Low-grade glial neoplasms of germ cell origin may represent maturation of embryonic-type neuroectodermal elements

Abstract

Aims: Glial tumours of germ cell origin are relatively rare in men, occurring predominantly after chemotherapy. Many exhibit low-grade histological features within a spectrum that includes teratomas with mature glial/ganglioglial elements and pure low-grade tumours with glial/ganglioglial phenotype (LGGT) that resemble gliomas/gangliogliomas of the central nervous system. Because foci of glial differentiation are very often seen in association with embryonic-type neuroectodermal tumour (ENT), we hypothesise that LGGTs may represent differentiation of embryonic-type neuroectodermal elements of teratoma and/or ENT.

Methods and results: To address this hypothesis, we compared LGGTs, ENT, non-teratomas, and teratomas using microRNA and DNA methylation analyses. Seven LGGTs underwent microRNA-371~373 analysis and genomic methylation profiling. Evidence of a prior or concurrent germ cell tumour component containing embryonic neuroectoderm (including overt ENT) was present in 4 LGGTs. None of the tested LGGTs were positive for miR-371a-3p, with three cases demonstrating low levels of expression within the so-called "grey zone". Unsupervised clustering based on microRNA-371~373 showed two clusters, one comprising non-teratomas and another including teratomas, ENTs, and LGGTs. Clustering according to top-differentially methylated probes did not demonstrate a clear separation according to histology. Genome-wide assessment of mean methylation levels using violin plots demonstrated that LGGT show a methylation profile "intermediate" between ENT and teratoma.

Conclusions: These results suggest that LGGTs of germ cell origin result from the maturation of ENT components.

Keywords: biomarkers; epigenetics; glial tumours; microRNA‐371‐373; teratoma; testicular germ cell tumours.

Figure 1

Histopathological features of the study cases. (A, B) Low grade glial tumours were composed of sheets and nests of cells with granular to fibrillary eosinophilic cytoplasm, round to oval nuclei, and centrally‐located nucleoli. Individual tumours showed areas with variable cellularity (best seen in A). Necrosis, brisk mitotic activity, and nuclear pleomorphism were not identified. (C) Some tumours contained aggregates of foamy and pigment‐laden macrophages. (D) Notice the prominent fibrillary neuropil‐like background and the presence of scattered developing ganglion cells, characterized by hyperchromatic nuclei and dense eosinophilic cytoplasm with delicate projections.

Figure 2

One tumour exhibited small scattered foci of embryonic type neuroectoderm, characterised clusters of primitive cells with scant cytoplasm and round hyperchromatic nuclei. Some of the neuroectodermal clusters are arranged in rosette‐like structures.

Figure 3

One tumour showed vague resemblance to gemistocytic astrocytomas, with sheets of large polygonal cells with abundant eosinophilic cytoplasm.

Figure 4

Levels of miR‐371a‐3p, miR‐372‐3p, and miR‐373‐3p in low grade glial/gliomatous tumours of germ cell origin (LGGT), embryonic‐type neuroectodermal tumour (ENT), and conventional testicular germ cell tumours (TGCTs). (A–C) Overall levels in LGGT versus non‐teratoma TGCT (considered as a single group). ****P < 0.0001. (D–F) Overall levels in LGGT compared with ENT and conventional histologic types of TGCT. (G–I) Levels of expression in individual LGGTs compared to median expression in ENT and conventional histologic types of TGCT. (J) Unsupervised clustering of samples based on microRNA 371~373 profiling, including LGGT, ENT, and conventional TGCTs (both teratoma and non‐teratoma). Results are plotted as 40‐Ct for readability. Median values are reported. Dashed lines define the categorisation of cases as positive (Ct < 28), grey zone (28 ≤ Ct ≤ 35), and negative (Ct > 35), as proposed by Lafin et al. The TCam‐2 seminoma‐like cell line is shown as a positive control. CH, choriocarcinoma; EC, embryonal carcinoma; ENT, embryonic type neuroectodermal tumour; LGGT, low‐grade glial/gliomatous tumour of germ cell origin; SE, seminoma; TE, teratoma; YST, yolk sac tumour.

Figure 5

Genomic DNA methylation profiling. (A) 2D UMAP projection visualising the clustering of samples. Each point represents an individual sample, and colours indicate their diagnosis: embryonic‐type neuroectodermal tumour (ENT; yellow), low grade glial tumour of germ cell origin (LGGT; blue), teratoma (orange), and teratoma with immature elements (purple). (B) Heatmap showign the results of unsupervised hierarchical clustering according to the top 1000 most variable probes. Each row represents a probe, and each column represents a sample. Beta values range from 0 to 1, with >0.8 as hypermethylated (red) and <0.2 as hypomethylated (blue). Diagnoses are annotated above the heatmap and colour‐coded: ENT (yellow), LGGT (orange), teratoma (blue), and teratoma with immature elements (purple). (C) Violin plots showing the distribution of average beta values per CpG site for the top 10,000 most variable probes in each histologic type (same probes used for UMAP dimensionality reduction).

Figure 6

Immunophenotype of embryonic‐type neuroectodermal tumour (ENT) and low grade glial tumours of germ cell origin (LGGT). (A) ENT (A–F), with typical morphology (A) and expression of SOX11 (B) and SOX2 (C). The tumour is negative for GFAP (D) and S100 (E), whereas synaptophysin is expressed in scattered foci (F). LGGT (G–L), with typical morphology (G) and absence of expression of SOX11 (H) and SOX2 (I). The tumour is diffusely positive for GFAP (J) and S100 (K). Synaptophysin highlights scattered ganglion cells (L).