Leukemia-initiating cells can originate from hematopoietic progenitor cells that have acquired self-renewal capacity upon transformation with leukemic fusion genes. In this issue of Cancer Cell, Kirstetter and colleagues describe a mouse model for the frequent CEBPA mutations in human acute myeloid leukemia that result in the synthesis of only the 30kDa isoform, but not the 42kDa isoform of C/EBPalpha. This mutation uncouples C/EBPalpha\u27s roles in myeloid differentiation and proliferation control. Furthermore, this mutation activates self-renewal in committed myeloid progenitor cells and induces myeloid malignancy with complete penetrance that is sustained by leukemia-initiating cells with a committed myeloid molecular signature

Castilla, Lucio H.

eScholarship@UMMS

C/EBPalpha in leukemogenesis: a matter of being in the right place with the right signals

Leukemia-initiating cells can originate from hematopoietic progenitor cells that have acquired self-renewal capacity upon transformation with leukemic fusion genes. In this issue of Cancer Cell, Kirstetter and colleagues describe a mouse model for the frequent CEBPA mutations in human acute myeloid leukemia that result in the synthesis of only the 30kDa isoform, but not the 42kDa isoform of C/EBPα. This mutation uncouples C/EBPα's roles in myeloid differentiation and proliferation control. Furthermore, this mutation activates self-renewal in committed myeloid progenitor cells and induces myeloid malignancy with complete penetrance that is sustained by leukemia-initiating cells with a committed myeloid molecular signature

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Cancer Cell
PreviewsC/EBPα in Leukemogenesis: A Matter of Being  
in the Right Place with the Right Signals
Lucio H. Castilla1,*
1Program in Gene Function and Expression, University of Massachusetts Medical School, Worcester, MA 01605, USA
*Correspondence: lucio.castilla@umassmed.edu
DOI 10.1016/j.ccr.2008.03.009
Leukemia-initiating cells can originate from hematopoietic progenitor cells that have acquired self-
renewal capacity upon transformation with leukemic fusion genes. In this issue of Cancer Cell, Kirstetter 
and colleagues describe a mouse model for the frequent CEBPA mutations in human acute myeloid 
leukemia that result in the synthesis of only the 30kDa isoform, but not the 42kDa isoform of C/EBPα. 
This mutation uncouples C/EBPα’s roles in myeloid differentiation and proliferation control. Further-
more, this mutation activates self-renewal in committed myeloid progenitor cells and induces myeloid 
malignancy with complete penetrance that is sustained by leukemia-initiating cells with a committed 
myeloid molecular signature.Acute myeloid leukemia (AML) rises 
from the accumulation of genetic 
mutations that alter the regulation of 
proliferation, survival, and differentia-
tion of hematopoietic progenitors. The 
hematopoietic system is a heteroge-
neous group of cells sustained by the 
hematopoietic stem cells (HSCs), a 
rare group of cells with the ability to 
self renew (i.e., the capacity to produce 
a daughter cell identical to the paren-
tal). Similarly, the leukemia tumor is a 
heterogeneous group of highly prolif-
erative cells with partial differentiation 
capacity that is sustained by the “leu-
kemia-initiating cells” (LIC or leukemia 
stem cells) (Passegue et al., 2003). The 
study of the origin and function of LICs 
is critical as they may be crucial targets 
in the treatment of leukemia.
The LICs can originate from HSCs that 
maintain self-renewal functions after 
transformation or from committed pro-
genitors that have regained self-renewal 
capacity. Expression of the leukemia 
fusion protein BCR-ABL from a retroviral 
vector induces leukemia only when intro-
duced in the HSCs, while MLL-ENL and 
MOZ-TIF2 can induce AML in committed 
granulocytic-macrophage progenitors 
(GMPs) (Cozzio et al., 2003; Huntly et 
al., 2004). It may therefore be a common 
theme that mutations found in AML may 
have the ability to reactivate self-renewal 
programs in short-lived progenitors.
The article published by Kirstetter 
and colleagues in this issue of Cancer 
Cell (Kirstetter et al., 2008) investi-gates alterations in hematopoietic dif-
ferentiation and transformation in mice 
in the context of Cebpa mutations. 
The CCAAT-enhancer binding protein 
alpha (C/EBPα) transcription factor is 
a critical regulator of proliferation and 
differentiation in multiple cell types. 
In hematopoiesis, C/EBPα regulates 
myeloid differentiation at multiple lev-
els, including the transition from com-
mon myeloid progenitors (CMPs) to 
the bilineage GMPs and granulopoiesis 
(Nerlov, 2004). The intronless CEBPA 
gene encodes an mRNA that is trans-
lated to a 42kDa (p42) and a 30kDa 
(p30) C/EBPα proteins from different 
translation start sites. C/EBPα modu-
lates differentiation and proliferation by 
direct interaction with coactivators and 
repressors in a cell-type-specific man-
ner. The N-terminal region unique to p42 
contains transactivation elements TE-I 
and TE-II that interact with TBP/TF-IIB 
and CBP/p300. The common C-ter-
minal region interacts with the SWI/
SNF complex at TE-III and with E2F1, 
GATA-1, PU.1, RUNX1, and c-JUN at the 
basic region DNA-binding and leucine 
zipper dimerization domain (bZIP).
The expression of C/EBPα is fre-
quently reduced in AML through dif-
ferent mechanisms (Nerlov, 2004). The 
expression of CEBPA mRNA can be 
repressed by the leukemia fusion pro-
tein AML1-ETO, and the expression of 
C/EBPα protein can be regulated by 
CBFβ-SMMHC and BCR-ABL. In addi-
tion, the function of C/EBPα can be Cancerinhibited by TRIB2. Mutations of CEBPA 
are present in approximately 9% of AML 
cases, commonly occurring in interme-
diate-risk karyotype and in the absence 
of AML1-ETO or CBFB-MYH11. CEBPA 
mutations frequently cluster in the 5′ 
region that specifically disrupts the 
synthesis of p42 while retaining the 
expression of p30. Another mutation 
hot spot is at the 3′ region that often 
results in creating dominant-negative 
proteins. In most cases, when CEBPA 
mutations occur in both alleles, at least 
one mutation is in the 5′ region. This 
observation strongly suggests that p30 
may be essential to trigger AML. This is 
consistent with findings that Cebpa null 
mice remain leukemia free (Heath et al., 
2004; Zhang et al., 2004) (Figure 1) and 
that BCR-ABL fails to produce AML in 
Cebpa null mice (Wagner et al., 2006).
Mimicking the 5′ region muta-
tions found in human AML, Kirstetter 
and colleagues created a Cebpa “L” 
allele in mice that expressed p30, but 
not p42, by introducing a nonsense 
codon between the two translation-
start sites. Careful characterization of 
CebpaL/L hematopoietic progenitors 
that preceded leukemic transformation 
revealed a differentiation block at the 
myeloblast stage and neutropenia that 
progressed to granulocyte hyperpro-
liferation. The GMPs and committed 
myeloid progenitors (cMP) were hyper-
proliferative with signs of self-renewal 
(Figure 1). Interestingly, serial replating 
of purified progenitors confirmed the  Cell 13, April 2008 ©2008 Elsevier Inc. 289
Cancer Cell
Previewsenhanced self-renewal in cMPs and not 
in HSCs. Although multiple modulators 
of cell-cycle progression and differen-
tiation are known to interact with C/
EBPα, the mechanism of action is still 
unclear, and competing models have 
been proposed (Nerlov, 2004). C/EBPα 
binds to E2F1 via the bZIP domain to 
repress c-Myc, thereby inhibiting pro-
liferation and promoting differentiation. 
The CebpaBRM2/BRM2 mice, carrying a 
point mutation in the bZIP domain that 
impairs E2F1 interaction, show simi-
lar preleukemic features as CebpaL/L 
but with the significant difference that 
CebpaBRM2/BRM2 mice do not develop 
myeloid leukemia (Porse et al., 2005). 
Considering that p30 maintains an 
intact bZIP domain but lacks the abil-
ity to repress E2F activity, E2F/c-Myc 
is probably a critical mediator of C/
EBPα modulation of proliferation and 
self-renewal programs. The rescue of 
myeloid differentiation by p30 shows 
an uncoupling of proliferation and dif-
ferentiation capacities and suggests 
that other partners that interact with 
p30 may be involved.
A critical finding of this pivotal study 
is that, unlike CebpaBRM2/BRM2 mice, 
CebpaL/L mice succumb to myeloid 
leukemia after 9 to 14 months. Trans-
plantation assays of immunofraction-
ated leukemia cells demonstrated that 
the CebpaL/L-LICs reside in the cMP 
compartment. The expression profile of 
CebpaL/L-LICs and its clustering with the 
expression profile of MLL-AF9-trans-
formed leukemia-GMPs (Krivtsov et 
al., 2006) revealed that LICs may share 
common and initiator-mutation-specific 
expression signatures that drive leu-
kemia development. The p30-specific 
gene set showed the dysregulation of 
genes encoding members of the C/ebp 
complex, with upregulation of the C/
ebp repressor Ddit3 and repression of 
Cebpb. Further dissection of these pro-
grams may provide valuable therapeutic 
targets.
This study demonstrates that a basal 
CEBPA function provided by p30, and 
not the loss of expression, is required 
for leukemia transformation. Consid-
ering the variety of partners known 
to associate with and to modulate (or 
be modulated by) C/EBPα, p30 may 
not just provide a basal C/EBPα func-290 Cancer Cell 13, April 2008 ©2008 ElseviFigure 1. Role of p30 Cebpα Protein in Myeloid Leukemia Development
Top: normal myeloid differentiation follows a hierarchy of progenitors that include hematopoietic stem 
cells (HSCs) with self-renewal capacity (blue) that progressively differentiate (yellow) to common 
myeloid progenitors (CMPs), granulocytic-macrophage progenitors (GMPs), and committed myeloid 
progenitors (cMP), which mature to granulocytes. Middle: hematopoiesis in Cebpa null mice show 
enhanced (capital letters) self-renewal capacity in HSCs and CMPs, hyperproliferation in CMPs, and 
differentiation block of CMPs. Bottom: hematopoiesis in CebpaL/L mice, which express p30, but not 
p42, has GMPs and cMPs with enhanced self-renewal and proliferation capacities. These abnormal 
progenitors acquire additional mutations (yellow lightning bolt) to develop myeloid leukemia (pink 
cells). The leukemia-initiating cells (red cell) are immunophenotypically a cMP.tion but rather a unique combination of 
domains that allow transformation. A 
precedent of this model in leukemia is 
GATA1, which is also involved in regu-
lation of myeloid differentiation. The 
GATA1 exon-2 mutations found in down 
syndrome-associated acute megakaryo-
blastic leukemia maintain the expression 
of the short GATA1s protein that lacks 
a transactivation domain but abrogates 
the expression of the full-length GATA1 
factor (Wechsler et al., 2002). Therefore, 
the increase in proliferation and self-
renewal within Cebpa null CMPs may 
present an unfavorable environment for 
transformation. Instead, p30-mediated 
differentiation to GMPs may permit a 
permissive cell environment for leukemia 
development. These cases suggest that 
reduction, but not loss, of transcription 
factor function may be a common theme 
in myeloid leukemia.
The CEBPA mutations are probably 
early events that occur in the HSCs. 
The CebpaL/L mice are the first Cebpa er Inc.mouse model of leukemia and repre-
sent a powerful tool to further dissect 
leukemogenesis. For example, how 
does p30 affect HSC function in an 
otherwise normal environment? What 
mutations cooperate with p30 expres-
sion in transformation of the cMPs? 
Is p30-expression just permitting dif-
ferentiation to MPs, or does it play a 
role in the acquisition of additional 
 mutations?
The elegant study reported by 
Kirstetter and colleagues emphasize 
the importance of understanding the 
specific effect of human-leukemia 
mutations in the etiology of leukemia. 
This model validates the causality 
of CEBPA mutations as observed in 
human AML and has brought together 
proliferation and self-renewal to the 
GMP compartment to trigger  trans-
formation. The identification of the 
committed myeloid LIC will also be 
valuable in the test for efficient drug 
therapy.
Cancer Cell
PreviewsREFEREnCES
Cozzio, A., Passegue, E., Ayton, P.M., Karsunky, 
H., Cleary, M.L., and Weissman, I.L. (2003).  Genes 
Dev. 17, 3029–3035.
Heath, V., Suh, H.C., Holman, M., Renn, K., 
Gooya, J.M., Parkin, S., Klarmann, K.D., Ortiz, 
M., Johnson, P., and Keller, J. (2004).  Blood 104, 
1639–1647.
Huntly, B.J., Shigematsu, H., Deguchi, K., Lee, 
B.H., Mizuno, S., Duclos, N., Rowan, R., Amaral, 
S., Curley, D., Williams, I.R., et al. (2004).  Cancer 
Cell 6, 587–596.
Kirstetter, P., Schuster, M.B., Bereshchenko, O., The epidermal growth factor receptor 
(EGFR/ErbB1/HER1) consists of an extra-
cellular ligand binding region followed 
by a single membrane-spanning helix, a 
cytoplasmic tyrosine kinase domain, and 
a C-terminal tail of ~230 amino acids (Bur-
gess et al., 2003). Ligand binding to the 
extracellular region promotes receptor 
dimerization, which in turn leads to acti-
vation of the cytoplasmic tyrosine kinase 
(Holbro and Hynes, 2004). When acti-
vated, the EGFR kinase phosphorylates 
several tyrosines in the EGFR C-terminal 
tail that then serve as docking sites for 
downstream signaling effectors that initi-
ate signaling cascades and stimulate cell 
growth and differentiation (Holbro and 
Hynes, 2004). Three EGFR homologs, 
HER2 (Neu/ErbB2), HER3 (ErbB3), and 
HER4 (ErbB4) are found in humans and, 
A Molecular View
Monoclonal Antib
Daniel J. Leahy1,*
1Department of Biophysics and Biophysical C
*Correspondence: dleahy@jhmi.edu
DOI 10.1016/j.ccr.2008.03.010
Abnormal activation of the epiderm
has been associated with many hum
effective anticancer therapies. Stru
about how they function. In this issu
of the anti-EGFR monoclonal antib
in a manner distinctive from that of
therapies with Matuzumab and otheMoore, S., Dvinge, H., Kurz, E., Theilgaard-Monch, 
K., Mansson, R., Pedersen, T.A., Pabst, T., et al. 
(2008). Cancer Cell, this issue. 
Krivtsov, A.V., Twomey, D., Feng, Z., Stubbs, M.C., 
Wang, Y., Faber, J., Levine, J.E., Wang, J., Hahn, 
W.C., Gilliland, D.G., et al. (2006). Nature 442, 
818–822.
Nerlov, C. (2004). Nat. Rev. Cancer 4, 394–400.
Passegue, E., Jamieson, C.H., Ailles, L.E., and 
Weissman, I.L. (2003). Proc. Natl. Acad. Sci. USA 
100 (Suppl 1), 11842–11849.
Porse, B.T., Bryder, D., Theilgaard-Monch, K., 
Hasemann, M.S., Anderson, K., Damgaard, I., Ja-Cancer
together with EGFR, make up the EGFR/
ErbB family of receptors. HER2 is an 
atypical member of this family in that 
it is not directly activated by ligand but 
rather serves as a universal heterodimeric 
partner for each of the other ErbB family 
members (Holbro and Hynes, 2004).
EGFR was the first cell-surface recep-
tor to be associated with cancer, and 
abnormal EGFR or HER2 function has 
subsequently been found to contribute 
to the severity of many human tumors 
(Hynes and Lane, 2005). For this reason, 
agents targeting EGFR or HER2 have 
been actively pursued as cancer thera-
pies. These agents fall into two general 
classes: monoclonal antibodies, which 
bind to receptor extracellular regions 
and will be discussed here, and small-
molecule kinase inhibitors that target the 
 of Anti-ErbB  
ody Therapy
hemistry, Johns Hopkins University School of M
al growth factor receptor (EGFR) a
an cancers, and monoclonal antibo
ctural studies of these receptors an
e of Cancer Cell, Schmiedel et al. rep
ody Matuzumab. They show that Ma
 other therapeutic anti-EGFR antibo
r antibodies may prove beneficial.cobsen, S.E., and Nerlov, C. (2005). J. Exp. Med. 
202, 85–96.
Wagner, K., Zhang, P., Rosenbauer, F., Drescher, 
B., Kobayashi, S., Radomska, H.S., Kutok, J.L., 
Gilliland, D.G., Krauter, J., and Tenen, D.G. (2006). 
Proc. Natl. Acad. Sci. USA 103, 6338–6343.
Wechsler, J., Greene, M., McDevitt, M.A., Anas-
tasi, J., Karp, J.E., Le Beau, M.M., and Crispino, 
J.D. (2002). Nat. Genet. 32, 148–152.
Zhang, P., Iwasaki-Arai, J., Iwasaki, H., Fenyus, 
M.L., Dayaram, T., Owens, B.M., Shigematsu, H., 
Levantini, E., Huettner, C.S., Lekstrom-Himes, 
J.A., et al. (2004). Immunity 21, 853–863. Cell 13, April 2008 ©2008 Elsevier Inc. 291
cytoplasmic kinase activity. To date, two 
monoclonal antibodies against EGFR, 
Cetuximab (Erbitux) and Panitumumab 
(Vectibix), have been approved by the 
FDA for treatment of colorectal and/or 
head-and-neck cancer, and two EGFR 
kinase inhibitors, erlotinib (Tarceva) and 
gefitinib (Iressa), have been approved for 
the treatment of lung cancer. A mono-
clonal antibody targeting HER2, Tras-
tuzumab (Herceptin), and a pan-ErbB 
kinase inhibitor, lapatinib (Tykerb), have 
also been approved for treatment of 
HER2-overexpressing breast cancers. 
Many other ErbB-targeted therapies are 
under development.
Beginning ~5 years ago, X-ray crystallo-
graphic studies of the extracellular regions 
of ErbB family members uncovered the 
basic mechanism by which ligand binding 
edicine, Baltimore, MD 21205, USA
nd its homolog HER2 (Neu/ErbB2) 
dies targeting EGFR and HER2 are 
d antibodies have revealed much 
ort structural and functional studies 
tuzumab binds and inhibits EGFR 
dies and suggest that combination 


C/EBPα in Leukemogenesis: A Matter of Being in the Right Place with the Right Signals 

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C/EBPalpha in leukemogenesis: a matter of being in the right place with the right signals

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