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Neuro-Oncology
19(9), 1206–1216, 2017 | doi:10.1093/neuonc/nox028 | Advance Access date 1 April 2017
Strong FGFR3 staining is a marker for FGFR3 fusions in
diffuse gliomas
Kirsi J. Granberg, Matti Annala, Birgitta Lehtinen, Juha Kesseli, Joonas Haapasalo,
Pekka Ruusuvuori, Olli Yli-Harja, Tapio Visakorpi, Hannu Haapasalo, Matti Nykter, and Wei Zhang
Corresponding Author: Kirsi Granberg, PhD, BioMediTech institute and Faculty of Medicine and Life Sciences, University of Tampere,
Lääkärinkatu 1, 33520 Tampere (kirsi.granberg@uta.fi); Wei Zhang, PhD, Department of Cancer Biology, Comprehensive Cancer
Center of Wake Forest Baptist Medical Center, 1 Medical Center Blvd., Winston-Salem, NC 27157 (wezhang@wakehealth.edu); Matti
Nykter, PhD, BioMediTech institute and Faculty of Medicine and Life Sciences, University of Tampere, Lääkärinkatu 1, 33520 Tampere,
Finland (matti.nykter@uta.fi).
Abstract
Background. Inhibitors of fibroblast growth factor receptors (FGFRs) have recently arisen as a promising treatment option for patients with FGFR alterations. Gene fusions involving FGFR3 and transforming acidic coiled-coil
protein 3 (TACC3) have been detected in diffuse gliomas and other malignancies, and fusion-positive cases have
responded well to FGFR inhibition. As high FGFR3 expression has been detected in fusion-positive tumors, we
sought to determine the clinical significance of FGFR3 protein expression level as well as its potential for indicating FGFR3 fusions.
Methods. We performed FGFR3 immunohistochemistry on tissue microarrays containing 676 grades II–IV astrocytomas and 116 grades II–III oligodendroglial tumor specimens. Fifty-one cases were further analyzed using targeted
sequencing.
Results. Moderate to strong FGFR3 staining was detected in gliomas of all grades, was more common in females,
and was associated with poor survival in diffuse astrocytomas. Targeted sequencing identified FGFR3-TACC3
fusions and an FGFR3-CAMK2A fusion in 10 of 15 strongly stained cases, whereas no fusions were found in 36
negatively to moderately stained cases. Fusion-positive cases were predominantly female and negative for IDH
and EGFR/PDGFRA/MET alterations. These and moderately stained cases show lower MIB-1 proliferation index
than negatively to weakly stained cases. Furthermore, stronger FGFR3 expression was commonly observed in
malignant tissue regions of lower cellularity in fusion-negative cases. Importantly, subregional negative FGFR3
staining was also observed in a few fusion-positive cases.
Conclusions. Strong FGFR3 protein expression is indicative of FGFR3 fusions and may serve as a clinically applicable predictive marker for treatment regimens based on FGFR inhibitors.
Key words
biomarker | gene fusion | glioblastoma | targeted sequencing
© The Author(s) 2017. Published by Oxford University Press on behalf of the Society for Neuro-Oncology.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License
(http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any
medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com
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BioMediTech Institute and Faculty of Medicine and Life Sciences, University of Tampere, Tampere, Finland (K.J.G.,
M.A., B.L., J.K., P.R., T.V., M.N.); Department of Signal Processing, Tampere University of Technology, Tampere,
Finland (K.J.G., M.A., B.L., O.Y-H.); Department of Pathology, The University of Texas MD Anderson Cancer Center,
Houston, Texas (K.J.G., W.Z.); Science Center, Tampere University Hospital, Tampere, Finland (K.J.G., M.N.);
Fimlab Laboratories Ltd., Tampere University Hospital, Tampere, Finland (J.H., T.V., H.H.); Unit of Neurosurgery,
Tampere University Hospital, Tampere, Finland (J.H.); Pori unit, Tampere University of Technology, Pori, Finland
(P.R.); Department of Pathology, University of Tampere, Tampere, Finland (H.H.); Department of Cancer Biology,
Comprehensive Cancer Center of Wake Forest Baptist Medical Center, Winston-Salem, North Carolina (W.Z.)
Granberg et al. Strong FGFR3 staining marks FGFR3 fusions
Glioma pathology has recently experienced great advancements as sequencing studies by the working group of
The Cancer Genome Atlas1,2 and others3,4 have identified
new molecular criteria for the diagnosis and stratification of gliomas. The World Health Organization recently
published new definition criteria for glioma diagnosis.5
Diffuse gliomas can be currently stratified into 3 main categories based on isocitrate dehydrogenase (IDH) mutation status and the presence of 1p/19q codeletion.1–4 IDH
wild-type tumors, which are present in all diffuse glioma
grades, have usually the poorest survival rates and are
generally less responsive to treatment,6 highlighting the
need for better treatment options for these patients. Since
the discovery of recurrent fibroblast growth factor receptor (FGFR) gene fusions in glioblastoma (GBM),7,8 targeted
treatment regimens using FGFR inhibitors have arisen as
a promising option for glioma patients with FGFR alterations. Of those, oncogenic FGFR3 fusions and various
FGFR1 alterations have been detected in brain tumors.7–14
In addition, FGFR fusions and other FGFR alterations have
been detected in several extracranial malignancies.10,15–20
FGFR3 is most commonly fused to the transforming acidic
coiled-coil protein 3 gene (TACC3), but other fusion partners also exist, such as recurrent FGFR3–BAIAP2L1 fusions
in bladder cancer.18 Several FGFR inhibitors are currently
being tested in clinical trials for different cancer types,21
and FGFR3 fusion-positive cells and tumors show the
best treatment responses.18,22 Responses to FGFR inhibitor treatment have also been reported in GBM.9,22 Tumors
that carry FGFR3 fusions or other responsive FGFR alterations represent a minority of cases in these malignancies,
which emphasizes the need for efficient patient stratification tools, as also stated by others.23
We have previously shown that FGFR3 protein expression is suppressed by miR-99a, one of the most abundantly
expressed miRNAs in gliomas and normal brain tissue.8
High fusion protein levels have been reported in GBM,8,9
most likely because the miR-99a binding site is removed
by genomic rearrangements that generate FGFR3 fusions.8
The miR-99a–mediated suppression of FGFR3 may explain
why activating FGFR3 mutations are not observed in diffuse gliomas. The relationship between FGFR3 immunostaining intensity and fusion status or FGFR3 expression
levels in fusion-negative cases has not been systematically
analyzed before. We wanted to determine whether strong
FGFR3 staining, detected using an antibody that recognizes
heterogeneity by revealing heterogeneous expression levels and FGFR3-negative subclones. Fusions
were only detected in aggressive isocitrate dehydrogenase wild-type tumors and were more common
in female patients. Interestingly, moderate to strong
FGFR3 staining was associated with a lower MIB-1
proliferation index without any prognostic benefit. The
obtained information is relevant when FGFR3 fusions
are targeted for therapeutic purposes and for better
understanding of FGFR3 fusion-driven tumor development and progression.
an epitope present in all reported gene fusions, could be
used as a marker for FGFR3 gene fusion. We therefore used
immunohistochemistry (IHC) to detect FGFR3 in 791 diffuse glioma cases and associated FGFR3 expression with
patients’ clinical features. Genetic alterations in FGFR3 and
other glioma-associated genes were identified by targeted
sequencing of selected cases. Our results demonstrate that
strong FGFR3 staining characterizes FGFR3 fusion-positive
diffuse gliomas. FGFR3 fusions involved several fusion
partners, junctions, and breakpoints, rendering them difficult to detect in a comprehensive manner with PCR-based
methods in a diagnostic context.
Materials and Methods
Patient Tissue Samples
The study design was approved by the ethical committee
of Tampere University Hospital and the National Authority
for Medicolegal Affairs in Finland. Diffuse glioma samples
had been obtained from tumor surgery patients at Tampere
University Hospital between 1983 and 2009 and diagnosed
according to the World Health Organization (WHO) 2007
classification for this study. We used 676 diffuse astrocytoma samples and 116 oligodendroglial tumors for FGFR3
detection (Table 1). Cause-specific survival association of
grades II–IV diffuse astrocytomas was analyzed using primary tumors (n = 533) (Table 1, Supplementary Table 1).
The samples were fixed in formaldehyde (buffered with 4%
phosphate), embedded in paraffin, and processed for tissue microarray (TMA).24
In the 2016 WHO classification, IDH mutation and 1p/19q
codeletion are used as additional criteria for more detailed
diffuse glioma diagnosis. In our cohort, 113 astrocytomas (20%, 562 analyzed) and 79 oligodendroglial tumors
(81%, 97 analyzed) were positive for IDH p.R132H. The
1p/19q codeletion was detected in 28 (90%, 31 analyzed)
of IDH p.R132H positive-staining oligodendroglial tumors
using fluorescence in situ hybridization.25 Our cohort thus
includes 96 IDH-mutant diffuse astrocytomas, 40 IDHmutant GBMs, and 28 oligodendrogliomas (IDH-mutant
and 1p/19q codeleted) according to the WHO 2016 criteria. The remaining tumors with inconclusive genetic testing are classified as diffuse astrocytoma, not otherwise
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FGFR3 gene fusions, initially discovered in glioblastoma, have since been reported in a wide spectrum
of other malignancies. FGFR3 fusion-positive tumors
have responded well to FGFR inhibitors in ongoing
clinical trials. We demonstrate that FGFR3 staining
is negative in most diffuse gliomas, whereas strong
FGFR3 staining is observed in all fusion-positive
cases, which facilitates stratification of FGFR3 fusionpositive gliomas. FGFR3 immunostaining does not
rely on prior knowledge about fusion breakpoints
or partners and allows evaluation of intratumoral
NeuroOncology
Importance of the study
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Granberg et al. Strong FGFR3 staining marks FGFR3 fusions
Table 1
Patient demographic and clinical characteristics in the astrocytic and oligodendroglial tumor cohorts*
Astrocytoma
Grade II
Grade III
Grade IV
All
No. of patients
114
59
503
676
Male
82
38
287
407
Female
32
21
216
269
Median (mean ± SD)
46 (46 ± 16)
50 (52 ± 17)
65 (65 ± 13)
Minimum
12
23
4
Maximum
83
84
90
No. of survivors at the end of follow-up
25
6
9
Follow-up duration for survivors, mo (median [mean ± SD])
92 (108 ± 77)
141 (142 ± 80)
59 (98 ± 69)
5-y survival rate, %
58
32
3.4
81
41
414
Sex (n)
Age, y
Tumors (n)
Primary
Second recidive
25
13
69
Third recidive
8
4
17
Fourth-sixth recidive
0
1
2
Oligodendroglial tumor
O II
O III
OA II
OA III
No. of patients
38
33
23
22
Male
19
17
12
8
Female
19
16
11
14
Median (mean ± SD)
45 (45 ± 12)
57 (57 ± 10)
52 (46 ± 17)
42 (46 ± 15)
Minimum
26
38
5
25
Maximum
69
72
64
74
25
9
9
4
Sex (n)
Age, y
Follow-up
No of survivors at the end of follow-up
Follow-up duration for survivors, mo (median [mean ± SD])
83 (106 ± 64)
96 (66 ± 50)
94 (161 ± 121)
203 (221 ± 114)
5-y survival rate, %
79
43
84
50
Primary
34
21
19
18
Second recidive
4
10
4
4
Third recidive
0
1
0
0
Fourth-sixth recidive
0
1
0
0
Tumors (n)
Abbreviations: O, oligodendroglioma, OA, oligoastrocytoma.
*Patient age and follow-up information were calculated using primary cases. Follow-up times are shown in months (mo).
specified (NOS) (80 tumors), oligodendroglioma, NOS (48
tumors), oligoastrocytoma, NOS (37 tumors), and glioblastoma, NOS (463 tumors).
Immunohistochemistry Staining
We stained 24 TMA slides using mouse monoclonal
FGFR3 antibody (1:600 dilution; #sc-13121, Santa Cruz
Biotechnology) together with the Dako EnVision+ kit
(K4006) and antibody diluent S0809. Antigen retrieval was
performed in sodium citrate buffer (pH 6.0), and immunoperoxidase reactions were visualized with 3,3'-diaminobenzidine using hematoxylin for counterstaining. IDH
p.R132H, MIB-1, and p53 staining have been reported previously.26,27 An experienced neuropathologist (H.H.) and
an experienced cell biologist (K.G.) independently scored
FGFR3 IHC staining intensities as 0 (no staining), 1 (weak),
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Follow-up
Granberg et al. Strong FGFR3 staining marks FGFR3 fusions
Statistical analyses were performed using R or IBM SPSS
statistics 21.0 software for Windows. For tables larger than
2 × 2, the P-values of Fisher’s exact test were calculated
using Monte Carlo simulation with 2.5*107replicates.
Targeted Sequencing
Fifty-one samples with staining data from whole-mount
tissue samples were analyzed using pulldown-based targeted sequencing. These included 15 strongly stained,
22 moderately stained, 6 weakly stained, and 8 negatively stained samples. Most samples were formalin
fixed and paraffin embedded (FFPE). The turXTRAC FFPE
DNA kit (Covaris) or AllPrep DNA/RNA Mini Kit (Qiagen)
was used for DNA isolation. The QIAamp DNA Mini Kit
(Qiagen) was used for 4 freshly frozen GBM samples that
were available. Target regions were enriched for sequencing from 1 μg of extracted DNA using the Sureselect XT
Target enrichment system and custom-designed RNA
probes (Supplementary Table 2). The sequencing libraries were prepared according to the kit (200 ng of DNA
samples) with a shorter DNA-shearing protocol (220 sec)
and were sequenced with an Illumina MiSeq instrument.
Reads were aligned against the GRCh37 human reference
genome using Bowtie-2.2.4.28
Somatic Mutation Calling and Fusion Detection
Somatic mutations were called by searching for variants
with an alternate allele fraction of at least 10%, and at least
5 supporting reads. Additionally, the allele fraction was
required to be 20 times higher than the background error
rate (ie, the average allele fraction across all nontumor
samples). Variants with a population frequency of 0.5% or
above in the 1000 Genomes or ESP6500 database were
filtered out. Protein-level outcomes were predicted using
ANNOVAR.29 To discover chromosomal rearrangements
for fusion detection, unaligned reads from each sample
were split into two 30 bp anchors (one from both ends)
that were aligned to the hg38 genome using Bowtie-1.1.2.
Discordant anchor pairs were grouped by position, and
groups with 8 or more supporting reads were flagged as
rearrangement candidates and manually curated using
IGV and BLAT.
Read counts were calculated using Bedtools-2.25.0.30
At least 25% of a read was required to be within the targeted capture region for the read to be counted. For each
sample, we calculated coverage log ratios at all captured
regions against a median reference derived from gendermatched blood samples from healthy patients (2 for each
gender). We then corrected for guanine-cytosine content
bias in coverage log ratios using a Theil–Sen estimator
(after observing that guanine-cytosine content and coverage log ratio bias had a linear relationship in this cohort)
(Supplementary Figure 1). Finally, we forced the median
log ratio of control baits in infrequently copy number
altered chromosomes 5, 8, 11, and 18 to zero by subtracting
the median from all log ratios. Copy numbers were called
by visual inspection of violin plots that visualize coverage
log ratios for each target gene.
Computational Analysis of FGFR3 IHC Staining
FGFR3 staining intensities were computationally analyzed
from whole-mount FFPE tissue samples of cases that
were used for targeted sequencing. One to 3 representative regions of interest (size 1.4–52 mm2, average 9.7 mm2)
were selected from scanned tissue images. Red versus
blue ratio, which was used as a readout of staining intensity, was calculated for each pixel in the regions of interest.
To remove background and nuclear hematoxylin staining,
only pixels with a red/blue ratio ≥1.00 were included in the
analysis. The mean of red/blue ratios was calculated from
these pixel locations.
Results
Moderate to Strong FGFR3 Staining Associated
with Poor Survival and Absence of IDH1 p.R132H
Mutation
We used an antibody that targets amino acids 25–124 in
the FGFR3 N-terminus to perform IHC staining on 676
diffuse astrocytomas and 116 grades II–III oligodendroglial tumors (Table 1). Moderate to strong FGFR3 staining was generally diffuse and cytosolic with occasional
nuclear staining, whereas membrane-restricted staining
was observed in some of the more weakly stained cases
(Fig. 1A, Supplementary Figure 2). Negatively stained
blood vessels provided an internal control for antibody
specificity (Supplementary Figure 3a). Normal brain tissue
was generally negative, with the exception of the cerebellar and cerebral molecular layers, where weak to moderate staining was observed (Supplementary Figure 3B–C).
Among astrocytomas, 575 (85%) cases were fully negative
for FGFR3. Among samples with FGFR3 immunoreactivity,
68 (10%) showed weak, 21 (3.1%) moderate, and 12 (1.8%)
strong positivity. The proportion of moderately to strongly
stained cases was within the range of previous estimates of
FGFR3 fusion frequency.1,2,7,8 FGFR3 staining was not associated with tumor grade (Fig. 1B). Among 116 oligodendroglial tumors, only 2 cases (1.7%) exhibited moderate and 1
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Statistical Methods
Copy Number Analysis
NeuroOncology
2 (moderate), or 3 (strong staining). Consensus interpretation was used in the case of discrepant interpretation.
MIB-1 staining on TMAs was used for the proliferation
analysis.
For antibody blocking, either peptide corresponding to fulllength epitope (amino acids 25–124 of FGFR3 of human origin)
(Biocenter Finland Protein Service) or peptide corresponding
to amino acids 40–64 (GPEPGQQEQLVFGSGDAVELSCPPP)
(GenScript) efficiently blocked antibody staining. Antibody
was incubated (30 min room temperature) with or without
the excess blocking peptide (5 times the antibody concentration in ng/µL units) in antibody diluent before adding the
mixture onto slides.
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Fig. 1 Moderate or strong FGFR3 staining was associated with worse survival and lack of IDH1 p.R132H mutation. (A) Representative staining
images. Proportion of samples for each staining category in grades II–IV astrocytomas is indicated below the representative images. Scale bars:
100 µm in 40×, 20 µm in 200×, and 10 µm in 400× images. (B) FGFR3 staining distribution in grades II–IV astrocytomas was not associated with tumor
grade (P = .523, Fisher’s exact test). (C) Moderate or strong FGFR3 staining was associated with poor cause-specific survival (n = 533, P = .042,
log-rank test). (D) Moderate to strong FGFR3 staining was more common in IDH1 p.R132H staining-negative cases. Each sample is marked with a
circle and colored according to FGFR3 staining. IDH1 p.R132H-negative GBM samples included 159 females and 215 males; thus, all the negatively
to weakly stained cases are not visualized separately. **P < .01, Fisher’s exact test. A: astrocytoma, O: oligodendroglioma, OA: oligoastrocytoma.
Granberg et al. Strong FGFR3 staining marks FGFR3 fusions
patients who lacked IDH1 p.R132H mutation, and all except
one of them suffered from GBM in the survival analysis
(Supplementary Figure 8). In the whole diffuse glioma
cohort, we found only 3 IDH1 p.R132H–positive cases with
moderate and none with strong FGFR3 staining (P = .0063,
Fisher’s exact test, for the IDH1 p.R132H staining association) (Fig. 2D, Supplementary Figure 9). The immunostaining results strongly coincide with previously reported
data on FGFR3 fusion-positive tumors (Supplementary
Figure 10),1,2,8,9 supporting the use of IHC for FGFR3 fusion
detection.
Next, we performed FGFR3 staining on all the available
whole-mount tissue samples with moderate to strong
FGFR3 staining on TMA. In addition, a subset (11%) of GBM
samples with negative to weak staining on TMA were analyzed using whole-mount tissue sections. Heterogeneous
FGFR3 staining of malignant tissue was generally observed
(Fig. 2A). Strong staining was subregional, and staining
intensities typically varied in a continuous fashion. Most
samples (65%, n = 114) had the same FGFR3 staining score
Fig. 2 FGFR3 staining is heterogeneous. (A) Representative images (50× magnification) of heterogeneously stained areas in 3 cases with strong
FGFR3 staining. In sample GBM-05, the staining pattern clearly differed between tumor sites. Scale bar 50 µm. (B) Summary of FGFR3 IHC staining
data from whole-mount tissue slides. Most samples (65% of all) retain their original FGFR3 staining score evaluated from TMA data, but especially
moderate scoring was observed in many samples (21% of all) with negative to weak staining on TMA. Numbers of TMA samples, whole-mount
tissue samples used for staining, IDH1 p.R132H staining-positive samples, and GBM samples are marked on the left-hand side. On the right, shown
are the number of whole-mount tissue samples and their proportion of all the stained slides with the specified staining score on TMAs.
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FGFR3 Staining Is Spatially Heterogeneous
NeuroOncology
case (0.9%) strong FGFR3 staining; all 3 cases were grade II
(Supplementary Figure 4).
An association analysis of astrocytic tumors (Supplementary Figure 5) revealed that moderate to strong
FGFR3 staining was more common in females (8.55%) than
in males (2.46%) (P < .001, Fisher’s exact test) and less common in samples with aberrant p53 expression (P < .010,
Fisher’s exact test) (Supplementary Figure 6). In the prognostic analysis, moderate and strong FGFR3 staining was
associated with a significantly shorter cause-specific survival duration than weak and negative staining (P = .0417,
n = 533, log-rank test) (Fig. 1C). Although the association
remained significant after adjustment for grade, proliferation, and IDH1 p.R132H mutation status (P = .0422, Cox
proportional hazards model) (Supplementary Table 3), no
significant associations were observed when analysis was
restricted to grade IV GBM (P = .203, n = 414, log-rank test)
or cases lacking IDH1 p.R132H mutation (P = .119, n = 370
for grades II–IV astrocytomas and P = .525, n = 343 for
GBMs, log-rank test) (Supplementary Figure 7), suggesting
that the reduced survival of moderately to strongly stained
cases is at least partly explained by their IDH1 mutation
status and high proportion of glioblastoma tumors. Indeed,
moderate to strong FGFR3 staining was only observed in
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on both the TMA and the whole-mount tissue slide (Fig. 2B),
but higher staining intensities were observed in one third
of the samples. It is thus better to perform FGFR3 staining
for diagnostic purposes using whole-mount tissue slides, as
they will be more representative than biopsies or TMAs.
fusion-negative cases (P < .0001, Kruskal–Wallis test)
(Fig. 3A–B). Fusion-negative cases that show only focal
strong FGFR3 staining (GBM-09, GBM-10, and ASTRO-02)
resemble more moderately stained cases in the computational analysis (Fig. 3A–B).
Cases with Strong FGFR3 Staining Harbor FGFR3
Fusion Rearrangements
FGFR3 Fusion-Positive Cases Are Predominantly
Female and Mutually Exclusive with EGFR/
PDGFRA/MET Alterations
All cases with FGFR3 fusions were IDH wild-type, which
is concordant with the results of previous reports.7,9 In
addition, FGFR3 fusion events were mutually exclusive
with amplification or mutation of epidermal growth factor receptor (EGFR)/platelet derived growth factor receptor alpha (PDGFRA)/MET (Fig. 3A, Supplementary Tables
4–5). Mutual exclusivity between EGFR amplification and
FGFR3-TACC3 fusions has been previously reported by us
and others.8,9
Nearly all patients (7 of 8) with FGFR3-TACC3 fusions
were female, the sole exception being a male patient
with grade II oligoastrocytoma. Alternative FGFR3CAMK2A and FGFR3-ELAVL31 fusions were observed
in male patients. A higher FGFR3 fusion frequency in
females (P = .028, Fisher’s exact test) (Fig. 3B) is consistent with sex-associated FGFR3 staining distribution in
our IHC cohort (Supplementary Figure 6a). In all previously reported cohorts, 14 females and 11 males carried
FGFR3 fusions.1,2,8,9,13,34 After combining our data and all
IDH wild-type cases with sex information in previous repo
rts,1,2,8,13,34 3.1% of males (8 of 255) and 8.4% of females (14
of 152) were FGFR3 fusion positive (P = .027, Fisher’s exact
test), suggesting a higher incidence among females.
Predominant FGFR3 Staining Typically Observed
in Fusion-Positive Tumors
As negative FGFR3 staining was commonly observed in
whole-mount tissue slide samples, we determined the proportion of positively (weakly to strongly) stained malignant
cells from the tissue slide samples. In 6 of 9 fusion-positive
patients, all malignant cells were FGFR3 positive (Fig. 4B).
Moderate to strong staining was observed in 4 cases and
weak to strong staining in 2 cases. Furthermore, both GBM04 and GBM-07 samples, derived from different surgeries
of one patient, had a small negatively stained tumor region
(<10% of malignant cells). There was a significant proportion of negatively stained malignant cells in 2 additional
fusion-positive patients (GBM-02 and GBM-05). These
cases harbored an FGFR3-CAMK2A fusion and an FGFR3TACC3 fusion, in which the breakpoint was in the 3' untranslated region (UTR) of FGFR3 but produced fusion protein,
as the splicing acceptor site had been deleted during the
rearrangement (Supplementary Figure 13). The proportion
of positively stained malignant cells was higher in fusionpositive than fusion-negative samples (P = 1.2 × 10–8,
Fisher’s exact test) (Fig. 4A), suggesting that positive
staining in the whole malignant tissue further supports
the presence of FGFR3 fusion. The majority of FGFR3positive malignant cells were moderately to strongly
stained in all fusion-positive cases, which is concordant
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To detect genomic breakpoints in cases with positive FGFR3
staining, we selected 51 diffuse glioma samples (including 43 GBMs, 5 grade II astrocytomas, 2 grade II oligoastrocytomas, and 1 grade II oligodendroglioma according
to initial diagnosis) for targeted sequencing. We analyzed
all the patients with moderate to strong FGFR3 staining in
whole-mount tissue slides and with enough tumor material for sequencing, but the cohort included also negatively
to weakly stained cases. In addition to FGFR3 and FGFR1,
the sequencing panel targeted genes with reported alterations in diffuse gliomas, such as IDH1, IDH2, TP53, ATRX,
CIC, FUBP1, CDKN2A, and RB1. FGFR3 rearrangements
that generated gene fusions were detected in 10 cases
(Fig. 3A, Supplementary Table 4), including 9 FGFR3-TACC3
fusions and 1 FGFR3-CAMK2A fusion. In addition, case
GBM-19 harbored an FGFR3 copy number gain (Fig. 3A)
and a rearrangement that joined the WHSC1 gene, located
downstream of FGFR3, to TACC3 (data not shown). The
DNA breakpoints and resulting FGFR3 fusion junctions
were distinct in each tumor (Fig. 3C, Supplementary
Figure 11), but the coiled-coil domain of TACC3 was conserved in FGFR3-TACC3 fusions. CAMK2A codes for a
subunit of calcium/calmodulin-dependent protein kinase
II (CAMK2).31,32 The C-terminal self-association domain of
CAMK2A is joined with nearly the full-length FGFR3 protein.31,32 This is also the case for the fusion of FGFR3 and
embryonic lethal abnormal vision (ELAV)–like RNA binding protein 3 (ELAVL3) previously reported in oligoastrocytoma1,33 (Supplementary Figure 10). It is thus likely that
ELAVL3 and CAMK2A facilitate spontaneous fusion protein
oligomerization and drive the oncogenic activity of FGFR3
fusion through increased kinase activity, as suggested for
previously reported FGFR3 fusions.18,19
Most positive cases were GBMs, but FGFR3-TACC3
fusion rearrangements were also found in one grade II
tumor with astrocytic morphology and one grade II tumor
with oligoastrocytic morphology. Both cases were IDH
wild-type in targeted sequencing, and thus represent
“diffuse astrocytoma, IDH wild-type” in the WHO 2016
classification.5 This is also the case for the IDH wild-type
grade II tumors ASTRO-02 and OASTRO-02, which showed
strong FGFR3 staining but did not carry detectable FGFR3
fusions.
All FGFR3-TACC3 and FGFR3-CAMK2A fusion-positive
samples were strongly FGFR3 positive on IHC (staining sensitivity 100% in sequencing cohort) (Fig. 3A,
Supplementary Figure 12). Five cases (out of 41, staining
specificity 88% in the sequencing cohort) lacked intergenic FGFR3 rearrangements but showed strong FGFR3
staining in subregions of whole tissue sections (Fig. 3A,
Supplementary Figure 12). Computationally analyzed
staining intensities were also significantly higher in fusionpositive cases than in negatively to moderately stained
Granberg et al. Strong FGFR3 staining marks FGFR3 fusions
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NeuroOncology
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Fig. 3 FGFR3 fusion-positive cases showed strong FGFR3 staining. Most were female and lacked alterations in IDH, EGFR, and PDGFRA genes.
(A) FGFR3 rearrangements and associated alterations were detected by targeted sequencing. FGFR3 staining scores (3: strong, 2: moderate, 1:
weak, 0: negative staining) on whole-mount tissue slides and computational analyzed staining intensities are marked above the aberration matrix.
The cases had been initially diagnosed as GBMs (one giant cell glioblastoma [GC] and 2 gliosarcomas [GS]), grade II astrocytomas (A), grade II
oligoastrocytomas (OA), and a grade II oligodendroglioma (O). M: male, F: female. (B) Computationally analyzed FGFR3 staining intensity is significantly higher in fusion-positive cases than in negatively to moderately stained fusion-negative cases (P < .0001, Kruskal–Wallis test). Cases with
focal strong FGFR3 staining are marked in red. (C) FGFR3 fusions were more commonly observed in females than in males (*P < .05, Fisher’s exact
test). (D) All FGFR3 fusion partners carried dimerization domains. Predicted fusion protein structures are shown in the figure. Intronic regions that
become part of the translated fusion protein are marked with gray. *In-frame fusion junction is located inside the marked exon.
1214
Granberg et al. Strong FGFR3 staining marks FGFR3 fusions
both fusion-positive and fusion-negative tumors, as only
red blood cell and hemosiderin staining were apparent
in samples that were stained by using a blocking peptide
(Supplementary Figure 14).
Higher FGFR3 Staining Was Associated with
Lower Proliferation Rate
Discussion
Fig. 4 (A) The proportion of negatively stained tumor cells was generally lower in fusion-positive cases. P = 1.2 × 10–8, Fisher’s exact test.
(B) Tumors with moderate to strong FGFR3 staining showed lower
proliferation rate than negatively to weakly stained tumors (P = .0048,
Kruskal–Wallis test). FGFR3 fusion-positive cases significantly differed from fusion-negative ones (P = .032, Mann–Whitney test). GBM
samples with whole-mount tissue staining of FGFR3 were included
in the analysis. Out of strongly stained samples, only fusion-positive
cases were included due to low number of fusion-negative ones. The
mean proliferation rate observed on TMA and SEM are shown. (C)
FGFR3 staining was associated with lower cellularity within malignant regions in whole-mount tissue sections used for targeted
sequencing, but this association was not observed in FGFR3 fusionpositive cases (P = .024, Fisher’s exact test).
with moderate to strong FGFR3 staining observed in all the
fusion-positive cases on TMA. The use of antibody blocking peptide diminished FGFR3 staining very efficiently in
Our results demonstrate that FGFR3 staining, as detected
using IHC, is indicative of FGFR3 gene fusion, which can be
further confirmed by PCR- or sequencing-based technologies. This method allows efficient patient selection, as most
cases are fully negative for FGFR3, and is fully compatible
with current clinical practices throughout the world. Even
if sequencing-based technologies are used as the primary
tool for FGFR3 fusion detection, information on fusion protein expression levels will be valuable when estimating
FGFR inhibitor treatment response. FGFR3 alterations will
not be informative if the altered protein is not expressed.
Actually, FGFR1 expression levels have been shown to
predict treatment responses better than genomic FGFR1
alterations in head and neck squamous cell cancers as well
as different lung cancers.35,36 The role of FGFR3 staining as
an independent predictive marker needs to be evaluated in
the future. We identified 2 IDH wild-type cases with strong
and widely positive FGFR3 staining but no evidence for
intergenic FGFR3 rearrangements (Fig. 3A). It is relevant
to determine whether these cases will benefit from treatment with FGFR inhibitors. Reason for strong FGFR3 staining in fusion-negative samples is unknown, but at least the
miR-99a gene was not altered in these tumors. One reason
might be suppressed miR-99a expression, despite miR-99a
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We next associated FGFR3 staining data with the proportion of MIB-1 immunopositive nuclei, which is commonly
used as a cell proliferation index and a key feature of
tumor aggressiveness. Surprisingly, astrocytoma cases
with moderate to strong FGFR3 staining exhibited lower
proliferation rates than negatively or weakly stained cases
independently of tumor grade (P < .05, Kruskal–Wallis test)
(Supplementary Figure 15a). This is somewhat contradictory to worse prognosis of moderately to strongly stained
cases in the same cohort (Fig. 1C). A similar pattern was
also observed in whole-mount tissue GBM samples used
for FGFR3 staining (P = .0048, Kruskal–Wallis test) (Fig. 4B,
Supplementary Figure 15b). Staining was not associated
with patient prognosis in this cohort (Supplementary
Figure 16). Lower proliferation rates of FGFR3 fusionpositive tumors may have led to underestimation of their
aggressiveness when diagnosis was based on WHO 2007
classification. More detailed inspection of intratumoral
FGFR3 staining patterns revealed that stronger FGFR3
staining was observed in less cellular tumor areas with
higher differentiation state in a large proportion of fusionnegative cases but not in fusion-positive cases (P = .024,
Fisher’s exact test) (Fig. 4C, Supplementary Figure 17). In
addition, stronger FGFR3 staining was perivascular in a
subpopulation of cases (Supplementary Figure 3a), but
this was not associated with fusion status.
Granberg et al. Strong FGFR3 staining marks FGFR3 fusions
Funding
Acknowledgments
We highly appreciate Marika Vähä-Jaakkola, Paula Kosonen,
Päivi Martikainen, Marja Pirinen, Katja Liljeström, Riina Kylätie,
Dr. Leena Latonen, Osku Alanen, Maria Laaksonen, Dr. Eloise
Mikkonen, Riitta Koivisto, Dr. Joanna Ilvesaro, and Satu Salo for
their help in sample handling. Personnel at Tampere University
Hospital and Fimlab laboratories are acknowledged for their
contribution to sample collection. We acknowledge the CSC
IT Centre for Science, Finland, for computational resources
and Ann M. Sutton (Department of Scientific Publications,
University of Texas MD Anderson Cancer Center) for editing this
manuscript.
Conflict of interest statement. The authors have no conflicts of
interest to disclose.
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This work was supported by grants from the Academy of
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