Abstract Date: 5/2/2015
Jamie Purzner, MD
Alex Brown, PhD
Matthew Scott, PhD
Yoon-Jae Cho, MD (Woodside, CA)
Despite the many cell types that utilize Hh signaling during development, relatively few are susceptible to oncogenic transformation with Hh pathway mutations. During cerebellar development GNPs proliferate in response to sonic hedgehog (Shh) and mutations that lead to unrestrained Hh signaling can result in medulloblastoma (MB). We investigated whether the epigenetic identify of GNPs may predispose the cell type to Hh mediated tumorigenesis.
We determined RNA abundance, histone marks, DNA accessibility, RNA polymerase II occupancy, DNA methylation and Gli1 binding genome wide in both GNPs and MB. We then used machine learning algorithms to identify frequently occurring patterns of epigenetic marks.
Comparison between GNPs and MB showed substantial alterations in histone marks associated with both enhancer and promoter regions. Numerous bivalent promoters, bearing both the H3K4me3 and H3K27me3 histone marks were identified in GNPs. Approximately 40 % of these transitioned to active in MB, suggesting a role in tumor susceptibility. To test the biological importance of loss of the H3K27me3 mark in GNPs we conditionally knocked out EZH2. EZH2 knockout led to transcriptional increase in bivalent genes, yet surprisingly, there was no increase in MB formation. Developmentally, EZH2 knockout led to a substantial reduction in GNP proliferation.
We have densely mapped the epigenetic landscape of Shh driven MB in comparison to their cell of origin. This revealed a complex interplay between multiple forms of transcriptional repression.
Article ID: AA-31562