and Staudt, 2013). Initial clinical

studies with some of these agents, partic-

ularly ibrutinib, a covalent inhibitor of

the Bruton tyrosine kinase (BTK), have

yielded substantial responses in different

lymphoid neoplasms, including diffuse

large B cell lymphoma (DLBCL), chronic

lymphocytic leukemia (CLL), and MCL

(Young and Staudt 2013; Wang et al.,

2013). However, the encouraging results

obtained for MCL were somewhat sur-

prising, because, contrary to DLBCL or

CLL, evidence supporting a pathogenic

role for BCR stimulation in MCL were

limited. Suggestive findings from recent

studies in MCL have identified a restricted

immunoglobulin gene repertoire and often

stereotyped configurations of the BCR,

supporting the role of antigen selection

in a subset of these tumors. In addition,

different kinases in BCR signaling,

including SYK, LYN, and BTK, may be

amplified or phosphorylated in primary

MCL, supporting the activation of this

pathway in certain tumors (reviewed in

Perez-Galan et al., 2011 and Jares et al.,

2012). A recent study by Wang et al.

(2013) showed complete or partial re-

sponses to ibrutinib in up to 68% of

patients who had failed to respond

to other therapies. This response was

higher than expected based on known

BCR alterations in MCL. Beyond the

positive clinical impact, this study high-

lighted our limited understanding of

the molecular bases for both the high

response rate and the resistance to

BTK inhibition in these tumors.

In a recent study in Nature Medicine,

Rahal et al. (2013) found answers to

some of the open questions surrounding

clinical experience with ibrutinib in MCL.

Using a large-scale pharmacological

Cancer Cell

profiling strategy across more than 100

hematological cell line models, the au-

thors identified 4 MCL lines that were

highly sensitive to the BCR signaling

inhibitors ibrutinib and sotrastaurin

and 6 lines that were resistant to the

treatments. Intriguingly, both subsets

of sensitive and resistant cell lines were

dependent on IKKb-NF-kB signaling,

confirming previously reported constitu-

tive NF-kB activation in MCL (Pham

et al., 2003). However, the sensitive, but

not the resistant, cells were addicted to

signaling through the CARD11-BCL10-

MALT1 (CBM) complex downstream of

BTK and PKC, which sustained activation

of the classical NF-kB pathway (Figure 1).

To understand the possible mechanisms

underlying NF-kB activation in resistant

cells, the authors performed RNA se-

quencing and found inactivating muta-

tions of TRAF2 and TRAF3 in two of the

resistant cell lines. TRAF2 and TRAF3

are negative regulators of the alternative

NF-kB signaling pathway interacting

with cIAP1 and cIAP2 (gene products

of BIRC2 and BIRC3, respectively)

to downregulate NIK (encoded by

MAP3K14), a central kinase in this

pathway that promotes processing of

the NF-kB precursor p100 into the active

p52 isoform (Figure 1). Loss of TRAF2

and TRAF3 were essential for survival of

the cells suggesting that NIK could be a

new target in MCL that specifically bear

these genetic alterations. Interestingly,

three of the other resistant MCL lines

in which the authors did not find similar

mutations are known to be infected

by Epstein-Barr virus (EBV), an activator

of the alternative NF-kB pathway. Intrigu-

ingly, there was one resistant cell line

without clear evidence of alternative

25, January 13, 2014 ª2014 Elsevier Inc. 7

Figure 1. Somatic Mutation and Activation of the NF-kB Pathway in MCLActivation of the classical and alternative NF-kB pathway in MCL may occur by chronic active BCR signaling and other mechanisms. Genomic sequencingstudies have identified mutations in several elements of this regulatory pathway (denoted by red asterisks). Chronic active BCR (A) or Toll-like receptor (TLR)(B) signaling activate classical NF-kB pathway. MCL that is dependent on BCR activation (A) may be blocked by BCR signaling inhibitors. Somatic mutationsin the inhibitors of the alternative pathways (C) cIAP1 and cIAP2 (gene products of BIRC2 and BIRC3, respectively) and TRAF2/3 activate the alternativeNF-kB pathway and confer resistance to inhibitors of the BCR signaling pathway. Somatic mutations in other elements of these pathways (CARD11, IKKb,encoded by IKBKB, TLR2, and NIK, encoded by MAP3K14) have been also found in MCL by whole exome or genome sequencing .

Cancer Cell

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NF-kB pathway activation as it was

negative for EBV and TRAF2/3mutations,

suggesting that other mechanisms

may create resistance to BCR inhibitors

in MCL.

Rahal et al. (2013) analyzed the pres-

ence of mutations in genes encoding

members of the alternative NF-kB path-

ways TRAF2, TRAF3, BIRC2, BIRC3,

and MAP3K14 across 165 primary MCLs

and found TRAF2 and BIRC3 mutations

in 6% and 10% of the cases, respectively.

These observations are concordant with

the findings in our recent whole genome

and exome sequencing study of MCL

in which we found BIRC3 mutations in

6% of cases (Bea et al., 2013). BIRC3 is

located at 11q22.2, a region frequently

deleted in MCL. In our study, virtually all

BIRC3 mutations were associated with

11q deletions, including a case without

8 Cancer Cell 25, January 13, 2014 ª2014 El

ATM mutations, suggesting that, in addi-

tion to the dominant negative effect of

these mutations suggested by Rahal

et al. (2013), the deletion of the normal

BIRC3 allele may confer an additional

advantage to the cells. Although we did

not observed TRAF2/3 mutations, we

found mutations in other genes of the

classical and alternative NF-kB pathway,

including recurrent activating mutations

in TLR2 as well as mutations in CARD11,

MAP3K14 (NIK), and IKBKB (IKKb)

(Figure 1). Although the functional impli-

cations of some of these mutations need

to be confirmed, together, the studies

suggest that genetic alterations in the

NF-kB pathway in MCL may be more

common than initially thought.

The observations of Rahal et al. (2013),

if confirmed in primary tumor cells, may

have important clinical implications. The

sevier Inc.

detection of RelB cleavage, as a down-

stream effect of PKC-CBM activity, and

low levels of p52 identified cells sensitive

to BCR inhibitors. In contrast, tumors

with high p52 levels and genetic lesions

in the alternative NFkB pathway predicted

resistance to these agents. Therefore,

these alterations may be useful bio-

markers for selecting targeted therapies

in MCL. The involvement of the alternative

NF-kB pathway in resistance to BCR

inhibitors may also be of interest for

other lymphoid neoplasms. Mutations in

genes of the alternative (BIRC3, TRAF3,

and MAP3K14) and classical (TNFAIP3

and IKBKB) NF-kB pathways have been

described in 36% of splenic marginal

zone lymphomas (Rossi et al., 2011) and

24% of advanced or chemoresistant

CLL (Rossi et al., 2012). The functional

relationship between these mutations

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and the response to new therapies in lym-

phomas deserves further exploration.

Additional mechanisms, such as muta-

tions in BTK or in other targets of the

new drugs, may also emerge and confer

resistance in MCL. The preclinical results

presented by Rahal et al. (2013) provide

critical insights into understanding the

mechanisms of response to new drugs

and suggest innovative therapeutic

strategies for tumors refractory to BCR

signaling inhibitors. The study also high-

lights the increasing clinical interest of

sequencing tumors to guide the selection

of new therapies in lymphoid neoplasms.

ACKNOWLEDGMENTS

We thank Dr. Xose A. Puente and Silvia Bea fortheir helpful comments. The work of the authors

is supported by Red Tematica de InvestigacionCooperativa en Cancer (RD12/0036) and PlanNacional (SAF10/21165, SAF12/38432); E.C. isan Institucio Catalana de Recerca i EstudisAvancats-Academia (ICREA) investigator.

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FromAnecdote to TargetedTherapy: TheCuriousCaseof Thalidomide in Multiple Myeloma

Jonathan D. Licht,1,* Jake Shortt,2,3 and Ricky Johnstone2,31Division of Hematology/Oncology, Robert H. Lurie Comprehensive Cancer Center, Northwestern University Feinberg School of Medicine,Chicago, IL 60611, USA2Gene Regulation Laboratory, Peter MacCallum Cancer Centre, Melbourne, VIC 3002, Australia3Sir Peter MacCallum Department of Oncology, University of Melbourne, Parkville, VIC 3052, Australia*Correspondence: j-licht@northwestern.eduhttp://dx.doi.org/10.1016/j.ccr.2013.12.019

Thalidomide and related drugs are key drugs for the treatment of multiple myeloma (MM). These agentsbind to cereblon, a component of a ubiquitin ligase complex, altering the specificity of the complex to inducethe ubiquitylation and degradation of Ikaros (IKZF1) and Aiolos (IKZF3), transcription factors essential forMM growth.

The story of the use of thalidomide and

related compounds in the treatment of

multiple myeloma (MM) represents a

remarkable case of bedside to bench

research. Thalidomide may have first

been discovered during World War II as

a potential antidote to nerve gas and

was developed as a sedative used for

morning sickness in the 1950s. This

culminated in a great medical tragedy

because of the extraordinary teratogenic

effects on limb development. The drug

was abandoned but was later explored

for the treatment of leprosy ulcers, HIV/

AIDS, and autoimmune diseases. These

anti-inflammatory effects were linked to

inhibition of tumor necrosis factor secre-

tion, providing an early indication that

thalidomide functioned as an immune

modulatory drug (IMiD). Dr. Judah Folk-

man and colleagues subsequently found

that thalidomide inhibited tumor associ-

ated angiogenesis. Folkman’s advice

to a patient to try thalidomide in MM

(http://www.nytimes.com/1999/11/18/us/

thalidomide-found-to-slow-a-bone-cancer.

html) led to a seminal study showing

a 32% response rate of MM patients to

a single agent thalidomide treatment,

including complete responses in several

patients refractory to all prior therapies

(Singhal et al., 1999). Thalidomide in

combination with dexamethasone as an

initial therapy yields response rates of

>60%, while lenalidomide, a chemically

similar IMiD with fewer constitutional

side effects, plus dexamethasone yields

responses in 80% of patients. The use

of these IMiDs along with the protea-

some inhibitor bortezomib has extended

the median survival of MM patients to

greater than 7 years. This remarkable

progress occurred in the absence of

clear molecular and biological mecha-

nisms of action of thalidomide,

25, January 13, 2014 ª2014 Elsevier Inc. 9