Antisense radiotherapy: targeting full‐sizemdr1mRNA with125I‐labelled oligonucleotides

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Abstract
Purpose: Antisense radiotherapy is an approach based on the targeting of mRNA of specific genes by complementary oligonucleotide probes labelled with an Auger‐electron‐emitting radioisotope. Decay of the Auger emitter should specifically destroy the targeted mRNA while producing minimal damage to the rest of mRNA pool and the nuclear DNA. The feasibility of this approach was investigated by using full‐length human multidrug‐resistance gene (mdr1) mRNA as a target. Materials and methods: Antisense oligonucleotides were labelled with [125I] I‐dCTP by primer extension and annealed to target mRNA. Breaks in the target mRNA were analysed by denaturing polyacrylamide gel electriphoresis. Results: The efficiency of 125I‐labelled antisense oligonucleotides in producing RNA strand breaks was tested on short synthetic RNA and DNA targets. The position and specificity of 125I‐induced breaks in the full‐length mRNA were then tested and compared with the cleavage of the target by RNase H. The distribution of the breaks in the longer mRNA is different from that in the short RNA targets, most likely due to a complex folding of RNA strands in the full‐length mRNA. Conclusions: The authors posit that 125I‐labelled antisense probes could be useful not only for targeting mRNA, but also as probes for mRNA folding in vivo.