Oligosarcomas, IDH-mutant are distinct and aggressive

Abstract

Oligodendrogliomas are defined at the molecular level by the presence of an IDH mutation and codeletion of chromosomal arms 1p and 19q. In the past, case reports and small studies described gliomas with sarcomatous features arising from oligodendrogliomas, so called oligosarcomas. Here, we report a series of 24 IDH-mutant oligosarcomas from 23 patients forming a distinct methylation class. The tumors were recurrences from prior oligodendrogliomas or developed de novo. Precursor tumors of 12 oligosarcomas were histologically and molecularly indistinguishable from conventional oligodendrogliomas. Oligosarcoma tumor cells were embedded in a dense network of reticulin fibers, frequently showing p53 accumulation, positivity for SMA and CALD1, loss of OLIG2 and gain of H3K27 trimethylation (H3K27me3) as compared to primary lesions. In 5 oligosarcomas no 1p/19q codeletion was detectable, although it was present in the primary lesions. Copy number neutral LOH was determined as underlying mechanism. Oligosarcomas harbored an increased chromosomal copy number variation load with frequent CDKN2A/B deletions. Proteomic profiling demonstrated oligosarcomas to be highly distinct from conventional CNS WHO grade 3 oligodendrogliomas with consistent evidence for a smooth muscle differentiation. Expression of several tumor suppressors was reduced with NF1 being lost frequently. In contrast, oncogenic YAP1 was aberrantly overexpressed in oligosarcomas. Panel sequencing revealed mutations in NF1 and TP53 along with IDH1/2 and TERT promoter mutations. Survival of patients was significantly poorer for oligosarcomas as first recurrence than for grade 3 oligodendrogliomas as first recurrence. These results establish oligosarcomas as a distinct group of IDH-mutant gliomas differing from conventional oligodendrogliomas on the histologic, epigenetic, proteomic, molecular and clinical level. The diagnosis can be based on the combined presence of (a) sarcomatous histology, (b) IDH-mutation and (c) TERT promoter mutation and/or 1p/19q codeletion, or, in unresolved cases, on its characteristic DNA methylation profile.

Keywords: 1p/19q; Codeletion; DNA methylation; Gliosarcoma; NF1; Oligodendroglioma; Oligosarcoma; Prognosis; SMA; Subtype; TERT; TP53; Type; Variant; YAP1.

Fig. 1

Oligosarcomas are epigenetically distinct. Unsupervised DNA methylation-based t-SNE for 24 oligosarcomas, 12 primary tumors of oligosarcomas and 166 reference cases. Each dot represents an individual tumor sample. RTK = receptor tyrosine kinase; wt = wildtype

Fig. 2

Histology and immunohistochemistry of oligosarcomas. HE staining of the primary tumor (a) and oligosarcoma (b) of patient 3. The oligosarcoma of patient 16 shows highly anaplastic tumor cells in HE staining (c). Reticular fibers are restricted to blood vessels in Gomori’s reticulin staining in the oligodendroglioma of patient 3 (d) but extensively present between tumor cells in the oligosarcoma (e). Groups of tumor cells of the tumor shown in c are also embedded in reticulin fibers (f). Patient 8 presented with an oligosarcoma (g; HE), the tumor cell nuclei showing high p53 (h) and retained H3K27me3 expression (i). Oligosarcoma with subtotal loss (j) or mostly retained (k) expression of GFAP. Lack of OLIG2 expression in oligosarcoma with single entrapped positive nuclei (l). Scale bar is 200 µm

Fig. 3

Oligosarcomas may lose 1p/19q codeletion but retain CN-LOH. Copy number profiles of the tumors of patient 11: The primary tumor shows a complete 1p/19q codeletion (a) while in the subsequent oligosarcoma 1p deletion is absent but homozygous CDKN2A/B loss is present (b). SNP-array data regarding chromosome 1 shows deviation of BAF in 1p while the log R ratio (LRR) shows presence of two copies of the arm demonstrating CN-LOH. Note also the partial trisomy of 1q

Fig. 4

Oligosarcomas have distinct copy number profiles. Summary of chromosomal copy number plots of oligosarcomas (a, n = 24), primary tumor of oligosarcoma (b, n = 12) and conventional oligodendroglioma (c, n = 21). In oligosarcomas, homozygous CDKN2A/B deletion and loss of chromosome 6q are frequent, and in a few cases 1p/19q loss is not observed (a). Copy number profiles of primary tumors of oligosarcomas and conventional oligodendrogliomas are similar (b, c)

Fig. 5

Oligosarcomas have a highly distinct proteome with evidence for smooth muscle differentiation. a Volcano plot of significantly differentially regulated proteins between oligosaroma and conventional grade 3 oligodendroglioma. Using the 500 most variable proteins of the dataset for principal component analysis (b) and unsupervised hierarchical clustering (c) differentiates oligosarcomas from conventional grade 3 oligodendrogliomas. d Heatmap shows upregulation of (smooth) muscle specific proteins in oligosarcomas (left) and immunohistochemistry confirms expression of CALD1 and ACTA2 (SMA) in tumor cells of oligosarcoma (right). Scale bar is 200 µm

Fig. 6

Oligosarcomas frequently lose NF1 and gain YAP1 expression. a Heatmap of selected cancer proteins in oligosarccomas shows downregulation of tumor suppressors (in green rectangle) and upregulation of oncoproteins (in red rectangle). b Immunohistochemistry shows retained expression of NF1 and lack of expression of YAP1 in an oligodendroglioma and loss of NF1 expression accompanied by strong YAP1 expression in an oligosarcoma. Scale bar is 200 µm

Fig. 7

Clinical behavior of oligosarcomas. a Oligosarcomas can arise from oligodendrogliomas after different time spans and treatment procedures but may also occur de novo. b Kaplan–Meier analyses of the overall survival of patients with oligosarcoma, calculated from the timepoint of first diagnosis of a brain tumor (green) and from timepoint of oligosarcoma manifestation (red). c Kaplan–Meier analyses of the overall survival after a first progression event in form of a grade 3 oligodendroglioma (green) or an oligosarcoma (red)