Nat Rev Cancer. Author manuscript; available in PMC 2020 Jan 7.
Published in final edited form as:
PMCID: PMC6946181
NIHMSID: NIHMS1064813
PTEN loss in the continuum of common cancers, rare syndromes and
mouse models
Medical Oncology Branch, Center for Cancer Research, National Cancer
Institute, 37 Convent Drive Room 1118B, Bethesda, Maryland 20892, USA.
The publisher's final edited version of this article is available at
Nat Rev CancerAbstract
PTEN is among the most frequently inactivated tumour
suppressor genes in sporadic cancer. PTEN has dual protein and lipid phosphatase
activity, and its tumour suppressor activity is dependent on its lipid
phosphatase activity, which negatively regulates the PI3K–AKT–mTOR
pathway1,2. Germline mutations in
PTEN have been described in a variety of rare syndromes
that are collectively known as the PTEN hamartoma tumour syndromes (PHTS).
Cowden syndrome is the best-described syndrome within PHTS, with approximately
80% of patients having germline PTEN mutations3. Patients with Cowden syndrome
have an increased incidence of cancers of the breast, thyroid and endometrium,
which correspond to sporadic tumour types that commonly exhibit somatic
PTEN inactivation. Pten deletion in mice
leads to Cowden syndrome-like phenotypes, and tissue-specific
Pten deletion has provided clues to the role of
PTEN mutation and loss in specific tumour types. Studying
PTEN in the continuum of rare syndromes, common cancers and mouse models
provides insight into the role of PTEN in tumorigenesis and will inform targeted
drug development.
The tumour suppressor PTEN was first identified in 1997 by
deletion mapping of brain, breast and prostate cancers4,5. Shortly
thereafter, germline PTEN mutations were linked to Cowden
syndrome6 and other
proliferative syndromes7. The term PTEN
hamartoma tumour syndrome (PHTS) is now used to unify these seemingly disparate clinical
syndromes into one entity (see the PHTS
GeneReview on the US National Library of Medicine website; see Further
information). Patients with PHTS are a rare but ideal population to study PTEN biology
and targeted drug development, as loss of PTEN function seems to be driving many of the
phenotypic features of this syndrome. As is common in most tumours, sporadic
(non-hereditary) tumours with somatic PTEN alteration also carry other
genetic alterations, making the role of PTEN more ambiguous. As discussed below, mouse
models have shown that Pten deletion alone is sufficient to cause
tumorigenesis in certain tissues but not in others. However, even when deletion of PTEN
alone has minimal effects, it frequently contributes to tumorigenesis in the context of
other genetic alterations. Efforts to compensate for loss of Pten by
inhibiting the PI3K–AKT–mTOR pathway through genetic or pharmacological
means can be investigated in genetically defined mouse models. PHTS provides a defined
population for clinical trials of pathway-targeted therapies. This Review focuses on
tumours types that occur in Cowden syndrome, that exhibit somatic PTEN
alterations and that develop in mouse models engineered to lose Pten.
The intersection of these three groups provides strong evidence for the functional
importance of PTEN alteration in specific tumour types.
PTEN biology
The PTEN gene spans 105 kb and includes nine exons on
chromosome 10q23. Tumour suppressor function requires both the phosphatase domain
and the C2 or lipid membrane-binding domain (), and mutations have been reported throughout the protein. The lipid
phosphatase activity of PTEN dephosphorylates the 3-phosphoinositide products of
PI3K. 3-phosphoinositides can activate important survival kinases, such as
phosphoinositide-dependent kinase 1 (PDK1; encoded by PDPK1) and
AKT, as well as other proteins that are not kinases (). PTEN therefore negatively regulates the AKT pathway, leading to
decreased phosphorylation of AKT substrates such as tuberous sclerosis 2 (TSC2) and
PRAS40 (encoded by AKT1S1) that control mTOR activity, p27 (encoded
by CDKN1B), p21 (encoded by CDKN1A), glycogen
synthase kinase 3 (GSK3A and GSK3B), BCL-2-associated agonist of cell death (BAD),
apoptosis signal regulating kinase 1 (MAP3K5; also known as ASK1), WT1 regulator
PAWR (also known as PAR4) and CHK1, as well as members of the fork-head
transcription factor family (for example, FOXO1, FOXO3 and FOXO4)8 and others. Changes in phosphorylation alter
the activity and/or localization of these proteins, which in turn affects processes
such as cell cycle progression, metabolism, migration, apoptosis, transcription and
translation.
Schematic of the PTEN protein.PTEN contains two key domains that are required for its tumour
suppressor function; the phosphatase (catalytic) domain (amino acids
14–185)165
with an active site included within the residues 123 and 130 (REF. 166), and the C2 (lipid membrane-binding) domain
(amino acids 190–350)167. The importance of other domains such as the PDZ-binding
domain (in grey; amino acids 401–403)168, which binds proteins containing PDZ
domains, and the carboxy-terminal region (amino acids 351–400), which
contains PEST sequences and may contribute to protein stability and
activity169, is less
defined in the tumour suppressor functions of PTEN.
Although the lipid phosphatase activity of PTEN is important for its tumour
suppressor functions, other functions of PTEN may also prove to be important. For
example, several studies have demonstrated that PTEN protein phosphatase activity is
important for its functions in cell cycle arrest and inhibition of cell invasion
in vitro9–13. The lipid
phosphatase activity of PTEN is thought to mostly occur at the cell membrane, but
PTEN has also demonstrated nuclear functions. The binding of PTEN to centromere
protein C1 (CENP-C1) is required for centrosome stability, and its nuclear
localization is required for DNA double-strand break (DSB) repair that is mediated
by DNA repair protein RAD51 (REF. 14). PTEN
also regulates the tumour suppressor function of anaphase-promoting complex (APC)
and its regulator E-cadherin (encoded by CDH1) in the nucleus,
independently of its lipid phosphatase activity15. Altered APC–CDH1 activity has been implicated in
multiple tumour types16.
PTEN mutations and cancer.
Germline mutations resulting in the loss of PTEN function or in reduced
levels of PTEN are found in approximately 80% of patients with Cowden
syndrome3, and PTEN
deletion, mutation or alteration occurs in many sporadic tumours17. The Sanger Institute
maintains a database of PTEN mutations with 1,904 annotated
mutations for 30 tumour types (see the Catalogue of Somatic Mutations in Cancer
(COSMIC) website; see Further information). From this database,
it is clear that in sporadic tumours, mutations, small insertions and deletions
occur throughout the length of PTEN, although there are higher
frequency mutations, known as mutation hotspots, at specific amino acids.
However, mutations at these hotspots are not specific for a particular type of
cancer. For example, more than 250 different PTEN mutations
have been described for endometrial tumours, but 19% of the 632 reported
mutations correspond to Arg130 within the phosphatase catalytic site. Mutations
in Arg130 occur in other tumour types (such as 4% of central nervous system
(CNS) tumours), but they are most frequent in endometrial and ovarian tumours
(19%). Mutant PTEN was reported in 18% of CNS tumours, with the
highest frequency (6% of PTEN mutations) corresponding to
Arg.
Germline PTEN mutations in PHTS are found throughout most of the
PTEN coding region, with the exception of exon 9, which
encodes the carboxy-terminal 63 amino acids18; 40% occur within exon 5, which encodes the phosphatase
domain18. In sporadic
tumours, only 2% of reported sporadic PTEN mutations occur
within exon 9 and 27% occur within exon 5. Correlations between specific
PTEN mutations and disease severity in PHTS have been
suggested3,19. However, larger data sets and more
detailed functional mapping of PTEN will certainly allow more informed models.
Allelic or total deletion of PTEN is a frequent occurrence in cancers such as
breast and prostate cancer, and melanoma and glioma (see the Tumorscape website; see Further information).
A subset of patients with Cowden syndrome carries germline mutations in the
PTEN promoter or in potential splice donor and acceptor
sites20. Splicing
alterations can lead to exon skipping that alters PTEN function, but promoter
methylation has been shown to decrease apparently normal PTEN21. In mice, decreasing PTEN
dosage correlates with increasing tumour susceptibility22,23. This suggests that reduced levels of normal PTEN are
insufficient for its tumour suppressor function and raises the possibility that
regulation of PTEN activity could be an important driving mechanism for
cancer.
PTEN dosage.
There are multiple mechanisms for the regulation of PTEN, including
transcription, mRNA stability, microRNA (miRNA) targeting, translation and
protein stability. PTEN is transcriptionally silenced by
promoter methylation in endometrial, gastric, lung, thyroid, breast and ovarian
tumours, as well as glioblastoma24–30. In
glioma, lung and prostate cancer, PTEN expression is decreased by overexpression
of miRNA 21 (miR-21), miR-25a, miR-22 or the miR-106b–25
cluster31–33. PTEN can also be
post-translationally regulated by phosphorylation, ubiquitylation, oxidation,
acetylation, proteosomal degradation and subcellular localization (reviewed in
REFS 34,35). Although many of these post-translational changes in PTEN have
been shown to alter various cellular phenotypes in vitro, most
have not been validated as key regulators of PTEN in human cancer or mouse
models. PTEN amino acids Lys13 and Lys289 are monoubiquitylated, which leads to
nuclear import in vitro, and Lys289 mutations have been
observed in Cowden syndrome and associated with nuclear exclusion36. No Lys289 mutations have been
reported in sporadic cancers, although Lys13 mutation was found in four of 632
endometrial cancers (see the COSMIC database; see Further information).
Cancers classically associated with PHTS
Germline PTEN mutation in Cowden syndrome can lead to
decreased or absent expression or activity of the mutant allele. Initial efforts to
model Cowden syndrome in mice used genetic deletion of a single allele of
Pten, as loss of both alleles is embryonic lethal. These
Pten heterozygous (Pten+/–)
mice recapitulated some of the neoplastic phenotypes observed in patients with
Cowden syndrome, such as breast and endometrial tumours and intestinal
polyps37–39. However, the genetic background of
Pten+/– mice is a strong determinant of
susceptibility to specific tumour types (BOX
1). Some strains exhibit tumour types that are not typically associated
with Cowden syndrome, such as prostate and adrenal tumours and lymphoma40, whereas other strains show a
reduced incidence of tumours types that are normally associated with Cowden
syndrome, such as breast and endometrial tumours41. Decreasing PTEN dosage has been shown to correlate with
increasing tumour formation in mice, supporting the value of
Pten+/– mice as models for Cowden
syndrome.
Box 1 |
What determines tumour risk in Cowden syndrome?
A limited number of mouse studies suggest that both the type of germline
Pten mutation and the genetic background can affect risk for
specific tumour types. Comparison of three different Cowden syndrome-specific
Pten mutations in the same mouse strain indicated that specific
Pten mutations may contribute to risk for specific tumour
types153. In this study,
specific mutations altered the relative frequency of uterus, prostate, thyroid and
mammary neoplasms but did not alter the range of tumour types. These types of
studies may help to stratify PTEN mutations in patients with Cowden
syndrome in order to identify those at the highest risk for specific tumour types.
Conversely, studies using Pten+/– and
PtenΔ5/+ (deletion of exon
5) mice indicate that genetic background is also a very strong determinant of tumour
susceptibility in mice153. Given
the diversity of the human genome, identification of risk factors that contribute to
tumour susceptibility in Cowden syndrome might help to predict the risk of specific
tumours in this population. For example, polymorphisms in caspase 8 have been
identified as risk factors for breast and ovarian cancers in tumour-prone
BRCA1 mutation carriers154. Naturally occuring polymorphisms within
PTEN itself are found at a disproportionately high rate in
patients with Cowden syndrome, even in the absence of apparent PTEN
mutation, suggesting that certain PTEN haplotypes might function as
risk-modifying factors20. However,
given the number of different PTEN mutations in Cowden syndrome
that may also affect risk even large genome-wide association studies (GWAS) might
have trouble detecting additional risk loci. Identification of risk-modifying loci
in inbred mouse models for Cowden syndrome could inform more targeted searches for
human risk factors. In addition, risk factors for Cowden syndrome tumours might also
prove to be risk factors for PTEN-mutant sporadic tumours. However, in Cowden
syndrome, PTEN alteration in non-tumour cell types, such as stroma, endothelial and
immune cells, may also contribute to increased tumour risk46,155,156 possibly exacerbating other
general risk factors.
Somatic PTEN alteration is common in many sporadic tumour
types42, some of which
also occur with germline PTEN alteration in Cowden syndrome (). This suggests that
PTEN alteration may be an aetiological factor in these tumour
types. Various tissue-specific and/or inducible homozygous deletions of
Pten have been generated in mice to model sporadic PTEN loss in
tumorigenesis. In the endometrium43, mammary gland44 and prostate45, and in T cells46, homozygous deletion of Pten led to rapid
tumour formation in the targeted tissue. Tumours took longer to develop after
Pten deletion in the liver47, bladder48 and lung49.
By contrast, when Pten was deleted in pancreatic
β-cells50 or the
intestine51, no malignant
tumours developed, although intestinal polyps were common, as observed in Cowden
syndrome. Loss of other tumour suppressors or the activation of oncogenes can
nonetheless combine with PTEN loss to cause cancer in these organs.
The following sections describe the intersection of PHTS, sporadic cancer and mouse
models to delineate the role of PTEN alteration in specific cancers.
Table 1 |
Summary of evidence for PTEN and Pten
alteration in specific cancers, by tissue
Tissue | PTEN alteration in human
cancer | Neoplasms and tumours in PHTS | Tumours in
Pten+/− mice | Mice: tissue-specific deletion outcome | Mice: enhanced tumours in the presence of
additional alterations* | Refs |
---|
Breast | Mutation <5%, LOH 40%, promoter
methylation 50% and loss of expression ~40% | 25–50% lifetime risk for women | Yes | Tumours | Wnt or Erbb2
transgenes | 39,44,57,58 |
Endometrium | Mutation 35–50% | Yes | Yes | NR | Mlh1−/−
accelerated Pten LOH in
Pten+/−
mice | 39,60–63,68 |
Thyroid | Homozygous deletion <10%, promoter
methylation >50%, and rearrangement in most papillary thyroid
carcinomas | Yes | Late onset and low frequency | Goiter and benign follicular adenomas in
females | Thyroid hormone receptor-β
(Thrb) transgene, with metastasis | 24,70–76 |
Prostate | Frequent LOH and miR-22 and miR-106b-25
cluster overexpression | NR | Late onset | Early onset of invasive, metastatic prostate
tumours | Cdkn1b+/−,
Nkx31−/−,
Tmprss2-Erg fusion protein and SV40 Tag | 42,74,89,91–94 |
Leukaemia or lymphoma | Deletion 10% of T-ALL and 27% mutation in
T-ALL | NR | Lymphoma and radiation decreases latency | Early onset lymphoma and autoimmunity (T cell
deletion) | NR | 40,124,126–128,212 |
Glioma | LOH >70%, mutation 44% (coincident with
LOH) and miR-26a amplification | Dysplastic gangliocytoma of the cerebellum in
LD | NR | Macrocephaly, seizures and benign cerebellar
abnormalities | Mutant Hras, SV40 Tag,
Trp53−/−,
Trp53+/− and
Nf1+/− | 31,77–81,83–87 |
Melanoma | LOH 30–60%, mutation 10–20%
(metastases) and >50% frequent promoter methylation in patients
with XP | NR | NR | No spontaneous melanoma but melanoma induced
by carcinogen in 50% | Braf−/− | 97,99–103,213,214 |
Lung cancer | Mutation infrequent, promoter methylation
frequent, miR-21 upregulation 74% and loss of PTEN 74% | Occasional | NR | Late-onset lung adenocarcinoma 87% and
increased carcinogen-induced lung tumours | Mutant Kras | 30,32,49,105–109,112 |
Liver | Mutation <5%, PTEN expression lost in
12% and PTEN expression lost in HepC HCC | NR | Infrequent | Fatty liver and insulin hypersensitivity | Vhl−/− | 116–118,215,216 |
Bladder | LOH 23%, homozygous deletion 6%, mutation 23%
(late stage) and decreased or absent PTEN expression 53% | NR | NR | Late-onset transitional cell carcinomas in
10% | Trp53−/− | 48,120–122 |
Kidney | LOH 25% | NR | NR | NR | NR | 120 |
Pancreas | Altered localization common | NR | NR | Metaplasia and carcinoma 20% | Smad4−/− | 113–115 |
Adrenal pheochromocytoma | LOH more common in malignant than in benign
tumours | NR | Yes | NR | Cdkn2a−/− | 39,103,123 |
Colon and intestine | Up to 18% mutated and up to 19% LOH depending
on tumour type | Yes and benign polyps in >90% | Hyperplastic changes | NR | Apc+/− | 37,217–219 |
Breast cancer.
Female patients with Cowden syndrome have a high risk (an estimated
25–50% risk) of developing breast cancer over the course of their
lifetime, and male patients with Cowden syndrome are also thought to be at an
increased risk52. PTEN loss
can also occur in other populations at a high risk of breast cancer, such as
those that carry germline mutations in BRCA1 in which
PTEN deletions have been described53, and can also occur in those at an
indeterminate risk. For example, despite the fact that less than 5% of sporadic
breast tumours harbour PTEN mutations, loss of PTEN
immunoreactivity is observed in nearly 40%54. This highlights the importance of immunohistochemistry
methodology in determining PTEN status55. Moreover, about 40% display loss of heterozygosity
(LOH) at 10q23 (REF. 56), and aberrant
promoter methylation was identified in nearly 50% of tumours25. As PTEN loss and
ERBB2 mutations both activate the AKT signalling pathway,
perhaps it is not surprising that many tumours that exhibit loss of
PTEN are also oestrogen receptor (ER)-positive and
ERBB2-negative54.
Pten+/– mice can develop mammary
tumours at high frequencies depending on their genetic background39. Deletion of both
Pten alleles in the mammary epithelium leads to altered
mammary development and high-frequency, early-onset tumours in mice44. Loss of a single
Pten allele accelerated tumorigenesis in a Wnt-induced
mammary tumour model, and most tumours lost the remaining Pten
allele57. Similar
results were observed when breast-specific Pten deletion was
coupled with overexpression of Erbb2 (REF. 58). In two other models, subtle decreases in
PTEN expression increased the risk of tumour formation in the absence of any
other introduced mutations22,23. These mouse studies suggest
that decreased PTEN expression leads to an increased risk of breast tumour
formation. Attenuated PTEN expression by gene mutation, LOH or promoter
methylation may indeed be a driving alteration in breast cancer, making PTEN
signalling pathways or pathways downstream of PTEN potential targets for breast
cancer therapy.
Endometrial cancer.
The lifetime risk of endometrial cancer for patients with Cowden
syndrome is estimated to be 5–10%52,59, and
35–50% of sporadic endometrial carcinomas have PTEN
mutations (). Mutations in PTEN
are also observed in endometrial hyperplasia, which is thought to be a precursor
lesion for endometrial carcinoma60–62. Many
endometrial tumours have short insertion or deletion frameshift mutations that
are typical of microsatellite instability. In particular, PTEN
frameshift mutations are observed in endometrial carcinomas that are associated
with hereditary non-polyposis colon cancer syndrome (HNPCC)63. In addition, polymorphisms in DNA
mismatch repair genes affect the risk of endometrial tumours64, suggesting that the alterations in
PTEN that contribute to endometrial tumours can arise as a
result of compromised DNA repair mechanisms. In endometrial tumours, activation
of AKT is associated with loss of PTEN65.
In mice, loss of Pten is sufficient to cause
endometrial carcinogenesis. Depending on strain background,
Pten+/– mice can develop endometrial
hyperplasia with high penetrance, which in some cases can progress to
endometrial carcinoma as the mice age39. In this model, most malignant tumours lose the
remaining Pten allele39, leading to AKT activation and subsequent ERα
phosphorylation and activation66. Consequently, ER antagonists can substantially decrease
hyperplasic lesions and tumour formation in these mice66. Likewise, inhibition of mTOR,
downstream of PTEN–AKT, can prevent the progression of endometrial
hyperplasia67.
The role of DNA repair in the maintenance of PTEN
integrity is also highlighted in mouse models of endometrial cancer. Familial
mutations in the DNA mismatch repair gene MLH1 underlie HNPCC,
and deletion of Mlh1 in
Pten+/– mice accelerated endometrial
carcinoma formation68.
Mlh1 deletion was associated with earlier LOH for the
remaining Pten allele68, suggesting that Pten may be
particularly susceptible to disruptions in DNA repair.
Thyroid cancer.
Thyroid tumours were one of the first tumour types to be associated with
Cowden syndrome69.
Subsequently, about 25% of benign thyroid adenomas and several sporadic
malignant thyroid tumour types were found to have PTEN LOH,
with PTEN mutations occurring less frequently70,71. Complete loss of PTEN expression occurs in less than
10% of thyroid tumours, but occurs at a higher frequency in the anaplastic
subtype72. A more
recent study found methylation of the PTEN promoter in more
than 50% of thyroid tumours of various histologies, particularly follicular
carcinoma, and loss of PTEN immunoreactivity correlated significantly with
promoter methylation24. In
addition, PTEN is rearranged in most papillary thyroid
carcinomas, and in a subset of normal thyroid samples, leading to putative
non-functional PTEN73.
Despite the high prevalence of PTEN alterations in
human tumours, Pten+/– mice
only develop thyroid lesions with late onset and low frequency74. However, homozygous deletion
of Pten in mouse thyroid cells led to the development of
goiters and benign follicular adenomas in female mice75. Decreased gene dosage of PTEN may
nonetheless promote thyroid carcinogenesis, because hemizygous deletion of
Pten accelerated thyroid adenocarcinoma formation that was
induced by a dominant-negative mutant thyroid hormone receptor-β, and
increased metastases to the lung76. In addition, hemizygous deletion of Pten
also cooperated with loss of p27 to accelerate thyroid tumorigenesis74. These data suggest that
Pten mutation alone may not drive thyroid carcinogenesis in
mice, but can contribute to the malignant phenotype in the setting of other
genetic alterations.
Central nervous system tumours.
PTEN loss is observed in benign and malignant brain tumours.
Lhermitte–Duclos disease is a rare benign tumour (a dysplastic
gangliocytoma of the cerebellum) that frequently occurs in patients with Cowden
syndrome and is associated with a high rate of morbidity52. PTEN LOH occurs in
more than 70% of glioblastomas, with mutation of the remaining
PTEN allele found in 44%77. Decreased PTEN expression is
characteristic of tumour progression, as lower grade gliomas express higher
levels of PTEN than glioblastomas78,79.
Independently of tumour grade, higher PTEN expression levels significantly
correlated with increased overall survival78. miR-26a, which targets PTEN mRNA for
degradation, is amplified in glioma and often associated with
PTEN LOH, suggesting that in this tumour type, multiple
mechanisms may coexist to attenuate PTEN expression31.
Pten+/– mice do not develop brain
tumours, but homozygous deletion of Pten in mouse brain
resulted in abnormalities that resembled those occurring in patients with
Lhermitte–Duclos disease80,81. Deletion was
associated with an increase in neural stem cells82 (BOX 2). Deletion of Pten alone in adult mouse glial
cells does not lead to glioma formation, but Pten deletion can
contribute to rapid glioma formation in the context of additional genetic
alterations. For example, Pten deletion accelerated high-grade
malignant astrocytoma formation in the presence of activated HRAS1 (REF. 83), and Pten
hemizygosity accelerated astrocytoma formation by SV40 T antigen84. Heterozygous deletion of
Pten also accelerated glioblastoma formation that is
induced by brain-specific heterozygous or homozygous deletion of
Trp53 (REFS 85,86) or heterozygous deletion of both
Trp53 and neurofibromatosis 1
(Nf1)87. Deletion of Pten accelerated glioma
progression that is induced by overexpression of platelet-derived growth factor
(PDFG). Overexpression of miR-26a also accelerated PDGF-induced glioma and
decreased survival. This effect was dependent on PTEN, validating the
Pten-targeting role of miR-26a in glioma31.
Box 2 |
The role of PTEN in the maintenance of tissue and cancer stem cells
The fact that loss of PTEN can cause or contribute to
tumorigenesis in several tissues suggests that PTEN might control tumour-initiating
cells. In fact, Pten deletion can increase the self-renewal
capacity of normal stem cells and increase the number of putative tumour-initiating
cells. In neural stem cells, Pten deletion increases self-renewal
capacity157, which was
further augmented by co-deletion of Trp53 (REF. 86). Pten deletion in the adult
subependymal zone also increased neural stem cell self-renewal, leading to enhanced
olfactory bulb mass and enhanced olfactory function158. Increased stem and progenitor cells have
been reported in Pten-deficient prostate, lung, intestinal and
pancreatic tissues before tumour formation49,114,159–161. In both haematopoietic cells and melanocytes,
Pten deletion leads to normal stem cell exhaustion102,162,163, but
paradoxically, in haematopoietic cancer stem cells, Pten deletion
leads to unlimited expansion162,164. Although still an emerging
concept, the role of tumour-initiating cells and control by PTEN is an area of
intense investigation.
Pten loss in non-PHTS-associated cancers
Prostate.
Prostate tumours have not been associated with Cowden syndrome, perhaps
owing to their high incidence in the general population. One of the early
cytogenetic abnormalities identified in prostate cancer was the deletion of
chromosome 10q88, and nearly a
decade later frequent PTEN loss in primary prostate cancer was
mapped to this region89.
Prostate cancer is the most common malignancy in men, and the role of PTEN in
prostate tumorigenesis and tumour progression has been extensively studied in
mice.
Pten+/– mice develop prostate tumours
from 9 months of age74.
Homozygous deletion of Pten in the mouse prostate led to
prostatic intraepithelial neoplasia (PIN) lesions at 6 weeks of age that
progressed to invasive and metastatic prostate carcinoma within a few
weeks45. In this
model, prostate tumours responded to androgen ablation, which prolonged
survival. However, highly proliferative prostate tumours were observed in these
mice at necropsy, suggesting that this is a faithful model of disease
progression in humans, in which androgen-independent tumours arise after
androgen-ablation therapy90.
Pten+/– mice have been crossed with
various other strains of genetically engineered mouse (GEM) models that
represent the genetic or phenotypic changes that are observed in human prostate
cancer. In many cases, concurrent Pten hemizygosity coupled
with deletions in other genes accelerates tumorigenesis. For example, concurrent
deletion of Cdkn1b, which is often lost in human prostate
tumours, accelerated prostate tumorigenesis74. Concurrent deletion of the transcription factor
Nkx3.1 decreased survival, increased metastasis and
resulted in tumours with androgen independence, which is associated with a poor
prognosis in patients with prostate cancer91. A Tmprss2–Erg translocation,
which was recently described in human prostate tumours92, in mice can cooperate with
Pten hemizygosity to accelerate invasive prostate
adenocarcinoma93,94. Heterozygous deletion of
Pten also accelerated prostate tumorigenesis and decreased
survival in the transgenic adenocarcinoma of the mouse prostate (TRAMP) mouse
model95. The use of
Pten hypomorphic alleles demonstrated that decreasing PTEN
levels correlate with increased progression of prostate tumours in the
mouse96, suggesting
that Pten may be haploinsufficient for prostate tumorigenesis
and/or prostate tumour progression.
Melanoma.
Despite the fact that melanomas have not been associated with Cowden
syndrome, sporadic melanomas frequently have a loss of PTEN
through LOH, deletion and mutation97. PTEN can also be epigenetically silenced
in melanoma, as decreased PTEN transcript levels were
associated with PTEN promoter methylation98. PTEN methylation also
correlated with decreased survival99. In another study, low PTEN expression was associated with
melanoma ulceration, which is characteristic of aggressive tumours, but did not
significantly correlate with overall survival100. A link between DNA damage and PTEN
mutation in melanoma has been suggested by Wang et
al.101, who
showed that more than 50% of the melanomas from patients with xeroderma
pigmentosum showed PTEN mutation types that are typically
associated with ultraviolet radiation exposure101.
In mice, Pten deletion in pigmented mouse cells does
not lead to the development of spontaneous melanoma, despite an increase in the
number of dermal melanocytes. However, in this model, topical carcinogen
treatment led to melanoma formation in nearly 50% of the mice within 20
weeks102. In
conjunction with Cdkn2a (encoding p14ARF) deletion, nearly 10%
of Pten+/– mice developed spontaneous
melanoma103.
Simultaneous activation of BRAF and deletion of Pten in
melanocytes leads to early onset spontaneous melanomas, with metastasis to the
lymph nodes and lung104.
Notably, the mTOR inhibitor rapamycin increased survival in these mice by more
than twofold104. These mouse
studies indicate that Pten is probably not a driving mutation
in melanoma, but can contribute to a malignant phenotype in the presence of
other genetic alterations.
Lung cancer.
Lung cancer has rarely been described in Cowden syndrome105 and somatic
PTEN mutations occur at a low frequency in small-cell lung
cancer (SCLC)106 and
non-small-cell lung cancer (NSCLC)107. However, other mechanisms to diminish PTEN function
may be more important in lung cancer. For example, 24% of early NSCLC samples
lack PTEN expression, which correlated with PTEN promoter
methylation30. In a
later study, PTEN protein expression was reduced or lost in 74%
of lung tumours, and was associated with low or aberrant TP53
staining108. Levels
of miR-21 were upregulated in lung tumours compared with normal lung tissue in
74% of cases and were correlated with decreased levels of PTEN
mRNA and advanced tumour stage32.
PTEN function may determine treatment outcome in lung cancer. Mutant
epidermal growth factor receptor (EGFR) is a frequent driving
mutation in lung cancer in never-smokers109, whose tumours initially respond to treatment with
EGFR inhibitors. However, resistant tumours emerge through multiple mechanisms,
one of which might be homozygous deletion of PTEN110. Regardless of
EGFR status, PTEN promoter methylation is
significantly associated with poor outcome in surgically treated early stage
lung cancer111.
Pten+/– mice have not been reported
to develop lung tumours. However, lung-specific homozygous deletion of
Pten in alveolar type II cells led to lung adenocarcinoma
in 87% of mice at 40–70 weeks of age, and increased both the number and
size of urethane-induced lung adenomas49. Lung-specific homozygous deletion in bronchiole
epithelium cells did not produce tumours in mice, but accelerated tumours driven
by mutant Kras, and dramatically decreased survival112.
Pancreatic cancer.
Pancreatic cancer is not associated with Cowden syndrome, and mutations
in PTEN are rare in sporadic cancers. However, pancreatic
tumours frequently have altered localization of PTEN, suggesting that
subcellular sequestration of PTEN may decrease its function113. In mice, homozygous deletion of
Pten in the pancreas leads to metaplasia, which progresses
to carcinoma in about 20% of mice114. Pten deletion in pancreatic
β-cells only, does not lead to tumour formation50. However, co-deletion of
Smad4, the common mediator of signal transduction by
transforming growth factor-β (TGFβ), does lead to tumour
formation, which is accompanied by increased active AKT and mTOR
signalling115. These
results suggest that PTEN might contribute to pancreatic cancer.
Studies of human cancer and mouse models suggest that alterations in
PTEN might have some role in pancreatic tumours113–115, liver tumours47,116–119,
bladder tumours48,120–122, adrenal pheochromocytomas123, leukaemia124,125 and lymphoma40,46,126–128. However, in most cases the available human data do
not support PTEN as a major factor in these tumour types.
Supporting data are included in .
Drug development for PTEN-deficient disorders
Mouse models of tumorigenesis and diseases such as Cowden syndrome can not
only help to discern cause–effect and mutation–disease relationships,
but can also be used for preclinical testing and to validate targets for cancer
therapy and prevention. For example, deletion of Akt1 in
Pten-heterozygous mice prevents endometrial and prostate
tumorigenesis, and heterozygous deletion of Mtor or
Mlst8 (a component of both mTOR TORC1 and TORC2 complexes)
prolongs the life of mice with prostate tumours that are associated with
prostate-specific deletion of Pten129,130. A hypomorphic mutation in Pdpk1 (REF. 131) and a pharmacological inhibition of
mTOR132 both prevent the
formation of multiple tumour types in Pten+/–
mice. These data suggest that inhibitors of pathway components such as AKT1, mTOR or
PDK1 might be developed for cancer prevention in or the treatment of patients with
germline or tumour-specific PTEN mutations. Inhibitors of mTOR,
such as rapamycin (also known as sirolimus) and its analogues, temsirolimus and
everolimus, can prevent tumorigenesis in multiple mouse models of cancer. For
example, everolimus reduced the progression of endometrial hyperplasia, and
sirolimus reversed premalignant lesions and/or decreased proliferation in prostate
tumours in Pten+/– mice67,133.
Metformin, an activator of AMP-activated protein kinase (AMPK) that leads to
inactivation of mTOR, delayed tumour onset in
Pten+/– mice134.
Several compounds that have been designed to inhibit the
PI3K–AKT–mTOR pathway in cancer are in clinical development, including
newer mTOR inhibitors that target the ATP-binding domain. Some of these have
cross-reactivity with class I PI3Ks and other proteins with PI3K domains (). These pathway inhibitors may be
useful in the prevention of malignancy or in treating existing tumours. Patients
with germline mutations of PTEN could be an ideal population to
test these inhibitors, as pathway activation is a feature of both benign and
malignant tumours in Cowden syndrome. Easily accessible benign tumours in the skin
and gastrointestinal tract of patients with Cowden syndrome could provide in
vivo evidence of target modulation and be a reliable surrogate for
cancer cells.
Table 2 |
Selected drugs targeting the PI3K–AKT–mTOR pathway that is
activated in tumours deficient for PTEN
Drug | Target | Human trials | Human results | Mouse results |
---|
XL147 | PI3K | Phase I/II | One partial response in NSCLC in Phase
Ia176 | Not reported |
GDC-0941 | PI3K | Phase I | One partial response in breast cancer in Phase
Ia177 | Growth inhibition but not regression in
xenografts178
and prolonged tumour regression in combination with imatinib179 |
PX-866 | PI3K | Phase I/II | Best response reported: stable disease in 7 of
31 evaluable patients in Phase Ia180 | Prevents TGFα-induced pulmonary
fibrosis in mice181 |
BKM120 | PI3K | Phase I/II | One partial response in triple-negative breast
cancer in Phase Ia182 | Prevents emergence of resistance to inhibitors
of SMO in medulloblastoma xenografts183 |
CAL-101 | PI3K (delta) | Phase I/II | Objective response rate 9 of 15 in indolent
NHL, 6 of 7 mantle cell lymphoma and 4 of 17 CLL in Phase Ia184 | Not reported |
BEZ235 | PI3K and mTOR (TORC1 and TORC2) | Phase I/II | Two partial responses in Cowden syndrome and
breast cancer in Phase Ia185 | Prevents emergence of resistance to inhibitors
of SMO in medulloblastoma xenografts183 |
SF1126 (h) | PI3K | Phase I | Best response reported: stable disease in
Phase Ia186 | Prevents tumour growth in xenografts187 |
GDC-0980 | PI3K and mTOR (TORC1 and TORC2) | Phase I | One partial response in mesothelioma in Phase
Ia188 | Not reported |
XL765 | PI3K and mTOR (TORC1 and TORC2) | Phase I/II | Best response reported: stable disease in
Phase Ia189 | Decreased xenograft growth and increased
survival in combination with temozolomide190 |
PKI-402 | PI3K and mTOR (TORC1 and TORC2) | | Not reported | Xenograft tumour regression with subsequent
regrowth191 |
PKI-587 (also known as PF-05212384) | PI3K and mTOR (TORC1 and TORC2) | Phase I | Not reported | Xenograft tumour regression192 |
Rapalogues (rapamycin, sirolimus, everolimus
and temsirolimus) | mTOR (TORC1) | Approved | Improved overall survival and progression-free
survival in RCC193,194, improved
progression-free survival in PNET195, 75% response rate in subependymal giant-cell
astrocytoma in TSC136, 40% response rate in MCL and lower in other tumour
types (reviewed in REF.
196) | Prevention of uterine and adrenal tumours in
Pten+/− mice132, prolonged survival in a mouse
model of Cowden syndrome197, decreased
Pten−/− prostate tumour
growth198,
prevention of lung tumours199, anal tumours200, lymphoma201, bladder tumours202, mammary
tumours203,
prostate tumours198
and regression of salivary gland tumours204 and PNET205 |
AZD8055 | mTOR | Phase I/II | Not reported | Growth inhibition or tumour regression in
xenografts206 |
Perifosine | AKT | Phase III | Improved TTP and overall survival in
randomized Phase II of capecitabine with or without perifosine in
refractory colorectal cancer207 | Growth inhibition and increased survival in
multiple myeloma xenograft208, growth inhibition in neuroblastoma
xenograft209 |
MK-2206 | AKT | Phase I/II | Best response reported: stable disease in
Phase Ia210 | Modest xenograft growth inhibition as a single
agent211 |
Of all of the pathway inhibitors in development, inhibitors of the TORC1
complex, such as sirolimus and its analogues, are the most developed and have
established safety profiles that are most relevant for rare syndromes. For example,
sirolimus was tested in a Phase II trial of patients with tuberous sclerosis, which,
like Cowden syndrome, is a highly morbid familial syndrome in which the loss of a
tumour suppressor gene leads to mTOR activation135. In patients with tuberous sclerosis, prolonged use of
sirolimus seemed to be safe and showed preliminary efficacy in shrinking
angiomyolipomas and improving pulmonary function135. Treatment with everolimus similarly caused
a sustained decrease in subependymal giant-cell astrocytomas (SEGAs) in patients
with tuberous sclerosis136. A
case report also showed that sirolimus decreased tumour burden in a child with
Proteus syndrome and a germline PTEN mutation137. Sirolimus is currently being tested in
patients with Cowden syndrome (clinical trial number: ).
In cancer, temsirolimus and everolimus are approved for the treatment of
advanced renal cell carcinoma, and are being tested as single agents, and in
combination, in various other malignancies. The activity of rapamycin analogues as
single agents in common cancers has been modest, however, which could be related to
feedback activation of AKT through insulin receptor substrate 1 (IRS1) or through
direct phosphorylation at Ser473 by TORC2 (REF.
138) (). Feedback activation
of AKT has been observed in PTEN-null glioblastoma biopsy samples
from patients treated with sirolimus, and was associated with a shorter time to
disease progression. Nonetheless, the modest results of clinical trials with TORC1
inhibitors in cancers in which PTEN inactivation is common suggest
that the inhibition of TORC1 alone is insufficient to induce meaningful tumour
regression139,140.
Canonical PTEN–PI3K–AKT–mTOR pathway.PTEN opposes PI3K function, leading to inactivation of AKT crucial
downstream target1. When PTEN
activity is decreased or absent, products of PI3K activate AKT through the
activation of its upstream kinase phosphoinositide-dependent kinase 1 (PDK1;
encoded by PDPK1)170. Other upstream regulators of the pathway include
receptor tyrosine kinases (RTKs) such as ERBB2 and epidermal growth factor
receptor (EGFR) that are important in breast and lung cancer, respectively
(reviewed in REF. 171). Important
downstream targets of AKT (such as p27, p21, FOXO and PAWR (also known as PAR4))
are involved in multiple functions that are crucial for tumour cell growth and
survival (reviewed in REF. 8). mTOR
activity is also increased when PTEN activity is lost, and mTOR itself has
important targets, including AKT, as well as proteins required for protein
translation such as ribosomal protein S6 kinase (S6K; encoded by
RPS6KB1 and TPS6KB2) and eukaryotic
initiation factor 4E binding protein (4EBP1; encoded by
EIF4EBP1)172. mTOR exists in two different protein complexes, TORC1 and
TORC2 (REF. 173). Inhibitors of TORC1 by
drugs such as rapamycin can activate AKT by deactivating a negative-feedback
loop mediated by S6K and insulin receptor substrate 1 (IRS1)174,175. Proteins that can be targeted by drugs (as outlined
in ) are indicated in red. BAD,
BCL-2-associated agonist of cell death; GSK3, glycogen synthase kinase 3;
MAP3K5, apoptosis signal regulator kinase 1.
The next generation of pathway inhibitors includes dual PI3K–mTOR
inhibitors, PI3K inhibitors, AKT inhibitors and mTOR complex catalytic site
inhibitors (reviewed in REFS 141–143). These compounds
may better compensate for the loss of PTEN by targeting more
upstream components of the pathway and may circumvent feedback AKT activation.
However, these agents are likely to be more toxic than the pure TORC1 inhibitors and
are also likely to be less useful for cancer prevention in patients with rare
syndromes.
Trial design considerations for PHTS and PTEN-deficient cancers.
Given the rarity of Cowden syndrome, cancer prevention trials pose a
challenge. Pilot studies using pathway inhibitors that focus on tissues at risk
for malignant transformation are more feasible. For example, a trial evaluating
the effects of a pathway inhibitor on endometrial hyperplasia or fibrocystic
changes of the breast in patients with Cowden syndrome would be a useful
proof-of-concept, but this would require multiple biopsies, which might be
objectionable to patients with Cowden syndrome who do not have cancer. Molecular
imaging to assess tumour metabolism using fluorodeoxyglucose
(FDG)–positron emission tomography (PET) or tumour cell proliferation
using deoxyfluorothymidine (FLT)–PET might be useful surrogates for
patients with Cowden syndrome who are unwilling or unable to undergo biopsies.
Trials in patients with Cowden syndrome could also test pathway inhibitors as a
means of ameliorating the severe but non-malignant manifestations of the
disease, such as Lhermitte–Duclos disease, in which improvement in
neurological function could be measured clinically. Selecting objective and
reliable clinical end points for these studies is challenging, but
pharmacodynamic end points and assays that are validated in trials of patients
with Cowden syndrome could be applied to general oncology trials.
The location of PTEN mutations or relevant epigenetic
modifications may assist the choice of therapy for
PTEN-deficient malignancies. For example, if mutations occur in
the C-terminal PEST domain and spare the phosphatase domain, treatment with a
proteasome inhibitor might rescue PTEN from degradation. Moreover, treatment
with statins might increase the expression of PTEN through peroxisome
proliferator-activated receptor-γ (PPARG)-mediated promoter
activation144, and
demethylating agents or histone deacetylase inhibitors might reverse epigenetic
silencing. Three recent studies suggest that PTEN is required for homologous
recombination, which could be exploited therapeutically. In one mouse study, T
cell-specific Pten deletion resulted in lymphomas with T cell
receptor (Tcr)–Myc translocations
resulting from aberrant Tcr recombination145. In PTEN-deficient endometrial cancer
cell lines, decreased homologous recombination underlies sensitivity to
polyadenosine diphosphate ribose polymerase (PARP) inhibitors146. Pten
deletion decreased homologous recombination in mouse astrocytes through the
downregulation of the DNA repair protein RAD51. These studies raise the
possibility that PARP inhibitors may have efficacy for PTEN-deficient
tumours147, owing to
generalized defects in homologous recombination.
As PTEN loss mediates resistance to targeted therapies against receptor
tyrosine kinases, combinations of PI3K or AKT inhibitors with cell surface
receptor inhibitors might be effective. For example, acquired resistance to EGFR
tyrosine kinase inhibitors in lung cancer and trastuzumab in
ERBB2-amplified breast cancer are associated with
Pten loss and/or maintenance of AKT activation110,148,149.
Inhibition of multiple nodes of the signalling cascade may effectively overcome
acquired resistance. Alternatively, targeting of parallel networks by targeting
the PI3K–AKT pathway and the MEK–ERK (MAPK1, MAPK3 and MAPK1)
pathway may also overcome acquired resistance, have antitumour activity and
ultimately accelerate the development of these agents to treat patients with
germline or somatic loss of PTEN.
Perspectives and conclusions
The comparison of sporadic tumours carrying PTEN alteration, tumours that
occur with germline PTEN mutation in Cowden syndrome, and tumours
that develop in Pten-deficient GEM strains provides evidence that
the development of many different tumour types seems to be driven by the loss of
PTEN function. mTOR inhibitors have been approved for the treatment of advanced
renal cell carcinoma and SEGA that is associated with tuberous sclerosis. Upstream
pathway inhibitors of PI3K and AKT are in clinical development, both in combination
with traditional chemotherapy and with inhibitors of parallel pathways such as
MEK–ERK. This is a reasonable approach as PTEN mutations and
subsequent activation of the AKT–mTOR pathway provide survival signals that
are associated with resistance to therapy. However, one key question that remains to
be answered is whether tumours that develop as a consequence of PTEN attenuation are
addicted to that signal. Given that PTEN alteration is so prevalent
in many human tumour types, validating PTEN as a target during different stages of
tumorigenesis is crucial to validating any downstream targets. Mouse models could be
used to show whether re-expression of Pten in
Pten-deficient tumours leads to tumour regression, as is the case
for Trp53-null lymphomas and sarcomas upon Trp53
re-expression150, or
whether it is context-dependent as is the case for the reconstitution of
Trp53 in lung tumours151,152.
Identification of novel PTEN functions and crucial signalling events downstream of
PTEN could provide additional targets and new therapeutic approaches.
It is becoming clear that PTEN may have many important functions, any or all
of which might contribute to its tumour suppressor activity. Pten
deletion clearly contributes to tumorigenesis in multiple tissues in mice. The
continued characterization of specific human PTEN mutations is
driving the discovery of novel PTEN functions that might correlate with specific
tumour risk in Cowden syndrome and might have implications for sporadic tumours.
At a glance
PTEN hamartoma tumour syndrome (PHTS) is a group of syndromes
characterized by benign growths and a high risk for cancers of the breast,
endometrium and thyroid. Cowden syndrome is the best characterized of these
and 85% of patients have germline PTEN mutations. The range
of abnormalities in patients with PHTS varies from patient to patient.
Somatic PTEN mutations and deletions, and
inactivation of PTEN by methylation or microRNA silencing,
are common in multiple tumour types. These include the classical
PHTS-associated tumours like breast, endometrium and thyroid, but also
tumours of the central nervous system, prostate, lung, pancreas, liver and
adrenal glands, as well as melanoma, leukaemia and lymphoma.
Mouse models of Cowden syndrome, in which a single allele of
Pten is deleted or mutated, exhibit characteristic
Cowden syndrome phenotypes. Tumour types are very much dependent on the
genetic background of the mice suggesting that there may be genetic risk
factors for PHTS penetrance in humans.
Tissue-specific deletion of Pten in mice can lead
to rapid, slow or no tumours, depending on the tissue type. In some cases,
tissue-specific Pten deletion can cooperate with other
genetic alterations to enhance tumorigenesis. These mouse models have
validated mutation or loss of PTEN as an aetiological
factor in similar human tumours.
PTEN is a lipid phosphatase that acts as a negative regulator of the
PI3K–AKT–mTOR pathway, which is an important regulator of cell
growth and survival. As such, pharmacological inhibition of this pathway may
be exploited for therapy of tumours with altered PTEN, or
for tumour prevention in patients with PHTS.
Acknowledgements
This Review is dedicated to T.S., a dear patient with Cowden syndrome. The
authors remain devoted to the study and cure of Cowden syndrome in her honour and
the honour of others who wrestle with the consequences of disease caused by the loss
of PTEN.
Footnotes
Competing interests statement
The authors declare no competing financial interests.
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