OS07.6.A INVESTIGATING REGIONAL HETEROGENEITY AND PROGRESSION-ASSOCIATED MARKERS IN OLIGODENDROGLIOMAS

Abstract

Abstract BACKGROUND Oligodendrogliomas are defined by IDH-mutation, 1p/19q-codeletion and TERT promoter mutation. Molecular stratification of oligodendrogliomas has so far proved difficult, despite recent progress in other brain tumours. Loss of H3K27 trimethylation (H3K27me3) is seen in many, but not all oligodendrogliomas, and we have observed that this trimethylation mark is lost predominantly in low-grade oligodendrogliomas, and more often retained in high-grade tumours. The key aims of this project are to establish the downstream effects of H3K27me3 regulation in oligodendrogliomas to identify progression-associated biomarkers. MATERIAL AND METHODS 78 oligodendrogliomas (CNS WHO grade 2: n=38, and grade 3: n=40) were tested for H3K27me3 expression. These tumours showed either down-regulation (H3K27me3-), retention of expression (H3K27me3+) or intratumoural heterogeneity (H3K27me3Het). 65 regions across these three types were micro-dissected for DNA methylation profiling and RNA sequencing. RESULTS Of the H3K27me3- tumours 85% (35/41) were grade 2, of the H3K27me3+ tumours 100% (10/10) were grade 3, and of the H3K27me3Het tumours 87% (20/23) were grade 3. With DNA methylation arrays and derived copy number data we have shown a different breakpoint of the 1p chromosome in H3K27me3+ samples or areas, and in 15 paired samples, 6 gains and 12 losses in H3K27me3+ regions, and overall 24 copy number differences between H3K27me3+ and me3- areas within the same tumours, indicative of spatial genetic heterogeneity. RNA sequencing suggests distinct expression profiles between H3K27me3+ and H3K27me3- areas. CONCLUSION We have shown a close correlation between areas with high-grade features and H3K27me3 expression both between and within tumours. We have shown that H3K27me3+ and me3- areas within the same tumours show copy number changes and have differential gene expression. Using pathway analysis we have identified pathways and genes involved in tumour progression and mediation of histone modification. We are currently using these targets for functional studies and protein level validation with the overall aim of developing immunohistochemistry biomarkers.