Immunity, Vol. 9, 47±57, J uly, 1998, Copyright 1998 by Cell Press
Mice Defective in Two Apoptosis Pathways
in the Myeloid Lineage Develop
Acute Myeloblastic Leukemia
Recent work has shown that the AML1/ETO fusion pro-
teinacts as a dominantnegative inhibitorofnormalAml1
function (Yergeauet al., 1997)and also up-regulates the
expression of the bcl-2 protooncogene (Klampfer et al.,
Bcl-2 was discovered upon characterization of a
t(14;18) chromosomal translocation found in follicular
lymphoma (Bakhshiet al., 1985; Cleary and Sklar, 1985)
and was subsequently shown to protect cells from a
wide variety of apoptotic cues (Vaux et al., 1988; re-
viewed in Yang and Korsmeyer, 1996). The role of Bcl-2
in oncogenesis has been confirmed mainly in lymphoid
cells, but recent work has suggested that deregulation
of the bcl-2gene may be important inthe transformation
of myeloid cells. Leukemic cells from mosthuman AMLs
have been found to express Bcl-2 at levels muchhigher
thantheirnormalcellularcounterparts (Delia et al., 1992;
Bensi et al., 1995), suggesting that deregulation of the
bcl-2 gene may be one of the critical events in the my-
eloid transformation process. The extension of cellular
survival by Bcl-2 may allow sufficient time forthe acqui-
sition of additional oncogenic mutations. This mecha-
nismmay underly the progression from chronic to acute
leukemia (Rabbitts, 1991, 1994).
To test directly the role of bcl-2 in the development
of myeloid leukemia, we have created a transgenic
mouse inwhichconstitutive expressionof thebcl-2gene
is targeted exclusively to myeloid cells by the hMRP8
promoter (Lagasse and Weissman, 1994). hMRP8bcl-2
mice develop a disease that in many ways is analogous
to human chronic myelomonocytic leukemia (CMML)
(Lagasse and Weissman, submitted). hMRP8bcl-2 trans-
genic animals develop monocytosis with age and be-
come neutropenic dueto ashiftingranulopoiesis toward
immature celltypes. Whereas monocytes fromwild-type
mice rapidly undergoapoptosis invitro, monocytes from
hMRP8bcl-2 mice are able to survive in the absence of
serum or exogenous growth factors. We have recently
foundthat monocytes frompatients withCMML similarly
have extended survival in vitro while monocytes from
healthy volunteers rapidly undergo apoptosis, suggesting
that a disorderin programmed cell death of monocytes
is a critical feature of CMML (Lagasse and Weissman,
submitted). hMRP8bcl-2 mice show decreased survival
compared to their littermate controls but rarely develop
acute malignancies (Lagasse and Weissman, submit-
ted). Despite the deregulation of Bcl-2 expressionfound
in many human cancers, overexpression of Bcl-2 alone
has beenfoundto be relativelybenign interms ofcellular
transformation in transgenic mouse models (Cory et al.,
1994). However, deregulated expression of Bcl-2 cou-
pled to additional mutations such as enforced expres-
sion of c-Myc can rapidly lead to the transformation of
cells of the B lymphoid lineage (Vaux et al., 1988). We
thus reasoned that additional mutations are likely to
act in concert with bcl-2 deregulation to promote acute
Loss-of-functionmutations inthe Fas receptor(CD95)
or in components of the Fas signaling pathway have
been implicated in several human AMLs (Robertson et
David Traver,*Koichi Akashi,
Irving L. Weissman, and Eric Lagasse
Department of Pathology
Department of Developmental Biology
Stanford University School of Medicine
Stanford, California 94305
Fas-deficient (Faslpr/lpr) mice constitutively expressing
Bcl-2 in myeloid cells by the hMRP8 promoter often
develop a fatal disease analogous to human acute
myeloblastic leukemia (AML-M2). Hematopoietic cells
from leukemic Faslpr/lprhMRP8bcl-2 animals form clo-
nogenic blastcolonies invitro andcan transferdisease
to wild-type mice. In vitro ligation of Fas on Fas?/?
hMRP8bcl-2 marrow cells depletes approximately 50%
of myeloid progenitor activity, demonstrating that Bcl-2
can only partially block Fas-mediated death signals in
myelomonocytic progenitors. Inaddition, Faslpr/lprmar-
row contains greatly increased numbers of myeloid
colony-forming cells as compared to Fas?/?controls.
Taken together, these data suggest that Fas has a
novel role in the regulation of myelopoiesis and that
Fas may act as a tumor suppressor to control leuke-
mogenic transformation in myeloid progenitor cells.
Acute myeloid leukemia (AML) accounts for over 80%
of all adult acute leukemias (Schiffer, 1997)and is char-
acterized by a clonal expansion of immature myeloid
cells in all hematopoietic tissues. Many patients pro-
gress to AML from preleukemic myelodysplastic syn-
dromes (MDS) or from chronic myelogenous leukemia
(CML).This progressionto blastcrisisis thought toresult
from the accumulation of genetic lesions in a single
self-renewing progenitor cell that prevents the normal
maturation of the mutant cell and its progeny (reviewed
in Sawyers et al., 1991).
Acutemyeloblastic leukemia withmaturation, orAML-
M2 as classified by the French-American-British (FAB)
system (Bennett et al., 1976), is the most common sub-
type of AML and is characterized by an accumulation of
granulocyte precursors in all hematolymphoid tissues.
Surveys aimed at identifying molecular abnormalities in
AML-M2 have found that approximately 40% of AML-
M2 patients possess a t(8;21) chromosomal transloca-
tion that juxtaposes the aml1 and eto genes (Miyoshi et
al., 1991).Aml1 is a transcriptionfactor of the core bind-
ing factor (CBF) family and has been shownto up-regu-
late the expression of several growth factor genes in-
cluding m-csf, gm-csf, and il-3 (reviewed in Tenen et
al., 1997), and is necessary forthe formationof allhema-
topoietic lineages (Okudaet al., 1996; Wang etal., 1996).
*To whom correspondence should be addressed (e-mail: dtraver@
Figure 1. Morphology of Leukemia in Faslpr/lprhMRP8bcl-2 Mice
(A±C) Preparations of marrow (A), spleen (B), and blood (C) cells of leukemic mice compared to controls. Note predominance of myeloblasts
in all hematopoietic tissues in leukemic Faslpr/lprhMRP8bcl-2 mouse (May-Gru Ènwald/Giemsa, all photos ?187.5 original magnification).
(D) Expression of human Bcl-2 in splenocytes. Expression is seen in all myeloid cells, including intermediate staining in leukemic blasts (?125
(E) Staining for NCAE in control (left) versus leukemic (right) splenocytes. Tumor cells stain slightly positive (red) for the granulocytic NCAE
marker (?125 original magnification).
Blockade of Myeloid Cell Death Leads to AML
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