CORRELATIVE IMMUNOHISTOCHEMISTRY AND MOLECULAR-GENETIC STUDY OF THE INACTIVATION OF THE P16(INK4A) GENES IN ASTROCYTOMAS

Loss of p16 expression can occur via homozygous deletion, point mutati on, or hypermethylation of exon 1. Astrocytomas representing all World Health Organization (WHO) grades of malignancy were analyzed in a cor relative study using multiplex polymerase chain reaction (PCR) analysi s to detect deletions of the p16 gene together with immunohistochemist ry to detect loss of the protein in archival specimens of the same tum ors. Homozygous deletions of p16 were detected in 29% (15 of 52) of WH O grade 3 and 4 tumors. Immunostaining for p16 protein was present in 26 tumors retaining the p16 gene and absent in 11 tumors with deletion s of the p16 gene. A close correlation was found between the two detec tion methods, with all tumors lacking immunostaining showing homozygou s loss of the p16 gene. Astrocytomas exhibiting inactivation of the p1 6 gene most often contained p53 gene mutations or amplified epidermal growth factor receptor genes, genetic characteristics associated with both the progressive and de novo tumor variants. Immunohistochemical e valuation may be a useful, rapid method to screen astrocytomas for los s of p16 gene expression, regardless of the underlying mechanism leadi ng to p16 gene inactivation.