Site-specific base changes in the coding or promoter region of the human beta- and gamma-globin genes by single-stranded oligonucleotides

Therapeutic strategies in Sickle Cell Anemia: The past present and future

Sickle Cell Anemia (SCA) was one of the first hemoglobinopathies to be discovered. It is distinguished by the mutation-induced expression of a sickle cell variant of hemoglobin (HbS) that triggers erythrocytes to take a characteristic sickled conformation. The complex physiopathology of the disease and its associated clinical complications has initiated multi-disciplinary research within its field. This review attempts to lay emphasis on the evolution, current standpoint and future scope of therapeutic strategies in SCA.

Transcription-replication collision increases recombination efficiency between plasmids

It has been proposed that the stalling of the replication forks can induce homologous recombination in several organisms, and that arrested replication forks may offer nuclease targets, thereby providing a substrate for proteins involved in double-strand repair. In this article, we constructed a plasmid with the potential for transcription-replication collision (TRC), in which DNA replication and RNA transcription occur on the same DNA template simultaneously. Theoretically, transcription will impede DNA replication and increase homologous recombination. To validate this hypothesis, another plasmid was constructed that contained a homologous sequence with the exception of some mutated sites. Co-transfection of these two plasmids into 293T cells resulted in increased recombination frequency. The ratio of these two plasmids also affected the recombination frequency. Moreover, we found high expression levels of RAD51, which indicated that the increase in the recombination rate was probably via the homologous recombination pathway. These results indicate that mutant genes in plasmids can be repaired by TRC-induced recombination.

Oligo/polynucleotide-based gene modification: strategies and therapeutic potential

Oligonucleotide- and polynucleotide-based gene modification strategies were developed as an alternative to transgene-based and classical gene targeting-based gene therapy approaches for treatment of genetic disorders. Unlike the transgene-based strategies, oligo/polynucleotide gene targeting approaches maintain gene integrity and the relationship between the protein coding and gene-specific regulatory sequences. Oligo/polynucleotide-based gene modification also has several advantages over classical vector-based homologous recombination approaches. These include essentially complete homology to the target sequence and the potential to rapidly engineer patient-specific oligo/polynucleotide gene modification reagents. Several oligo/polynucleotide-based approaches have been shown to successfully mediate sequence-specific modification of genomic DNA in mammalian cells. The strategies involve the use of polynucleotide small DNA fragments, triplex-forming oligonucleotides, and single-stranded oligodeoxynucleotides to mediate homologous exchange. The primary focus of this review will be on the mechanistic aspects of the small fragment homologous replacement, triplex-forming oligonucleotide-mediated, and single-stranded oligodeoxynucleotide-mediated gene modification strategies as it relates to their therapeutic potential.

Genetic modification through oligonucleotide-mediated mutagenesis. A GMO regulatory challenge?

In the European Union, the definition of a GMO is technology-based. This means that a novel organism will be regulated under the GMO regulatory framework only if it has been developed with the use of defined techniques. This approach is now challenged with the emergence of new techniques. In this paper, we describe regulatory and safety issues associated with the use of oligonucleotide-mediated mutagenesis to develop novel organisms. We present scientific arguments for not having organisms developed through this technique fall within the scope of the EU regulation on GMOs. We conclude that any political decision on this issue should be taken on the basis of a broad reflection at EU level, while avoiding discrepancies at international level.

The potential of oligonucleotides for therapeutic applications

Viral-derived particles have been widely used and described in gene therapy clinical trials. Although substantial results have been achieved, major safety issues have also arisen. For more than a decade, oligonucleotides have been seen as an alternative to gene complementation by viral vectors or DNA plasmids, either to correct the genetic defect or to silence gene expression. The development of RNA interference has strengthened the potential of this approach. Recent clinical trials have also tested the ability of aptamer molecules and decoy oligonucleotides to sequestrate pathogenic proteins. Here, we review the potential of oligonucleotides in gene therapy, outline what has already been accomplished, and consider what remains to be done.

Third strand-mediated psoralen-induced correction of the sickle cell mutation on a plasmid transfected into COS-7 cells

A significant level of correction of the mutation responsible for sickle cell anemia has been achieved in monkey COS-7 cells on a plasmid containing a beta-globin gene fragment. The plasmid was treated in vitro with a nucleic acid 'third strand' bearing a terminal photoreactive psoralen moiety that binds immediately adjacent to the mutant base pair. Following covalent attachment of the psoralen by monoadduct or diadduct formation to the mutant T-residue on the coding strand, the treated plasmid was transfected into the cells, which were then incubated for 48 h to allow the cellular DNA repair mechanisms to remove the photoadducts. Upon re-isolation and amplification of the transfected plasmid, sickle cell mutation correction, as determined by sequence analysis of both complementary strands, was established in a full 1%. This result encourages extension of the approach to correct the mutation directly on the chromosome.

Protein misfolding disorders: pathogenesis and intervention

Newly synthesized proteins in the living cell must go through a folding process to attain their functional structure. To achieve this in an efficient fashion, all organisms, including humans, have evolved a large set of molecular chaperones that assist the folding as well as the maintenance of the functional structure of cellular proteins. Aberrant proteins, the result of production errors, inherited or acquired amino acid substitutions or damage, especially oxidative modifications, can in many cases not fold correctly and will be trapped in misfolded conformations. To rid the cell of misfolded proteins, the living cell contains a large number of intracellular proteases, e.g. the proteasome, which together with the chaperones comprise the cellular protein quality control systems. Many inherited disorders due to amino acid substitutions exhibit loss-of-function pathogenesis because the aberrant protein is eliminated by one of the protein quality control systems. Examples are cystic fibrosis and phenylketonuria. However, not all aberrant proteins can be eliminated and the misfolded protein may accumulate and form toxic oligomeric and/or aggregated inclusions. In this case the loss of function may be accompanied by a gain-of-function pathogenesis, which in many cases determines the pathological and clinical features. Examples are Parkinson and Huntington diseases. Although a number of strategies have been tried to decrease the amounts of accumulated and aggregated proteins, a likely future strategy seems to be the use of chemical or pharmacological chaperones with specific effects on the misfolded protein in question. Positive examples are enzyme enhancement in a number of lysosomal disorders.