Emerging clinical applications of RNA

Immunotherapy of Hematologic Malignancy

Over the past few years, improved understanding of the molecular basis of interactions between antigen presenting cells and effector cells and advances in informatics have both led to the identification of many candidate antigens that are targets for immunotherapy. However, while immunotherapy has successfully eradicated relapsed hematologic malignancy after allogeneic transplant as well as virally induced tumors, limitations have been identified in extending immunotherapy to a wider range of hematologic malignancies. This review provides an overview of three immunotherapy strategies and how they may be improved.

In Section I, Dr. Stevenson reviews the clinical experience with genetic vaccines delivered through naked DNA alone or viral vectors, which are showing promise in clinical trials in lymphoma and myeloma patients. She describes efforts to manipulate constructs genetically to enhance immunogenicity and to add additional elements to generate a more sustained immune response.

In Section II, Dr. Molldrem describes clinical experience with peptide vaccines, with a particular focus on myeloid tissue-restricted proteins as GVL target antigens in CML and AML. Proteinase 3 and other azurophil granule proteins may be particularly good targets for both autologous and allogeneic T-cell responses. The potency of peptide vaccines may potentially be increased by genetically modifying peptides to enhance T-cell receptor affinity.

Finally, in Section III, Dr. Heslop reviews clinical experience with adoptive immunotherapy with T cells. Transferred T cells have clinical benefit in treating relapsed malignancy post transplant, and Epstein-Barr virus associated tumors. However, T cells have been less successful in treating other hematologic malignancies due to inadequate persistence or expansion of adoptively transferred cells and the presence of tumor evasion mechanisms. An improved understanding of the interactions of antigen presenting cells with T cells should optimize efforts to manufacture effector T cells, while manipulation of lymphocyte homeostasis in vivo and development of gene therapy approaches may enhance the persistence and function of adoptively transferred T cells.

Functional repair of a mutant chloride channel using a trans-splicing ribozyme

RNA repair has been proposed as a novel gene-based therapeutic strategy. Modified Tetrahymena group I intron ribozymes have been used to mediate trans-splicing of therapeutically relevant RNA transcripts, but the efficiency of the ribozyme-mediated RNA repair process has not been determined precisely and subsequent restoration of protein function has been demonstrated only by indirect means. We engineered a ribozyme that targets the mRNA of a mutant canine skeletal muscle chloride channel (cClC-1) (mutation T268M in ClC-1 causing myotonia congenita) and replaces the mutant-containing 3' portion by trans-splicing the corresponding 4-kb wild-type sequence. Repair efficiency assessed by quantitative RT-PCR was 1.2% +/- 0.1% in a population of treated cells. However, when chloride channel function was examined in single cells, a wide range of electrophysiological activity was observed, with 18% of cells exhibiting significant functional restoration and some cells exhibiting complete rescue of the biophysical phenotype. These results indicate that RNA repair can restore wild-type protein activity and reveal considerable cell-to-cell variability in ribozyme-mediated trans-splicing reaction efficiency.

Aptamer rivalry

A controllable, intracellular RNA aptamer could be an invaluable therapeutic or discovery reagent. A report published in this journal shows that aptamer behavior can be regulated by a small molecule, paving the way for the development of more sophisticated ligand-regulated aptamers.