Isocitrate dehydrogenase 1 (IDH1) mutations are frequent in
astrocytomas, oligoastrocytomas and
oligodendrogliomas. We previously reported the generation of a mutation-specific antibody that specifically detects R132H mutated IDH1
protein (clone H09). Here, we investigate the feasibility of H09 immunohistochemistry to differentiate between
oligodendrogliomas/oligoastrocytomas and other
tumors with similar morphology. A total of 274
brain tumors presenting with focal or extensive clear cell morphology were investigated. High numbers of H09-positive cases were observed in adult grade II
oligodendrogliomas (67 of 74, 91%), grade III
oligodendrogliomas (65 of 69, 94%), grade II oligoastrocytomas (11 of 14, 79%) and grade III oligoastrocytomas (10 of 11, 91%). All cases of pediatric
oligodendrogliomas (n = 7),
neurocytomas (n = 41, 35 central, 4 extraventricular, 2 cerebellar liponeurocytomas), dysembryoplastic
neuroepithelial tumors (n = 21), clear cell
ependymomas (n = 8),
clear cell meningiomas (n = 9) as well as 12 primary
glioblastomas with oligodendroglial differentiation and 5
pilocytic astrocytomas with oligodendroglial-like differentiation were negative for H09 immunohistochemistry. Three
oligodendrogliomas with neurocytic differentiation had evidence of IDH1/IDH2 mutations either by H09 immunohistochemistry or direct sequencing. We conclude that in
tumors with an
oligodendroglioma-like morphology, binding of H09 is highly specific for
oligodendrogliomas or oligoastrocytomas and substantially helps in the discrimination from other clear cell
tumors. Negative H09 immunohistochemistry of an
adult oligodendroglioma or oligoastrocytoma should prompt the consideration of other clear cell
neoplasms. Further, our observations firmly assign
oligodendrogliomas with neurocytic differentiation to the group of
oligodendrogliomas and demonstrate that H09 is especially helpful for the difficult discrimination of such lesions from extraventricular
neurocytomas.