RNAi meets imatinib mesylate (STI571)

RNA interference (RNAi) describes a highly conserved mechanism of sequence-specific posttranscriptional gene silencing triggered by double-stranded RNA (dsRNA). DsRNA in the form of small interfering RNA (siRNA) can trigger RNAi in mammalian cells without activation of the nonspecific interferon response. With different tools to initiate RNAi now available, an increasing number of reports use this process for functional genomics in several organisms. Such studies include human cells where RNAi can inhibit both normal and aberrant gene expression arising from viral infection or cellular mutations.

One such mutation is the bcr-abl oncogene involved in leukemogenesis of chronic myeloid leukemia (CML) and a subset of acute lymphoblastic leukemia (ALL). Bcr-Abl tyrosine kinase activity is required for cellular transformation and can be inhibited by imatinib mesylate (STI571), which now represents an effective strategy to treat bcr-abl–positive leukemias. However, imatinib mesylate resistance does occur and this point is addressed in the study by Wohlbold and colleagues (page 2236). The authors used repeated electroporation to deliver anti–bcr-abl siRNA into cell lines that express either wild-type or imatinib mesylate-resistant mutants of Bcr-Abl. They demonstrate that inhibition of bcr-abl gene expression by siRNA and that of Bcr-Abl tyrosine kinase activity by imatinib mesylate can cooperate to inhibit proliferation and survival of bcr-abl–positive cells. Both strategies are nonoverlapping but rather complementary since suitable siRNA can inhibit both wild-type and mutant bcr-abl gene expression, resulting in comparable reduction of cell survival. Finally, bcr-abl gene silencing may depend on intracellular siRNA levels, suggesting some pharmacologic aspects of siRNA in common with more conventional drugs, at least in this specific model.

While these results suggest that cellular oncogenes may effectively be targeted by application of RNAi, either alone or in combination with other therapeutics, a number of important challenges remain. The first and most obvious is the effective delivery of siRNAs to human cells in a clinical setting. Retroviral or lentiviral gene transfer of suitable expression cassettes allow stable intracellular transcription of RNAi triggers and represent an alternative to physico-chemical transfection procedures. Additionally, stable expression of RNAi triggers may overcome the transient nature of RNAi in mammalian cells after a single siRNA application. Furthermore, potential side effects of RNAi or of specific RNAi triggers such as off-target gene silencing have to be considered carefully. These aspects of RNAi will certainly be studied in the future and help to better define the therapeutic potential of RNAi-based gene silencing in hematopoietic cells.