OLIG2 is a useful immunohistochemical marker in differential diagnosis of clear cell primary CNS neoplasms.

Aims: To analyse OLIG2 expression in clear cell primary central nervous system (CNS) tumours to clarify the diagnostic usefulness of OLIG2 immunohistochemistry in this subset of brain tumours.
Methods and Results: We analysed OLIG2 expression in 60 oligodendroglial neoplasms (57 with and three without chromosome 1p aberration), 10 central neurocytomas, 10 clear cell ependymomas, nine dysembryoplastic neuroepithelial tumours (DNTs) and two clear cell meningiomas using immunohistochemistry. Additionally, we analysed oligodendroglial neoplasms with numerous gliofibrillary and minigemistocytic oligodendrocytes for OLIG2/glial fibrillary acidic protein (GFAP) coexpression and central neurocytoma for coexpression of neurone-specific nuclear protein (NeuN) and OLIG2 using double immunofluorescent labelling and confocal laser scanning microscopy. All oligodendroglial neoplasms and DNTs showed widespread OLIG2 expression. Eight of 10 central neurocytomas, all clear cell meningiomas and 8/10 clear cell ependymomas were negative for OLIG2. Two of 10 central neurocytomas and 2/10 clear cell ependymomas showed focal OLIG2 expression. We found prominent coexpression of GFAP and OLIG2 in gliofibrillary and minigemistocytic oligodendrocytes. Further, we found coexpression of NeuN and OLIG2 in single cells in central neurocytoma.
Conclusions: Widespread OLIG2 expression discriminates oligodendroglial neoplasms or DNTs from other clear cell primary brain tumour types. In clear cell primary brain tumours lacking OLIG2 expression, differential diagnosis may require additional immunohistochemical markers.