High-grade neuroepithelial tumors with the BCOR alteration (HGNET-BCOR) were isolated
by a distinct methylation profile from a series of central nervous system (CNS) primitive
neuroectodermal tumors (PNET) [6]. These tumors are mainly (94%, 45/48 with available
molecular data) characterized by a recurrent internal tandem duplication (ITD) of
the BCOR (BCL6 Corepressor) gene [1–4, 6, 9]. In rare cases, HGNET-BCOR presented
a deletion of BCOR (3%, 1/48) or a mutation of the BCOR gene (3%, 1/48) [6]. In one
case, molecular analyses failed to reveal any alteration of BCOR [6]. The cIMPACT-NOW
update 6 recommends the new terminology of CNS tumor with BCOR ITD to designate this
entity [5]. Here we report two tumors with a HGNET-BCOR methylation class (MC) but
harboring a BCOR fusion with the EP300 gene (encoding the protein p300 which is an
acetyltransferase histone implicated in controlling cell growth and differentiation).
The aim of our work was to compare the clinical, radiological and histopathological
features of these two previously published HGNET-BCOR cases with ITD.
The two observations concerned a 13-year old boy (Case #1) and a 27-year-old man (Case
#2). Tumors were located in the right temporal lobe (Case #1) and in the left frontal
lobe (Case #2). Central neuroradiological review revealed large and well-circumscribed
tumors with a meningeal attachment but without peri-lesional edema (Figs. 1 and 2).
They appeared as solid hypercellular masses with a restricted apparent diffusion coefficient
(ADC) in the main part of the tumors (Figs. 1 and 2). They displayed a heterogeneous
enhancement after contrast injection (Figs. 1 and 2). These imaging characteristics
were similar to HGNET-BCOR radiological data descriptions such as large and well-circumscribed
tumors with a meningeal attachment, no peri-lesional edema, solid and hypercellular
masses and a heterogeneous enhancement after a contrast injection [9]. Histopathological
review revealed that both tumors presented the same features (Figs. 1 and 2). These
tumors were mainly well-circumscribed from the brain parenchyma (with few infiltrating
isolated cells at the periphery of the tumors). Pseudo-rosettes and microcysts were
constantly observed. These microcysts contained a myxoid substance or occasional floating
neurons. One case presented calcifications. There was intra-tumoral hetereogeneity
in terms of cytology, with oligo-like, embryonal, or ependymal features. Malignancy
was obvious with necrosis (calcified), high mitotic count and proliferation index,
and microvascular proliferation in both cases. Immunohistochemical findings are summarized
in Additional file 1: Table S1, and main features are presented in Figs. 1 and 2.
There was preserved expression of H3K27me3, INI1 and ATRX in the two cases, expression
of GFAP was absent, whereas Olig2 was diffusely expressed in both cases. Expression
of at least one neuronal marker was present in both cases. All these results were
in line with the reported HGNET-BCOR with ITD (25/43 reported cases were initially
diagnosed as PNET) (Table 1) [1, 2, 6, 9]. Using the Heidelberg DNA methylation classifier,
our two cases were classified as HGNET-BCOR (with calibrated max-scores of 0.6 and
0.9). RNA sequencing analysis of the two cases showed a fusion between EP300 and BCOR
genes, with intra exonic breakpoints (in exon 31 for EP300, and exon 4 for BCOR) (Fig. 3).
None of our cases exhibited an overexpression of BCOR (Fig. 3) contrarily to 100%
of reported HGNET with BCOR ITD [1, 2, 9]. The fusion EP300:BCOR causes the loss of
the first 3 exons of BCOR and a part of the exon 4 encoding the Nter domain of the
protein (Fig. 3). As the BCOR antibody is designed against the 300 first residues
of the native protein, this epitope is missing in the resulting chimeric fusion protein
and not detected by immunohistochemistry (Fig. 3).
Fig. 1
Radiological and histopathological features of #case 1. a Computed tomography scan
showing a large and calcified tumor of the right temporal lobe. b T2-weighted MRI
sequence reveals leptomeningeal attachment but no peri-lesional edema. c T1-weighted
image, d T1-weighted image after injection of gadolinium showing a heterogeneous enhancement
diffusion-weighted images. e Cerebral blood flow was low using arterial spin labeling.
f Diffusion was restricted in a large part of the tumor and g apparent diffusion coefficient
was low. h Compact tumor with delicate branching vessels exhibiting a chicken-wire
pattern mimicking ependymoma (HPS, magnification ×200) with some calcifications (i,
HPS, magnification ×200). j Microcyst formation in the tumor (HPS, magnification ×200),
k containing occasional neuronal cells (arrowheads, HPS, magnification ×400). l High
mitotic index (circles, HPS, magnification ×400) and m elevated MIB1 labeling index
(magnification ×400). n Necrosis with calcifications, and microvascular proliferation
(arrowheads, HPS, magnification ×200). o Well-circumscribed tumor on neurofilament
staining (magnification ×100). p Diffuse expression of Olig2 (magnification ×400)
whereas q GFAP was not expressed by tumor cells, with internal positive control (scattered
astrocyte remnants designated by arrowheads) (magnification ×400). r NeuN expression
by tumor cells (magnification ×400). s Intense EGFR expression (magnification ×400).
Black scale bars represent 100 µm (h–j, n), and 50 μm (k–m, p–s) and 250 µm (o). HPS
Hematoxylin phloxin saffron
Fig. 2
Radiological and histopathological features of #case 2. a Coronal T2-weighted sequence
showing a large tumor without peri-lesional edema in the left frontal lobe. b Axial
T1-weighted image showing a left frontal mass with leptomeningeal attachment and heterogeneous
enhancement after gadolinium injection. c T1-weighted image after injection of gadolinium
showing a heterogeneous enhancement. d Flair sequence showing hyperintensity. e Compact
tumor with delicate branching vessels exhibiting a chicken-wire pattern (HPS, magnification
×200) with oligo-like features (f, HPS, magnification ×200). g Microcyst with a sometimes
myxoid background (HPS, magnification ×200) and h containing some neuronal cells (arrowheads,
HPS, magnification ×400). i Area with dense cellularity and high mitotic index (arrowheads,
HPS, magnification ×400) and j elevated MIB1 labeling index (magnification ×400).
k Palisading necrosis (HPS, magnification ×400) and microvascular proliferation (l,
HPS, magnification ×400). m The tumor is well-circumscribed from brain parenchyma,
as seen on GFAP staining, without expression in the tumor (magnification ×100). (n)
Diffuse expression of Olig2 (magnification ×400). o Neurofilament expression by tumor
cells (magnification ×400) and p cytoplasmic expression of EMA (magnification ×400).
Black scale bars represent 100 µm (e–g, k,l), and 50 μm (h,i, n–p) and 250 µm (m)
Table 1
Comparison of clinical, histopathological and molecular data according to methylation
classes and diagnoses
HGNET-BCOR ITD (n = 29)
HGNET-BCOR EP300:BCOR/BCORL1 fusions (n = 3)
GLIOMAS EP300:BCOR fusion (n = 4)
Location
Infratentorial (52%)
Supratentorial (100%)
Supratentorial (100%)
Age
Median age = 3.5 YO (0;22)
Median age = 27 YO (13;72)
Median age = 12 YO (10;18)
Sex
Male (54%)
Male (100%)
Male (66%)
Radiology
Large, well-circumscribed, solid with meningeal attachment; T1 hypointense, T2 hyperintense,
low ADC, heterogeneous enhancement
Large, well-circumscribed, solid with meningeal attachment; T1 hypointense, T2 hyperintense,
low ADC, heterogeneous enhancement
Limited data: no meningeal attachment, not well circumscribed, T2 hyperintense, mild
enhancement
Histopathology
High-grade solid tumor with perivascular pseudorosettes and microcysts
High-grade solid tumor with perivascular pseudorosettes and microcysts
Infiltrative tumor Variable grade (low in 2 cases, high in 2 cases)
Immunohistochemistry
GFAP-/Olig2+/EMA-/Neuronal markers+/BCOR+
GFAP-/Olig2+/EMA-/Neuronal markers+/BCOR-
GFAP+/Olig2+/Neuronal markers-/BCOR+
DNA-methylation class
HGNET-BCOR
HGNET-BCOR
LGG-MYB/MYBL1
Outcome
65% recurrences
Median PFS = 12.5 months 30% dead at the end of follow-up
Median OS = 26 months
0% recurrences
0% dead at the end of follow-up
Median OS = 27 months
100% recurrences
Median PFS = 4.0 months
0% dead at the end of follow-up
Median OS = 7 months
ADC apparent diffusion coefficient, ITD internal tandem duplication, OS overall survival,
PFS progression-free survival, YO years-old
Fig. 3
Fusion EP300:BCOR and correlation with immunohistochemistry. a RNAseq analysis highlights
a fusion between EP300 (pink) and BCOR (blue) genes, respectively located on chr22q13.2
and chrXp11.4. As the breakpoints are intra exonic (in exon 31 for EP300, and exon
4 for BCOR), the fusion point can easily been detected by split and span reads encompassing
the rearrangement with a good coverage. Localized on minus strand (inverse orientation),
the DNA sequence of BCOR is switched in frame with EP300 (b Circos plot). This aberration
causes the loss of the first 3 exons of BCOR and a part of the exon 4 encoding the
Nter domain of the protein (dark blue). As the BCOR antibody is designed against the
300 first residues of the native protein and since this epitope is missing in the
resulting chimeric fusion protein, it cannot be used for EP300-BCOR detection by IHC.
c Conserved domains in the fusion protein. d Absence of expression of BCOR by immunohistochemistry
with positive internal control (tumor of methylation class HGNET-BCOR with BCOR internal
tandem duplication, insert) (magnification ×400). Black scale bars 50 μm (D)
Interestingly, this same fusion was previously reported in gliomas [7] but these cases
were distinct of our cases from radiology (infiltrative pattern), histopathology and
immunohistochemistry (infiltrative proliferation with calcifications, composed of
GFAP positive cells without expression of neuronal markers) [7]. Moreover, gliomas
described by Torre et al. were in close vicinity to LGG with an MYB/MYBL1 alteration
by t-Distributed Stochastic Neighbor Embedding plot (t-SNE) analysis whereas our cases
were classified into the MC HGNET-BCOR and clearly clustered with HGNET-BCOR by t-SNE
analysis (Fig. 4) [7]. Despite constant malignant histopathological features and a
high rate of recurrences (65%, 17/26 cases), the prognosis of HGNET-BCOR with ITD
remains unclear with a mortality rate of 30% (7/23 cases) [1–4, 9]. Mean/median progression-free
survival (PFS) were 24.4/12.5 months and mean/median overall survival (OS) were 38.9/26.0 months
in reported HGNET-BCOR with ITD [1–4, 9]. Notably, some reported cases were alive
more than ten years after the initial diagnosis [2, 4]. In our cases, after total
resection, patient #1 was treated by chemotherapy only and patient #2 was treated
by chemotherapy and focal irradiation. Neither have presented a recurrence and are
alive, 16 and 27 months after the initial diagnosis.
Fig. 4
Methylation-based t-SNE distribution. The two tumors with EP300:BCOR fusion were compared
with 147 reference samples from the Heidelberg cohort belonging to the HGNET-BCOR,
HGNET-MN1, LGG-MYB/MYBL, EPN-RELA, EPN-YAP methylation classes which constitute histopathological
differential diagnoses. The two cases of this study are indicated as grey dots and
shown by arrows. HGNET-BCOR, high-grade neuroepithelial tumors with BCOR alteration
(red dots); HGNET-MN1, high-grade neuroepithelial tumors with MN1 alteration (yellow
dots); LGG-MYB/MYBL1, low-grade gliomas with MYB or MYBL1 alteration (blue dots);
EPN-RELA, ependymomas with RELA fusion (pink dots); EPN-YAP, ependymomas with YAP
fusion (gree dots)
To conclude, we presented for the first time two supratentorial tumors with EP300:BCOR
fusion sharing clinico-radiological, histopathological, immunohistochemical, and methylome
homologies with HGNET-BCOR with ITD while they did not share similarities with the
previous reported gliomas harboring this same fusion. Consequently, the EP300:BCOR
fusion expands the spectrum of the alterations encountered in the MC HGNET-BCOR, and
therefore, the terminology “CNS tumors with BCOR ITD” seems to be too restrictive.
This finding echoes the data published in small round cell sarcomas of soft tissue,
which may harbor BCOR fusions (mainly with CCNB3 gene) and BCOR ITD [8]. Because the
BCOR immunohistochemistry does not allow detections of HGNET-BCOR with fusion, we
recommand searching for alternative alterations of the BCOR gene in the event of radiological
and histopathological suspicion of this diagnosis when ITD is absent.
Supplementary information
Additional file 1: Table S1. Immunohistochemical findings of our cases of HGNET-BCOR
with EP300:BCOR fusion.