GSE124210
HERV-K ENV expression is a therapeutic target in atypical teratoid rhabdoid tumors
Human Endogenous Retroviruses (HERVs) are exogenous viral elements that integrated into the germline millions of years ago.  They comprise 8% of the human genome and play a critical role in cellular differentiation during development.  HERV-K(HML-2) is the most recently integrated and actively transcribed HERV. It is found to be highly expressed in pluripotent stem cells and strongly downregulated during cellular differentiation. Moreover, expression of HERV-K in terminally differentiated neurons results in cytotoxicity. Expression of HERVs has been implicated in carcinogenesis and is suggested to confer stem cell like features important for cell growth. Atypical Teratoid / Rhabdoid Tumor (AT/RT) is a rare pediatric brain cancer that results from incomplete neuronal differentiation during embryonic development.  In this paper, we investigated the cytopathic effect of Ouabain, a cardiac glycoside that induces cytotoxicity in stem cells, on AT/RT cells. We characterized four AT/RT cell lines by immunostaining and observed variable levels of stem cell, Oct4 and Pax6, and neuronal differentiation markers, TUBB3 and MAP2, as well as HERV-K envelope (env), polymerase (pol), and gag transcripts.  We stained formalin-fixed brain tissues from 20 AT/RT patients and 5 unaffected controls for HERV-K ENV protein expression. 40% of AT/RT patient tissues showed ENV expression.  In contrast, no ENV immunostaining was observed in all 5 control brain tissues. We used Ouabain, known to be cytotoxic to stem cells, observed up to 50% toxicity at 24 hr and 80% toxicity at 48 hr after treatment.  While AT/RT primarily exist as large cell aggregates in suspension, Ouabain treatment downregulated HERV-K ENV protein and caused disruption of cell-to-cell interactions. Similarly, when AT/RT cells were transfected with a CRISPR/Cas9 construct designed to repress HERV-K promotor activity, cell aggregates separated and cell viability decreased significantly. Using a luciferase reporter construct under the control of a HERV-K promotor, we observed a significant decrease in HERV-K promotor activity at 8 to 72 hr post treatment with 0.5 uM Ouabain.  Given that downregulation of HERV-K alone was sufficient to cause cytotoxicity in the cells, HERV-K expression may be essential for AT/RT growth and survival.  Ouabain and other compounds that modulate HERV expression may provide an alternative or adjunctive therapeutic option for treatment of embryonal tumors. Overall design: RNA-Seq profiles for 4 human Atypical Teratoid Rhabdoid Tumor (ATRT) cell lines available from ATCC (www.atcc.org): CHLA 02, CHLA 04, CHLA 05, CHLA 06.  Two of the cell lines, CHLA 04 and CHLA 06, were seqeunced twice; providing for a pair of technical profiles each.