Ribozyme trans-splicing and RNA tagging: following the messenger

Molecular therapy and prevention of liver diseases

Molecular analyses have become an integral part of biomedical research as well as clinical medicine. The definition of the molecular and genetic basis of many human diseases has led to a better understanding of their pathogenesis and has in addition offered new perspectives for their diagnosis, therapy and prevention. Genetically, liver diseases can be classified as hereditary monogenic, acquired monogenic, complex genetic and diseases. Based on this classification, gene therapy is based on six concepts: gene repair, gene substitution, cell therapy, block of gene expression or function, DNA vaccination as well as gene augmentation. While recent developments are promising, various delivery, targeting and safety issues need to be addressed before gene therapy will enter clinical practice. In the future, molecular diagnosis and therapy liver diseases will be part of our patient management and complement existing diagnostic, therapeutic and preventive strategies.

Oligonucleotide-mediated gene therapy for muscular dystrophies

Several new approaches to gene therapy for the muscular dystrophies involve oligonucleotides as targeting vectors. These oligonucleotides are designed to repair genetic mutations, to modify genomic sequences in order to compensate for gene deletions, or to modify RNA processing in order to ameliorate the effects of the underlying gene mutation. Among the various approaches currently under investigation for dystrophin mutations that cause Duchenne muscular dystrophy is the use of chimeric RNA/DNA oligonucleotides ("chimeraplasts") to repair point mutations. Studies in the mdx mouse and the GRMD dog have demonstrated that point mutations in the dystrophin gene can be corrected by chimeraplasts that have been injected into muscles. The scope of this review includes a summary of the current status of chimeraplast-mediated gene repair for dystrophin mutations, ongoing studies to apply chimeraplast-mediated gene repair to frame-shift deletions of the dystrophin gene, and major hurdles that need to be overcome to translate current experimental successes into a viable therapeutic modality for Duchenne muscular dystrophy.

Messenger RNA repair and restoration of protein function by spliceosome-mediated RNA trans-splicing

The functional repertoire of the human genome is amplified by the differential assortment of exons. Spliceosome-mediated RNA trans-splicing can mobilize these packets of genetic information to reprogram mRNAs. In principle, this process could repair defective transcripts in loss-of-function genetic disorders in humans. We developed a tractable lacZ repair system to serve as a model for these genetic disorders. Targeted pre-trans-splicing RNA molecules efficiently and specifically repaired mutated lacZ transcripts and restored enzymatic activity in human cells. The development of this model confirms the potential for spliceosome-mediated RNA trans-splicing in genetic repairs and provides a powerful tool for rational design and in vitro evolution of pre-trans-splicing molecules.

Design of highly specific cytotoxins by using trans-splicing ribozymes

We have designed ribozymes based on a self-splicing group I intron that can trans-splice exon sequences into a chosen RNA target to create a functional chimeric mRNA and provide a highly specific trigger for gene expression. We have targeted ribozymes against the coat protein mRNA of a widespread plant pathogen, cucumber mosaic virus. The ribozymes were designed to trans-splice the coding sequence of the diphtheria toxin A chain in frame with the viral initiation codon of the target sequence. Diphtheria toxin A chain catalyzes the ADP ribosylation of elongation factor 2 and can cause the cessation of protein translation. In a Saccharomyces cerevisiae model system, ribozyme expression was shown to specifically inhibit the growth of cells expressing the virus mRNA. A point mutation at the target splice site alleviated this ribozyme-mediated toxicity. Increasing the extent of base pairing between the ribozyme and target dramatically increased specific expression of the cytotoxin and reduced illegitimate toxicity in vivo. Trans-splicing ribozymes may provide a new class of agents for engineering virus resistance and therapeutic cytotoxins.

Expression of ribozymes in gene transfer systems to modulate target RNA levels

The possibility of designing ribozymes to cleave any specific target RNA has rendered them valuable tools in both basic research and therapeutic applications. In the therapeutics area, they have been exploited to target viral RNAs in infectious diseases, dominant oncogenes in cancers and specific somatic mutations in genetic disorders. Most notably, several ribozyme gene therapy protocols for HIV patients are already in Phase 1 trials. More recently, ribozymes have been used for transgenic animal research, gene target validation and pathway elucidation.