Pathologic diversity of glioneuronal tumor with neuropil-like islands: a histological and immunohistochemical study with a special reference to isocitrate dehydrogenase 1 (IDH1) in 5 cases.

Glioneuronal tumor with neuropil-like islands (GTNI) is featured by "neuropil-like islands (NIs)" within dominating astroglial components. Isocitrate dehydrogenase (IDH) mutations, particularly IDH1 R132H (G395A), are found in WHO Grade II and III diffuse gliomas as well as secondary, but not primary, glioblastomas. We reviewed 5 cases of GTNI, and assessed histology and immunohistochemistry with various antibodies, including those for IDH1 R132H, as well as direct DNA sequencing for IDH1 G395A. NIs were variable in morphology, and constantly synaptophysin-positive and glial fibrillary acidic protein-negative. The glioma components were primary glioblastoma in 2 cases, anaplastic astrocytoma in 1 and anaplastic oligoastrocytoma in 2. The IDH1 R132H was expressed in the 2 cases with oligoastrocytoma: In 1, NIs and the astrocytoma-like area as well as the oligodendroglioma-like area were positive. In the other, only the oligodendrogliomalike area was positive. The mutation analysis performed on the latter case with DNA separately sampled from the oligodendroglioma- like area and the astrocytoma-like area detected IDH1 G395A in both areas. We have shown diverse pathologic aspects of GTNI. Also, we have shown that the expression of IDH1 R132H in GTNI is largely concordant with that in diffuse gliomas, and that it can be dependent on each histologic component although the mutant IDH1 gene is ubiquitously present within the tumor.