Abstract LB-208: Using toxic siRNAs to treat cancer

Abstract

Abstract Targeted cancer therapy has failed to produce cures. Rather it extends patients' lifespans a few months. Novel approaches are needed to effectively kill cancer cells. We recently showed that over 80% of multiple tested siRNAs and shRNAs targeting the death receptor CD95 or its ligand CD95L induce a form of cell death characterized by simultaneous activation of multiple cell death pathways preferentially killing transformed and cancer stem cells (1,2). We demonstrated that these si/shRNAs kill all cancer cells through canonical RNAi by targeting the 3'UTR of critical survival genes in a unique form of off-target effect we call DISE (death induced by survival gene elimination) (3,4). Drosha and Dicer deficient cells, devoid of most miRNAs, were hypersensitive to DISE, suggesting cellular miRNAs protect cells from this form of cell death. We demonstrated that DISE can be triggered in vivo by delivering CD95L-derived siRNAs via HDL mimetic bioactive nanoparticles to treat ovarian cancer in mouse xenografts (6) without any signs of toxicity. Remarkably, cancer cells did not become resistant to the DISE mechanism. We subsequently found that other genes in the human genome also contain sequences that when converted to shRNA are toxic to all cancer cells (5). Together with the recognition that DISE-inducing si/shRNAs cause a form of cell death that is similar in all cancer cells this suggests that toxic RNAs could be embedded in the genome to kill cancer cells (4). Based on this hypothesis we most recently discovered another class of toxic siRNAs which are derived from triplet nucleotide repeat (TNR) expansions known to be the cause of a number of degenerative diseases (7). A prominent TNR expansion involves the triplet CAG in the huntingtin gene (HTT) which causes Huntington's disease (HD). We have now identified a family of TNR-based siRNAs - which includes the CAG repeat found in HD - to be 10-100 times more toxic to cancer cells than any tested DISE-inducing si/shRNA. Our data suggest this super toxicity is caused by targeting multiple complementary triplet repeat expansions present in the open reading frames (ORFs) of multiple genes, rather than in their 3'UTRs. siCAG/CUG could be safely administered to mice to slow down growth of xenografted ovarian cancer cells again with no toxicity to the animals. Our data on the toxicity of CAG triplet derived siRNAs for cancer cells but not normal cells and the reported decreased incidence rate for different types of cancer in patients with CAG expansions suggest that TNR-based siRNAs may be useful for cancer therapy. In my talk I will present a vision to develop toxic siRNAs into a new form of cancer therapy.