Immunolocalization of the oligodendrocyte transcription factor 1 (Olig1) in brain tumors.

Recent in situ hybridization studies showed that mRNA levels of OLIG1 and OLIG2 transcription factors are elevated in oligodendrogliomas. We raised polyclonal antibodies against a synthetic peptide homologous to the human transcription factor Olig1 and studied by immunohistochemistry the expression of Olig1 in 84 brain tumors and in non-neoplastic brain tissues. All oligodendrogliomas, oligoastrocytomas, and dysembryoplastic neuroepithelial tumors showed moderate to strong intranuclear immunoreactivity in cells morphologically identified as oligodendrocytes. In addition, some astrocytomas showed a slight to moderate intranuclear immunoreactivity. None of the other neuroepithelial and non-neuroepithelial tumors showed nuclear immunoreactivity. Double immunostaining of oligodendrogliomas, oligoastrocytomas, and glioblastoma multiforme (GBM) using antibodies against Olig1 and GFAP showed the presence of 3 different cell populations: 1) immunopositive for Olig1 and immunonegative for GFAP, histologically identified as oligodendrocytes; 2) immunopositive only for GFAP, histologically identified as astrocytes; and 3) immunonegative for both antibodies ("null cells"), histologically observed as a population of cells usually with round nuclei and a small amount of cytoplasm. The use of double immunostaining facilitated the distinction among these 3 different tumors. In summary, the use of immunohistochemistry using Olig1 antibodies alone or in combination with anti-GFAP antibody, which can be performed in the routine diagnostic setting, may help in the diagnosis of neuroepithelial tumors.