CANCER CELL : MARCH 2004 203
As the final episode of Sex and the City
drew ever closer, the legion of viewers
who had followed the tempestuous rela-
tionships of Carrie Bradshaw focused on
a single question.Would she end up with
Mr. Big, Petrovsky, a dark horse, or none
of the clever and handsome men who
populated her universe? Carrie and the
fans knew that her choice of partner was
what ultimately mattered. In this issue of
Cancer Cell, Roumiantsev and cowork-
ers find that this principle also explains
major biologic and phenotype features of
The FGFR1 gene, which encodes the
fibroblast growth receptor-1 tyrosine
kinase, is located on chromosome band
8p11. Translocations that join FGFR1
with a number of binding partners gener-
ate fusion proteins with aberrant tyrosine
kinase activity in patients with “8p11
Affected individuals develop a myelopro-
lferative disorder (MPD), and are strongly
predisposed to lymphoblastic T cell lym-
phoma bearing the same 8p11 transloca-
tion, implying a common stem cell origin.
In addition to this association with EMS, a
few patients have been described in
whom FGFR1 is fused to the BCR gene.
BCR is famous for another role in
leukemia biology—it causes chronic
myeloid leukemia (CML) with another
partner gene, the ABL tyrosine kinase.
Interestingly, patients with BCR-FGFR1
fusions develop a CML-like MPD, but
lymphomas have not been reported.
This new work provides a satisfying
molecular explanation for these clinical
differences, and identifies key amino
acids within BCR-FGFR1 and the EMS-
associated ZNF198-FGFR1 protein that
contribute to specific disease pheno-
types. Roumiantsev et al. utilized retrovi-
ral gene transfer into mouse bone
marrow followed by adoptive transfer into
irradiated recipients to investigate how
the ZNF198-FGFR1 and BCR-FGFR1
fusion genes perturb hematopoietic
growth control. Mice transplanted with
cells engineered to express a chimeric
ZNF198-FGFR1 protein consistently
developed both overproliferation of
myeloid cells and T cell lymphomas of
the gastrointestinal tract that closely
model human EMS disease.These data
and a recent report in which a FOP-
FGFR1 protein was expressed in
murine bone marrow (Guasch et al.,
2004) firmly establish a central patho-
genic role of EMS-associated fusions.
Importantly, Southern blot analysis
revealed that ZNF198-FGFR1-infected
marrow generated clonal or oligoclonal
tumors. This indicates intense in vivo
selection, as the fatal malignancies that
arise in recipient animals are derived
from one (or a few) of many thousands of
infected cells. The most likely explana-
tion is that ZNF198-FGFR1 requires one
or more additional mutations to induce
leukemia, which is also a feature of
transgenic and “knockin” strains of mice
that express many other leukemia-asso-
ciated fusions such as PML-RARA and
AML1-ETO (Grisolano et al., 1997;
Higuchi et al., 2002). An intriguing possi-
bility is that retroviral insertions con-
tribute directly to leukemogenesis. This
idea has been used with great success
to perform forward genetic screens in
mice to uncover genes that are mutated
in human hematologic malignancies (Li
et al., 1999). A recent analysis of two
children with X-linked severe combined
immunodeficiency who developed T cell
leukemia after receiving autologous
hematopoieitc cells that had been trans-
duced ex vivo with a retrovirus contain-
ing the IL2RG
gene found LMO1
insertions in both cases (Hacein-Bey-
Abina et al., 2003).Based on an analysis
of retrovirally induced murine T cell
leukemias, Dave and associates (Dave
et al., 2004) proposed that high level
IL2RG expression from the retroviral
long terminal repeat rendered the vector
oncogenic, and that insertion into
Lmo1 provided a cooperating mutation.
By analogy, cloning ZNF198-FGFR1
insertions from murine leukemias and
lymphomas may identify genes and
pathways that contribute to EMS.
Compared with transgenic or gene-
targeting technologies, both of which
require extensive screening and breed-
ing of mice, retroviral vector strategies
sacrifice precise oncogene regulation for
experimental efficiency. The tractable
nature of marrow transduction/transplan-
tation systems permits investigators to
generate allele series in order to analyze
functional domains that are required for
transformation in vivo. Roumiantsev et
al. employed this strategy to show that
FGFR1 kinase activity is essential for
leukemogenesis, and that a phospholi-
pase C-γ1 (PLCγ1) binding site located
at Tyr-766 contributes to the EMS pheno-
type. Interestingly, mice transplanted
with bone marrow cells engineered
to express the latter mutation show
enhanced survival and less myeloprolif-
eration, and succumb from lymphomas
with a more mature immunophenotype.
Whereas primary lymphoma cells from
mice transplanted with ZNF198-FGFR1-
expresing cells showed elevated levels
of activated (phosphorylated) PLCγ1,
this was not true of lymphomas from
recipients that expressed the Tyr-766
mutation.Together, these studies identify
FGFR1 and PLCγ1 as rational biochemi-
cal targets for small molecule therapeu-
tics in EMS.
The authors next found that mice
transplanted with bone marrow engi-
neered to express the BCR-FGFR1
fusion, which is associated with a CML-
like disease in patients, succumbed
from an aggressive polyclonal MPD that
closely mimics the disease induced by
BCR-ABL. Thus, the FGFR1 partner
gene has a major effect on disease phe-
notype. Over a decade ago, Tyr-177 of
BCR was identified as important for
BCR-ABL-induced transformation of
cultured cells by regulating phosphory-
lation of Grb-2 and Ras activation
(Pendergast et al., 1993). Recent work
P R E V I E W S
The sum is greater than the FGFR1 partner
Cancer-associated chromosomal translocations create chimeric oncoproteins that contribute to aberrant growth by domi-
nant or dominant negative mechanisms. Interestingly, genes such as MLL, RARA, and EWS are fused to multiple partners.
This molecular promiscuity can provide important functional information, as specific translocations may be associated
with discrete clinical and molecular features. In this issue of Cancer Cell, Roumiantsev et al. (2004) use a murine retroviral
transduction/transplantation system to analyze two FGFR1 fusions found in hematologic malignancies.Their results show
that these chromosomal rearrangements play a central role in pathogenesis, underscore the role of partner genes in mod-
ulating disease phenotypes, and uncover potential therapeutic targets.
204 CANCER CELL : MARCH 2004
in hematopoietic cells showed that the
Gab-2 adaptor is recruited to Tyr-177,
and is essential for efficiently phospho-
rylating Grb-2, for activating signaling
cascades downstream of Ras-GTP, and
for induction of MPD by BCR-ABL
(Sattler et al., 2002). Similarly, mutating
Tyr-177 in the BCR-FGFR1
markedly attenuated the MPD, and
transplanted mice died from a clonal
EMS-like disease with modest leukocy-
tosis and T cell lymphoma. Interestingly,
somatic activation of a latent mutant
Kras allele also induces a fulminant
MPD (Braun et al., 2004). The aggres-
sive MPDs that result from expressing
BCR-ABL, BCR-FGFR1, or Kras in pri-
mary mouse bone marrow cells argue
that high level, constitutive hyperactiva-
tion of Ras signaling is critical for this
phenotype.This idea does not exclude a
role for hyperactive Ras in the EMS-like
disease; indeed, Guasch et al. (2004)
have reported that the FOP-FGFR1
fusion activates the ERK and phosphati-
dynlinositol-3-kinase cascade in cul-
tured cells. Instead, the specific disease
phenotype may be dictated, in part, by
the degree to which Ras and its effec-
tors are deregulated.
insights regarding the role of FGFR1
fusions in hematologic malignancies,
Roumiantsev et al. have generated
reagents that might be harnessed to
develop treatments for EMS. The Ba/F3
system has been used to test inhibitors
of BCR-ABL, FLT3, and FIP1L1-
PDGFRA and to characterize mutations
that confer resistance to targeted thera-
peutics (Cools et al., 2003; Levis et al.,
2002;Shah et al., 2002).Similarly, Ba/F3
cells engineered to express various
EMS-associated FGFR1 chimeric pro-
teins will be useful for screen for
inhibitors of FGFR1 or PLCγ1.Promising
compounds could be further tested in
mice transplanted with ZNF198-FGFR1-
expressing bone marrow cells to assess
therapeutic index in vivo.
How might candidate small molecule
inhibitors be identified for testing?
Because EMS is an uncommon cancer,
no commercial efforts will be launched to
discover FGFR1 or PLCγ1 inhibitors for
this indication. However, pharmaceutical
companies are performing large-scale
screens to identify kinase inhibitors.
Small molecules that block the FGFR1
kinase might emerge from these efforts,
which could be developed further in part-
nership with academic investigators
and/or the National Cancer Institute.
Moreover, a molecule that is marketed
for another indication might also inhibit
FGFR1 due to the extensive structural
similarity between the active sites of
many tyrosine kinases. For example,
imatinib mesylate was developed as a
BCR-ABL inhibitor but also blocks patho-
genic kinases in gastrointestinal stromal
tumors and idiopathic hypereosinophilic
syndrome (Cools et al., 2003;Joensuu et
al., 2001). Indeed, imatinib mesylate is
now a first line treatment for these rare
malignancies. The tractable experimen-
tal systems developed by Roumiantsev
et al. now set the stage for translating
mechanistic insights into improved thera-
pies for individuals with EMS.
Benjamin S. Braun and
Department of Pediatrics and
Comprehensive Cancer Center
University of California San Fransisco
San Francisco, California 94143
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P R E V I E W S