Cancer Sci|October 2009| vol. 100| no. 10|1786–1793doi: 10.1111/j.1349-7006.2009.01257.x
© 2009 Japanese Cancer Association
Blackwell Publishing Asia
Protein tyrosine phosphatase SHP-2: A proto-
oncogene product that promotes Ras activation
Takashi Matozaki,1 Yoji Murata, Yasuyuki Saito, Hideki Okazawa and Hiroshi Ohnishi
Laboratory of Biosignal Sciences, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi, Gunma, Japan
(Received May 22, 2009/Accepted June 15, 2009/Online publication July 20, 2009)
SHP-2 is a cytoplasmic protein tyrosine phosphatase (PTP) that
contains two Src homology 2 (SH2) domains. Although PTPs are
generally considered to be negative regulators on the basis of their
ability to oppose the effects of protein tyrosine kinases, SHP-2 is
unusual in that it promotes the activation of the Ras-MAPK
signaling pathway by receptors for various growth factors and
cytokines. The molecular basis for the activation of SHP-2 is also
unique: In the basal state, the NH2-terminal SH2 domain of SHP-2
interacts with the PTP domain, resulting in autoinhibition of PTP
activity; the binding of SHP-2 via its SH2 domains to tyrosine-
phosphorylated growth factor receptors or docking proteins,
however, results in disruption of this intramolecular interaction,
leading to exposure of the PTP domain and catalytic activation.
Indeed, SHP-2 proteins with artificial mutations in the NH2-terminal
SH2 domain have been shown to act as dominant active mutants in
vitro. Such activating mutations of PTPN11 (human SHP-2 gene)
were subsequently identified in individuals with Noonan syndrome,
a human developmental disorder that is sometimes associated with
juvenile myelomonocytic leukemia. Furthermore, somatic mutations
of PTPN11 were found to be associated with pediatric leukemia.
SHP-2 is also thought to participate in the development of other
malignant disorders, but in a manner independent of such activating
mutations. Biochemical and functional studies of SHP-2 and genetic
analysis of PTPN11 in human disorders have thus converged to
provide new insight into the pathogenesis of cancer as well as potential
new targets for cancer treatment. (Cancer Sci 2009; 100: 1786–1793)
in colon and pancreatic cancers.(1) Ras was first implicated in
oncogenesis by pioneering studies in the 1980s by Weinberg and
colleagues, who showed that transfection of NIH 3T3 mouse
fibroblasts with DNA derived from human bladder carcinoma
cell lines (T24 and EJ) resulted in their transformation.(2)
Furthermore, they as well as other groups found that the DNA
indeed contained the H-Ras gene with a point mutation at codon
12. Mutations of Ras genes were subsequently identified in a
variety of sporadic human cancers.(1) In parallel with these
genetic studies of human cancer, it was shown that Ras genes
encode small GTP-binding proteins and that the mutations
found in human malignancies result in constitutive activation of
these proteins.(1,2) Such gain-of-function mutations of Ras were
thus concluded to be important for cell transformation (Fig. 1).
In the early 1990s, it became evident that Ras is an essential
component of the signaling pathway by which growth factors
stimulate cell proliferation. The binding of growth factors to
their receptor tyrosine kinases (RTKs) triggers receptor auto-
phosphorylation and the consequent recruitment of an adaptor
protein, designated growth factor receptor-bound protein 2 (Grb2),
which forms a constitutive complex with Son of Sevenless
utations of Ras genes are highly prevalent (30%) in human
malignancies, occurring at particularly high frequencies
(Sos), a guanine nucleotide exchange factor that catalyzes
conversion of the inactive, GDP-bound form of Ras to the
active, GTP-bound form.(3,4) The GTP-bound form of Ras in turn
activates the Raf-MEK-MAPK cascade, which promotes cell
proliferation, differentiation, or survival (Fig. 1). The combination
of genetic analysis of Ras genes in cancer and biochemical
characterization of Ras proteins provided important insight
both into a signaling pathway that mediates the stimulatory
effect of growth factors on cell proliferation as well as into the
mechanism of cell transformation by specific Ras gene mutations.
The role of deregulated activation of Ras in cancer develop-
ment was also revealed by genetic analysis of neurofibromatosis
type 1 (NF1), a genetic disorder characterized by an increased
susceptibility to malignancies including neurofibrosarcoma,
pheochromocytoma, and juvenile leukemia.(5) The gene respon-
sible for this condition, NF1, is a tumor suppressor gene that
encodes the protein neurofibromin, the central domain of which
shows extensive sequence similarity to the catalytic domain of
GTPase-activating proteins (GAPs) for Ras, such as p120
RasGAP, which negatively regulate Ras by increasing the rate of
its intrinsic GTPase activity.(4) Loss of neurofibromin would thus
be expected to mimic the effect of an activating mutation of Ras
in malignant transformation of cells (Fig. 1). However, other
observations suggest that the tumor suppressor function of
neurofibromin is more complex.(5)
Recent studies have revealed a third mechanism for deregulation
of Ras in the development of malignancies. Germline mutations
of the gene PTPN11, which encodes SHP-2, a cytoplasmic protein
tyrosine phosphatase (PTP), have been identified in individuals
with Noonan syndrome (NS), a human developmental disorder
that is sometimes associated with juvenile myelomonocytic
leukemia (JMML) (Fig. 1). Furthermore, somatic mutations of
PTPN11 were also identified in individuals with pediatric leukemia.
In contrast to Ras and neurofibromin, the biochemical analysis
of SHP-2, which was found to promote Ras activation, took
place before the genetic analysis of the corresponding gene and
its role in human disease. Indeed, mutations of SHP-2 that were
shown to result in constitutive activation of this PTP by bio-
chemical studies were subsequently found to coincide with those
identified in individuals with NS or leukemia.
Physiological functions of SHP-2
SHP-2 was identified in the early 1990s by several groups on
the basis of its sequence similarity to the catalytic domain of
known PTPs and with the use of PCR amplification.(6–10) SHP-
2 contains two tandem Src homology 2 (SH2) domains, a single
1To whom correspondence should be addressed.
Matozaki et al.Cancer Sci|October 2009|vol. 100|no. 10| 1787
© 2009 Japanese Cancer Association
PTP domain, and a COOH-terminal hydrophobic tail with two
tyrosine phosphorylation sites (Fig. 2a). Exposure of cells to a
variety of extracellular stimuli triggers the binding of SHP-2
via its SH2 domains both to tyrosine-phosphorylated receptors
for growth factors such as platelet-derived growth factor
(PDGF) as well as to tyrosine-phosphorylated docking proteins
including insulin receptor substrates (IRSs), signal regulatory
protein α (SIRPα; also known as SHP substrate-1 ([SHPS-1]),
Grb2-associated binder proteins (Gabs), and fibroblast growth
factor receptor substrate (FRS) (Fig. 2b).(11–17) Such interactions
are important both for activation of the PTP activity of SHP-2
(see below) and for its recruitment to sites near the plasma
membrane where potential substrate proteins may be located.(18)
Although PTPs are conventionally thought to be negative
regulators on the basis of the fact that they reverse the effects
of protein tyrosine kinases, biochemical and genetic analyses
indicate that SHP-2 promotes Ras activation by growth factors
and cytokines. The first indication of such a role for SHP-2 in
vertebrates came from studies showing that forced expression
of a catalytically inactive mutant of SHP-2 prevented activation
of Ras(19) and MAPK in cultured mammalian cells as well as in
Xenopus.(19–22) In addition, Drosophila (Csw) and Caenorhabditis
elegans (PTP-2) orthologs of SHP-2 were implicated as mediators
of Ras activation downstream of RTKs.(23,24) Homozygous SHP-2
mutant mice, in which amino acid residues 46 to 110, including
most of the NH2-terminal SH2 domain of the protein, were
deleted, were found to die as embryos as a result of a defect in
gastrulation and abnormal mesoderm patterning.(25,26) Fibroblast
growth factor- or epidermal growth factor-induced activation of
MAPK was also found to be attenuated in fibroblasts from
these mice.(25,27) In addition, this same deletion mutation of the
NH2-terminal SH2 domain of SHP-2 suppressed the development
of embryonic stem cell-derived hematopoietic cells.(28) These
observations thus supported the notion that SHP-2 positively
regulates cell growth and differentiation by promoting activation
of the Ras–MAPK pathway (Fig. 2b).
The PTP activity of SHP-2 is indeed now thought to be
required for full activation of Ras, with SHP-2 being thought to
regulate an upstream element necessary for Ras activation.(19,29)
However, the precise mechanism by which SHP-2 promotes
Ras activation remains unclear. Three principal models have
been proposed to date (Fig. 2c). The first model (model a,
Fig. 2c), which is based on observations with cultured mamma-
lian cells, proposes that SHP-2 promotes Ras activation through
dephosphorylation of tyrosine-phosphorylated sites required for
the binding of p120 RasGAP either to RTKs for growth factors
such as PDGF and epidermal growth factor or to the docking
protein Gab-1, thereby preventing inhibition by p120 RasGAP
of Ras activation.(30–32) Similarly, Csw (Drosophila SHP-2), a
Drosophila ortholog of SHP-2, has been proposed to promote
Ras activation through dephosphorylation of the binding site
for RasGAP on the RTK Torso.(33) The second model (model b,
Fig. 2c) for Ras activation by SHP-2 proposes that SHP-2
promotes the activation of Src family kinases (SFKs) through
dephosphorylation either of Csk binding protein (Cbp; also known
as phosphoprotein associated with glycosphingolipid-enriched
membrane microdomains [PAG]), a membrane-anchored phos-
phoprotein that binds COOH-terminal Src kinase (Csk),(34,35) or
of paxillin, thereby preventing the access of Csk, a negative
regulator of SFKs, to these enzymes.(36,37) The third model (model
c, Fig. 2c) proposes that SHP-2 promotes Ras-MAPK activation
through dephosphorylation of Sprouty, a negative regulator of
cell proliferation, differentiation, or survival. Growth factors stimulate the tyrosine kinase (TK) activity and consequent autophosphorylation of
their receptors, resulting in recruitment of the adaptor protein Grb2, which forms a constitutive complex with Sos, a guanine nucleotide
exchange factor that catalyzes conversion of the GDP-bound (inactive) form of Ras to the GTP-bound (active) form. Activated Ras in turn induces
activation of the Raf-MEK-MAPK cascade, which leads to changes in gene expression that are required for the induction of cell proliferation,
differentiation, or survival. Neurofibromin (encoded by the tumor suppressor gene NF1) shows sequence similarity to the catalytic domain of
GTPase-activating proteins (GAPs) that negatively regulate Ras by increasing its intrinsic GTPase activity. SHP-2, a cytoplasmic protein tyrosine
phosphatase (PTP), promotes the activation of Ras by regulating signaling upstream of Ras. Mutations of Ras genes that result in constitutive
activation of the encoded protein (gain-of-function mutation) induce activation of the Raf-MEK-MAPK cascade in the absence of growth factor
stimulation, resulting in cell transformation and development of cancer. Loss of NF1 (loss-of-function mutation) also induces constitutive
activation of Ras and gives rise to neurofibromatosis type 1 (NF1) as well as to cancer. Mutations of PTPN11 (human SHP-2 gene) that result in
constitutive activation of the encoded phosphatase (gain-of-function mutation) promote Ras activation and cause Noonan syndrome (NS) as well
The function of Ras and its deregulation. Ras is an essential component of the signaling pathway that underlies growth factor-induced
© 2009 Japanese Cancer Association
the Ras-MAPK pathway whose tyrosine phosphorylation is
indispensable for its inhibitory effect.(38) Tyrosine-phosphorylated
Sprouty binds Grb2, preventing recruitment of the Grb2-Sos
complex to FRS.(38,39) SHP-2 dephosphorylates Sprouty in response
to growth factor stimulation, thereby preventing its inhibitory
effect on Ras-MAPK activation. Such regulation of Sprouty by
Csw (Drosophila SHP-2) has also been demonstrated.(40)
In addition to the regulation of mitogenic signaling, SHP-2 is
implicated in the regulation of cell adhesion and migration, in
part through control of the activity of the small GTP-binding
protein Rho (Fig. 2b).(41–46) Studies with dominant negative,
dominant active, or loss-of-function mutants of SHP-2 have
thus shown that the regulation by SHP-2 of RTK- or integrin-
dependent cell adhesion and migration is mediated, at least in
part, by Rho, and that SHP-2 negatively regulates Rho activity
in these processes.(44,45,47) SHP-2 was also found to function in
a spatially and temporally specific manner as a positive or
negative regulator of Rho activity in integrin-mediated cell
adhesion and migration.(46) Although the mechanism by which
SHP-2 regulates Rho activity is not completely understood, it is
thought to involve regulation by SHP-2 of Vav2, a guanine
nucleotide exchange factor for Rho family proteins.(44)
Intramolecular regulation of the PTP activity of SHP-2
Biochemical and enzymatic studies have revealed that SHP-2
possesses only low PTP activity in its basal state. In contrast,
synthetic phosphopeptides corresponding to the binding sites
for the SH2 domains of SHP-2 on the PDGF receptor or IRS-1
were shown to markedly increase the PTP activity of SHP-2
in vitro,(48–50) suggesting that the activity of SHP-2 is regulated
by an autoinhibitory mechanism mediated by its SH2 domains
(Fig. 3a). Consistent with this notion, the crystal structure of
SHP-2 indicated that the NH2-terminal SH2 domain indeed
interacts with the PTP domain in the basal state, likely resulting
in autoinhibition of PTP activity (Fig. 3b).(51) This conformation
is maintained by hydrogen bonding between D61 and C459
residues through one water molecule as well as involving N58,
G60, A72, G503, and Q506.(51) Furthermore, occupation of the
NH2-terminal SH2 domain of SHP-2 by phosphotyrosine-
containing ligands (such as tyrosine-phosphorylated RTKs or
docking proteins) was proposed to disrupt the intramolecular
interaction between the SH2 domain and the PTP domain,
resulting in the latter domain becoming available for interaction
with substrate (Fig. 3a). The residues of the NH2-terminal SH2
Domain organization of human SHP-2. SHP-2
contains two tandem SH2 domains (N-SH2 and C-
SH2), a single protein tyrosine phosphatase (PTP)
domain, and a COOH-terminal hydrophobic tail
that includes tyrosine phosphorylation sites. The
residue numbers of amino acids that delineate
the various domains(51) or correspond to the
phosphorylation sites are indicated. (b) In
response to extracellular stimuli, SHP-2 binds via
its SH2 domains either to autophosphorylated
growth factor receptors (such as that for
platelet-derived growth factor [PDGF]) or to
docking proteins (such as insulin receptor
substrates [IRSs], Grb2-associated binder proteins
[Gabs], fibroblast growth
substrate [FRS], and signal regulatory protein α
[SIRPα; also known as SHP substrate-1, SHPS-1])
that are tyrosine-phosphorylated by activated
receptor tyrosine kinases (RTKs) or by Src family
kinases (SFKs). Such interactions result in the
activation of SHP-2
promotion of Ras activation, leading to cell
growth or differentiation. SHP-2 also participates
in the regulation of cell adhesion and migration
by controlling the activity of Rho. (c) Models
proposed for the activation of Ras by SHP-2. In
model a, SHP-2 promotes Ras activation by
dephosphorylating tyrosine-phosphorylated sites
of growth factor receptors that bind p120
RasGAP (GAP). Dephosphorylation of these sites
prevents inhibition of Ras activation by p120
RasGAP. According to model b, SHP-2 promotes
the activation of SFKs by dephosphorylating Cbp/
PAG. Dephosphorylation of Cbp prevents the
access of Csk (a negative regulator of SFKs) to
SFKs, which may promote Ras activation by an as
yet unclear mechanism (dotted arrow). In model
c, SHP-2 promotes Ras-MAPK activation by
dephosphorylating Sprouty, a negative regulator
of Ras. Tyrosine-phosphorylated Sprouty binds
the Grb2–Sos complex and thereby prevents its
interaction with Ras. SHP-2 dephosphorylates
Sprouty in response to growth factor stimulation,
thereby preventing its interaction with Grb2.
Structure and function of SHP-2. (a)
and its consequent
Matozaki et al.Cancer Sci| October 2009| vol. 100|no. 10|1789
© 2009 Japanese Cancer Association
domain that interact with the PTP domain were found not to
overlap with those essential for phosphotyrosine binding.(51,52)
Indeed, two independent forms of SHP-2 (D61A and E76A)
with mutations of residues in the NH2-terminal SH2 domain
that interact with the catalytic domain manifested an increased
PTP activity in an in vitro assay as well as functioned as
dominant active mutants in elongation of animal caps of
Association of PTPN11 mutations with NS and
In 2001, PTPN11 (human SHP-2 gene) was identified as the
susceptibility gene for NS.(53) NS is an autosomal dominant
disorder with an estimated prevalence of 1 in 1000 to 2500 live
births.(54) The main clinical features of NS are short stature,
facial dysmorphia, and congenital heart defects. NS is also
associated with two childhood leukemias, JMML and acute
lymphoblastic leukemia, although these leukemias affect only a
small percentage of NS patients. Missense mutations of PTPN11
have been found to be present in ~50% of individuals with a
clinical diagnosis of NS.(53,54) Furthermore, the residues of SHP-2
commonly mutated in NS (including G60, D61, Y62, Y63,
T73, Q79, N308, and G503) (Fig. 3c) either participate directly
in the interaction between the NH2-terminal SH2 domain and
the PTP domain or are located in close proximity to these
interacting residues, suggesting that the pathogenesis of NS is
related to a loss of autoinhibition of PTP activity resulting from
disruption of the intramolecular interaction between the SH2
and PTP domains (Fig. 4a). Furthermore, in addition to the
tyrosine phosphatase (PTP) activity of SHP-2 and
the distribution of SHP-2 mutations associated
with Noonan syndrome
myelomonocytic leukemia (JMML). (a) Mechanism
for regulation of the PTP activity of SHP-2. In the
basal state, the NH2-terminal SH2 domain of SHP-2
interacts with the PTP domain (closed form),
resulting in autoinhibition of PTP activity. In
response to extracellular stimuli, SHP-2 binds via
its SH2 domains to tyrosine-phosphorylated
growth factor receptors or docking proteins such
as insulin receptor substrate (IRS), resulting in its
adoption of an open conformation (open form)
that is catalytically
homology domain. (b) A ribbon diagram of the
crystal structure of human SHP-2 is shown in the
left panel. The NH2- and COOH-terminal SH2
domains are shown in brown and green,
respectively. The PTP domain is shown in blue.
The circled region in the left panel is depicted in
the right panel, which shows amino acids that
participate in the formation of hydrogen bonds
(dotted lines) that mediate the interaction of
the NH2-terminal SH2 domain with the PTP
domain. Red sphere, oxygen; white sphere,
carbon; blue sphere, nitrogen; yellow sphere,
sulfur. (c) Distribution of residues of SHP-2 that
are frequently mutated in NS or JMML.
Intramolecular regulation of the protein
(NS) or juvenile
active. PH, pleckstrin
© 2009 Japanese Cancer Association
Fig.4. Roles of SHP-2 in human cancer. (a) Mutations
of SHP-2 associated with Noonan syndrome (NS)
or juvenile myelomonocytic leukemia (JMML)
disrupt the intramolecular interaction between
the NH2-terminal SH2 domain and the protein
tyrosine phosphatase (PTP) domain and thereby
result in a loss of autoinhibition of PTP activity.
The constitutive activation of SHP-2 in the
absence of growth factor stimulation results in
aberrant activation of the Ras-MAPK pathway,
which in turn leads to the development of NS or
leukemia. (b) Gab-2, a pleckstrin homology (PH)
domain-containing docking protein, binds and
activates SHP-2 in response to a variety of
cytokines and growth factors. Overexpression of
Gab-2 in human breast cancer may result in
hyperactivation of SHP-2
aberrant activation of the Ras-MAPK pathway.
(c) CagA in Helicobacter pylori (H. pylori)-infected
gastric epithelial cells
phosphorylation by Src family kinases (SFKs).
docking protein for SHP-2 and thereby triggers
aberrant activation of SHP-2 and the Ras-MAPK
pathway and subsequent development of gastric
CagA serves as a
Matozaki et al.Cancer Sci|October 2009|vol. 100|no. 10|1791
© 2009 Japanese Cancer Association
germline mutations of PTPN11 associated with NS, somatic
mutations of PTPN11 have been identified in a substantial
proportion (34%) of JMML patients without NS.(55) Somatic
mutations of PTPN11 are also found in a small percentage of
children with myelodysplastic syndrome (~10%), acute myeloid
leukemia (AML) (~5%), or B-precursor acute lymphoblastic
leukemia (~7%).(55–58) The residues of SHP-2 commonly mutated
in JMML (D61, E69, A72, and E76 in the NH2-terminal SH2
domain) partly overlap with but are not identical to those
associated with NS (Fig. 3c).(54) In contrast to the childhood
condition, PTPN11 mutations appear to be rare in adult AML.(54,59)
Enzymatic analysis has revealed that the PTP activity of
either NS- or leukemia-associated SHP-2 mutants is indeed greater
than that of wild-type SHP-2.(55,60–63) Moreover, SHP-2 mutants
associated with sporadic JMML as well as those related to
NS-associated JMML manifest higher PTP activity than do
those related to NS alone.(55,61–63) Forced expression of these
various SHP-2 mutants in cultured mammalian cells together
with growth factor stimulation resulted in prolonged MAPK
activation in some studies(55,60) but not in others.(61,64,65) Mutations
of PTPN11 that result in constitutive activation of the PTP
activity of SHP-2 thus appear to induce aberrant activation of
Ras and development of NS (Fig. 4a). The higher level of PTP
activity conferred by certain mutations of PTPN11 may result
in a higher level of Ras activation, leading to the development
of JMML or other types of pediatric leukemia. Such a notion is
also supported by evidence that gain-of-function mutations of
K-Ras or N-Ras, or a homozygous loss of NF1, are also associated
with NS or sporadic JMML.(66–69)
Further insight into the molecular mechanism by which
mutation of PTPN11 causes NS or pediatric leukemia has been
provided by the generation of ‘knock-in’ mice expressing the
NS-associated mutation D61G.(65) Homozygous D61G mice were
found to die in utero, whereas heterozygous (D61G/+) mice
exhibited features characteristic of NS, including short stature,
craniofacial abnormalities, and multiple cardiac defects.(65) The
D61G/+mice also developed mild myeloproliferative disease
with peripheral blood leukocytosis as well as mild myeloid
hyperplasia in the spleen and bone marrow (BM). In addition,
BM from D61G/+mice yielded growth factor-independent
myeloid colonies, and BM progenitor cells showed increased
sensitivity to interleukin (IL)-3 and granulocyte-macrophage
colony-stimulating factor (GM-CSF) in a colony formation
assay.(65) Given that the D61G mutation is found not only in NS
but also (albeit infrequently) in sporadic JMML,(54,70) these
observations provided the first evidence that mutations of
PTPN11 indeed cause NS as well as myeloproliferative disease
in vivo. Moreover, forced expression of leukemia-associated
mutants of SHP-2 (E76K or D61Y), but not that of wild-type
SHP-2, was found to promote transformation of BM cells or
fetal liver cells.(71,72) The transformation potency of SHP-2 mutants
associated with leukemia (either NS-related or sporadic) was
shown to be much higher than that of NS-specific mutants.(71)
Forced expression of leukemia-associated SHP-2 mutants also
conferred the property of cytokine-independent formation of
myeloid colonies as well as hypersensitivity to GM-CSF or IL-3
stimulation in hematopoietic cells.(71–73) Furthermore, forced
expression of SHP-2 mutants (D61V, D61Y, or E76K) in macro-
phage progenitors enhanced the GM-CSF-induced activation of
MAPK,(73) and mast cells derived from BM expressing mutant
SHP-2 (D61Y or E76K) showed increased basal and IL-3-induced
activity of MAPK and Akt or hyperphosphorylation of signal
transducer and activator of transcription 5 (Stat5).(71) The trans-
formation activity of the leukemia-associated SHP-2 mutant
E76K was shown to require PTP activity as well as the ability
of the SH2 domains to bind a tyrosine-phosphorylated target
such as Gab-2.(71) Activating mutations of Ras or NF1 deficiency
also cause JMML-like myeloproliferative disease as well as
hypersensitivity of hematopoietic cells to GM-CSF and IL-3.(74,75)
Together, these various observations indicate that hyperactiva-
tion by aberrantly activated Ras of GM-CSF- or IL-3-induced
signaling pathways is responsible, at least in part, for the develop-
ment of NS-associated as well as sporadic JMML.
Recent evidence suggests that the effect of leukemia-associated
SHP-2 mutants on myeloid cell transformation involves inacti-
vation of interferon consensus sequence-binding protein (ICSBP),
also known as interferon regulatory factor (IRF)-8, a transcription
factor that positively regulates NF1 transcription.(76) The tyrosine
phosphorylation of ICSBP is required for its effect on NF1
transcription, and ICSBP was shown to be a substrate for a
constitutively active SHP-2 mutant.(77) Dephosphorylation of
ICSBP by constitutively active mutants of SHP-2 in myeloid
progenitors may thus inhibit the transactivation activity of ICSBP
and thereby down-regulate the abundance of NF1, resulting in
hyperactivation of Ras signaling and excessive cell proliferation.(77)
Although ICSBP deficiency itself induces myeloproliferative
disease that progresses to AML over time in mice, expression
of an active mutant (E76K) of SHP-2 cooperates with ICSBP
deficiency to accelerate progression of AML.(76)
Cancer development associated with up-regulation of
SHP-2 docking proteins
SHP-2 is also implicated in cancer development by a mechanism
different from that for leukemia caused by PTPN11 mutation
(Fig. 4b). Gab-2, a pleckstrin homology domain-containing
docking protein in hematopoietic cells, binds and activates
SHP-2 in response to a variety of cytokines and is important for
recruitment of SHP-2 to sites near the plasma membrane.(16)
Forced expression of Gab-2 promotes proliferation of MCF10A
human mammary cells, and expression of Gab-2 with an activated
form of human EGFR-related 2 (HER2), a receptor-type protein
tyrosine kinase, confers an invasive phenotype on these cells.(78)
Such effects of Gab-2 require its binding site for SHP-2 and
activation of MAPK. Furthermore, the expression of Gab-2 is
increased in human breast cancer,(78,79) whereas mutation of
PTPN11 is infrequent in this and other types of solid tumor.(59,80)
Hyperactivation of SHP-2 due to an increased abundance of
Gab-2 might thus give rise to breast cancer as a result of
aberrant activation of the Ras-MAPK pathway.
Another example of cancer development attributable to increased
expression of a SHP-2 docking protein might be that of gastric
cancer associated with CagA-positive Helicobacter pylori
(H. pylori) (Fig. 4c).(81) Infection with CagA-positive H. pylori
is a risk factor for the development of gastric cancer, and the
CagA protein, which is directly injected by the bacterium into
gastric epithelial cells and undergoes tyrosine phosphorylation
at its EPIYA motifs by SFKs, behaves as a docking protein, like
Gab or IRS, for SHP-2.(82) Indeed, forced expression of CagA
in gastric epithelial cells promotes sustained activation of
MAPK.(83) Transgenic mice expressing CagA manifest gastric
hyperplasia, with some of these animals also developing polyps
or adenocarcinomas in the stomach.(84) These transgenic mice
also manifest leukocytosis as well as IL-3 and GM-CSF hyper-
sensitivity, and some of the animals develop myeloid leukemia(84)
phenotypes similar to those of mice transplanted with BM cells
expressing leukemia-associated mutants of SHP-2.(71)
The convergence of biochemical analysis of SHP-2 with genetic
analysis of PTPN11 in NS and leukemia has shown that SHP-2
is an important regulator of Ras, acting downstream of growth
factor or cytokine receptors, and that gain-of-function mutations
of SHP-2 give rise to human cancer. Following the discovery of
mutations of PTPN11 in NS, Aoki’s group has found germline
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© 2009 Japanese Cancer Association
mutations of H-Ras in Costello syndrome and those of K-Ras or
BRAF in cardio-facio-cutaneous (CFC) syndrome, respectively.(85,86)
Both syndromes are autosomal dominant disorders characterized
by symptoms, such as facial dysmorphia, congenital heart defects,
and mental retardation, that overlap with those of NS.(1,87)
Moreover, patients with Costello syndrome have an increased
risk for malignancy development.(87) Germline loss-of-function
mutations of SPRED1 are also associated with an autosomal
dominant disorder in humans that resembles NF1.(88) Spred1 is a
member of the Sprouty-Spred family of proteins that act as
negative regulators of the Ras–MAPK pathway. On the basis of
these various observations, it has been proposed that disorders
attributable to germline mutations of molecules that participate
in the Ras-MAPK pathway should be termed ‘Ras-MAPK
syndromes’.(87) The discovery of germline mutations of PTPN11
associated with NS has been key to the development of this
It is now clear that the activity of Ras is aberrantly up-regulated
as a consequence of various gene alterations both in sporadic
cancers (such as pancreatic, lung, and colon cancers) and in Ras-
MAPK syndromes (such as NS, NF1, and Costello syndrome).
However, the reason why the different gene mutations that
underlie the aberrant activation of Ras give rise to such different
conditions remains unclear. Given that Ras activates multiple
downstream signaling pathways in addition to the Raf-MEK-
MAPK cascade, including those mediated by phosphoinositide
3-kinase or Rho family proteins,(89) the pathways activated by
Ras in response to different gene mutations might be distinct
and thereby give rise to the development of different conditions.
Identification of aberrantly activated signaling molecules
downstream of Ras in the various types of sporadic cancer or
Ras-MAPK syndromes might provide new therapeutic targets
for cancer treatment.
We thank Dr Yoko Aoki (Tohoku University, Japan) for her critical reading
and comments for the manuscript.
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