PFA ependymomas are pediatric brain tumors with very poor prognosis. They represent about 10% of all pediatric tumors of the central nervous system (CNS), and up to 30% of CNS tumors in children under 3. (1) 9.4% of PFA tumors aberrantly express EZHIP (aka CXorf67), a protein that inhibits Polycomb repressive complex 2 (PRC2). PRC2 has an important function in silencing genes through trimethylating lysine 27 of histone H3, H3K27me3. (2, 3) PFA tumor cells show a reduction of this important epigenetic mark. The decrease in H3K27me3 results in deregulated gene expression and leads to cancer initiation and progression. (3) EZHIP deletion using CRISPR knockout leads to the restoration of H3K27me3. (2) These finding points at a causal relationship between EZHIP’s expression and the lack of H3K27me3, driving a subset of PFA ependymomas.
The goal of my project is to remove EZHIP from cancer cells to restore normal levels of H3K27me3. I want to use proteolysis-targeting chimeras (PROTAC) to degrade EZHIP to stop its interaction with the PRC2 complex. Below is a schematic to show the way a PROTAC would send EZHIP to the proteasome for degradation. (4)
PROTACs may work only if there is at least one domain in EZHIP that is folded into a structural module where the PROTAC can bind. To determine if that is the case, I will express and purify EZHIP, and test whether it is folded using different assays, such as differential scanning fluorimetry (5).
- Pajtler,K.W., Witt,H., Sill,M., Jones,D.T.W., and Hovestadt,V. (2015) Molecular Classification of Ependymal Tumorsacross All CNS Compartments,Histopathological Grades, and Age Groups. Cancer cell, 27, 728–743
- Pajtler,K.W., Wen,J., Sill,M., Lin,T., and Orisme,W. (2018) Molecular heterogeneity and CXorf67 alterations in posterior fossa group A (PFA) ependymomas. Acta Neuropathologica, 136, 211–226.
- Hubner,J., Müller,T., Papageorgiou,D.N., Mauermann,M,. and Krijgsveld,J. (2019) EZHIP/CXorf67 mimics K27M mutated oncohistones and functions as an intrinsic inhibitor of PRC2 function in aggressive posterior fossa ependymoma. Neurooncology, 21, 878–889.
- Schapira,M., Calabrese,M., Bullock,A.N., Crews,C.M. (2019) Targeted protein degradation: expanding the toolbox. Nature Reviews Drug Discovery.
- Differential Scanning Fluorimetry (DSF). Center For Macromolecular Interactions, Harvard Medical School. ( https://cmi.hms.harvard.edu/differential-scanning-fluorimetry