2012 EDUCATIONAL BOOK - American Society of Clinical Oncology

HEDGEHOG PATHWAY IN PEDIATRIC CANCER
Diffuse Intrinsic Pontine Glioma
In a report by Monje and colleagues, early postmortem
tissue from patients with diffuse intrinsic pontine glioma
(DIPG) was used to establish in vivo xenograft models of
DIPG. 19 Subsequent analysis of the established tumors
showed that the HH signaling pathway is active in DIPG
tumor cells and that DIPG self-renewal capacity in neurosphere
culture is significantly reduced with inhibition of the
HH signaling pathway.
Juvenile Pilocytic Astrocytoma
Rush and colleagues 20 demonstrated that mRNA expression
levels of members of the HH pathway were elevated in
45% of juvenile pilocytic astrocytoma specimens analyzed
and that the expression of the HH pathway correlated
inversely with patient age. Immunohistochemical (IHC)
staining for PTCH1, GLI1, and the proliferation marker
Ki67 demonstrated that patients diagnosed before the age of
10 years had an increased frequency and level of marker
immunopositivity compared with those diagnosed after 10
years of age. 20 A significant correlation was also observed
between Ki67, PTCH1, and GLI1 positive staining, with 86%
of Ki67-positive cells also expressing PTCH1.
Non-CNS Pediatric Cancer
Rhabdomyosarcoma
Rhabdomyosarcoma (RMS) is the most common soft tissue
sarcoma in children and comprises two major histologic
subtypes: alveolar rhabdomyosarcoma (ARMS) and embryonal
rhabdomyosarcoma (ERMS). Although RMS is frequently
said to be associated with activating mutations in
the HH pathway, the role of the HH pathway in sporadic
human RMS is not clear, and linkage to specific HH pathway
mutations has not been confirmed. 3 Although one study
reported mutations in PTCH1 and SUFU in RMS, most of
the analysis was restricted to loss of heterozygosity (LOH)
analysis of PTCH1 and SMO in rhabdomyoma, and in only
one case of RMS. 21 Since this region contains other potential
tumor suppressor candidate genes, definitive conclusion
from LOH analysis alone cannot be drawn. Soft tissue
sarcomas that are very similar to human RMS are seen in
Ptc1� mice, but the incidence of these tumors is strongly
influenced by the murine genetic background. 3
The initial descriptions of Gorlin syndrome identified an
association of the disease with benign fetal rhabdomyoma,
as well as rare instances of RMS. Tostar and colleagues
reported overexpression of PTCH1 and GLI1 mRNA, as
determined by in situ hybridization, in sporadic RMS. 21
Using array comparative genomic hybridization to examine
a specific subset of clinically defined intermediate-risk
ERMS tumors, Paulson and colleagues similarly demonstrated
that over 50% of tumors had low-level gains of a
region containing GLI1 along with a gene expression signature
consistent with HH-pathway activation. 22 Zibat and
colleagues reported that PTCH1, GLI1, GLI3, and MYF5 are
expressed at significantly higher levels in ERMS and that
GLI1 expression consistently correlates with PTCH1 expression
in ERMS, as well as a specific subset of ARMS. 23 This
study also showed that high PTCH1 expression significantly
correlated with reduced survival in a subset of RMS. Likewise,
Pressey and colleagues demonstrated that expression
of GLI1, with or without PTCH1, is detectable in substantial
subsets of ERMS, but only rarely in ARMS tumors. 24 Importantly,
neither PTCH1 mutations nor activating SMO mutations
were detected in ERMS tumors with high GLI1
expression, and in contrast to other reports, HH pathway
activity in ERMS tumors did not correlate with a unique
clinical phenotype. Furthermore, the gene expression patterns
in ERMS indicated that approximately 29% exhibited
evidence of HH pathway activity, yet this pattern was
always coassociated with either p53 or RB pathway signatures.
25 Finally, Oue and colleagues examined 18 RMS by
IHC and showed that the majority of the tumors were
immunopositive for SHH (78%), PTCH1 (100%), and GLI1
(78%). 18 In contrast to the study by Pressey and colleagues,
marker expression was higher in ARMS than in ERMS.
Caution should be taken in interpreting the clinical significance
of these results, and attempts to clinically translate
these findings to therapeutic interventions would be premature
as the presence of the HH pathway signature does
not necessarily mean that the tumor cells are dependent on
SMO activity. Cyclopamine studies in the Ptc1� mice are
an example of this, whereby loss of ptc1 may contribute
to tumor initiation, but is not required for tumor maintenance.
26
Neuroblastoma
Neuroblastoma (NB) is a heterogeneous pediatric malignancy,
with variable differentiation and growth potential,
that shares a common origin arising from neural crest cells
in the sympathetic nervous system. Using IHC, Souzaki and
colleagues examined 82 NB and 10 ganglioneuroblastoma
(GNB) and demonstrated tumor immunopositivity for SHH,
GLI1, and PTCH1 in 67 (73%), 62 (67%), and 73 (79%),
respectively. 27 Most NBs without MYCN amplification were
positive for all three HH pathway markers. Only two (10%)
of 20 cases with MYCN amplification were also positive for
SHH and GLI1, and four (20%) were positive for PTCH1.
The percentage of GLI1-positive cells without MYCN amplification
was significantly higher than those with MYCN
amplification, and the prognosis of the GLI1-positive cases
was significantly better than that of the GLI1-negative
cases. In tumors without MYCN amplification, high expression
of GLI1 was significantly associated with early clinical
stage, more differentiated tumors, and a good prognosis.
Oue and colleagues examined 25 NB by IHC and similarly
showed that 24 (96%), 17 (68%), and 25 (100%) stained
positive for SHH, PTCH, and GLI1, respectively. 18 Likewise,
all of the Gli1-negative tumors were poorly differentiated
and exhibited advanced-stage disease. In this study, there
was no significant relationship between GLI1 expression
and MYCN amplification or prognosis. This study also
showed that GLI1 transduction of NB cells inhibited proliferation
in vitro and induced a gene expression pattern that
resembled benign differentiated ganglioneuroma. 28 Notably,
GLI1 transduction did not induce MYCN expression in NB
cells.
Renal Tumors
IHC analysis of seven Wilms’ tumors showed that five
(71%), seven (100%), and three (43%) stained positive for
SHH, PTCH1, and GLI1, respectively. 18 Two renal clear cell
sarcoma and three rhabdoid tumors of the kidney showed
very high expression of HH pathway markers, suggesting
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