Morphological and molecular features of astroblastoma, including BRAFV600E mutations, suggest an ontological relationship to other cortical-based gliomas of children and young adults

Abstract

Background: Astroblastomas (ABs) are rare glial tumors showing overlapping features with astrocytomas, ependymomas, and sometimes other glial neoplasms, and may be challenging to diagnose.

Methods: We examined clinical, histopathological, and molecular features in 28 archival formalin-fixed, paraffin-embedded AB cases and performed survival analyses using Cox proportional hazards and Kaplan-Meier methods.

Results: Unlike ependymomas and angiocentric gliomas, ABs demonstrate abundant distinctive astroblastic pseudorosettes and are usually Olig2 immunopositive. They also frequently exhibit rhabdoid cells, multinucleated cells, and eosinophilic granular material. They retain immunoreactivity to alpha thalassemia/mental retardation syndrome X-linked, are immunonegative to isocitrate dehydrogenase-1 R132H mutation, and only occasionally show MGMT promoter hypermethylation differentiating them from many diffuse gliomas. Like pleomorphic xanthoastrocytoma, ganglioglioma, supratentorial pilocytic astrocytoma, and other predominantly cortical-based glial tumors, ABs often harbor the BRAF^V600E mutation, present in 38% of cases tested (n = 21), further distinguishing those tumors from ependymomas and angiocentric gliomas. Factors correlating with longer patient survival included age less than 30 years, female gender, absent BRAF^V600E , and mitotic index less than 5 mitoses/10 high-power fields; however, only the latter was significant by Cox and Kaplan-Meier analyses (n = 24; P = .024 and .012, respectively). This mitotic cutoff is therefore currently the best criterion to stratify tumors into low-grade ABs and higher-grade anaplastic ABs.

Conclusions: In addition to their own characteristic histological features, ABs share some molecular and histological findings with other, possibly ontologically related, cortical-based gliomas of mostly children and young adults. Importantly, the presence of BRAF^V600E mutations in a subset of ABs suggests potential clinical utility of targeted anti-BRAF therapy.

Keywords: BRAFV600E mutation; MGMT promoter hypermethylation; IDH1R132H mutant protein expression; Olig2 protein expression; astroblastoma.

Fig. 1

Astroblastic pseudorosettes and ependymal rosettes. (A) Astroblastic pseudorosette with stout columnar-like cells occasionally demonstrating expanded endfeet. (B) Astroblastic pseudorosette displaying tapered cells. Note “bell-bottom” endfeet. (C) Astroblastic pseudorosette with cuboidal cells. Similar perivascular pseudorosettes may be seen in some ependymomas. In contrast to AB, however, ependymomas are Olig2 negative and lack other AB histological features. (D) Infrequently, astroblastomas may contain astroblastic pseudorosettes exhibiting spindly perivascular cells necessitating further distinction of such tumors from angiocentric gliomas, pilocytic/pilomyxoid astrocytomas, tanycytic ependymomas, and myxopapillary ependymomas. Even spindly cells within astroblastic pseudorosettes occasionally show flared endfeet, however, which are usually lacking in the latter tumors. In practice, individual ABs frequently demonstrate multiple astroblastic pseudorosette cell types both within a given astroblastic pseudorosette and between individual pseudorosettes. (E) Typical classic ependymal perivascular pseudorosette showing a fibrillary perivascular area lacking the more distinct cell borders seen in ABs. Note that as in astroblastic pseudorosettes, cell processes are radially oriented (depicted by the faint gray lines in the anuclear perivascular zone). (F) True ependymal rosettes lacking central blood vessels may be seen in classic ependymomas and some cortical ependymal tumors and are only rarely seen in ABs. Cartoons are not drawn to scale.

Fig. 2

Astroblastoma histological features. Case 1: (A) Astroblastic pseudorosette in longitudinal section. Numerous signet ring or adipocyte-like cells are present. Case 2: (B) Astroblastic pseudorosette cells are cuboidal to stout columnar in shape. (C) An area of rhabdoid cells is depicted. Case 3: (D) Astroblastic pseudorosettes. Necrosis is present on the left (arrows). Note retinoblastoma florette-like or “cartwheel” appearance of the astroblastic pseudorosettes. Case 4: (E) Astroblastic pseudorosette in anaplastic AB showing multinucleated “light bulb” cells (arrows). Abundant atypical mitoses and rare pleomorphic nuclei were also present (not shown). This case nevertheless demonstrated a sharp border with brain tissue. Case 18: (F) Astroblastic pseudorosettes in longitudinal section and EGB (arrow). Case 10: (G) AB showing extensive vascular sclerosis. Rhabdoid cells with pale cytoplasmic bodies (arrows) and apparent acellular pale hyaline spherical bodies (arrowheads) were noted. Multinucleated cells (gray arrow) and occasional cells with nuclear pseudoinclusions (inset, same scale as main panel) were present. Recurrent tumor lacked this extensive sclerosis. Case 20: (H) Large astroblastic pseudorosettes. A nuclear pseudoinclusion is indicated by the black arrow. Frequent multinucleate cells (arrowheads) and pale spherical bodies (gray arrows) were present. Large clear cells reminiscent of those seen in PXAs are also seen in the photomicrograph, as well as smaller cells with perinuclear clearing resembling oligodendrocytes. Case 28: (I) Anaplastic AB composed mainly of astroblastic pseudorosettes. Perivascular cells were frequently tapered and showed occasional flared endfeet. Rhabdoid cells contained brightly eosinophilic inclusions (arrows). (J) A smaller component of the tumor consisted of small undifferentiated-appearing cells demonstrating mitotic activity (arrow). (K) Pseudopalisading necrosis encompassing area of small cells. Case 19: (L) Brightly eosinophilic granules appeared to be both intracellular and extracellular. (M) GFAP immunohistochemical stain demonstrating stout perivascular cells in an anaplastic astroblastoma. Cells between astroblastic pseudorosettes were mostly GFAP negative. Case 26: (N) Anaplastic AB showing numerous small undifferentiated-appearing cells between astroblastic pseudorosettes. (O) Astroblastic pseudorosettes demonstrating retraction of perivascular cells from central vascular proliferation. Note that although very narrow, the cellular processes are of mixed columnar and tapered forms. The arrow indicates a thrombosed astroblastic pseudorosette central vessel. Case 22: (P) Large astroblastic pseudorosette in an anaplastic astroblastoma demonstrating retraction from a folded central vessel and smaller vessels. Tall perivascular cells with flared endfeet are separated from the vessel by a lymphocytic infiltrate. (Q) Collagen IV immunohistochemical stain demonstrating immunopositivity of basal lamina attached to retracted perivascular cells (arrows). (R) Scattered rhabdoid (arrows) and multinucleate cells (arrowhead) were present. The upper right inset depicts a gemistocytic cell. The lower right inset shows apparent extracellular eosinophilic granular material (insets same scale as main panel). (S) Tumor infiltrated brain parenchyma along blood vessels, but retained radially oriented cell rosetting. (T) Primitive-appearing small cells were focally present, here in the subarachnoid space abutting well-differentiated subpial tumor. Increased mitotic activity and occasional apoptotic cells were noted among these small cells. (U) Olig2 immunoreactivity was of variable intensity. All scale bars = 100 μm.

Fig. 3

Histology of BRAF^V600E positive astroblastomas. Case 5: (A) Large astroblastic pseudorosette within an anaplastic astroblastoma demonstrating a small perivascular lymphocytic infiltrate and intravascular thrombosis. (B) Eosinophilic granular body near astroblastic pseudorosette (arrow). (C) Large cells with bubbly cytoplasm. (D) Astroblastic pseudorosettes associated with a rhabdoid cell (arrow) and membrane bound pale spherical bodies possibly representing transected rhabdoid cells (arrowheads). Case 6: (E) Astroblastic pseudorosettes and rhabdoid cells. Clear and vacuolated cells are also present. Case 9: (F) Astroblastoma with stout columnar perivascular cells. Occasional rhabdoid cells are present. Case 11: (G) Papillary-like arrangement of astroblastic pseudorosettes with mostly polygonal perivascular cells. (H) Longitudinally oriented astroblastic pseudorosette demonstrating perivascular cells with flared endfeet (arrows). (I) Rare large vacuolated tumor cell nucleus reminiscent of those seen in PXA. A cell demonstrating a single nuclear pseudoinclusion (black arrow) and a gemistocytic cell with eosinophilic granular cytoplasm (gray arrow) are also present. The inset depicts a large cell with bubbly cytoplasm and multiple peripherally located nuclei (same scale as main panel). Case 17: (J) Astroblastic pseudorosette with nearby gemistocytic cell containing eosinophilic granules (arrow). (K) Recurrent tumor demonstrating longitudinally oriented astroblastic pseudorosette displaying a multinucleated perivascular cell and perinuclear clearing (arrows). Case 23: (L) Pseudopapillary structure of anaplastic astroblastoma. (M) Rhabdoid cells and gemistocytic cell containing eosinophilic granules (center). Case 12: (N) Astroblastic pseudorosettes demonstrating retraction from the central vessel with cells apparently attached to the basement membrane (gray arrows). Some perivascular cells show perinuclear clearing (black arrows). Pale bodies are also present (arrowhead). Case 24: (O) Papillary-like astroblastic pseudorosettes. A multinucleated perivascular cell is denoted by the black arrow. Apparent astroblastic pseudorosette cell retraction from the central vessel is indicated by the gray arrow. (P) Focal areas of rhabdoid cells were present. (Q) Rhabdoid cells were vimentin immunopositive. All scale bars = 100 μm.

Fig. 4

Correlates of AB patient survival. Kaplan–Meier analysis curves are shown for (A) patient age, (B) patient gender, (C) histological grade, and (D) mitotic index. Although all plots appear to depict clear trends, only mitotic index showed a statistically significant relationship to patient survival using the Mantel–Cox log rank test.