The potential of extrachromosomal replicating vectors for gene therapy

Bovine papillomavirus type 1 genomes and the E2 transactivator protein are closely associated with mitotic chromatin

The bovine papillomavirus type 1 E2 transactivator protein is required for viral transcriptional regulation and DNA replication and may be important for long-term episomal maintenance of viral genomes within replicating cells (M. Piirsoo, E. Ustav, T. Mandel, A. Stenlund, and M. Ustav, EMBO J. 15:1-11, 1996). We have evidence that, in contrast to most other transcriptional transactivators, the E2 transactivator protein is associated with mitotic chromosomes in dividing cells. The shorter E2-TR and E8/E2 repressor proteins do not bind to mitotic chromatin, and the N-terminal transactivation domain of the E2 protein is necessary for the association. However, the DNA binding function of E2 is not required. We have found that bovine papillomavirus type 1 genomes are also associated with mitotic chromosomes, and we propose a model in which E2-bound viral genomes are transiently associated with cellular chromosomes during mitosis to ensure that viral genomes are segregated to daughter cells in approximately equal numbers.

Identification of autonomously replicating sequence (ARS) elements in eukaryotic cells

Autonomously replicating sequence (ARS) elements were first identified in the budding yeast Saccharomyces cerevisiae as chromosomal DNA fragments that promoted high frequency of transformation and extrachromosomal maintenance of plasmid DNA. These specific sequence elements were subsequently shown to function as origins of DNA replication. Detailed analysis of the structure and function of ARS elements has been limited largely to S. cerevisiae and more recently the fission yeast Schizosaccharomyces pombe. Characterization of ARS activity in other eukaryotes is far less complete. Here we describe the ARS assay developed in yeast and its application to the study of origin function in other eukaryotes. Other available methods for detecting autonomous replication in these systems are also presented.

Mammalian artificial chromosomes--vectors for somatic gene therapy

Mammalian artificial chromosomes might prove to be useful vectors for somatic gene therapy. The functional elements of such an artificial chromosome are telomeres, a centromere and a replication origin. Recent progress in the characterization of these functional elements of the eukaryotic chromosome will be described. Attempts to construct artificial chromosomes for mammalian cells and their use for somatic gene therapy are discussed.

Delivery of tumor suppressor genes to reverse the malignant phenotype

Despite early enthusiasm and excitement, the treatment of cancer via gene therapy is a long way from reaching fruition. The objective of this review is to describe the rationale as to why the delivery of genes encoding functional proteins whose activity has been lost during the initiation and development of cancer may be a feasible therapeutic option. In addition we will evaluate the limitations of the current delivery systems and discuss how these limitations have impacted upon the progress of gene therapy. Finally, we will describe and discuss the most recent attempts to deliver tumor suppressor genes to rodent models of human cancer and perhaps more importantly to human patients. As will become apparent during this review the excitement and enthusiasm for gene therapy remains high, however, this should not diminish the challenges that must be overcome before gene therapy becomes routine.

In vivo adenovirus-mediated gene transfer to newborn rat retinal pigment epithelial cells

A successful surgical access to the subretinal space is critical for achieving adenovirus-mediated gene transfer to the retinal pigment epithelial (RPE) cells or photoreceptor cells. We report a novel surgical approach allowing an efficient delivery of recombinant replication-deficient adenoviral vectors into the subretinal space of newborn rats. Our data suggest that this method may be useful for infecting reproducibly large area of the RPE cell layer of normal newborn rats and should be applicable to RCS pups. We also show the feasibility of infecting ex vivo RPE cells in culture using the same recombinant adenoviral vector.

Autonomous plasmid replication in Aspergillus nidulans: AMA1 and MATE elements

With few exceptions, in eukaryotic organisms the presence of a chromosomal replicator on a circular vector molecule is not sufficient to confer on it the ability to persist and replicate extrachromosomally. However, it is possible to isolate from genomes of some filamentous fungi DNA fragments which can provide extrachromosomal maintenance of plasmids. In Aspergillus nidulans, two functional classes of such sequences can be distinguished: effective plasmid replicators (e.g., AMA1) and transformation enhancers (e.g., ANS1 or MATEs), which apparently are able to initiate aberrant replication, leading to vector rearrangement and multimerization and eventually resulting in chromosomal integration. We discuss the similarity of these events to DNA amplification in other eukaryotes. A model is suggested which accounts for the formation of effective replicating plasmids as a result of sequence amplification. The model is based on the observation that in some organisms, including A. nidulans and Schizosaccharomyces pombe, duplication of an inefficient replicator enhances its efficiency dramatically. Some structural traits of transformation enhancers in A. nidulans imply a role for topoisomerases in amplification and replication of circular DNA molecules. We discuss practical applications of replicative vectors for gene cloning and expression studies.