Efficient protein expression from the endogenous RNA polymerase I promoter using a human ribosomal DNA targeting vector

The nucleolus as a genomic safe harbor for strong gene expression in Nannochloropsis oceanica

Microalgae are used in food and feed, and they are considered a potential feedstock for sustainably produced chemicals and biofuel. However, production of microalgal-derived chemicals is not yet economically feasible. Genetic engineering could bridge the gap to industrial application and facilitate the production of novel products from microalgae. Here, we report the discovery of a novel gene expression system in the oleaginous microalga Nannochloropsis that exploits the highly efficient transcriptional activity of RNA polymerase I and an internal ribosome entry site for translation. We identified the nucleolus as a genomic safe harbor for Pol I transcription and used it to construct transformant strains with consistently strong transgene expression. The new expression system provides an outstanding tool for genetic and metabolic engineering of microalgae and thus will probably make substantial contributions to microalgal research.

PiggyBac transposase and transposon derivatives for gene transfer targeting the ribosomal DNA loci of CHO cells

Integrative non-viral vectors such as transposons engineered to mediate targeted gene transfer into safe harbor sites in the genome may be a promising approach for the production of therapeutic proteins or for gene therapy in an efficient and secure way. In this context, we designed and evaluated two strategies for targeting the nuclear ribosomal DNA (rDNA) loci. One approach relied on the co-location of the transposase and transposon near transcriptionally active rDNA copies using a nucleolar localization signal (NoLS). Another one consisted of targeting the 18S-coding region in the rDNA loci using a NoLS-FokI-dCas9 endonuclease to perform targeted transgene knock-in. We show that integration into the rDNA of Chinese hamster ovary (CHO) cells can be achieved at a high frequency using the piggyBac transposon system, indicating that the rDNA is highly accessible for transposition. Consistently, rDNA-targeted transposition events were most frequently obtained when both the piggyBac transposon DNA and the transposase were nucleoli-targeted, yielding cells displaying stable and homogeneous expression of the transgene. This approach thus provides an alternative strategy to improve targeted transgene delivery and protein expression using CHO cells.

A non-viral vector for potential DMD gene therapy study by targeting a minidystrophin-GFP fusion gene into the hrDNA locus

Gene therapy has emerged as a promising approach for the lethal disorder of Duchenne muscular dystrophy (DMD). Using a novel non-viral delivery system, the human ribosomal DNA (hrDNA) targeting vector, we targeted a minidystrophin-GFP fusion gene into the hrDNA locus of HT1080 cells with a high site-specific integrated efficiency of 10(-5), in which the transgene could express efficiently and continuously. The minidystrophin-GFP fusion protein was easily found to localize on the plasma membrane of HT1080 cells, indicating its possible physiologic performance. Our findings showed that the hrDNA-targeting vector might be highly useful for DMD gene therapy study.