Anomalous p27kip1 expression in a subset of malignant gliomas

p27^Kip1 (p27) expression was immunohistochemically investigated in 28 astrocytic tumors, and compared with the cell proliferation index (MIB-1 staining index). Normal rat brains and surgical specimens from human nonneoplastic brain lesions were used as controls. In the rat brains, the astrocytes were exclusively p27-positive. The reactive astrocytes in various disease processes sometimes lacked p27 expression. The distribution of p27-positive cells was uniform in low-grade astrocytomas and heterogeneous in high-grade tumors. Double staining of p27 and MIB-1 showed a reciprocal pattern in most cases. The frequency of p27 expression was inversely correlated with MIB-1 staining index and tumor grade. However, several malignant gliomas showed high p27 expression in spite of high MIB-1 staining indices. In such cases, MIB-1-positive cells were occasionally p27-positive. In this paper we discuss the etiology of the anomalous p27 expression in a subset of malignant gliomas.