Double-stranded RNA molecules that are specifically designed to block gene expression — termed short interference RNA (siRNAs) — promise to be a useful tool in cancer research. Less than one year after the first demonstration of how siRNAs could silence gene expression in mammalian cell culture, Agami and colleagues, reporting in Cancer Cell, show that oncogenic KRAS can be specifically and stably inactivated through the use of a viral RNA interference vector.

The guanine-nucleotide-binding proteins that are encoded by RAS are integral to cellular signal-transduction pathways. They regulate proliferation, differentiation and cell survival, and are frequently mutated in human cancers, particularly in pancreatic (85%) and colon carcinomas (40%). Mutant RAS oncogenes often contain point mutations, such as glycine to valine at codon 12 (KRASV12), and these lead to constitutively active RAS proteins. Wild-type KRAS seems to be required for viability, and antisense strategies to KRAS — which cannot distinguish between the wild-type and oncogenic forms of the gene — have not been successful.

Brummelkamp et al. used a retroviral vector (pRETROSUPER) carrying the expression cassette of the plasmid pSUPER — developed recently by the same group — that directs the synthesis of siRNAs in mammalian cells. When a human pancreatic cell line (CAPAN-1) was infected with the pRETROSUPER vector that contains sequences that spanned the mutation region of KRASV12, expression of KRASV12 was inhibited. This suppression was specific — use of a TP53 sequence in the vector did not cause a decrease in KRAS. Infecting the EJ bladder cell line — which has mutated HRAS but wild-type KRAS — with the mutant KRAS vector also had no effect on the expression of KRAS.

In addition, the authors showed that the specific suppression of KRASV12 leads to loss of tumorigenicity. When KRASV12 expression was downregulated in CAPAN-1 cells, the cells could not form tumours in nude mice, whereas mice injected with CAPAN-1 cells that had been infected with a control vector all developed tumours.

So, how can we make use of viral RNA interference vectors in cancer research? They are a powerful tool for investigating the genetic events that are required for a tumorigenic phenotype and for further elucidating the events that are needed to maintain that phenotype. In addition, siRNA-based gene therapy — shown to be a possibility by Brummelkamp et al. in both cell culture and animal models — should be investigated further. Issues that will need to be addressed include overcoming the poor penetration of tumours, possible neutralization of siRNA by the immune system and the possibility of escape from siRNA-mediated attack.