Particularly in small brain biopsies, it might be difficult to distinguish reactive
astrogliosis from low-grade or infiltration zones of high-grade
astrocytomas. So far no immunohistochemical marker allows a reliable distinction. Recently, the over-expression of Wilms' tumor gene product WT1 was reported in astrocytic
tumor cells. However, no sufficient data on WT1 expression in normal or reactive astrocytes are available. Therefore, we investigated WT1 expression in
paraffin-embedded brain sections from 28 controls, 48 cases with
astrogliosis of various etiology and 219
astrocytomas [World Health Organization (WHO) grades I-IV] by immunohistochemistry. In normal brains and in
astrogliosis, expression of WT1 was restricted to endothelial cells. In
astrocytomas, WT1-positive
tumor cells were found in
pilocytic astrocytomas (66.7% of cases), diffuse
astrocytomas (52.7%) WHO grade II (52.7%),
anaplastic astrocytomas (83.4%) and
glioblastomas (98.1%). Overall, the majority of all astrocytic
neoplasms (84.5%) expressed WT1. Establishing a cut-off value of 0% immunoreactive
tumor cells served to recognize neoplastic astrocytes with 100% specificity and 68% sensitivity and was associated with positive and negative predictive values of 1 and 0.68, respectively. Therefore, WT1 expression in astrocytes indicates a neoplastic origin and might represent an important diagnostic tool to differentiate reactive from neoplastic astrocytes by immunohistochemistry.