Integration of multicopy extrachromosomal transgenes into defined loci without phenotypes

Streamlined single shot safe harbor transgene integration in C. elegans using unc-119 rescue

Transgenic animals are an invaluable tool in model organism genetics. The ease of modifying the C. elegans genome through high-copy integration of transgenes facilitates the investigation of diverse and fundamental biological processes. However, generation of new multicopy integrated transgenes is limited by the time and labor cost. Further, many transgenes are integrated using non-specific DNA damaging agents. These DNA damaging agents cause unwanted mutations during the integration process and may have deleterious effects. A recently described method for CRISPR/Cas9-based integration of multicopy transgenes at safe harbor loci using Fluorescent Landmark Interference (FLInt) greatly increases the efficiency of multicopy transgene integration and mitigates issues related to off-target mutagenesis during integration. unc-119 rescue is a simple and widely used phenotypic marker in C. elegans transgenesis and genome engineering. To streamline generation of multicopy transgenes via FLInt, we have generated a set of strains suitable for FLInt-mediated integration of transgenes using rescue of the unc-119 mutant phenotype to select transgenic animals. We demonstrate the utility of this approach and outline a protocol that uses unc-119 rescue as a selection marker for streamlined integration of multicopy transgenes at safe harbor loci.

Modular safe-harbor transgene insertion for targeted single-copy and extrachromosomal array integration in Caenorhabditis elegans

Efficient and reproducible transgenesis facilitates and accelerates research using genetic model organisms. Here, we describe a modular safe-harbor transgene insertion (MosTI) for use in Caenorhabditis elegans which improves targeted insertion of single-copy transgenes by homology directed repair and targeted integration of extrachromosomal arrays by nonhomologous end-joining. MosTI allows easy conversion between selection markers at insertion site and a collection of universal targeting vectors with commonly used promoters and fluorophores. Insertions are targeted at three permissive safe-harbor intergenic locations and transgenes are reproducibly expressed in somatic and germ cells. Chromosomal integration is mediated by CRISPR/Cas9, and positive selection is based on a set of split markers (unc-119, hygroR, and gfp) where only animals with chromosomal insertions are rescued, resistant to antibiotics, or fluorescent, respectively. Single-copy insertion is efficient using either constitutive or heat-shock inducible Cas9 expression (25-75%) and insertions can be generated from a multiplexed injection mix. Extrachromosomal array integration is also efficient (7-44%) at modular safe-harbor transgene insertion landing sites or at the endogenous unc-119 locus. We use short-read sequencing to estimate the plasmid copy numbers for 8 integrated arrays (6-37 copies) and long-read Nanopore sequencing to determine the structure and size (5.4 Mb) of 1 array. Using universal targeting vectors, standardized insertion strains, and optimized protocols, it is possible to construct complex transgenic strains which should facilitate the study of increasingly complex biological problems in C. elegans.