MB-29DISSECTING THE ROLE OF EPIGENETIC MODULATORS IN SHH-DRIVEN MEDULLOBLASTOMA

The Repressor Element-1 Silencing Transcription Factor (REST) negatively regulates neurogenesis. It epigenetically silences the expression of neuronal differentiation genes through modulation of histone acetylation and histone methylation. REST levels are elevated in over 80% of human medulloblastomas, and spans all four molecular sub-groups. Interestingly, a subset of patients with sonic hedgehog (Shh)-driven medulloblastomas with high REST expression (+3/ + 4 on a scale of 0-4) had a significantly elevated incidence of metastatic disease compared to patients with low-REST or no-REST expressing Shh-driven tumors. To understand if REST is a driver of metastatic disease, we generated a novel REST knock-in (RESTKI) mouse model in which transgene expression is conditionally elevated in cerebellar granule neuron progenitors (GNPs), the cells of origin of Shh-driven medulloblastomas. RESTKI mice were then crossed with mice exhibiting constitutive activation of Shh signaling (Ptc +/−), and REST transgene expression was induced in the progeny (RESTKI/Ptc +/−) on postnatal day 2. While RESTKI mice exhibited pre-neoplastic lesions, both Ptc +/− and RESTKI/Ptc +/− mice formed cerebellar tumors. However, a significant increase in tumor penetrance (100%) was seen in RESTKI/Ptc +/− mice compared to 20% in Ptc +/− mice. A substantial decrease in tumor latency (10-40 days) was also observed in RESTKI/Ptc +/− in contrast to 3-7 months in Ptc +/− mice. Histopathological analyses of mice brains revealed leptomeningeal and infiltrative medulloblastoma with significant changes in the tumor microenvironment in RESTKI/Ptc +/− mice. In contrast, cerebella of Ptc +/− mice exhibited localized disease. Importantly, RESTKI/ Ptc +/− tumors expressed very high levels of neural stem cell (NSC) markers such as Nestin, GFAP and Vimentin when compared with Ptc +/− tumors, suggesting that REST elevation in GNPs may have resulted in their de-differentiation into a more primitive state, and driven medulloblastoma genesis in the context of Shh hyperactivation. Our studies implicate REST in the modulation of the oncogenic potential of Shh signaling and medulloblastoma development.