EPCO-33. DIFFERENTIAL EXPRESSION OF AN ENDOGENOUS RETROVIRAL ELEMENT [HERV-K(HML-6)] IS ASSOCIATED WITH REDUCED SURVIVAL IN GLIOBLASTOMA PATIENTS

INTRODUCTION

Comprising approximately 8% of our genome, Human Endogenous RetroViruses (HERVs) represent a class of germline retroviral infections that are regulated through epigenetic modifications. In cancer cells, which often have epigenetic dysregulation, HERVs have been implicated as potential oncogenic drivers. However, their role in gliomas is not known.Given the link between HERV expression in cancer cell lines and the distinct epigenetic dysregulation in gliomas, we utilized a tailored bioinformatic pipeline to characterize and validate the glioma retrotranscriptome and correlate HERV expression with locus-specific epigenetic modifications.Method:A custom workflow was used to quantify HERV expression in our cell lines of interest. Cell-line methylation was quantified using a custom script. We generated primers specific for the Human endogenous Mouse mammary tumor (MMTV)-Like virus 6 (HML-6). Visualization of RNA transcripts was performed using RNA-scope. Clinical data was obtained using the R package, TCGABiolinks.

RESULTS

The A172 cell line had significantly higher mean overall HERV expression relative to the M059J and H4 cell lines (p< 0.0001 for both). A172 cells had significantly lower mean number of CpG islands relative to M059J cells and H4 cells (p< 0.0001 for both). There was a significant inverse correlation between mean beta value and FC HERV expression (R=-0.57, p=0.01). qPCR confirmed robust expression of the HML-6 locus in cell culture and neurospheres. Elevated ERVK3-1 expression was associated reduced survival among IDHwt GBM patients (18.3 vs. 15.1 months, p=0.039). This was preserved among IDH mutant (IDHm) GBM as well (17.9 months vs. 14.0 months, p=0.0088).

CONCLUSION

In gliomas, HERV expression correlates with loss of DNA methylation at HERV loci. HML-6 is overexpressed in highly invasive glioblastoma cell lines and patient-derived neurospheres. We have demonstrated a potential survival detriment associated with elevated expression of the HML-6 product, ERVK3-1.

EXTH-76. PRMT5 INHIBITION SENSITIZES GLIOBLASTOMA NEUROSPHERES TO TEMOZOLOMIDE

INTRODUCTION

The median survival of Glioblastoma (GBM) patients is less than two years with the standard of care of maximal surgical resection, radiation, and temozolomide (TMZ) chemotherapy. Protein Arginine Methyltransferase 5 (PRMT5), which regulates cellular functions by symmetrically di-methylating arginine residues, is overexpressed in GBM. Inhibiting PRMT5 induces apoptosis in differentiated and senescence in stem-like GBM tumor cells. We inhibited PRMT5 in GBM neurospheres to determine if PRMT5 inhibition would enhance TMZ’s antitumor effect.

METHODS

We depleted PRMT5 activity, in vitro, using target-specific siRNA or LLY-283 and combined these with TMZ treatment. We evaluated the antitumor effect of this combination using cell viability assay, cell cycle analysis, apoptosis assay, and western blot.

RESULTS

TMZ reduced the viability of GBMNS with PRMT5 knockdown significantly more than the viability of PRMT5 intact GBMNS. The combination of TMZ and PRMT5 knockdown elevated the expression of cleaved caspase 3 and caspase3/7 indicating that PRMT5 knockdown enhanced the apoptotic effects of TMZ. Cell cycle analysis showed that depleting PRMT5 abrogated TMZ-induced G2/M cell cycle arrest. Further, treatment of PRMT5-depleted GBMNS with TMZ increased ɣ-H2AX expression compared PRMT5 intact GBMNS treated with TMZ, suggesting that PRMT5 depletion enhanced TMZ-induced DNA damage. PRMT5 knockdown also inhibited the symmetric di-methylation of RUVBL1 that is required for homologous recombination repair of TMZ treatment-related DNA damage.

CONCLUSION

Overall, PRMT5 inhibition sensitized GBMNS to TMZ and enhanced TMZ-related DNA damage and cytotoxicity. These findings support further development of this potential therapeutic combination.

STEM-07. DEFINING THE ROLE OF HUMAN ENDOGENOUS RETROVIRUS-K (HML-2) IN THE GLIOBLASTOMA STEM-CELL NICHE

Human Endogenous Retrovirus (HERV) are ancestral viral relics that comprise nearly 8% of the human genome. Although silenced in normal tissues, the most recently integrated provirus HERV-K (HML-2) can be pathologically reactivated in certain cancers. Here, we report pathological expression of HML-2 transcripts in human malignant gliomas in cerebrospinal fluid (HERV-K DNA/RPP30=35.2±8.8 vs 23.1±6.7, n=18, p=0.02) and tumors (HERV-K RNA/HPRTmean=1.15±0.2 vs. 0.5±0.2, p=0.01, n=20) compared to epilepsy controls. Aberrant HML-2 expression corresponded to a unique stem-cell niche using multivoxel automated segmentation. Using a tailored single-cell RNA sequencing pipeline to detect retrotransposons, we identified glioblastoma cellular populations with elevated HML-2 transcripts in neural progenitor-like cells that can drive cellular plasticity (ANOVA, multiple-testing correction, p< 0.001). Using CRISPR technology, we demonstrate that HML-2 is critical to maintenance of glioblastoma stemness and tumorigenesis in both glioblastoma neurospheres and intracranial orthotopic murine models (OS: 26 days vs. 18.6, p=0.0008, n=20). Downregulation of HERV-K using CRISPRi reduces the glioblastoma neurosphere formation (2-way ANOVA, p< 0.0001). and HERV-K env, Polymerase, OCT4 and Nestin transcripts (2-way ANOVA p< 0.001). Using Chromatin Immunoprecipitation, we determined that this interaction in gliomas is likely mediated by the nuclear transcription factor OCT4 which binds to an HML-2 specific Long-Terminal Repeat (LTR5Hs). Moreover, using Transmission Electron Microscopy, we discovered that some glioma stem-cells form immature retroviral virions in glioblastoma. Inhibiting HML-2 expression with nucleoside reverse transcriptase inhibitors reduces extracellular reverse transcriptase (One-way ANOVA, p< 0.05), tumor viability (IC50=75.8-123.1 uM), and pluripotency (One-way ANOVA, p< 0.01). Our results suggest that HML-2 is overexpressed in the cancer stem-cell niche of glioblastoma. Since persistence of glioblastoma stem-cells is considered responsible for treatment resistance and recurrence, HML-2 may serve as a unique therapeutic target in glioblastoma.