Factors affecting SFHR gene correction efficiency with single-stranded DNA fragment

Gene correction by 5'-tailed duplexes with short editor oligodeoxyribonucleotides

Various diseases, including cancer, are caused by genetic mutations. A 5'-tailed duplex (TD) DNA, consisting of a long single-stranded (ss) editor DNA and a short (∼35-base) ss assistant oligodeoxyribonucleotide, can introduce a base-substitution in living cells and thus correct mutated genes. Previously, several hundred-base DNAs were employed as the editor DNAs. In this study, 5'-TDs were prepared from various editor DNAs with different lengths and examined for their gene correction abilities, using plasmid DNA bearing a mutated copepod green fluorescent protein (copGFP) gene, in human cells. High-throughput analysis was performed by the reactivated fluorescence of the wild-type protein encoded by the corrected gene as the indicator. The analysis revealed that 5'-TDs with ∼100-base ss editor DNAs enabled gene editing at least as efficiently as those with longer editor DNAs. Moreover, the antisense strand was more effective as the editor than the sense strand, in contrast to the 5'-TDs with longer editor strands. These results indicated that the 5'-TD fragments with shorter editor strands than those used in previous studies are useful nucleic acids for gene correction.

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.

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.

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.

Sequence-specific correction of genomic hypoxanthine-guanine phosphoribosyl transferase mutations in lymphoblasts by small fragment homologous replacement

Oligo/polynucleotide-based gene targeting strategies provide new options for achieving sequence-specific modification of genomic DNA and have implications for the development of new therapies and transgenic animal models. One such gene modification strategy, small fragment homologous replacement (SFHR), was evaluated qualitatively and quantitatively in human lymphoblasts that contain a single base substitution in the hypoxanthine-guanine phosphoribosyl transferase (HPRT1) gene. Because HPRT1 mutant cells are readily discernable from those expressing the wild type (wt) gene through growth in selective media, it was possible to identify and isolate cells that have been corrected by SFHR. Transfection of HPRT1 mutant cells with polynucleotide small DNA fragments (SDFs) comprising wild type HPRT1 (wtHPRT1) sequences resulted in clones of cells that grew in hypoxanthine-aminopterin-thymidine (HAT) medium. Initial studies quantifying the efficiency of correction in 3 separate experiments indicate frequencies ranging from 0.1% to 2%. Sequence analysis of DNA and RNA showed correction of the HPRT1 mutation. Random integration was not indicated after transfection of the mutant cells with an SDF comprised of green fluorescent protein (GFP) sequences that are not found in human genomic DNA. Random integration was also not detected following Southern blot hybridization analysis of an individual corrected cell clone.

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.

[Development of multifunctional envelope type artificial viral-like gene delivery system]

This review introduces a new concept "Programmed Packaging" to develop a non-viral gene delivery system. Based on this concept, multifunctional envelope type nano devices (MEND) were developed for in vitro, in situ and in vivo conditions. A quantitative study to identify a rate limiting step in intracellular trafficking was also shown between viral and non-viral vectors, which indicated an important role of controlled intranuclear disposition for development a safe and efficient non-viral gene delivery system. This review will provide a future direction of non-viral gene delivery system.

Gel purification of genomic DNA removes contaminating small DNA fragments interfering with polymerase chain reaction analysis of small fragment homologous replacement

Oligonucleotides can mediate sequence-specific gene modification that results in the correction and/or alteration of genomic DNA. There is evidence to suggest that the polymerase chain reaction (PCR)-based analytical methods usually used to analyze oligonucleotide-mediated modification can generate artifacts. To investigate the conditions under which a PCR artifact can be generated and eliminated when analyzing small fragment homologous replacement (SHFR)-mediated modification, cells homozygous for the DeltaF508 mutation (CFBE41o-) were mixed with small DNA fragments (SDFs) containing the wild-type CFTR (wt-CFTR) sequence. An artifact could be generated after wild-type allele-specific PCR (wtAS-PCR) if the genomic DNA was not gel purified. Without gel purification, the amount of SDF/cell required to generate the artifact was dependent to the AS primer pairs used. When the genomic DNA was gel purified, no artifact could be detected with any of the wtAS-PCR primers whether the SDF was mixed with the cells or transfected into the cells. Furthermore, treatment of cellular mRNA with DNase was sufficient to eliminate potential artifacts in the reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Thus, it is critical to gel purify genomic DNA and DNase treat mRNA when analyzing SFHR-mediated modification by PCR.