Jason B. Bell

Gene transfer into genomes of human cells by the sleeping beauty transposon system.

The Sleeping Beauty (SB) transposon system, derived from teleost fish sequences, is extremely effective at delivering DNA to vertebrate genomes, including those of humans. We have examined several parameters of the SB system to improve it as a potential, nonviral vector for gene therapy. Our investigation centered on three features: the carrying capacity of the transposon for efficient integration into chromosomes of HeLa cells, the effects of overexpression of the SB transposase gene on transposition rates, and improvements in the activity of SB transposase to increase insertion rates of transgenes into cellular chromosomes. We found that SB transposons of about 6 kb retained 50% of the maximal efficiency of transposition, which is sufficient to deliver 70-80% of identified human cDNAs with appropriate transcriptional regulatory sequences. Overexpression inhibition studies revealed that there are optimal ratios of SB transposase to transposon for maximal rates of transposition, suggesting that conditions of delivery of the two-part transposon system are important for the best gene-transfer efficiencies. We further refined the SB transposase to incorporate several amino acid substitutions, the result of which led to an improved transposase called SB11. With SB11 we are able to achieve transposition rates that are about 100-fold above those achieved with plasmids that insert into chromosomes by random recombination. With the recently described improvements to the transposon itself, the SB system appears to be a potential gene-transfer tool for human gene therapy.

Harnessing a high cargo-capacity transposon for genetic applications in vertebrates.

Viruses and transposons are efficient tools for permanently delivering foreign DNA into vertebrate genomes but exhibit diminished activity when cargo exceeds 8 kilobases (kb). This size restriction limits their molecular genetic and biotechnological utility, such as numerous therapeutically relevant genes that exceed 8 kb in size. Furthermore, a greater payload capacity vector would accommodate more sophisticated cis cargo designs to modulate the expression and mutagenic risk of these molecular therapeutics. We show that the Tol2 transposon can efficiently integrate DNA sequences larger than 10 kb into human cells. We characterize minimal sequences necessary for transposition (miniTol2) in vivo in zebrafish and in vitro in human cells. Both the 8.5-kb Tol2 transposon and 5.8-kb miniTol2 engineered elements readily function to revert the deficiency of fumarylacetoacetate hydrolase in an animal model of hereditary tyrosinemia type 1. Together, Tol2 provides a novel nonviral vector for the delivery of large genetic payloads for gene therapy and other transgenic applications.

Prolonged expression of a lysosomal enzyme in mouse liver after Sleeping Beauty transposon-mediated gene delivery: implications for non-viral gene therapy of mucopolysaccharidoses

The Sleeping Beauty (SB) transposon system is a non‐viral vector system that can integrate precise sequences into chromosomes. We evaluated the SB transposon system as a tool for gene therapy of mucopolysaccharidosis (MPS) types I and VII.

Preferential delivery of the Sleeping Beauty transposon system to livers of mice by hydrodynamic injection

Nonviral, DNA-mediated gene transfer is an alternative to viral delivery systems for expressing new genes in cells and tissues. The Sleeping Beauty (SB) transposon system combines the advantages of viruses and naked DNA molecules for gene therapy purposes; however, efficacious delivery of DNA molecules to animal tissues can still be problematic. Here we describe the hydrodynamic delivery procedure for the SB transposon system that allows efficient delivery to the liver in the mouse. The procedure involves rapid, high-pressure injection of a DNA solution into the tail vein. The overall procedure takes <1 h although the delivery into one mouse requires only a few seconds. Successful injections result in expression of the transgene in 5–40% of hepatocytes 1 d after injection. Several weeks after injection, transgene expression stabilizes at ∼1% of the level at 24 h, presumably owing to integration of the transposons into chromosomes.

Systemic Correction of Storage Disease in MPS I NOD/SCID Mice Using the Sleeping Beauty Transposon System

The Sleeping Beauty (SB) transposon system is a nonviral vector that directs transgene integration into vertebrate genomes. We hydrodynamically delivered SB transposon plasmids encoding human alpha-L-iduronidase (hIDUA) at two DNA doses, with and without an SB transposase gene, to NOD.129(B6)-Prkdc(scid) IDUA(tm1Clk)/J mice. In transposon-treated, nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice with mucopolysaccharidosis type I (MPS I), plasma IDUA persisted for 18 weeks at levels up to several hundred-fold wild-type (WT) activity, depending on DNA dose and gender. IDUA activity was present in all examined somatic organs, as well as in the brain, and correlated with both glycosaminoglycan (GAG) reduction in these organs and level of expression in the liver, the target of transposon delivery. IDUA activity was higher in the treated males than in females. In females, omission of transposase source resulted in significantly lower IDUA levels and incomplete GAG reduction in some organs, confirming the positive effect of transposition on long-term IDUA expression and correction of the disease. The SB transposon system proved efficacious in correcting several clinical manifestations of MPS I in mice, including thickening of the zygomatic arch, hepatomegaly, and accumulation of foamy macrophages in bone marrow and synovium, implying potential effectiveness of this approach in treatment of human MPS I.

Structure-based prediction of insertion-site preferences of transposons into chromosomes

Mobile genetic elements with the ability to integrate genetic information into chromosomes can cause disease over short periods of time and shape genomes over eons. These elements can be used for functional genomics, gene transfer and human gene therapy. However, their integration-site preferences, which are critically important for these uses, are poorly understood. We analyzed the insertion sites of several transposons and retroviruses to detect patterns of integration that might be useful for prediction of preferred integration sites. Initially we found that a mathematical description of DNA-deformability, called Vstep, could be used to distinguish preferential integration sites for Sleeping Beauty (SB) transposons into a particular 100 bp region of a plasmid [G. Liu, A. M. Geurts, K. Yae, A. R. Srinivassan, S. C. Fahrenkrug, D. A. Largaespada,J. Takeda, K. Horie, W. K. Olson and P. B. Hackett (2005) J. Mol. Biol., 346, 161–173 ]. Based on these findings, we extended our examination of integration of SB transposons into whole plasmids and chromosomal DNA. To accommodate sequences up to 3 Mb for these analyses, we developed an automated method, ProTIS©, that can generate profiles of predicted integration events. However, a similar approach did not reveal any structural pattern of DNA that could be used to predict favored integration sites for other transposons as well as retroviruses and lentiviruses due to a limitation of available data sets. Nonetheless, ProTIS© has the utility for predicting likely SB transposon integration sites in investigator-selected regions of genomes and our general strategy may be useful for other mobile elements once a sufficiently high density of sites in a single region are obtained. ProTIS analysis can be useful for functional genomic, gene transfer and human gene therapy applications using the SB system.

Identification of Rtl1 , a Retrotransposon-Derived Imprinted Gene, as a Novel Driver of Hepatocarcinogenesis

We previously utilized a Sleeping Beauty (SB) transposon mutagenesis screen to discover novel drivers of HCC. This approach identified recurrent mutations within the Dlk1-Dio3 imprinted domain, indicating that alteration of one or more elements within the domain provides a selective advantage to cells during the process of hepatocarcinogenesis. For the current study, we performed transcriptome and small RNA sequencing to profile gene expression in SB–induced HCCs in an attempt to clarify the genetic element(s) contributing to tumorigenesis. We identified strong induction of Retrotransposon-like 1 (Rtl1) expression as the only consistent alteration detected in all SB–induced tumors with Dlk1-Dio3 integrations, suggesting that Rtl1 activation serves as a driver of HCC. While previous studies have identified correlations between disrupted expression of multiple Dlk1-Dio3 domain members and HCC, we show here that direct modulation of a single domain member, Rtl1, can promote hepatocarcinogenesis in vivo. Overexpression of Rtl1 in the livers of adult mice using a hydrodynamic gene delivery technique resulted in highly penetrant (86%) tumor formation. Additionally, we detected overexpression of RTL1 in 30% of analyzed human HCC samples, indicating the potential relevance of this locus as a therapeutic target for patients. The Rtl1 locus is evolutionarily derived from the domestication of a retrotransposon. In addition to identifying Rtl1 as a novel driver of HCC, our study represents one of the first direct in vivo demonstrations of a role for such a co-opted genetic element in promoting carcinogenesis.

Sex bias occurrence of hepatocellular carcinoma in Poly7 molecular subclass is associated with EGFR

Hepatocellular carcinoma (HCC) is one of the deadliest solid cancers and is the third leading cause of cancer‐related death. There is a universal estimated male/female ratio of 2.5, but the reason for this is not well understood. The Sleeping Beauty (SB) transposon system was used to elucidate candidate oncogenic drivers of HCC in a forward genetics screening approach. Sex bias occurrence was conserved in our model, with male experimental mice developing liver tumors at reduced latency and higher tumor penetrance. In parallel, we explored sex differences regarding genomic aberrations in 235 HCC patients. Liver cancer candidate genes were identified from both sexes and genotypes. Interestingly, transposon insertions in the epidermal growth factor receptor (Egfr) gene were common in SB‐induced liver tumors from male mice (10/10, 100%) but infrequent in female mice (2/9, 22%). Human single‐nucleotide polymorphism data confirmed that polysomy of chromosome 7, locus of EGFR, was more frequent in males (26/62, 41%) than females (2/27, 7%) (P = 0.001). Gene expression–based Poly7 subclass patients were predominantly male (9/9) compared with 67% males (55/82) in other HCC subclasses (P = 0.02), and this subclass was accompanied by EGFR overexpression (P < 0.001). Conclusion: Sex bias occurrence of HCC associated with EGFR was confirmed in experimental animals using the SB transposon system in a reverse genetic approach. This study provides evidence for the role of EGFR in sex bias occurrences of liver cancer and as the driver mutational gene in the Poly7 molecular subclass of human HCC. (HEPATOLOGY 2013)

Modeling hepatitis B virus X-induced hepatocellular carcinoma in mice with the Sleeping Beauty transposon system.

The mechanisms associated with hepatitis B virus (HBV)–induced hepatocellular carcinoma (HCC) remain elusive, and there are currently no well‐established animal models for studying this disease. Using the Sleeping Beauty transposon as a delivery system, we introduced an oncogenic component of HBV, the hepatitis B virus X (HBx) gene, into the livers of fumarylacetoacetate hydrolase (Fah) mutant mice via hydrodynamic tail vein injections. Coexpression of Fah complementary DNA from the transposon vector allowed for the selective repopulation of genetically corrected hepatocytes in Fah mutant mice. The process of hydrodynamic delivery induced liver inflammation, and the subsequent selective repopulation of hepatocytes carrying the transgene(s) could provide useful genetic information about the mechanisms of HBV‐induced hyperplasia. Short hairpin RNA directed against transformation‐related protein 53 (shp53) or other tumor suppressor genes and oncogenes [e.g., constitutively active neuroblastoma RAS viral (v‐ras) oncogene homolog with Gly12Val substitution (NRASG12V)] could also be codelivered with HBx by this system so that we could determine whether oncogenic cooperation existed. We found that the expression of HBx induced the activation of β‐catenin expression in hydrodynamically injected livers, and this indicated its association with the Wnt signaling pathway in HBV‐induced hyperplasia. HBx coinjected with shp53 accelerated the formation of liver hyperplasia in these mice. As expected, constitutively active NRASG12V alone was sufficient to induce liver hyperplasia, and its tumorigenicity was augmented when it was coinjected with shp53. Interestingly, HBx did not seem to cooperate with constitutively active NRASG12V in driving liver tumorigenesis. Conclusion: This system can be used as a model for studying the various genetic contributions of HBV to liver hyperplasia and finally HCC in an in vivo system. (HEPATOLOGY 2010;.)

Gene Expression in Lung and Liver After Intravenous Infusion of Polyethylenimine Complexes of Sleeping Beauty Transposons

Two methods of systemic gene delivery have been extensively explored, using the mouse as a model system: hydrodynamic delivery, wherein a DNA solution equivalent in volume to 10% of the mouse weight is injected intravenously in less than 10 sec, and condensation of DNA with polyethylenimine (PEI) for standard intravenous infusion. Our goal in this study was to evaluate quantitatively the kinetics of gene expression, using these two methods for delivery of Sleeping Beauty transposons. Transposons carrying a luciferase expression cassette were injected into mice either hydrodynamically or after condensation with PEI at a PEI nitrogen-to-DNA phosphate ratio of 7. Gene expression in the lungs and liver after hydrodynamic delivery resulted in exponential decay with a half-life of about 35-40 hr between days 1 and 14 postinjection. The decay kinetics of gene expression after PEI-mediated gene delivery were more complex; an initial decay rate of 6 hr was followed by a more gradual loss of activity. Consequently, the liver became the primary site of gene expression about 4 days after injection of PEI-DNA, and by 14 days expression in the liver was 10-fold higher than in the lung. Overall levels of gene expression 2 weeks postinjection were 100- to 1000-fold lower after PEI-mediated delivery compared with hydrodynamic injection. These results provide insight into the relative effectiveness and organ specificity of these two methods of nonviral gene delivery when coupled with the Sleeping Beauty transposon system.