Novel piggyBac transposase vectors for safer gene addition into mammalian genomes

Abstract

An extensive range of treatments of both inherited and acquired diseases are now possible due to our ability to permanently introduce foreign genes into chromosomes. However, the uncontrolled nature of random vector gene insertion presents a mutagenic risk. In this dissertation, a novel piggyBac (pB) transposon system has been developed to address potential genotoxicity issues. We hypothesized that modifications to the pB transposase could facilitate safer gene addition into mammalian genomes. In Chapter 2, we present a single-plasmid system, termed GENIE, that incorporates all of the required components for integration. These vectors are able to inactivate the pB gene after excision of the delivery transposon from the plasmid. This feature eliminates potential negative consequences that could arise from the persistence of an active pB transposase inadvertently taken up by the genome. In Chapter 3, we explore an application of our GENIE vector system by demonstrating effective knockdown of telomerase reverse transcriptase (TERT) in human immortalized cell lines. A significant hurdle for safer therapeutic gene addition is developing a method for controlling the precise location of insertion. We have developed a targetable transposase system by fusing DNA binding domains (DBDs) to pB in order to localize insertions near specific recognition sequences. In Chapter 4, we improved the GENIE vector system by fusing a GAL4 DBD to the pB transposase and demonstrated the ability of our vectors to target transgenes to predetermined sites. Gal4 recognition sites found on episomal plasmids and on target sequences introduced into the human genome were preferentially targeted by our chimeric Gal4-pB transposase. Furthermore, a genome-wide integration analysis revealed the ability of our fusion constructs to bias integrations near endogenous Gal4 recognition sequences. In Chapter 5, we demonstrated site-specific and user-defined transposition to the CCR5 genomic safe harbor. Custom TALE DBDs were designed to bind the first intron of the human CCR5 gene. These TALE proteins were incorporated into a variety of novel targeting vectors that used both plasmid-DNA and transposase-protein relocalization to the target sequence. We used these vectors to isolate single-copy clones harboring targeted integrations.