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The Journal of Clinical Investigation http://www.jci.org Volume 122 Number 6 June 2012
Clinical and epidemiological features
Squamous cell carcinoma of the head and neck (HNSCC) is the
sixth most common cancer worldwide, affecting 600,000 new
patients each year. In the United States, 50,000 new cases are diag-
nosed, and nearly 10,000 deaths are attributable to this disease
annually. Despite their common origin in the squamous mucosa
of the upper aerodigestive tract, there are several sources for the het-
erogeneity of HNSCC that complicate their consideration as a single
disease entity. Anatomically, the head and neck region consists of
several distinct structures (e.g., lip, tongue, nasopharynx, orophar-
ynx, larynx, hypopharynx) with distinct microscopic features and
lymphatic and venous drainage. As a result, treatment approaches
and outcomes vary significantly among the different subsites. In
addition, etiologic risk factors vary among the different subsites,
and HNSCC can occur in young patients without any known risk
factors. Biologic heterogeneity independent of subsite has been sug-
gested by molecular analyses that have uncovered distinct classes of
HNSCC with unique mRNA expression profiles or patterns of DNA
copy number alterations that correlate with clinical behavior (1–4).
Although the most important risk factors are tobacco use and alco-
hol consumption, infection with high-risk types of human papilloma
viruses (HPVs) has also been recognized as an increasingly important
risk factor, particularly for oropharyngeal squamous cell carcino-
ma (OPSCC). Importantly, survival for patients with HPV-positive
OPSCC is significantly better than for HPV-negative cancers, irre-
spective of treatment modality (5–8). Possible explanations include
both host-intrinsic factors (healthier patients with fewer comorbidi-
ties) and tumor-intrinsic factors (distinct genetic pathogenesis yield-
ing enhanced treatment sensitivity). The relatively good prognosis
of HPV-related cancers highlights an unfortunate paradox — despite
improvements in outcome for patients with oropharynx cancers, little
or no improvement has been observed for patients with HPV-negative
tumors over the past 30 years (9). This fact is attributable at least in
part to our limited understanding of molecular pathways that both
promote HSNCC pathogenesis and can be targeted therapeutically.
Linking pathways to genetics
Recent advances in our understanding of the molecular pathogenesis
of HNSCC were provided by whole-exome sequencing (i.e., sequenc-
ing exons of all known protein-coding genes) conducted on a total of
approximately 100 HNSCC specimens independently by two groups
(10, 11). While the two studies analyzed etiologically similar tumors
with related sequencing platforms, there was a five-fold difference in
the average number of mutations reported per tumor. This differ-
ence likely reflects distinct bioinformatic and validation approaches
used in the studies, and therefore a subset of identified changes may
represent “passenger” mutations (as a result of increased mutation
rates in cancer cells, or even mutation “miscalls”) rather than true
“driver” mutations with an etiologic role in HNSCC. Nevertheless,
several key findings were shared by these studies. This work, together
with a large body of previous genomic and functional analyses of
HNSCC, highlights the relatively small number of oncogenes tar-
geted by activating mutations and supports the fundamental roles
of tumor suppressor pathways including p53, Rb/INK4/ARF, and
Notch in disease pathogenesis. These and other bona fide HNSCC
cancer genes play major roles in at least four key functional path-
ways: cellular proliferation, squamous epithelial differentiation, cell
survival, and invasion/metastasis, with many of the genes impacting
more than a single pathway (Figure 1). These pathways are critical
to the pathogenesis of HNSCC and, not surprisingly, reflect normal
developmental programs within the stratified squamous epithelium.
Given the paucity of driver oncogenes in HNSCC, targeting these
pathways therapeutically represents a substantial and critical chal-
lenge for improving outcomes of this disease.
Cellular proliferation and p53/Rb/CDKN2A/CCND1
Mutation of the TP53 tumor suppressor gene is the most common
and among the earliest identified genetic alterations in HNSCC,
occurring in more than half of all cases (2). As in other human can-
cers, missense mutations primarily within the DNA binding domain
account for 75% of all mutations in the TP53 gene and confer both
dominant negative and poorly understood gain-of-function proper-
ties (12–15). In many of the remaining HNSCC tumors in which p53
is wild-type, p53 function may be inactivated by other mechanisms.
The molecular pathogenesis of head and neck
squamous cell carcinoma
S. Michael Rothenberg and Leif W. Ellisen
Massachusetts General Hospital Cancer Center and Harvard Medical School, Boston, Massachusetts, USA.
Squamous cell carcinoma of the head and neck (HNSCC) is a relatively common human cancer
characterized by high morbidity, high mortality, and few therapeutic options outside of surgery,
standard cytotoxic chemotherapy, and radiation. Although the most important risk factors are
tobacco use and alcohol consumption, the disease is also linked to infection with high-risk types
of human papilloma viruses (HPVs). Recent genetic analyses have yielded new insights into the
molecular pathogenesis of this disease. Overall, while somatic activating mutations within classi-
cal oncogenes including PIK3CA and RAS occur in HNSCC, they are relatively uncommon. Instead
genetic data point to a contribution of multiple tumor suppressor pathways, including p53,
Rb/INK4/ARF, and Notch, in tumor initiation, progression, and maintenance. The increasingly refined knowledge
of HNSCC genetics, combined with ever-more-sophisticated animal models and newer drug targeting strategies,
should promote novel therapeutic approaches and improved disease outcomes.
Conflict of interest: The authors have declared that no conflict of interest exists.
Citation for this article: J Clin Invest. 2012;122(6):1951–1957. doi:10.1172/JCI59889.
science in medicine
1952 The Journal of Clinical Investigation http://www.jci.org Volume 122 Number 6 June 2012
Hallmarks of head and neck squamous tumorigenesis. (A) The normal process of squamous morphogenesis in the adult mucosa is controlled in
part by TP63 and NOTCH1. The former is expressed in keratinocytes of the basal layer, where it maintains their proliferative potential and controls
expression of basal markers (e.g., keratins 5/14 [K5/14]); expression of the latter results in terminal differentiation into cells of the spinous (K1/10)
and granular layers. Rare stem cells in the basal layer (light blue) undergo terminal differentiation through asymmetric cell division. Abnormal
proliferation is prevented primarily by differentiation-associated cell cycle exit and by apoptosis. (B) Pathways altered in HNSCC pathogenesis
identified in whole-exome sequencing studies. Red: putative and established tumor suppressors; green: oncogenes; brown: other relevant genes/
proteins; blue: viral proteins. Loss of TP53 and CDKN2A, amplification of CCND1, and loss of TGFBR2/SMAD4 permit abnormal proliferation and
decrease apoptosis. However, abnormal cell cycling may still be restrained by intact differentiation and apoptotic programs. Loss of NOTCH1 and/
or abnormal expression of TP63, together with alterations in “survival” genes (e.g., CASP8, PIK3CA, EGFR), may remove additional barriers to
tumor cell proliferation and survival. Loss of cell adhesion genes (e.g., FAT1) could permit release of cells from the mucosal lining, while invasion
through the basement membrane is promoted by TGFB1 (and SMAD3). (C) Schematic of HNSCC hallmarks. The precise order of acquisition of
distinct alterations is not clear. In addition, several genes (e.g., TP53, TP63, NOTCH1) may contribute to more than one hallmark.
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The Journal of Clinical Investigation http://www.jci.org Volume 122 Number 6 June 2012
These include expression of the HPV E6 protein (which binds p53
and targets it for proteasomal degradation), overexpression/ampli-
fication of MDM2 (which also mediates p53 proteasomal degrada-
tion), and deletion of CDKN2A, which may eliminate p14/ARF, a
negative regulator of MDM2 (16–19). Overall, the data suggest that
the p53 pathway is downregulated in at least 80% of HNSCCs (2).
The finding that TP53 is mutated in both leukoplakia (a his-
tologically recognizable precursor lesion) and benign-appearing
mucosa has led to a “patch-field” progression model of HNSCC
development, in which the index squamous carcinoma (as well as
subsequent tumors) develops from a field of genetically abnor-
mal mucosa, itself the result of expansion of a clonal patch aris-
ing from a putative stem cell containing a mutated TP53 gene
(20). Interestingly, in some cases the TP53 mutations found in
the tumor and adjacent mucosa are different, implying a distinct
clonal origin for multiple patches and suggesting that metachro-
nous tumors from the same patient (e.g., primary versus locally
recurrent) could in fact develop from unique clones through inde-
pendent acquisition of additional alterations (21). In addition to
tumor initiation, TP53 inactivation also contributes to the clinical
behavior of tumors, at least in part independent of an influence on
the response to genotoxic therapy. Thus, truncating and function-
disrupting mutations of TP53 are significantly associated with
decreased survival — after primary surgery with or without post-
operative radiotherapy — compared with either non-disruptive
mutations or no mutation at all (22, 23).
The essential role of the retinoblastoma (Rb) pathway is evi-
denced by the finding of inactivation of CDKN2A, encoding the
cell cycle regulators p16/INK4A and p14/Arf/INK4B, in HNSCC.
CDKN2A mutations were found in approximately 7% of tumors by
exome sequencing, with copy number losses in another 20%–30%
(10, 11). It has been previously shown that CDKN2A inactivation
by mutation is significantly more rare than deletion or epigenetic
inactivation, which together account for inactivation of the gene in
up to 75% of HNSCCs (24–26). Although p16/INK4A loss (whether
genetic or functional) has been repeatedly demonstrated to cor-
relate with indicators of worse prognosis, data on p14/Arf/INK4B
loss (e.g., by methylation, when the genomic locus itself is not
deleted) is conflicting, with one study suggesting worsened prog-
nosis, while two others suggested improved prognosis, perhaps
a result of increased radiation sensitivity (27–29). In the case of
HPV+ HNSCC, inactivation of the Rb pathway is achieved through
expression of the HPV E7 protein, which binds RB1 and abrogates
the requirement for p16/INK4A silencing. As a result, assaying p16
protein expression in tumor cells by immunohistochemistry (IHC)
is of clinical value in determining HPV+ status (30).
Amplification of a discrete, approximately 5-Mb region of chro-
mosome 11q13 containing the CCND1 gene (encoding cyclin D1)
occurs in approximately one-third of HNSCCs, and perhaps even
more frequently in HPV-negative tumors (4, 31). Furthermore, over-
expression of cyclin D1 has been observed in up to 80% of HNSCCs
(2). This high frequency is remarkable given that CDKN2A loss or
CCND1 amplification would seem to be redundant mechanisms
to promote cell cycle progression through activation of G1 phase
cyclin-dependent kinases (CDKs) 4 and 6. Nevertheless, these two
genetic events are not mutually exclusive in HNSCC, potentially
reflecting either qualitatively or quantitatively different effects.
For example, cyclin D1 may indirectly stimulate CDK2 activity by
sequestering the CDK2 inhibitors p21 and p27, and alternatively,
cyclin D1 may function as a cofactor independent of its role in cell
cycle regulation, through binding to transcription factors (e.g.,
PPARγ) or DNA repair proteins (e.g., BRCA2, Rad51) (refs. 32, 33;
reviewed in ref. 34). In keeping with their distinct contributions,
loss of p16 expression and overexpression of cyclin D1 are inde-
pendent predictors of death from tongue cancer, and the loss of
p16 together with overexpression of cyclin D1 confers significantly
worse 5 year survival than either condition observation alone (35).
Terminal differentiation and the Notch/p63 axis
Perhaps the most novel finding to emerge from the whole-exome
sequencing studies of HNSCC is the discovery of mutations with-
in the NOTCH1 gene in 12%–15% of cases, and within additional
NOTCH family members in 3%–5% (10, 11). Although Notch sig-
naling had previously been implicated as pro-tumorigenic — by
virtue of activating mutations and translocations observed in the
genes for Notch receptors or their regulators in T cell acute lym-
phoblastic leukemia, chronic lymphocytic leukemia, and diffuse
large B-cell lymphoma (36–40) — several of the NOTCH family
mutations in HNSCC (and in chronic myelomonocytic leukemia,
a rare myeloproliferative disease) encode inactivating mutations,
suggesting a tumor suppressor function (41). The physiologic rel-
evance of these findings is supported by animal models in which
NOTCH activation in hematopoietic cells leads to T cell leukemias
and inactivation in squamous epidermis promotes skin tumori-
genesis (refs. 42–44; reviewed in ref. 45).
Notch signaling has been linked to multiple biological func-
tions, including regulation of self-renewal capacity, cell cycle exit
(in part through upregulation of p21/CDKN1A expression), and
cell survival (46–48). In the stratified epithelium, Notch has a cen-
tral role in promoting terminal differentiation (48, 49), which is
mediated through both direct effects (e.g., on activation of supra-
basal keratins) and indirect effects on the Wnt, hedgehog, and
interferon response pathways (43, 47, 50, 51). Additionally, Notch
activity has been linked to suppression of HPV E6 and E7 protein
expression, potentially providing additional selective pressure for
loss of Notch in HPV+ HNSCC (52, 53).
Further supporting a role for Notch in squamous epithelial differ-
entiation is its control by the p53-related transcription factor p63,
a master regulator of proliferative potential, lineage specification,
and differentiation in stratified epithelia. Constitutive knockout of
Tp63 in mouse models results in complete failure of normal epider-
mal development (54, 55). In mature epithelium, expression of p63
is highest in basal epithelial cells, where it functions as an inhibi-
tor of NOTCH1 expression, and becomes downregulated during
terminal differentiation coincident with NOTCH1 upregulation
(Figure 1A and ref. 56). Reactivation of p63 expression is observed
in the suprabasal layers of dysplastic mucosa, and overexpression
and/or genomic amplification of the TP63 locus is observed in the
majority of invasive HNSCCs (57, 58). TP63 gives rise to two major
isoform classes, TAp63 and ΔNp63, which differ in the presence and
absence, respectively, of an N-terminal transcriptional transactiva-
tion domain. Although tumor incidence data from Tp63-heterozy-
gous mice are conflicting, ΔNp63 isoforms, which are selectively
overexpressed in HNSCC, are likely oncogenic (59, 60). Importantly,
TP63 was found to be mutated or amplified in 8% of samples in one
of the sequencing studies (11). Consistent with a contribution of
ΔNp63 in these tumors, two of the mutations uncovered are pre-
dicted to alter the function of TAp63 (including a nonsense muta-
tion) but not ΔNp63. In addition to its contribution through Notch
suppression, ΔNp63 has been demonstrated to control other key
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1954 The Journal of Clinical Investigation http://www.jci.org Volume 122 Number 6 June 2012
tumor-relevant pathways, including cell survival (in part through
suppression of the proapoptotic p53-related protein p73), senes-
cence suppression (through suppression of p16/INK4A expression),
and growth factor signaling (through induction of EGFR) (61–64).
Cell survival through EGFR/Ras/PIK3CA/PTEN/CASP8
The PI3K signaling pathway is commonly activated in HNSCC,
as evidenced by recurrent alterations of two central regulators:
PTEN, encoding a negative regulator, and PIK3CA, encoding a
positive regulator of this pathway. PTEN is subject to frequent
loss of heterozygosity in a variety of cancers, including up to 40%
of HNSCCs, although biallelic inactivation occurs less frequently
(65–68). Loss of just a single PTEN allele in the remaining samples
may contribute to tumorigenesis, however, since recent data sug-
gesting a gene dosage effect for PTEN (69). Activating mutations
in two “hot spot” regions of the PIK3CA gene occur in 6%–11% of
HNSCCs, with a potential enrichment in tumors originating from
the pharynx (70, 71). The latter finding is particularly noteworthy
given the increased incidence of PIK3CA mutations in HPV-related
versus non-HPV-related tumors observed in both exome sequenc-
ing studies. This observation suggests that PIK3CA mutations
may cooperate with HPV E6 and E7 proteins in the development
of invasive OPSCC, as has been suggested for cervical carcinomas
(72). The prominent role of the PI3K pathway in HNSCC has
potentially important clinical implications, given that numerous
targeted inhibitors of this pathway are currently being evaluated
in clinical trials (reviewed in ref. 73).
Activating missense mutations causing single amino acid substi-
tutions in one of three positions (codon 12, codon 13, and codon
61) in the HRAS gene were uncovered in 3%–5% of samples in both
whole-exome sequencing studies. While it is currently unknown
whether HRAS-dependent signals function in collaboration with
or independently of PI3K activation in HNSCC, several findings
underscore the importance of this particular RAS family member
to the pathogenesis of the disease. These include the more fre-
quent occurrence of HRAS than KRAS mutations in HNSCC, par-
ticularly in relationship to tobacco history, whereas the reverse is
true for several other malignancies (74, 75); the presence of HRAS
mutations in HPV-driven tumors, suggesting potential cooperativ-
ity in tumor promotion (76); and the more frequent association of
HRAS versus KRAS mutation in squamous cell carcinomas arising
in the setting of tobacco exposure in humans and chemical car-
cinogen exposure in mice (77). Although Ras proteins themselves
have proven difficult to target directly, therapeutic strategies that
target downstream effectors of Ras proteins or the synthetic lethal
dependencies that result from their mutational activation have
already been successful in preclinical models (78–80).
Upstream signaling to both Ras and PI3K pathways may occur
through activation or overexpression of receptor tyrosine kinases
(RTKs) including EGFR. Although it is often considered to be
among the most important therapeutic targets in HNSCC (81),
our understanding of the role of EGFR is evolving with the
appreciation that EGFR activating mutations are rare in HNSCC
and that the reported frequency of EGFR gene amplification in
HNSCC varies widely, in part due to varying definitions of the
degree and size of copy number gain that constitute amplification
(82, 83). Furthermore, although copy number gain of EGFR has
been suggested to correlate with poor prognosis in HNSCC (83,
84), in general gain of EGFR has not been clearly demonstrated to
predict improved outcomes following EGFR-directed therapy (85,
86). Similarly, therapeutic agents that inhibit EGFR, including the
small molecule inhibitors gefitinib and erlotinib and the therapeu-
tic antibody cetuximab, have modest activity in HNSCC, with little
or no correlation with EGFR status (87–91).
Two other genetic abnormalities affecting RTK signaling have
received less attention but have potential near-term clinical impact.
Expression of EGFRvIII, a variant EGFR protein that results from
the in-frame genomic deletion of exons 2–7 and is present in glio-
mas and lung squamous cancers, was recently reported in 42% of
HNSCCs (92). Importantly, an antibody thought to be specific for
EGFRvIII (e.g., rather than full-length EGFR) was used to initially
identify cases; this finding was not reproduced in another study
that sequenced the full-length EGFR cDNA (93). It will be impor-
tant to resolve whether EGFRvIII is expressed with any apprecia-
ble frequency in HNSCC, as EGFR kinase inhibitors have dem-
onstrated clinical activity against tumors expressing this variant
(94). Mutation or amplification of the MET (c-Met) RTK gene has
been reported in some HNSCC cases (95). This finding is of clini-
cal interest both because MET amplification is thought to confer
resistance to EGFR-directed therapy (96) and because the small
molecule therapeutic crizotinib, which inhibits both the MET and
ALK kinases, has recently been FDA approved for use in lung can-
cers harboring ALK translocations (97).
While each of the above genes and pathways are associated with
activities that may indirectly prevent programmed cell death, sev-
eral constituents of the apoptotic signaling cascade may also have
an important role in HNSCC. These include caspase-8 (CASP8),
encoding the critical proapoptotic enzyme that initiates a cascade
of proteolysis responsible for executing apoptosis and found to be
mutated in 8% of samples in one exome sequencing study (11, 98,
99); and BCL2, encoding a key antiapoptotic regulator reported
to be overexpressed in a fraction of HNSCC cell lines, particularly
those with reduced expression of p63 (63).
Adhesion and invasion signaling through TGF-β/SMAD
Inactivation of TGF-β signaling components is well established
in human cancer, including HNSCC, most commonly through
loss of TGF-β receptor (TGFBR2) and SMAD genes as a result
of chromosome 18q deletion (100). Notably, although missense
mutations in TGFBR2 have been previously described in primary
HNSCCs (101), and SMAD2 and SMAD4 mutations have been
reported in HNSCC cell lines (102), no point mutations in these
genes were found through exome sequencing, perhaps due to the
low frequency of these events. The TGF-β pathway is a pleiotropic
regulator in human cancer, as mutational inactivation of its sig-
naling components is associated with tumor initiation, while
activation of the pathway is known to promote metastasis. Thus,
genetic loss of TGF-β pathway factors would at first glance seem
at odds with a contribution of this pathway to invasion and metas-
tasis in HNSCC. Recent mouse models, however, suggest a more
complex interaction. Conditional deletion of Smad4 in the mouse
stratified epithelium led to HNSCC in association with increased
genomic instability and increased inflammation, the latter attrib-
uted in part to elevation of TGF-β1 and activation of other SMADs
in stroma, mucosal epithelia, and tumor cells (103). In addition,
Tgfbr2 deletion within the mouse head and neck epithelia is insuf-
ficient to cause HNSCC, but cooperates with activated Kras to
promote squamous carcinomas that metastasize to local lymph
nodes (104). TGF-β1 itself has also been associated with epithelial-
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The Journal of Clinical Investigation http://www.jci.org Volume 122 Number 6 June 2012
mesenchymal transition (EMT) and metastasis, the latter in the
absence of functional TGF-βRII (105).
New insight into potential mechanisms of HNSCC invasion and
metastasis was provided by the identification of mutations in the
FAT1 gene in nine HNSCC samples (12%) in one of the two exome
sequencing studies (11). Six nonsense mutations and a seventh
frameshift were noted, suggesting FAT1 may function as a tumor
suppressor. Notably, focal, intragenic homozygous deletions of FAT1
have previously been described in oral cancer (106). As it is a mem-
ber of the cadherin superfamily of cell membrane proteins that have
demonstrated roles in the establishment of cell polarity and mediate
cell-cell contacts, loss of FAT1 might be predicted to permit loosen-
ing of the adhesions that normally restrain growth and/or migration
of cells in an epithelial sheet. Similarly, mutations in genes encoding
other membrane-associated proteins with a role in the establishment
of polarity and cell adhesion have been described in HNSCC (107).
Several genes with unclear roles in HNSCC were found to be
mutated at appreciable frequencies in at least one of the exome
sequencing studies. Although the functional significance of the
identified missense mutations is not clear (and some may merely
represent passenger mutations), recurrent inactivating mutations
were observed in several additional genes, suggestive of tumor
suppressor activity. These include MLL2 (11) and NSD1 (11), both
encoding histone methyltransferases, and SYNE1 (10, 11), a nucle-
ar envelope protein. Mutations within MLL2 have recently been
described in non-Hodgkin’s lymphoma, and mutations within
several other histone-modifying enzymes have been identified in
renal cell carcinoma and diverse human cancers, suggesting a role
for chromatin-mediated gene expression deregulation in cancer
pathogenesis (108–110). Although SYNE1 loss has been previously
described in ovarian cancers and gliomas, this genomic locus spans
more than 0.5 Mb (the longest isoform comprises 146 exons) and
is subject to copy number variation in normal tissues. As such, this
locus could be expected to accumulate relatively frequent passen-
ger mutations, resulting in an overestimate of the significance of
mutations if gene size is not taken into account (111–113).
Implications for therapy
The majority of patients with HNSCC come to clinical attention
before the development of metastases, and as a result they have the
potential to be cured with aggressive multimodality therapy (e.g., sur-
gery, radiation, and/or chemotherapy). However, although improve-
ments in surgical techniques, chemotherapy and radiation delivery,
and supportive care have improved quality of life for patients with
HNSCC, survival as a whole has not markedly improved (9).
As noted above, activating mutations in tyrosine kinases are rela-
tively rare in HNSCC, and thus only one kinase-targeted therapy,
cetuximab, is FDA approved for patients with HNSCC. This agent
has generally modest efficacy, and efforts to identify a subset of
patients whose tumors are “addicted” to EGFR signaling have not
been successful (86, 87). One intriguing possibility is that produc-
tion of EGFR activating ligands by tumor cells or tumor-associated
stroma may increase sensitivity to EGFR inhibitors in HNSCC (114).
The recognition of HPV-related OPSCC as a distinct clinical entity
with a better prognosis has led to the hypothesis that less-intensive
therapy might mitigate toxicity without affecting survival. Clinical
trials to test this hypothesis are now underway (115), but it should
be weighed against the possibility that the primary survival benefit
of intensive therapy may lie with HPV-positive patients (116). Also
relevant is the demonstration that human cancer cell lines derived
from HPV-positive HNSCCs may be addicted to the continuous
activity of the viral E6 and E7 proteins, suggesting that the latter
represent important therapeutic targets. In addition, endogenous
expression of E6 and E7 depends in part on the activity of transcrip-
tion factors (including Notch), which themselves are controlled by
potentially targetable upstream signaling pathways (52, 53).
The convergence of many HNSCC cancer genes on the activity
of the G1 cyclin-dependent kinases (e.g., CDK2, -4, and -6) sug-
gests that targeting these kinases could have potent efficacy. A
number of direct CDK inhibitors are currently being evaluated in
clinical trials; several appear particularly effective in combination
with cytotoxic agents with established efficacy in HNSCC (e.g., cis-
platin, taxol; reviewed in ref. 34). A potentially novel approach to
targeting the cell cycle in HSNCC has been suggested by the find-
ing that inhibitors of the deubiquitinating enzyme USP2 could
promote the proteasomal degradation of cyclin D1 (117, 118).
The ability to selectively target tumors with decreased p53 activ-
ity could have major implications for the majority of HNSCC
patients. Unfortunately, the majority of p53-selective compounds
currently in early phase clinical trials are designed to induce the
activation of wild-type p53 in cells in which p53 activity is down-
modulated by alterations in endogenous p53 regulators. These
agents are unlikely to be effective in tumors in which p53 activ-
ity is abrogated primarily through mutation. An exception could
be the large class of structural mutations that affect p53 protein
stability; in these cases, decreasing p53 degradation could lead to
restoration of some p53 activity (16). In addition, a variety of cre-
ative preclinical strategies have been undertaken to target tumor
cells expressing mutated p53. These include the development of
compounds that stabilize p53 through allosteric binding and syn-
thetic lethal screens to identify candidate agents (including polo-
like kinase inhibitors, some of which are in clinical trials) that
selectively induce toxicity in p53-deficient cells (119).
As noted above, the relative lack of readily targeted oncogenes in
these tumors has challenged traditional approaches to drug develop-
ment for HNSCC. Nevertheless, the increasingly refined knowledge
of epithelial biology and HNSCC genetics, combined with newer
approaches aimed at targeting proteins previously thought undrug-
gable, could lead to novel therapies with greater potency and less
toxicity for these cancers (120). These conceptual advances will allow
the development of ever-more-sophisticated animal models, which
in turn will be essential for testing the emerging host of candidate
targeted therapeutics, and for determining which of these may have
the most clinical viability. A strong biologic rationale will be essential
if progress against this disease is to be accelerated in the near term.
The authors wish to thank Lori Wirth and members of the Ellisen
laboratory for helpful discussions and review of the manuscript.
This work was supported by NIH R01 DE-015945 (to L.W. Ellisen)
and by NIH K08 DE-020139 (to S.M. Rothenberg).
Address correspondence to: Leif W. Ellisen, MGH Cancer Center,
GRJ-904, 55 Fruit Street, Boston, Massachusetts 02114, USA.
Phone: 617.726.4315; Fax: 617.726.8623; E-mail: ellisen@helix.
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1956 The Journal of Clinical Investigation http://www.jci.org Volume 122 Number 6 June 2012
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