Promoter hypermethylation and aberrant expression of EMP3at 19q13.3 in oligodendrogliomas and astrocytomas

Allelic losses on the long arm of chromosome 19 are found in the majority of oligodendroglial tumors and approximately 20–50% of astrocytic tumors. Losses on 19q are one of the earliest events in oligodendrogliomas, whereas such losses are linked to progression in astrocytomas. Due to a t(1;19) (q10;p10) translocations oligodendrogliomas show a loss of the entire long arm of chromosome 19, whereas astrocytomas exhibit interstitial and telomeric deletions without translocations. However, altered tumor suppressor genes (TSG) on 19q are still elusive in gliomas. We used cDNA microarray expression profiling to identify differentially expressed TSG candidates between gliomas with and without losses on 19q. Thereby, we found that 19q-deleted gliomas, when compared to gliomas without 19q deletion, showed lower expression of the epithelial membrane protein 3 (EMP3) gene at 19q13.3. Recently, EMP3 has been postulated as a hypermethylated TSG candidate in neuroblastomas and astrocytomas.

To further investigate the role of EMP3 as a putative tumor suppressor candidate gene in gliomas, we performed RT-PCR on a series of oligodendrogliomas and oligoastrocytomas as well as astrocytomas of various grade with or without loss of heterozygosity at 19q13 for transcriptional silencing. Next, we analyzed these tumors for EMP3 promoter hypermethylation. Genomic mutations were screened by Single-strand conformation polymorphism analysis.

In oligodendroglial tumors WHO grade II & III, promoter hypermethylation of EMP3 correlated significantly with either the 19q status (Student's t-test, p<0.0001) or with the mRNA expression of the gene (p<0.05). In contrast, astrocytomas of WHO grade II and III and secondary glioblastomas (sGBM) exhibited EMP3 promoter methylation independently of the 19q status in 83 to 89%. However, in primary glioblastomas (pGBM), EMP3 methylation was found in only 17%. Mutation analysis of EMP3 in 132 gliomas did not reveal any somatic mutation.

Our data indicate that EMP3 on 19q13.3 is a potential TSG candidate for oligodendroglial tumors with combined losses on 1p and 19q. The high frequency of EMP3 promoter hypermethylation in astrocytomas of WHO grade II & III and sGBM may imply an important role in induction of these astrocytomas as well. Furthermore, the low frequency of EMP3 promoter hypermethylation in pGBM suggest that pGBM and sGBM are not only characterized by distinct genetic profiles but also differ in their epigenetic aberrations.