Targeted Gene Repair: The Ups and Downs of a Promising Gene Therapy Approach

CRISPR/Cas9 therapeutics for liver diseases

The development of innovative genome editing techniques in recent years has revolutionized the field of biomedicine. Among the novel approaches, the clustered regularly interspaced short palindromic repeat/CRISPR-associated protein (CRISPR/Cas9) technology has become the most popular, in part due to its matchless ability to carry out gene editing at the target site with great precision. With considerable successes in animal and preclinical studies, CRISPR/Cas9-mediated gene editing has paved the way for its use in human trials, including patients with a variety of liver diseases. Gene editing is a logical therapeutic approach for liver diseases because many metabolic and acquired disorders are caused by mutations within a single gene. In this review, we provide an overview on current and emerging therapeutic strategies for the treatment of liver diseases using the CRISPR/Cas9 technology.

Synthesis of oligonucleotides containing novel G-clamp analogue with C8-tethered group in phenoxazine ring: Implication to qPCR detection of the low-copy Kemerovo virus dsRNA

Nowadays modified oligonucleotides are widely used in diagnostics and as novel therapeutics. Introduction of modified or unnatural residues into oligonucleotides allows fine tuning of their binding properties to complementary nucleic acids and leads to improved stability both in vitro and in vivo. Previously it was demonstrated that insertion of phenoxazine nucleotides with various groups in C9-position into oligonucleotides leads to a significant increase of duplex stability with complementary DNA and RNA. Here the synthesis of a novel G-clamp nucleoside analogue (G^8AE-clamp) bearing 2-aminoethyl tether at C8-atom is presented. Introduction of such modified residues into oligonucleotides lead to enhanced specificity of duplex formation towards complementary DNA and RNA targets with increased thermal and 3'-exonuclease stability. According to CD-spectroscopy studies G^8AE-clamp does not substantially disrupt helix geometry. Primers containing G^8AE-clamp demonstrated superior sensitivity in qPCR detection of dsRNA of Kemerovo virus in comparison to native oligonucleotides.

Insertion and Deletion Mismatches Distant from the Target Position Improve Gene Correction with a Tailed Duplex

A 5'-tailed duplex (TD) DNA corrects a base-substitution mutation. In this study, the effects of insertion and deletion (indel) mismatches distant from the target position on the gene correction were examined. Three target plasmid DNAs with and without indel mismatches ∼330 bases distant from the correction target position were prepared, and introduced into HeLa cells together with the TD. The indel mismatches improved the gene correction efficiency and specificity without sequence conversions at the indel mismatch site. These results suggested that the gene correction efficiency and specificity are increased when an appropriate second mismatch is introduced into the TD fragment.

New approaches to the treatment of orphan genetic disorders: Mitigating molecular pathologies using chemicals

With the advance and popularization of molecular techniques, the identification of genetic mutations that cause diseases has increased dramatically. Thus, the number of laboratories available to investigate a given disorder and the number of subsequent diagnosis have increased over time. Although it is necessary to identify mutations and provide diagnosis, it is also critical to develop specific therapeutic approaches based on this information. This review aims to highlight recent advances in mutation-targeted therapies with chemicals that mitigate mutational pathology at the molecular level, for disorders that, for the most part, have no effective treatment. Currently, there are several strategies being used to correct different types of mutations, including the following: the identification and characterization of translational readthrough compounds; antisense oligonucleotide-mediated splicing redirection; mismatch repair; and exon skipping. These therapies and other approaches are reviewed in this paper.

Liver-targeted gene therapy: Approaches and challenges

The liver plays a major role in many inherited and acquired genetic disorders. It is also the site for the treatment of certain inborn errors of metabolism that do not directly cause injury to the liver. The advancement of nucleic acid-based therapies for liver maladies has been severely limited because of the myriad untoward side effects and methodological limitations. To address these issues, research efforts in recent years have been intensified toward the development of targeted gene approaches using novel genetic tools, such as zinc-finger nucleases, transcription activator-like effector nucleases, and clustered regularly interspaced short palindromic repeats as well as various nonviral vectors such as Sleeping Beauty transposons, PiggyBac transposons, and PhiC31 integrase. Although each of these methods uses a distinct mechanism of gene modification, all of them are dependent on the efficient delivery of DNA and RNA molecules into the cell. This review provides an overview of current and emerging therapeutic strategies for liver-targeted gene therapy and gene repair.

Pharmacologic management of Duchenne muscular dystrophy: target identification and preclinical trials

Duchenne muscular dystrophy (DMD) is an X-linked human disorder in which absence of the protein dystrophin causes degeneration of skeletal and cardiac muscle. For the sake of treatment development, over and above definitive genetic and cell-based therapies, there is considerable interest in drugs that target downstream disease mechanisms. Drug candidates have typically been chosen based on the nature of pathologic lesions and presumed underlying mechanisms and then tested in animal models. Mammalian dystrophinopathies have been characterized in mice (mdx mouse) and dogs (golden retriever muscular dystrophy [GRMD]). Despite promising results in the mdx mouse, some therapies have not shown efficacy in DMD. Although the GRMD model offers a higher hurdle for translation, dogs have primarily been used to test genetic and cellular therapies where there is greater risk. Failed translation of animal studies to DMD raises questions about the propriety of methods and models used to identify drug targets and test efficacy of pharmacologic intervention. The mdx mouse and GRMD dog are genetically homologous to DMD but not necessarily analogous. Subcellular species differences are undoubtedly magnified at the whole-body level in clinical trials. This problem is compounded by disparate cultures in clinical trials and preclinical studies, pointing to a need for greater rigor and transparency in animal experiments. Molecular assays such as mRNA arrays and genome-wide association studies allow identification of genetic drug targets more closely tied to disease pathogenesis. Genes in which polymorphisms have been directly linked to DMD disease progression, as with osteopontin, are particularly attractive targets.

Enhanced H-bonding and π-stacking in DNA: a potent duplex-stabilizing and mismatch sensing nucleobase analogue

Combining enhanced π-stacking, H-bonding and electrostatic attraction in a single C-monomer greatly increases DNA duplex stability and massively destabilises mismatches.

Small fragment homologous replacement: evaluation of factors influencing modification efficiency in an eukaryotic assay system

Homologous Replacement is used to modify specific gene sequences of chromosomal DNA in a process referred to as “Small Fragment Homologous Replacement”, where DNA fragments replace genomic target resulting in specific sequence changes. To optimize the efficiency of this process, we developed a reporter based assay system where the replacement frequency is quantified by cytofluorimetric analysis following restoration of a stably integrated mutated eGFP gene in the genome of SV-40 immortalized mouse embryonic fibroblasts (MEF-SV-40). To obtain the highest correction frequency with this system, several parameters were considered: fragment synthesis and concentration, cell cycle phase and methylation status of both fragment and recipient genome. In addition, different drugs were employed to test their ability to improve technique efficiency. SFHR-mediated genomic modification resulted to be stably transmitted for several cell generations and confirmed at transcript and genomic levels. Modification efficiency was estimated in a range of 0.01–0.5%, further increasing when PARP-1 repair pathway was inhibited. In this study, for the first time SFHR efficiency issue was systematically approached and in part addressed, therefore opening new potential therapeutic ex-vivo applications.

Correction of frameshift mutations with tailed duplex DNAs

Tailed duplex (TD) DNAs, prepared by annealing an oligonucleotide to a several-hundred-base single-stranded (ss) DNA fragment, correct a base-substitution mutation with high efficiency. In the present study, the abilities of TD fragments to correct single-base insertion and deletion mutations were examined, using hygromycin-resistance and enhanced green fluorescent protein fusion (Hyg-EGFP) genes inactivated by +G and -C frameshift mutations. The 5'-TD and 3'-TD DNA fragments were co-transfected with plasmid DNA containing the inactivated Hyg-EGFP gene into CHO-K1 cells, and the gene correction efficiencies were determined by introducing the plasmid DNA recovered from the transfected cells into Escherichia coli cells. In contrast to their efficiencies for the substitution mutation, the gene correction abilities of the TD fragments were relatively low. The correction efficiencies by the TD fragments were apparently higher than that by a ss DNA fragment, one of the DNA fragments employed for gene correction. These results suggest that the TD fragments have the potential to correct frameshift mutations, although further improvement is required.

DNA duplexes and triplex-forming oligodeoxynucleotides incorporating modified nucleosides forming stable and selective triplexes

We have previously reported DNA triplexes containing the unnatural base triad G-PPI·C3, in which PPI is an indole-fused cytosine derivative incorporated into DNA duplexes and C3 is an abasic site in triplex-forming oligonucleotides (TFOs) introduced by a propylene linker. In this study, we developed a new unnatural base triad A-ψ·C(R1) where ψ and C(R1) are base moieties 2'-deoxypseudouridine and 5-substituted deoxycytidine, respectively. We examined several electron-withdrawing substituents for R1 and found that 5-bromocytosine (C(Br)) could selectively recognize ψ. In addition, we developed a new PPI derivative, PPI(Me), having a methyl group on the indole ring in order to achieve selective triplex formation between DNA duplexes incorporating various Watson-Crick base pairs, such as T-A, C-G, A-ψ, and G-PPI(Me), and TFOs containing T, C, C(Br), and C3. We studied the selective triplex formation between these duplexes and TFOs using UV-melting and gel mobility shift assays.

Towards artificial metallonucleases for gene therapy: recent advances and new perspectives

The process of DNA targeting or repair of mutated genes within the cell, induced by specifically positioned double-strand cleavage of DNA near the mutated sequence, can be applied for gene therapy of monogenic diseases. For this purpose, highly specific artificial metallonucleases are developed. They are expected to be important future tools of modern genetics. The present state of art and strategies of research are summarized, including protein engineering and artificial ‘chemical’ nucleases. From the results, we learn about the basic role of the metal ions and the various ligands, and about the DNA binding and cleavage mechanism. The results collected provide useful guidance for engineering highly controlled enzymes for use in gene therapy.

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.

Targeted gene correction using psoralen, chlorambucil and camptothecin conjugates of triplex forming peptide nucleic acid (PNA)

Gene correction activation effects of a small series of triplex forming peptide nucleic acid (PNA) covalently conjugated to the DNA interacting ligands psoralen, chlorambucil and camptothecin targeted proximal to a stop codon mutation in an EGFP reporter gene were studied. A 15-mer homopyrimidine PNA conjugated to the topoisomerase I inhibitor camptothecin was found to increase the frequency of repair domain mediated gene correctional events of the EGFP reporter in an in vitro HeLa cell nuclear extract assay, whereas PNA psoralen or chlorambucil conjugates both of which form covalent and also interstrand crosslinked adducts with dsDNA dramatically decreased the frequency of targeted repair/correction. The PNA conjugates were also studied in mammalian cell lines upon transfection of PNA bound EGFP reporter vector and scoring repair of the EGFP gene by FACS analysis of functional EGFP expression. Consistent with the extract experiments, treatment with adduct forming PNA conjugates (psoralen and chlorambucil) resulted in a decrease in background correction frequencies in transiently transfected cells, whereas unmodified PNA or the PNA-camptothecin conjugate had little or no effect. These results suggest that simple triplex forming PNAs have little effect on proximal gene correctional events whereas PNA conjugates capable of forming DNA adducts and interstrand crosslinks are strong inhibitors. Most interestingly the PNA conjugated to the topoisomerase inhibitor, camptothecin enhanced repair in nuclear extract. Thus the effects and use of camptothecin conjugates in gene targeted repair merit further studies.

Analysis of illegitimate genomic integration mediated by zinc-finger nucleases: implications for specificity of targeted gene correction

BACKGROUND

Formation of site specific genomic double strand breaks (DSBs), induced by the expression of a pair of engineered zinc-finger nucleases (ZFNs), dramatically increases the rates of homologous recombination (HR) between a specific genomic target and a donor plasmid. However, for the safe use of ZFN induced HR in practical applications, possible adverse effects of the technology such as cytotoxicity and genotoxicity need to be well understood. In this work, off-target activity of a pair of ZFNs has been examined by measuring the ratio between HR and illegitimate genomic integration in cells that are growing exponentially, and in cells that have been arrested in the G2/M phase.

RESULTS

A reporter cell line that contained consensus ZFN binding sites in an enhanced green fluorescent protein (EGFP) reporter gene was used to measure ratios between HR and non-homologous integration of a plasmid template. Both in human cells (HEK 293) containing the consensus ZFN binding sites and in cells lacking the ZFN binding sites, a 3.5 fold increase in the level of illegitimate integration was observed upon ZFN expression. Since the reporter gene containing the consensus ZFN target sites was found to be intact in cells where illegitimate integration had occurred, increased rates of illegitimate integration most likely resulted from the formation of off-target genomic DSBs. Additionally, in a fraction of the ZFN treated cells the co-occurrence of both specific HR and illegitimate integration was observed. As a mean to minimize unspecific effects, cell cycle manipulation of the target cells by induction of a transient G2/M cell cycle arrest was shown to stimulate the activity of HR while having little effect on the levels of illegitimate integration, thus resulting in a nearly eight fold increase in the ratio between the two processes.

CONCLUSIONS

The demonstration that ZFN expression, in addition to stimulating specific gene targeting by HR, leads to increased rates of illegitimate integration emphasizes the importance of careful characterization of ZFN treated cells. In order to reduce off-target events, reversible cell cycle arrest of the target cells in the G2/M phase is an efficient way for increasing the ratio between specific HR and illegitimate integration.

Targeted approaches for gene therapy and the emergence of engineered meganucleases

BACKGROUND

In spite of significant advances in gene transfer strategies in the field of gene therapy, there is a strong emphasis on the development of alternative methods, providing better control of transgene expression and insertion patterns.

OBJECTIVE

Several new approaches consist of targeting a desired transgene or gene modification in a well defined locus, and we collectively refer to them as 'targeted approaches'. The use of redesigned meganucleases is one of these emerging technologies. Here we try to define the potential of this method, in the larger scope of targeted strategies.

METHODS

We survey the different types of targeted strategies, presenting the achievements and the potential applications, with a special emphasis on the use of redesigned endonucleases.

CONCLUSION

redesigned endonucleases represent one of the most promising tools for targeted approaches, and the opening of a clinical trial for AIDS patients has recently shown the maturity of these strategies. However, there is still a 'quest' for the best reagents, that is the endonucleases providing the best efficacy:toxicity ratio. New advances in protein design have allowed the engineering of new scaffolds, such as meganucleases, and the landscape of existing methods is likely to change over the next few years.

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.

Structural analysis of the human Rad51 protein-DNA complex filament by tryptophan fluorescence scanning analysis: transmission of allosteric effects between ATP binding and DNA binding

Human Rad51 (HsRad51) catalyzes the strand exchange reaction, a crucial step in homologous recombination, by forming a filamentous complex with DNA. The structure of this filament is modified by ATP, which is required and hydrolyzed for the reaction. We analyzed the structure and the ATP-promoted conformational change of this filament. We systematically replaced aromatic residues in the protein, one at a time, with tryptophan, a fluorescent probe, and examined its effect on the activities (DNA binding, ATPase, ATP-promoted conformational change, and strand exchange reaction) and the fluorescence changes upon binding of ATP and DNA. Some residues were also replaced with alanine. We thus obtained structural information about various positions of the protein in solution. All the proteins conserved, at least partially, their activities. However, the replacement of histidine at position 294 (H294) and phenylalanine at 129 (F129) affected the ATP-induced conformational change of the DNA-HsRad51 filament, although it did not prevent DNA binding. F129 is considered to be close to the ATP-binding site and to H294 of a neighboring subunit. ATP probably modifies the structure around F129 and affects the subunit/subunit contact around H294 and the structure of the DNA-binding site. The replacement also reduced the DNA-dependent ATPase activity, suggesting that these residues are also involved in the transmission of the allosteric effect of DNA to the ATP-binding site, which is required for the stimulation of ATPase activity by DNA. The fluorescence analyses supported the structural change of the DNA-binding site by ATP and that of the ATP-binding site by DNA. This information will be useful to build a molecular model of the Rad51-DNA complex and to understand the mechanism of activation of Rad51 by ATP and that of the Rad51-promoted strand exchange reaction.

Improved gene correction efficiency with a tailed duplex DNA fragment

A 606-base single-stranded (ss) DNA fragment, prepared by restriction enzyme digestion of ss phagemid DNA, corrects a hygromycin resistance and enhanced green fluorescent protein (Hyg-EGFP) fusion gene more efficiently than a PCR fragment, which is the conventional type of DNA fragment used in gene correction. Here, a tailed duplex, obtained by annealing an oligonucleotide to the ss DNA fragment, was used in the correction. The tailed duplex may be a good substrate for the RAD51 protein, an important enzyme in homologous recombination, which could be the gene correction pathway. The annealing of the oligonucleotides enhanced the correction efficiency of the Hyg-EGFP gene, especially when annealed in the 3'-region of the ss DNA fragment. Both the length and backbone structure of the oligonucleotides affected the gene correction efficiency. This type of gene correction device was also effective for another target gene, the rpsL gene. The results obtained in this study indicate that tailed duplex DNA fragments are effective nucleic acids for gene correction.

Simultaneous targeted exchange of two nucleotides by single-stranded oligonucleotides clusters within a region of about fourteen nucleotides

Background

Transfection of cells with gene-specific, single-stranded oligonucleotides can induce the targeted exchange of one or two nucleotides in the targeted gene. To characterize the features of the DNA-repair mechanisms involved, we examined the maximal distance for the simultaneous exchange of two nucleotides by a single-stranded oligonucleotide. The chosen experimental system was the correction of a hprt-point mutation in a hamster cell line, the generation of an additional nucleotide exchange at a variable distance from the first exchange position and the investigation of the rate of simultaneous nucleotide exchanges.

Results

The smaller the distance between the two exchange positions, the higher was the probability of a simultaneous exchange. The detected simultaneous nucleotide exchanges were found to cluster in a region of about fourteen nucleotides upstream and downstream from the first exchange position.

Conclusion

We suggest that the mechanism involved in the repair of the targeted DNA strand utilizes only a short sequence of the single-stranded oligonucleotide, which may be physically incorporated into the DNA or be used as a matrix for a repair process.

Gene prophylaxis by a DNA repair function

Gene therapy, the treatment of disorders or pathophysiologic states on the basis of the transfer of genetic information, has been thoroughly investigated for the treatment of lung illnesses, e.g. cystic fibrosis, alpha1-antitrypsin deficiency-related emphysema and cancer. Transfer of genetic information may be further used to elevate the level of protection of normal lung tissues in at risk individuals, with preventing purposes. This concept can be described by the term "gene prophylaxis". Lying at the gas-exchange interface, lung epithelia may be at risk of oxidation-induced mutagenesis. Further, inflammation processes possibly consequent on smoking liberate reactive oxygen species (ROS) that multiply the carcinogenic effects of tobacco. Some studies report in lung cancer patients an high frequency of variations of the 8-oxoguanine DNA glycosylase (hOGG1) gene that encodes a sluggish glycosylase for oxidized purines. Unlike dietary interventions with antioxidant drugs that only allow temporary oxy-radical scavenging, reinforcing the DNA repair capacity in lung epithelia may afford long-term, steady protection from ROS-generated mutagenesis and carcinogenesis. In this regard, the Escherichia coli formamidopyrimidine DNA glycosylase (FPG) is a possible tool. FPG is 80-fold faster than hOGG1 in repairing oxidized purines and has broader substrate specificity. Cell culture studies have shown that FPG can be expressed in mammalian cells where it accelerates DNA repair and abates mutagenicity of a wide range of DNA damaging agents. Spontaneous mutagenesis drops too. Prophylaxis of oxidative DNA damage and mutation could be achieved in lung epithelia and other tissues of at-risk individuals by FPG expression. Currently available vehicles for this peculiar type of gene therapy are briefly surveyed.