Gene targeting in mosquito cells: a demonstration of 'knockout' technology in extrachromosomal gene arrays

High-throughput enhancer trap by remobilization of transposonMinos inCiona intestinalis

The enhancer trap approach utilizing transposons yields us information about gene functions and gene expression patterns. In the ascidian Ciona intestinalis, transposon-based transgenesis and insertional mutagenesis were achieved with a Tc1/mariner transposon Minos. We report development of a novel technique for enhancer trap in C. intestinalis. This technique uses remobilization of Minos in the Ciona genome. A Minos vector for enhancer trap was constructed and a tandem array insertion of the vector was introduced into the Ciona genome to create a mutator line. Minos was remobilized in Ciona chromosomes to create new insertions by providing transposases. These transposase-introduced animals were crossed with wild-type animals. Nearly 80% of F1 families showed novel GFP expression patterns. This high-throughput enhancer trap screen will be useful to create new marker transgenic lines showing reporter gene expression in specific tissues and to identify novel patterns of gene expression.

Oligonucleotides and derivatives as gene-specific control agents

The current achievement of genome sequence projects of a dozen eukaryote organisms (including human genome) and the development of functional genomics are providing the basic knowledge required to utilize gene-specific reagents for both basic understanding of cell physiology and therapeutical development. The field of chemical genomics has the ambitious goal of designing molecules that could act selectively on every single gene or gene product in a cell and in vivo. The progress in oligonucleotide-based approaches will be the topic of this review, however, other nucleic acid- and SELEX-based approaches as well as high sequence-specific low molecular weight DNA-specific ligands will also be discussed.

Functional genomics of parasitic worms: the dawn of a new era

The study of gene function in parasitic worms is technically demanding due to difficulties associated with life-cycle propagation and, hence, molecular genetics. Exploitation of the free-living nematode, Caenorhabditis elegans, coupled with recent major advances in molecular studies of parasitic nematodes, have opened up new avenues for understanding the biology of these parasites and present opportunities for novel strategies of therapeutic intervention and control.

A new method (GOREC) for directed mutagenesis and gene repair by homologous recombination

Directed mutagenesis in mammalian cells has been the focus of intense research because of its promising application for gene correction and engineering. Both natural and modified oligonucleotides (ODN), RNA-DNA chimeric oligonucleotide (RDO) and small fragment DNA (SFHR), as well as vector DNA were used for promoting homologous replacement with varying success. It was recently shown that a triple helix-forming oligonucleotide (TFO) tethered to an oligonucleotide (donor DNA) can enhance mutagenesis by homologous recombination in cells. The basic idea is to accelerate homology search by oligonucleotide-directed triple helix formation in the vicinity of the target site for donor DNA. Here we describe a new method named GOREC (guided homologous recombination) which shares similar gene targeting, but has notable difference in the concept with the previous method. It is made of a homing device (TFO) and a donor DNA for effecting distinct functions. They are linked together by non-covalent or covalent interaction. This modular concept allows guidance of either an oligonucleotide (ODN, RDO) or a small DNA fragment to the target site for homologous replacement. Therefore, the triple helix site can be hundreds of base pairs away from the target site. An episomal assay for proof-of-principle study will be presented and discussed.

A sensitive and rapid assay for homologous recombination in mosquito cells: impact of vector topology and implications for gene targeting

BACKGROUND

Recent progress in insect transgenesis has been dramatic but existing transposon-based approaches are constrained by position effects and potential instability. Gene targeting would bring a number of benefits, however progress requires a better understanding of the mechanisms involved. Much can be learned in vitro since extrachromosomal recombination occurs at high frequency, facilitating the study of multiple events and the impact of structural changes among the recombining molecules. We have investigated homologous recombination in mosquito cells through restoration of luciferase activity from deleted substrates. The implications of this work for the construction of insect gene targeting vectors are discussed.

RESULTS

We show that linear targeting vectors are significantly more efficient than circular ones and that recombination is stimulated by introducing double-strand breaks into, or near, the region of homology. Single-strand annealing represents a very efficient pathway but may not be feasible for targeting unbroken chromosomes. Using circular plasmids to mimic chromosomal targets, one-sided invasion appears to be the predominant pathway for homologous recombination. Non-homologous end joining reactions also occur and may be utilised in gene targeting if double-strand breaks are first introduced into the target site.

CONCLUSIONS

We describe a rapid, sensitive assay for extrachromosomal homologous recombination in mosquito cells. Variations in substrate topology suggest that single-strand annealing and one-sided invasion represent the predominant pathways, although non-homologous end joining reactions also occur. One-sided invasion of circular chromosomal mimics by linear vectors might therefore be used in vitro to investigate the design and efficiency of gene targeting strategies.