EXTH-61. RPP25L IS A NOVEL DEPENDENCY IN GLIOBLASTOMAS WITH LOW RPP25 EXPRESSION

Genetic redundancy is a property whereby two or more genes encode for the same biological function, and inactivation of one redundant gene has little effect on the overall phenotype. In recent years, genetic redundancy has been exploited as a cancer therapeutic strategy, as individual paralogs of essential genes are occasionally lost in cancers because of genomic deletions. Inactivation of the non-deleted paralog can therefore be selectively toxic to tumor cells. To identify cancer cells that may become reliant on the non-methylated paralog of essential genes, we performed an in-silico analysis of the correlation between DNA methylation and genetic dependency across large scale CRISPR knockout screens that revealed novel paralog dependencies. Through this analysis, we identified glioblastoma cell lines with hypermethylation of the RPP25 promoter and with sensitivity to loss of RPP25L. RPP25 is a known structural component of RNAse P, a key enzyme involved in tRNA maturation. While the biological function of RPP25L is unknown, it has significant sequence homology to RPP25. We used CRISPR editing and CRISPR interference to inactivate RPP25L in a panel of glioblastoma cell lines and demonstrated that cell lines that do not express RPP25 (SF295, GB1, LN18) exhibit a rapid reduction in viability upon RPP25L loss, whereas cell lines with retained RPP25 expression or re-expressed RPP25 are not affected. RPP25L dependency is associated with a marked reduction in nascent polypeptide formation as determined by click chemistry. Lastly, we validated RPP25L dependency in glioblastoma patient-derived neurospheres with loss of RPP25 expression. These results indicate that RPP25L is a promising novel therapeutic target in glioblastomas with hypermethylation of the RPP25 promoter.