Methylation of the p73 gene in gliomas

The p73 gene encodes a protein structurally and functionally homologous to TP53, and maps to chromosomal band 1p36.33, where loss of heterozygosity has been observed in up to 90% of oligodendrogliomas and in 10-25% of diffuse astrocytomas. We assessed the methylation status of the CpG islands in the promoter region of the p73 gene by methylation-specific PCR in 117 glioma biopsies. Methylation was detected in 5 out of 28 (18%) glioblastomas and in 4/26 (15%) anaplastic oligodendrogliomas (WHO grade III) but not in grade II oligodendrogliomas, low-grade diffuse astrocytomas (grade II), and anaplastic astrocytomas (grade III). To assess whether p73 methylation leads to loss of expression, we carried out reverse transcription-PCR and methylation-specific PCR in 10 glioblastoma cell lines. Two lines (U87MG and T98G) showed p73 methylation. U87MG had no unmethylated p73 (complete methylation), and showed loss of expression. T98G had methylated and unmethylated p73 (partial methylation), and retained p73 expression. A third cell line (LN-308) showed loss of p73 expression without p73 methylation. These results suggest that complete p73 methylation is associated with loss of expression, but that additional mechanisms may cause loss of p73 expression. Analysis of a polymorphic site in exon 2 further showed that p73 was mono-allelically expressed in 6 out of 7 primary gliomas with heterozygous GC/TA polymorphism.