Establishment of a pig fibroblast-derived cell line for locus-directed transgene expression in cell cultures and blastocysts

Generation of minipigs with targeted transgene insertion by recombinase-mediated cassette exchange (RMCE) and somatic cell nuclear transfer (SCNT)

Targeted transgenesis using site-specific recombinases is an attractive method to create genetically modified animals as it allows for integration of the transgene in a pre-selected transcriptionally active genomic site. Here we describe the application of recombinase-mediated cassette exchange (RMCE) in cells from a Göttingen minipig with four RMCE acceptor loci, each containing a green fluorescence protein (GFP) marker gene driven by a human UbiC promoter. The four RMCE acceptor loci segregated independent of each other, and expression profiles could be determined in various tissues. Using minicircles in RMCE in fibroblasts with all four acceptor loci and followed by SCNT, we produced piglets with a single copy of a transgene incorporated into one of the transcriptionally active acceptor loci. The transgene, consisting of a cDNA of the Alzheimer’s disease-causing gene PSEN1M146I driven by an enhanced human UbiC promoter, had an expression profile in various tissues similar to that of the GFP marker gene. The results show that RMCE can be done in a pre-selected transcriptionally active acceptor locus for targeted transgenesis in pigs.

Genetically modified pigs for biomedical research

During the last two decades, pigs have been used to develop some of the most important large animal models for biomedical research. Advances in pig genome research, genetic modification (GM) of primary pig cells and pig cloning by nuclear transfer, have facilitated the generation of GM pigs for xenotransplantation and various human diseases. This review summarizes the key technologies used for generating GM pigs, including pronuclear microinjection, sperm-mediated gene transfer, somatic cell nuclear transfer by traditional cloning, and somatic cell nuclear transfer by handmade cloning. Broadly used genetic engineering tools for porcine cells are also discussed. We also summarize the GM pig models that have been generated for xenotransplantation and human disease processes, including neurodegenerative diseases, cardiovascular diseases, eye diseases, bone diseases, cancers and epidermal skin diseases, diabetes mellitus, cystic fibrosis, and inherited metabolic diseases. Thus, this review provides an overview of the progress in GM pig research over the last two decades and perspectives for future development.

FMR1 CGG repeat lengths mediate different regulation of reporter gene expression in comparative transient and locus specific integration assays

The Fragile X mental retardation (FMR1) gene contains a polymorphic CGG trinucleotide repeat in the 5'-untranslated region. The repeat length in the normal population is between 5 and 54 repeats. A repeat length between 55 and 200 is defined as the pre-mutation repeat size. Elderly carriers of the pre-mutation can develop the progressive neurodegenerative disease Fragile X-associated tremor/ataxia syndrome (FXTAS). In FXTAS the FMR1 mRNA levels are increased and it is hypothesized that FXTAS is caused by a RNA gain of function mechanism. Repeat lengths beyond 200 CGGs are defined as the full-mutation and causes Fragile X-syndrome which is the most common inherited form of mental retardation. The full-mutation results in the absence of the FMR1 mRNA and protein, FMRP, through abnormal CpG methylation and FMR1 gene silencing. In this report we have used the Flp-In T-REx system to generate locus directed stable cell lines harboring the FMR1 5'-UTR with varying CGG repeat lengths in front of a reporter gene. By this system the influence of various CGG repeat lengths for reporter gene expression can be comparatively examined in cell lines where the only genetic difference is CGG repeat lengths. In such cell lines we find that a full-mutation CGG repeat confers inhibition of reporter gene expression, whereas a pre-mutation CGG repeat did not increase reporter gene expression. In transient transfection assays using the same expression vectors the pre-mutation and full-mutation CGG repeats increased reporter gene expression. This study shows that locus directed integration of model FMR1 CGG transgenes could be a new basic tool to further elucidating the basic molecular mechanisms behind transcriptional deregulation of the FMR1 gene in fragile X-syndrome and FXTAS.

Strategies for selection marker-free swine transgenesis using the Sleeping Beauty transposon system

Swine transgenesis by pronuclear injection or cloning has traditionally relied on illegitimate recombination of DNA into the pig genome. This often results in animals containing concatemeric arrays of transgenes that complicate characterization and can impair long-term transgene stability and expression. This is inconsistent with regulatory guidance for transgenic livestock, which also discourages the use of selection markers, particularly antibiotic resistance genes. We demonstrate that the Sleeping Beauty (SB) transposon system effectively delivers monomeric, multi-copy transgenes to the pig embryo genome by pronuclear injection without markers, as well as to donor cells for founder generation by cloning. Here we show that our method of transposon-mediated transgenesis yielded 38 cloned founder pigs that altogether harbored 100 integrants for five distinct transposons encoding either human APOBEC3G or YFP-Cre. Two strategies were employed to facilitate elimination of antibiotic genes from transgenic pigs, one based on Cre-recombinase and the other by segregation of independently transposed transgenes upon breeding.